Search

Mutations shape genetic architecture and thus influence the evolvability, adaptation and diversification of populations. Mutations may have different and even opposite effects on separate fitness components, and their rate of origin, distribution of effects and variance-covariance structure may depend on environmental quality. We performed an approximately 1,500-generation mutation-accumulation (MA) study in diploids of the yeast Saccharomyces cerevisiae in stressful (high-salt) and normal environments (50 lines each) to investigate the rate of input of mutational variation (Vm) as well as the mutation rate and distribution of effects on diploid and haploid fitness components, assayed in the normal environment. All four fitness components in both MA treatments exhibited statistically significant mutational variance and mutational heritability. Compared to normal-MA, salt stress increased the mutational variance in growth rate by more than sevenfold in haploids derived from the MA lines. This increase was not detected in diploid growth rate, suggesting masking of mutations in the heterozygous state. The genetic architecture arising from mutation (M-matrix) differed between normal and salt conditions. Salt stress also increased environmental variance in three fitness components, consistent with a reduction in canalization. Maximum-likelihood analysis indicated that stress increased the genomic mutation rate by approximately twofold for maximal growth rate and sporulation rate in diploids and for viability in haploids, and by tenfold for maximal growth rate in haploids, but large confidence intervals precluded distinguishing these values between MA environments. We discuss correlations between fitness components in diploids and haploids and compare the correlations between the two MA environmental treatments.

Mutations shape genetic architecture and thus influence the evolvability, adaptation and diversification of populations. Mutations may have different and even opposite effects on separate fitness components, and their rate of origin, distribution of effects and variance-covariance structure may depend on environmental quality. We performed an approximately 1,500-generation mutation-accumulation (MA) study in diploids of the yeast Saccharomyces cerevisiae in stressful (high-salt) and normal environments (50 lines each) to investigate the rate of input of mutational variation (Vm) as well as the mutation rate and distribution of effects on diploid and haploid fitness components, assayed in the normal environment. All four fitness components in both MA treatments exhibited statistically significant mutational variance and mutational heritability. Compared to normal-MA, salt stress increased the mutational variance in growth rate by more than sevenfold in haploids derived from the MA lines. This increase was not detected in diploid growth rate, suggesting masking of mutations in the heterozygous state. The genetic architecture arising from mutation (M-matrix) differed between normal and salt conditions. Salt stress also increased environmental variance in three fitness components, consistent with a reduction in canalization. Maximum-likelihood analysis indicated that stress increased the genomic mutation rate by approximately twofold for maximal growth rate and sporulation rate in diploids and for viability in haploids, and by tenfold for maximal growth rate in haploids, but large confidence intervals precluded distinguishing these values between MA environments. We discuss correlations between fitness components in diploids and haploids and compare the correlations between the two MA environmental treatments.

PREMISE OF THE STUDY Plant phenological responses to climate change now constitute one of the best studied areas of the ecological impacts of climate change. Flowering time responses to climate change of wind-pollinated species have, however, been less well studied. A novel source of flowering time data for wind-pollinated species is allergen monitoring records. METHODS We studied the male flowering time response to climatic variables of two wind-pollinated genera, Betula (Betulaceae) and Populus (Salicaceae), using pollen count records over a 17-year period. KEY RESULTS We found that changes in the pollen release envelope differed between the two genera. Over the study period, the only month with a significant rise in temperature was April, resulting in the duration of pollen release of the April-flowering Populus to shorten and the start and peak of the May-flowering Betula to advance. The quantity of pollen released by Betula has increased and was related to increases in the previous year's August precipitation, while the quantity of pollen released by Populus has not changed and was related to the previous year's summer and autumn temperatures. CONCLUSIONS Our findings suggest that taxa differ in the reproductive consequences of environmental change. Differing shifts in phenology among species may be related to different rates of change in climatic variables in different months of the year. While our study only considers two genera, the results underscore the importance of understanding non-uniform intra-annual variation in climate when studying the ecological implications of climate change.

Much recent attention has focused on the apparent benefit accrued by a plant through its partial destruction by grazing (Owen 1980; Belsky 1986; McNaughton 1986; Crawley 1987; Paige and Whitham 1987; Vail 1992; Belsky et al. 1993; Mathews 1994; Tuomi et al. 1994; Jaremo et al. 1996; Nilsson et al. 1996; Lennartsson et al. 1997). The subject has been characterized by heated debate on two related issues: the empirical question of whether fitness is, in fact, increased by the partial destruction of plant tissue (overcompensation; Paige and Whitham 1987; Bergelson and Crawley 1992; Crawley 1993; Bergelson et al. 1996) and, if overcompensation exists, whether claims that grazing is thus beneficial to a plant are justified (Crawley 1987; Belsky et al. 1993; Vail 1993, 1994; Mathews 1994). Empirical studies have concentrated on testing a plant’s capacity for compensation by comparing the performance of grazed plants to ungrazed controls (Paige and Whitham 1987; Maschinski and Whitham 1989; Bergelson and Crawley 1992; Paige 1992; Wegener and Odasz 1997) and the claim that plants benefit from herbivore damage has been made (Owen 1980; Paige and Whitham 1987; Paige 1992). As a result of differing interpretations of what constitutes a “benefit” (Crawley 1987), this claim is viewed by some as a paradox. A benefit of being partially destroyed might be observed in a population for two different reasons. First, grazing of aboveground tissue might alter the plant’s architecture such that fitness is increased by the removal of constraints. If partial destruction is truly beneficial to the plant through the removal of such obstacles as architectural constraints, the plant-herbivore association might be regarded as mutualistic (mutualism hypothesis).

