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The study of phenotypic divergence of, and selection on, functional traits in closely related taxa provides the opportunity to detect the role of natural selection in driving diversification. If the strength or direction of selection in field populations differs between taxa in a pattern that is consistent with the phenotypic difference between them, then natural selection reinforces the divergence. Few studies have sought evidence for such concordance for physiological traits.Herbarium specimen records were used to detect phenological differences between sister taxa independent of the effects on flowering time of long-term variation in the climate across collection sites. In the field, physiological divergence in photosynthetic rate, transpiration rate, and instantaneous water-use efficiency were recorded during vegetative growth and flowering in 13 field populations of two taxon pairs of Clarkia, each comprising a self-pollinating and a outcrossing taxon.Historically, each selfing taxon flowered earlier than its outcrossing sister taxon, independent of the effects of local long-term climatic conditions. Sister taxa differed in all focal traits, but the degree and (in one case) the direction of divergence depended on life stage. In general, self-pollinating taxa had higher gas exchange rates, consistent with their earlier maturation. In 6 of 18 comparisons, patterns of selection were concordant with the phenotypic divergence (or lack thereof) between sister taxa.Patterns of selection on physiological traits measured in heterogeneous conditions do not reliably reflect divergence between sister taxa, underscoring the need for replicated studies of the direction of selection within and among taxa.

PREMISE Strong correlations between traits can obscure their independent effects on components of reproduction. Style length (SL) and petal area (PA) vary within species, for example, but their independent effects on the opportunity for selection among pollen genotypes are poorly understood. Previous work in Clarkia detected a positive effect of SL on pollen receipt, potentially intensifying selection. However, this apparent effect of SL may be influenced by a correlated trait, such as PA. Here, we examine the independent effects of these two traits on pollen receipt and performance. METHODS We collected petals and styles from wild populations of two insect-pollinated Clarkia taxa and estimated the independent and combined effects of SL and PA on pollen receipt and performance. RESULTS In both taxa, SL and PA are positively correlated. In C. unguiculata, both traits positively and independently affect pollen receipt, but in C. xantiana ssp. xantiana, the two traits act only in combination to affect pollen receipt. In both taxa, pollen receipt positively affects the numbers of pollen tubes entering and penetrating the style, as well as pollen tube attrition. CONCLUSIONS The effects of SL and PA on pollen receipt and performance are taxon specific. In C. unguiculata, both traits may be independent targets of selection due to their effects on pollen receipt. In C. xantiana ssp. xantiana, by contrast, the combined (but not independent) effects of SL and PA influence pollen receipt. Ecological differences between these taxa require exploration to understand the mechanisms by which these traits affect pollinator behavior.

Author(s): Peach, Kristen; Mazer, Susan J | Abstract: Premise of the studyWild plant species that require the services of pollen-feeding insects for reliable pollination may evolve features that attract and reward their mutualistic partners. Heterantherous species have been proposed to exhibit a "division of labor" whereby "feeding anthers" (which produce pollen that may be consumed by an insect) are distinguished from "reproductive anthers" (which produce pollen more likely to contribute to reproduction). In some heterantherous species, including Clarkia unguiculata (Onagraceae), these two anther types differ with respect to stamen length, anther size, pollen production, and pollen color.MethodsThe primary goal of this study was to test one component of the "division of labor" hypothesis by comparing the performance of the pollen produced by each type of anther in C. unguiculata. To achieve this goal, under greenhouse conditions, we hand pollinated and assessed pollen performance (using epifluorescence microscopy) within ~228 flowers.Key resultsThe pollen produced by the two anther types differed significantly with respect to both stigma and style penetration. The inner series of anthers produce pollen with higher performance than the outer series of longer, dark red anthers.ConclusionsThese findings contradict previous descriptions of the genus, reporting that the inner diminutive series of anthers in Clarkia produce "abortive and nonfunctional" pollen. We outline the future research required to demonstrate the ecological function of heteranthery in this iconic wildflower group.

Ecologists and evolutionary biologists have been interested in the functional biology of pollen since the discovery in the 1800s that pollen grains encompass tiny plants (male gametophytes) that develop and produce sperm cells. After the discovery of double fertilization in flowering plants, botanists in the early 1900s were quick to explore the effects of temperature and maternal nutrients on pollen performance, while evolutionary biologists began studying the nature of haploid selection and pollen competition. A series of technical and theoretic developments have subsequently, but usually separately, expanded our knowledge of the nature of pollen performance and how it evolves. Today, there is a tremendous diversity of interests that touch on pollen performance, ranging from the ecological setting on the stigma, structural and physiological aspects of pollen germination and tube growth, the form of pollen competition and its role in sexual selection in plants, virus transmission, mating system evolution, and inbreeding depression. Given the explosion of technical knowledge of pollen cell biology, computer modeling, and new methods to deal with diversity in a phylogenetic context, we are now more than ever poised for a new era of research that includes complex functional traits that limit or enhance the evolution of these deceptively simple organisms.