Stronger pollen limitation should increase competition among plants, leading to stronger selection on traits important for pollen receipt. The few explicit tests of this hypothesis, however, have provided conflicting support. Using the arithmetic relationship between these two quantities, we show that increased pollen limitation will automatically result in stronger selection (all else equal) although other factors can alter selection independently of pollen limitation. We then tested the hypothesis using two approaches. First, we analysed the published studies containing information on both pollen limitation and selection. Second, we explored how natural selection measured in one Ontario population of Lobelia cardinalis over 3 years and two Michigan populations in 1 year relates to pollen limitation. For the Ontario population, we also explored whether pollinator-mediated selection is related to pollen limitation. Consistent with the hypothesis, we found an overall positive relationship between selection strength and pollen limitation both among species and within L. cardinalis. Unexpectedly, this relationship was found even for vegetative traits among species, and was not found in L. cardinalis for pollinator-mediated selection on nearly all trait types.

Premise of the Study Herbarium specimens are increasingly used in phenological studies. However, natural history collections can have biases that influence the analysis of phenological events. Arctic environments, where remoteness and cold climate govern collection logistics, may give rise to unique or pronounced biases. Methods We assessed the presence of biases in time, space, phenological events, collectors, taxonomy, and plant traits across Nunavut using herbarium specimens accessioned at the National Herbarium of Canada (CAN). Results We found periods of high and low collection that corresponded to societal and institutional events; greater collection density close to common points of air and sea access; and preferences to collect plants at the flowering phase and in peak flower, and to collect particular taxa, flower colours, growth forms, and plant heights. One‐quarter of collectors contributed 90% of the collection. Discussion Collections influenced by temporal and spatial biases have the potential to misrepresent phenology across space and time, whereas those shaped by the interests of collectors or the tendency to favour particular phenological stages, taxa, and plant traits could give rise to imbalanced phenological comparisons. Underlying collection patterns may vary among regions and institutions. To guide phenological analyses, we recommend routine assessment of any herbarium data set prior to its use.

Latitudinal gradients in biotic interactions have been suggested as causes of global patterns of biodiversity and phenotypic variation. Plant biologists have long speculated that outcrossing mating systems are more common at low than high latitudes owing to a greater predictability of plant–pollinator interactions in the tropics; however, these ideas have not previously been tested. Here, we present the first global biogeographic analysis of plant mating systems based on 624 published studies from 492 taxa. We found a weak decline in outcrossing rate towards higher latitudes and among some biomes, but no biogeographic patterns in the frequency of self-incompatibility. Incorporating life history and growth form into biogeographic analyses reduced or eliminated the importance of latitude and biome in predicting outcrossing or self-incompatibility. Our results suggest that biogeographic patterns in mating system are more likely a reflection of the frequency of life forms across latitudes rather than the strength of plant–pollinator interactions.

The transfer of nitrogen (N) from legumes to grasses is an important process in low-input forage production systems, and may be improved by selecting compatible species and cultivars. This study so...

Measures of selection on floral traits in flowering plants are often motivated by the assumption that pollinators cause selection. Flowering plants experience selection from other sources, including herbivores, which may enhance or oppose selection by pollinators. Surprisingly, few studies have examined selection from multiple sources on the same traits. We quantified pollinator-mediated selection on six floral traits of Lobelia cardinalis by comparing selection in naturally and supplementally (hand-) pollinated plants. Directional, quadratic and correlational selection gradients as well as total directional and quadratic selection differentials were examined. We used path analysis to examine how three herbivores--slugs, weevils and caterpillars--affected the relationship between floral traits and fitness. We detected stronger total selection on four traits and correlational selection (γ(ij)) on three trait combinations in the natural pollination treatment, indicating that pollinators caused selection on these traits. Weak but statistically significant selection was caused by weevil larvae on stem diameter and anther-nectary distance, and by slugs on median-flower date. In this study, pollinators imposed stronger selection than herbivores on floral traits in L. cardinalis. In general, the degree of pollen limitation and rate of herbivory are expected to influence the relative strength of selection caused by pollinators or herbivores.

We studied the development of 26 flower traits under natural conditions in three clades of the genus Amsinckia (Boraginaceae). Each clade contained both a derived highly self-fertilizing taxon and an ancestral more highly outcrossing taxon. The more outcrossing taxa contained two flower morphs—pins and thrums—with opposite positioning of the sex organs (heterostyly). The highly selfing taxa had smaller flowers with sex organs in close proximity (homostyly). Growth trajectories were quantified over the entire or nearly the entire period from primordium initiation to flower opening. These trajectories were compared in the heterochronic framework and, in contrast with previous studies, character size was tracked over time rather than relative to another character. We focused on three hypotheses: (1) The distinct developmental trajectories leading to pins and thrums should be similar in all clades, while the trajectories leading to homostylous flowers might differ among clades. This was supported. Specifically, contrasting growth rates of stamen and pistil heights in heterostylous flowers caused pin and thrum flowers to have the reciprocal arrangement of anther and stigma heights. From the viewpoint of heterochrony, the decreased size (paedomorphosis) of the homostylous morph, compared to pins and thrums, resulted from decreased growth rate (neoteny) and earlier offset (progenesis) in all clades. Nevertheless, multiple heterochronic processes were involved in the mosaic development and evolution of homostylous flowers. (2) We tested the hypothesis that small, self-fertilizing flowers have reduced development times, one of the proposed selective advantages of increased self-fertilization rates. We found in contrast that developmental duration of homostylous flowers was either the same (two clades) or longer (one clade) compared to duration of pins and thrums. (3) Finally, we tested von Baer’s Law, which proposes that developmental differences among closely related taxa should arise later in development than differences among more distantly related taxa. Von Baer’s Law was supported strongly among homostyles, moderately among thrums and weakly among pins.

Self-fertilization is expected to reduce genetic diversity within populations and consequently to limit adaptability to changing environments. Little is known, however, about the way the evolution of self-fertilization changes the amount or pattern of the components of genetic variation in natural populations. In this study, a reciprocal North Carolina II design and maximum-likelihood methods were implemented to investigate the genetic basis of variation for 15 floral and vegetative traits in four populations of the annual plant Amsinckia spectabilis (Boraginaceae) differing in mating system. Six variance components were estimated according to Cockerham and Weir's "bio" model c. Compared to the three partially selfing populations, we found significantly lower levels of nuclear variance for several traits in the nearly completely self-fertilizing population. Furthermore, for 11 of 15 traits we did not detect nuclear variation to be significantly greater than zero. We also found high maternal variance in one of the partially selfing populations for several traits, and little dominance variance in any population. These results are in agreement with the evolutionary dead-end hypothesis for highly self-fertilizing taxa.

The survival of populations of rare plant species may be affected both by total levels of seed production and by inbreeding depression expressed upon self-fertilization. We examined natural seed production and rates of self-fertilization in a Nova Scotian lakeshore population of the globally rare Atlantic Coastal Plain plant, Sabatia kennedyana Fernald (Gentianaceae), and determined the effect of self and cross pollinations on fitness at early plant life stages. Seed production was not limited by natural pollination levels. Furthermore, flowers from which pollinators were excluded produced 66% as many seeds as unprotected, naturally pollinated plants, indicating that substantial seed set can occur in the absence of pollinators. The selfing rate of naturally pollinated flowers was estimated as 21% using the inbreeding depression method. Stage-specific inbreeding depression was 19% at germination, and cumulative inbreeding depression from fertilization through germination was 23%. Seed formation (s...

▪ Abstract Quantifying the extent to which seed production is limited by the availability of pollen has been an area of intensive empirical study over the past few decades. Whereas theory predicts that pollen augmentation should not increase seed production, numerous empirical studies report significant and strong pollen limitation. Here, we use a variety of approaches to examine the correlates of pollen limitation in an effort to understand its occurrence and importance in plant evolutionary ecology. In particular, we examine the role of recent ecological perturbations in influencing pollen limitation and discuss the relation between pollen limitation and plant traits. We find that the magnitude of pollen limitation observed in natural populations depends on both historical constraints and contemporary ecological factors.

Age and size at reproduction are important components of fitness, and are variable both within and among angiosperm species. The fitness consequences of such life-history variation are most readily studied in organisms that reproduce only once in their lifetime. The timing of the onset of reproduction (bolting) in the monocarpic perennial, Lobelia inflata, occurs over a range of dates within a season, and may be postponed to a later year. Empirical relationships among life-history traits, derived from over 950 wild-growing and experimentally manipulated plants in the field, are used to model an optimal changing size threshold (norm of reaction) for bolting over the growing season. Comparisons are made between observed and expected norms of reaction governing bolting. An apparently suboptimal bolting schedule that precludes bolting beyond an early (conservative) date is observed, and is found to be qualitatively consistent with conservative bet hedging under unpredictable season lengths. On this basis we propose the schedule of bolting as a plausible example of a conservative bet-hedging strategy. The results underscore the critical need for long-term studies of fluctuating selection to distinguish suboptimality from bet hedging.

This issue of GENETICS features an article that signals the Editorial Board’s intent for the journal to increase its presence in the human genetics arena. In its 98-year history GENETICS has featured many articles in which the subject species was Homo sapiens , but until recently those were

Reproductive success in flowering plants is influenced by the morphology and timing of reproductive structures as well as the density of surrounding conspecifics. In species with separate male and female flower phases, successful pollen transfer is also expected to vary with the density and ratio of surrounding male and female flowers. Increased density of surrounding flowers may increase pollinator visitation rates, but the densities of male and female flowers will determine the availability of pollen and the strength of competition for pollen receipt. Here we (1) quantify the influence of surrounding plant density on total seasonal fruit and seed production, (2) quantify the influence of sexual neighborhood (surrounding sex ratio and densities of male- and female-phase flowers) on fruit and seed production for individual flowers presented within the season, and (3) compare the influence of plant density on fitness to that of focal plant phenotype, specifically stigma-nectary distance and plant height, in a natural population of the pollen-limited, hummingbird-pollinated hermaphrodite Lobelia cardinalis. These relationships were examined at four spatial scales (10, 20, 50, and 100 cm). By examining temporal and spatial scales we found that (1) total seed production per plant decreased with increasing plant density at the smallest scale but increased with increasing density at all larger scales; (2) at any given time, a female-phase flower benefited from a higher density of surrounding male-phase flowers and a lower density of surrounding female-phase flowers; (3) when sex ratio was explicitly analyzed, a female-phase flower benefited from a lower proportion of surrounding female flowers as well as a lower total flower density; and (4) at the whole-plant level, taller plants were more likely to produce fruit (even when accounting for total number of flowers produced), consistent with pollinator preference for taller floral displays. Our results suggest that the local density of male and female flowers (and surrounding sex ratio) influences successful pollen transfer, implying that the local floral environment may shape how attraction traits like plant height are related to fitness.

The evolution of plant morphology is the result of changes in developmental processes. Heterochrony, the evolutionary change in developmental rate or timing, is a major cause of ontogenetic modification during evolution. It is responsible for both interspecific and intraspecific morphological differences. Other causes include heterotopy, the change of structural position, and homeosis, the replacement of a structure by another. This paper discusses and reviews the role of heterochrony in plant evolution at the organismal, organ, tissue, cellular, and molecular levels, as well as the relationships among heterochrony, heterotopy, and homeosis. An attempt has been made to include all published studies through late 1999. It is likely that most heterochronic change involves more than one of the six classic pure heterochronic processes. Of these processes, we found neoteny (decreased developmental rate in descendant), progenesis (earlier offset), and acceleration (increased rate) to be more commonly reported than hypermorphosis (delayed offset) or predisplacement (earlier onset). We found no reports of postdisplacement (delayed onset). Therefore, although rate changes are common (both neoteny and acceleration), shifts in timing most commonly involve earlier termination in the descendant (progenesis). These relative frequencies may change as more kinds of structures are analyzed. Phenotypic effects of evolutionary changes in onset or offset timing can be exaggerated, suppressed, or reversed by changes in rate. Because not all developmental changes responsible for evolution result from heterochrony, however, we propose that plant evolution be studied from a viewpoint that integrates these different developmental mechanisms.

Seed germination constitutes an important event in the life cycle of plants. Two related seed traits affect fitness: seed size and the timing of seed germination. In three sets of experiments, we (1) partition the sources of seed-size variance in Lobelia inflata into components attributable to fruit size, relative fruit position, and parental identity; (2) examine the influence of pregermination conditions and seed size on time to germination; and (3) assess the fitness consequences of seed size and germination timing under seminatural, harsh conditions. Seed-size variance is attributable to both parental identity and fruit position within an individual. Distal fruits produce larger but fewer seeds. No significant correlation exists between fruit size and seed size, but a trade-off is found between the number and size of seeds contained in a fruit after correcting for fruit size. The timing of germination is influenced by seed size, light conditions before winter, and winter duration. Germination timing influences survival, and despite small seed size in this species (2 × 10 g/seed), seed size has a persistent and significant association with both final plant size and the probability of survival to autumn.

We carried out phenotypic selection analyses to assess the relative importance of several floral traits, as well as microhabitat, in determining both male and female pollination success in a nonrewarding orchid (Cypripedium acaule Ait.) in two Nova Scotia populations. Fruit production in these one-flowered, cosexual plants was strongly pollen limited within a season: 100% of individuals set fruit following hand-pollination, but only 5% and 13% did so under natural circumstances in the two populations. Male and female reproductive success were highly correlated, with >90% of plants that received a pollinium also dispersing one. Despite very low visitation rates, the chance of a removed pollinium fertilizing another ovary was 36% and 51%, respectively, in the two populations. There was no evidence for selection acting differently, either through the two sexual functions or in the two populations. In contrast, the strength of directional selection on several traits differed among microhabitats within each population. Univariate and multivariate regressions indicated statistically significant directional selection acting on flower-opening date through male, female, and total (combined) pollination success in one population. For female, male, and total success in this population, we also detected correlational selection acting on opening date and flower height. There was also negative correlational selection acting on flower height and labellum length through female success. Microhabitats within the populations had a larger effect on both female and male success than did floral traits. In particular, the presence of ericaceous shrubs and an open canopy appeared to be closely linked with higher pollination rates.

The relation between inbreeding depression and rate of self-fertilization was studied in nine natural populations of the annual genus Amsinckia. The study included two clades (phylogenetic lineages) in which small-flowered, homostylous populations or species are believed to have evolved from large-flowered, heterostylous, self-compatible ones. In one lineage the small-flowered species is tetraploid with disomic inheritance. Rates of self-fertilization were 25% to 55% in the four large-flowered, heterostylous populations; 72% in a large-flowered but homostylous population; and greater than 99.5% in the four small-flowered, homostylous populations, which produce seed autonomously. When present, inbreeding depression occurred in the fertility but not the survival components of fitness. Using a cumulative fitness measure incorporating both survival and fertility (flower number), we found inbreeding depression to be lower in the four very highly self-fertilizing populations than in the five intermediate ones. The Spearman rank correlation between inbreeding depression and selfing rate for the nine populations was -0.50, but was not statistically significant (P = 0.12). Inbreeding depression was greater in the two tetraploid populations than in the very highly self-fertilizing, diploid ones. Phenotypic stability of progeny from self-fertilization tended to be higher in populations with lower inbreeding depression. We conclude that levels of self-fertilization and inbreeding depression in Amsinckia are determined more by other factors than by each other. Estimates of mutation rates and dominance coefficients of deleterious alleles, obtained from a companion study of the four highly self-fertilizing populations, suggest that a strong relationship may not be expected. We discuss the relationship of the present results to current theory of the coevolution of self-fertilization and inbreeding depression.

Theories about the evolution of sex and the effects of inbreeding depend on knowledge of the mutation rate and dominance level of deleterious alleles affecting total fitness. In two species of largely self-fertilizing annual plants, minimal estimates of such mutation rates were found to be 0.24 to 0.87 per sporophyte genome per generation, but confidence intervals exceeded 1.0 in each of the four populations. Dominance levels were near zero in one species and intermediate (0.28 to 0.35) in the other. These results suggest that the detrimental effects of inbreeding are a result of new partially recessive mutations rather than overdominance.

Theory has shown that the fitness consequences of inbreeding can play a major role in the evolution of sex and reproduction (Maynard Smith 1977; Lloyd 1979; Shields 1982; Kondrashov 1985; Lande and Schemske 1985; Holsinger 1988; Charlesworth et al. 1991; Uyenoyama and Waller 1991 a and references therein). This recognition has sparked increasing interest among experimentalists in estimating inbreeding depression, particularly in plants with their diverse array of reproductive systems. In plants, inbreeding depression is typically expressed as one minus the ratio of the phenotypic values of offspring produced through self-fertilization and random outcrossing (Lande and Schemske 1985). These phenotypic values may be total lifetime fitness, fitness components or other phenotypes. In this note, we discuss a number of statistical issues that arise in both estimation and hypothesis tests involving inbreeding depression.

The number of ovules per flower varies over several orders of magnitude among angiosperms. Here we consider evidence that stochastic uncertainty in pollen receipt and ovule fertilization has been a selective factor in the evolution of ovule number per flower. We hypothesize that stochastic variation in floral mating success creates an advantage to producing many ovules per flower because a plant will often gain more fitness from occasional abundant seed production in randomly successful flowers than it loses in resource commitment to less successful flowers. Greater statistical dispersion in pollination and fertilization among flowers increases the frequency of windfall success, which should increase the strength of selection for greater ovule number per flower. We therefore looked for evidence of a positive relationship between ovule number per flower and the statistical dispersion of pollen receipt or seed number per flower in a comparative analysis involving 187 angiosperm species. We found strong evidence of such a relationship. Our results support the hypothesis that unpredictable variation in mating success at the floral level has been a factor in the evolution of ovule packaging in angiosperms.

Inbreeding depression, or the decreased fitness of progeny derived from self-fertilization as compared to outcrossing, is thought to be the most general factor affecting the evolution of self-fertilization in plants. Nevertheless, data on inbreeding depression in fitness characters are almost nonexistent for perennials observed in their natural environments. In this study I measured inbreeding depression in both survival and fertility in two sympatric, short-lived, perennial herbs: hummingbird-pollinated Lobelia cardinalis (two populations) and bumblebee-pollinated L. siphilitica (one population). Crosses were performed by hand in the field, and seedlings germinated in the greenhouse. Levels of inbreeding depression were determined for one year in the greenhouse and for two to three years for seedlings transplanted back to the natural environment. Fertility was measured as flower number, which is highly correlated with seed production under natural conditions in these populations. Inbreeding depression was assessed in three ways: 1) survival and fertility within the different age intervals; 2) cumulative survival from the seed stage through each age interval; and 3) net fertility, or the expected fertility of a seed at different ages. Net fertility is a comprehensive measure of fitness combining survival and flower number. In all three populations, selfing had nonsignificant effects on the number and size of seeds. Lobelia siphilitica and one population of L. cardinalis exhibited significant levels of inbreeding depression between seed maturation and germination, excluding the consideration of possible differences in dormancy or longterm viability in the soil. There was no inbreeding depression in subsequent survival in the greenhouse in any population. In the field, significant survival differences between selfed and outcrossed progeny occurred only in two years and in only one population of L. cardinalis. For both survival and fertility there was little evidence for the expected differences among families in inbreeding depression. Compared to survival, inbreeding depression in fertility (flower number) tended to be much higher. By first-year flower production, the combined effects on survival and flower number caused inbreeding depression in net fertility to reach 54%, 34% and 71% for L. siphilitica and the two populations of L. cardinalis. By the end of the second year of flowering in the field, inbreeding depression in net fertility was 53% for L. siphilitica and 54% for one population of L. cardinalis. For the other population of L. cardinalis, these values were 76% through the second year of flowering and 83% through the third year. Such high levels of inbreeding depression should strongly influence selection on those characters affecting self-fertilization rates in these two species.

There is increasing evidence that human disturbance can negatively impact plant–pollinator interactions such as outcross pollination. We present a meta-analysis of 22 studies involving 27 plant species showing a significant reduction in the proportion of seeds outcrossed in response to anthropogenic habitat modifications. We discuss the evolutionary consequences of disturbance on plant mating systems, and in particular whether reproductive assurance through selfing effectively compensates for reduced outcrossing. The extent to which disturbance reduces pollinator versus mate availability could generate diverse selective forces on reproductive traits. Investigating how anthropogenic change influences plant mating will lead to new opportunities for better understanding of how mating systems evolve, as well as of the ecological and evolutionary consequences of human activities and how to mitigate them.

Environmental variation that is not predictably related to cues is expected to drive the evolution of bet-hedging strategies. The high variance observed in the timing of seed germination has led to it being the most cited diversification strategy in the theoretical bet-hedging literature. Despite this theoretical focus, virtually nothing is known about the mechanisms responsible for the generation of individual-level diversification. Here we report analyses of sources of variation in timing of germination within seasons, germination fraction over two generations and three sequential seasons, and the genetic correlation structure of these traits using almost 10,000 seeds from more than 100 genotypes of the monocarpic perennial Lobelia inflata. Microenvironmental analysis of time to germination suggests that extreme sensitivity to environmental gradients, or microplasticity, even within a homogeneous growth chamber, may act as an effective individual-level diversification mechanism and explains more than 30% of variance in time to germination. The heritability of within-season timing of germination was low (h(2) = 0.07) but significant under homogeneous conditions. Consistent with individual-level diversification, this low h(2) was attributable not to low additive genetic variance, but to an unusually high coefficient of residual variation in time to germination. Despite high power to detect additive genetic variance in within-season diversification, it was low and indistinguishable from zero. Restricted maximum likelihood detected significant genetic variation for germination fraction (h(2) = 0.18) under homogeneous conditions. Unexpectedly, this heritability was positive when measured within a generation by sibling analysis and negative when measured across generations by offspring-on-parent regression. The consistency of dormancy fraction over multiple delays, a major premise of Cohen's classic model, was supported by a strong genetic correlation (r = 0.468) observed for a cohort's germination fraction over two seasons. We discuss implications of the results for the evolution of bet hedging and highlight the need for further empirical study of the causal components of diversification.

Currently, the types of factors that impact the mutation rate is a controversial issue. The marked attention towards identifying the factors that impact the genomic mutation rate is justified because mutations are the source of genetic variation underlying evolution and because many mutations have deleterious effects and can cause diseases. Although data showing correlations between germ cell division number and mutation rates (from epidemiological studies and molecular evolutionary rate analyses) have suggested that most mutations in animals are replication errors, this notion is highly debated and inconsistencies in the correlations suggest that other, replication-independent factors, could play an important role. Likely candidates include environmental parameters and cell age, but these issues have proved to be difficult to study using animals and in vitro systems, and consequently, very few or no data currently exist. The specific features of plants that make them powerful model systems for revealing the influence of the environment (natural environmental factors) and cell age on the spontaneous genomic mutation rate are discussed here. Overall, the evidence suggests that plants could be key biological systems for advancing our knowledge about how and why heritable mutations arise.

A mutation is a heritable change in the genetic material that is not due to genetic recombination. Mutation alters the structure or number of genes or entire chromosomes. Keywords: evolution; selection; variation; human disorders; sex; recombination; self-fertilization; polyploidy; aneuploidy

MANY of us have long railed against the journal impact factor. We continue to rail with the publication of the San Francisco Declaration on Research Assessment [1][1], signed by me and others, and endorsed by the Genetics Society of America (and other august groups). We rail for good reason

Although there is substantial evidence that, in animals, male-inherited neutral DNA evolves at a higher rate than female-inherited DNA, the relative evolutionary rate of male- versus female-inherited DNA has not been investigated in plants. We compared the substitution rates at neutral sites of maternally and paternally inherited organellar DNA in gymnosperms. The analysis provided substantial support for the presence of a higher evolutionary rate in both the mitochondrial and chloroplastidial DNA when the organelle was inherited paternally than when inherited maternally. These results suggest that, compared with eggs, sperm tend to carry a greater number of mutations in mitochondrial and chloroplastidial DNA. The existence of a male mutation bias in plants is remarkable because, unlike animals, the germ-lines are not separated from the somatic cells throughout an individual's lifetime. The data therefore suggest that even a brief period of male and female germ-line separation can cause gender-specific mutation rates. These results are the first to show that, at least in some species, germ-lines influence the number of mutations carried in the gametes. Possible causes of male mutation bias in plants are discussed.

In mammals, p53 is crucial for inducing the genes that lead to G1 arrest following DNA damage, enabling DNA repair. However, the possibility that such a system exists in plants has attracted little attention. Even though some plant cDNA sequences with partial homology to p53 have been reported recently, there has been little analysis of how these molecules might relate to DNA damage. The lack of investigation into whether a DNA-damage-induced, p53-mediated G1-arrest pathway might exist in plants is remarkable given that plant DNA, like that of all organisms, is continually under the threat of attack.

The timing of reproduction is an important life-history variable, especially for organisms that die following a single reproductive episode, such as the monocarp Lobelia inflata. The propensity to initiate flowering (to bolt) under a given set of conditions is expected to be shaped by natural selection acting on the norms of reaction for bolting behaviour over, for example, changing photoperiods. We study the genetic basis of bolting and of the plasticity of bolting using three continuously changing photoperiod regimes over two generations in a growth chamber experiment. Multiple genotypes from three populations are tested under three different photoperiod treatments mimicking early, mid, and late ‘summer’ during both generations. The frequency of bolting ranges from 88% under long days to 1% under short days. The overall heritability (h2) of bolting is found to be high, and increases later in the flowering season. Genetic variance for bolting is explained by genetic variance for threshold size itself, rather than for capacity to attain a fixed threshold size: genotypes that bolt most readily tend to be those that bolt at a smaller rosette size. No significant heritability of the plasticity of bolting behaviour is detected. Similarly to within populations, variation at the among-population level exists for bolting behaviour. There is no evidence for genetic population differentiation with respect to plasticity for bolting: although plasticity differs among populations within a generation, this population effect is not consistent between the two generations of the experiment.

Sex-allocation theory predicts that the evolution of increased rates of self-fertilization should be accompanied by decreased allocation to male reproduction (sperm production and broadcast). This prediction has found support in plants but has not previously been tested in animals, which, in contrast to biotically pollinated plants, are free of complications associated with incorporating the costs of attractive structures such as petals. Here we report rates of self-fertilization as well as proportional allocation to male reproductive tissues within populations of the simultaneous hermaphrodite Utterbackia imbecillis , a freshwater mussel. Individuals from populations with higher selfing rates devoted a lower proportion of reproductive tissue to sperm production (correlation = −0.99), in support of theory.

The evolutionarily stable rate of self-fertilzation is studied in phenotypic models that incorporate pollination ecology as well as the correlated evolution of inbreeding depression and the population mean selfing rate. Inbreeding depression is assumed to be caused by continual mutation to deleterious, partially recessive alleles. Several mutation rates and dominance levels are included. Two separate ecological cases are studied: how selfing rate affects proportion of ovules fertilized (pollination assurance, seed discounting) and how selfing rate affects male outcrossing success through pollen discounting. Evolutionarily stable rates are invariably zero or intermediate in two circumstances, namely when increased selfing causes (1) a decrease in the proportion of ovules fertilized or (2) an increase in pollen discounting and, therefore, a disproportionate decrease in male outcrossing success. Complete selfing is stable when selfing increases the proportion of ovules fertilized for all selfing rates. Stable selfing is zero or one in cases where the selfing rate has no effect on the proportion of ovules fertilized or when pollen discounting does not increase with selfing. Higher inbreeding depression tends to decrease the optimal selfing rate, and lower inbreeding depression (higher dominance coefficients and lower mutation rates) is more favorable to the existence of stable intermediate selfing rates. Approaches such as this that explicitly incorporate the interdependence of selfing, ovule fertilization, and male outcrossing may help explain the persistence of intermediate selfing rates in animal-pollinated plants.

A survey of restriction site variation in the chloroplast genome of the annual plant genus Amsinckia, together with estimation of outcrossing rates, was conducted to analyze the evolutionary history of the mating system. Species, and in some cases populations within species, differ markedly in their mating system. Five taxa are distylous and predominantly outcrossing, or show mixed mating systems, while the remaining taxa are homostylous and pre- dominantly self-fertilizing. Reconstruction of the molecular phylogeny of the group places different distylous and homostylous taxa at four separate branch tips. When distyly is treated as ancestral in the group, or when the loss of distyly is assumed to be more common than its gain, the results of the phylogenetic analysis support the hypothesis that the self-fertilizing taxa are of recent origin from outcrossing relatives. These findings are discussed with respect to theory for the evolution and breakdown of distyly and the probability of extinction of selfing lineages.

The Genetics Society of America annually honors members who have made outstanding contributions to genetics. The Thomas Hunt Morgan Medal recognizes a lifetime contribution to the science of genetics. The Genetics Society of America Medal recognizes particularly outstanding contributions to the

[Figure][1] Robert Mortimer ![Figure][1] Andre Goffeau THE 2002 Beadle Medal of the Genetics Society of America is awarded to Robert Mortimer and Andre Goffeau for their outstanding contributions to the development of the yeast Saccharomyces cerevisiae as an experimental

Environmental variation that is not predictably related to cues is expected to drive the evolution of bet-hedging strategies. The high variance observed in the timing of seed germination has led to it being the most cited diversification strategy in the theoretical bet-hedging literature. Despite this theoretical focus, virtually nothing is known about the mechanisms responsible for the generation of individual-level diversification. Here we report analyses of sources of variation in timing of germination within seasons, germination fraction over two generations and three sequential seasons, and the genetic correlation structure of these traits using almost 10,000 seeds from more than 100 genotypes of the monocarpic perennial Lobelia inflata. Microenvironmental analysis of time to germination suggests that extreme sensitivity to environmental gradients, or microplasticity, even within a homogeneous growth chamber, may act as an effective individual-level diversification mechanism and explains more than 30% of variance in time to germination. The heritability of within-season timing of germination was low (h(2) = 0.07) but significant under homogeneous conditions. Consistent with individual-level diversification, this low h(2) was attributable not to low additive genetic variance, but to an unusually high coefficient of residual variation in time to germination. Despite high power to detect additive genetic variance in within-season diversification, it was low and indistinguishable from zero. Restricted maximum likelihood detected significant genetic variation for germination fraction (h(2) = 0.18) under homogeneous conditions. Unexpectedly, this heritability was positive when measured within a generation by sibling analysis and negative when measured across generations by offspring-on-parent regression. The consistency of dormancy fraction over multiple delays, a major premise of Cohen's classic model, was supported by a strong genetic correlation (r = 0.468) observed for a cohort's germination fraction over two seasons. We discuss implications of the results for the evolution of bet hedging and highlight the need for further empirical study of the causal components of diversification.

In temporally varying environments selection will often act, at the expense of expected fitness in any given generation, so as to maximize geometric mean fitness across a number of generations (Gillespie 1977). Such "bet hedging" (Slatkin 1974) may occur in two ways (Seger and Brockmann 1987). In conservative bet hedging a genotype minimizes the risk of a very low fitness bout by producing a narrowly unimodal distribution of "safe" trait values. In contrast, a genotype practising diversification bet hedging spreads the risk by producing traits with increased variance (see Philippi and Seger 1989 for a full discussion). A classical example of such diversification both in theoretical and empirical studies is that of seed germination behaviour (Cohen 1966, Janzen 1977, Venable and Lawlor 1980, Cooper and Kaplan 1982, Bulmer 1984, Ellner 1985, Le6n 1985, Venable 1985, Kalisz 1986, Bull 1987, Venable and Brown 1988, Biere 1991), but diversification or multiple strategies (Lloyd 1984) could be equally relevant to other processes such as insect diapause (Tuljapurkar and Istock 1993). Little is known, however, about how such within-genotype diversification might be produced. Here we propose that diversification bet hedging may be attained through a mechanism traditionally considered to be exclusively detrimental: developmental instability. We discuss the plausibility of this perspective, present testable hypotheses that emerge directly if bet hedging is achieved through instability of development, and suggest appropriate tests of these hypotheses. In this paper we discuss the mechanisms underlying diversification bet hedging although these mechanisms should apply to other, nonbet-hedging situations in which within-genotype trait variance is shown to be adaptive (see Geritz 1995 for density-dependent selection of seeds under spatial heterogeneity). There exists considerable confusion surrounding the use of terms related to the degree of variability inherent to developmental processes (Zakharov 1992). The relative fidelity of development to a program is known variously as developmental stability (Mather 1953, Waddington 1957, Parsons 1992, Moller 1995), homeorhesis (Waddington 1957), homeostasis (Lerner 1954, Orzack 1985, Parsons 1992, Moller 1995), buffering capacity (Van Valen 1962), and developmental canalization (Mather 1953, Thoday 1958, Waddington 1960, Levin 1988). The mechanisms which these terms describe all have the effect of decreasing trait variance. Developmental instability, environmental sensitivity (Jinks and Pooni 1988), and phenotypic plasticity (Bradshaw 1965, Via and Lande 1985, Schlichting 1986) are all means by which trait variance is increased. There is considerable overlap in the definitions of some of these terms, and their usage may vary by author (Zakharov 1992). Phenotypic variance expressed among individuals of a genotype is usually attributed to two general sources: plasticity and noise. Plasticity is measured as the phenotypic expression of genotypes across an environmental gradient, whereas developmental noise is assumed to result from random errors of development, but in fact is variance resulting from all sources unknown. Developmental noise, then, may include plasticity in response to environmental variables that have not been identified (Bradshaw 1965). There is ample evidence for the existence of genetic variation for plasticity (Bradshaw 1965, Perkins and Jinks 1971, Schlichting and Levin 1986, Scheiner et al. 1991, Oyama 1994). Although phenotypic plasticity is the variable expression of a genotype under differing environmental circumstances, it is quite possible to have well-canalized reaction norms, and the degree of plasticity is a genotype-level property. There is no reason to suppose that, like plasticity, developmental noise could not be maintained at some optimal level by stabilizing selection. The genetic basis of developmental stability is not yet well established (see Clarke 1993 for a discussion of competing hypotheses). Developmental instability may be assessed through the occurrence of phenodeviants or, in (normally) bilaterally symmetrical organisms, as the fluctuating asymmetry (FA) of paired characters

Determining whether seed production is pollen limited has been an area of intensive empirical study over the last two decades. Yet current evidence does not allow satisfactory assessment of the causes or consequences of pollen limitation. Here, we critically evaluate existing theory and issues concerning pollen limitation. Our main conclusion is that a change in approach is needed to determine whether pollen limitation reflects random fluctuations around a pollen–resource equilibrium, an adaptation to stochastic pollination environments, or a chronic syndrome caused by an environmental perturbation. We formalize and extend D. Haig and M. Westoby's conceptual model, and illustrate its use in guiding research on the evolutionary consequences of pollen limitation, i.e., whether plants evolve or have evolved to ameliorate pollen limitation. This synthesis also reveals that we are only beginning to understand when and how pollen limitation at the plant level translates into effects on plant population dynamics...