The shift from outcrossing to self-fertilization is among the most commonly observed evolutionary transitions in fl ower-ing plants ( Darwin, 1876 ; Stebbins, 1957 , 1970 ; Barrett, 2002 ). Although an estimated 39% of species are obligate outcrossers ( Igic et al., 2008 ), ~20% predominantly self-fertilize ( Barrett and Eckert, 1990 ; Barrett, 2002 ), an additional ~42% regularly engage in mixed mating (producing seeds through a combina-tion of outcrossing and selfi ng) ( Goodwillie et al., 2005 ). The widespread evolution of selfing has generated long-standing interest among evolutionary biologists because of the potentially harmful consequences of self-fertilization. In many outcrossing species, selfi ng is associated with high levels of inbreeding depression ( Darwin, 1876 ; Charlesworth et al., 1993 ; Keller and Waller, 2002 ; but also see Byers and Waller, 1999 ), which may impede or prevent the evolution of selfi ng when it arises as a mating strategy in habitually outcrossing taxa and populations ( Lande and Schemske, 1985 ; Goodwillie et al., 2005 ). Additionally, self-fertilizing populations and lineages often harbor lower levels of genetic variation than their out-crossing counterparts ( Tantawy, 1956b ; Loveless and Hamrick, 1984 ; Awadalla and Ritland, 1997 ), which may hinder their ability to adapt to changing environmental conditions. Indeed, selfi ng may lead to a greater risk of extinction ( Tantawy, 1956a ; Bijlsma et al., 2000 ; Wright et al., 2008 ). A great deal of research on plant mating systems has focused on identifying the ecological and genetic conditions under which selfi ng may evolve and remain stable. For example, in the absence of inbreeding depression, because selfi ng geno-types can transmit their genes to future generations in more ways than outcrossers (i.e., in selfi ng genotypes, pollen may contribute genes through both self-fertilization and outcross-ing), selfi ng may evolve rapidly if it arises in outcrossing pop-ulations ( Fisher, 1941 ). Depending on the magnitude of inbreeding depression, the advantages of self-fertilization may outweigh the genetic risks that it imposes ( Lande and Schemske, 1985 ). In addition, in habitats where pollinators are scarce or unreli-able, selection may favor self-fertilization if it provides repro-ductive assurance ( Baker, 1955 ; Goodwillie et al., 2005 ; Morgan and Wilson, 2005 ; Eckert et al., 2006 , 2010 ). Several studies have evaluated the reproductive assurance hypothesis ( Pannell and Barrett, 1998 ; Kalisz et al., 2004 ; Herlihy and Eckert, 2005 ; Moeller and Geber, 2005 ; Geber and Moeller, 2006 ; Fishman and Willis, 2008 ; Brys et al., 2011 ; Brys and Jacquemyn, 2011 ; Roels and Kelly, 2011 ), corroborating its potential to explain the widespread evolution of self-fertilization in fl owering plants.

 Premise of the study : The study of geographic variation in ecologically important traits within and among taxa is a fi rst step toward understanding the environmental factors that contribute to population differentiation and species divergence. This study examines variation in mean sex allocation per fl ower (androecium mass/gynoecium mass) among 49 wild populations representing 12 Pedicularis species across an elevation gradient on the eastern Tibetan Plateau.  Methods : We used population means to evaluate sources of variation in per-fl ower sex allocation within and across species. In particular, we evaluate the relative infl uence of intrinsic (i.e., plant size, estimated as aboveground stem biomass) vs. extrinsic factors affecting mean sex allocation among populations.  Key results : Mean sex allocation per fl ower (the relative investment in male fl oral organs) is negatively correlated with mean plant size; populations of large plants produce relatively female-biased fl owers. This relationship between mean plant size and mean sex allocation is not statistically signifi cant, however, when the effect of elevation is controlled statistically. Among populations within and across species, mean sex allocation increases with elevation. This relationship persists even when the effect of mean plant size is controlled statistically. Factors associated with increasing elevation appear to favor genotypes and/ or taxa with male-biased fl owers.  Conclusion : Extrinsic environmental conditions may be more important than intrinsic resource status in determining patterns of geographic variation in mean sex allocation among populations or species of Pedicularis . We cannot conclude whether the effect of elevation on mean sex allocation is the result of environmentally induced plasticity, genetically based adaptation, or species sorting, but it is only partly mediated by mean plant size.

AJB Advance Article published on June 9, 2015, as 10.3732/ajb.1400557. The latest version is at http://www.amjbot.org/cgi/doi/10.3732/ajb.1400557 RESEARCH ARTICLE A M E R I C A N J O U R N A L O F B OTA N Y S EASONAL CHANGES IN PHYSIOLOGICAL PERFORMANCE IN WILD C LARKIA XANTIANA POPULATIONS : I MPLICATIONS FOR THE EVOLUTION OF A COMPRESSED LIFE CYCLE AND SELF - FERTILIZATION 1 L EAH S. D UDLEY 2,5 , A LISA A. H OVE 2 , S IMON K. E MMS 3 , A MY S. V ERHOEVEN 3 , AND S USAN J. M AZER 4 2 Warren Wilson College, P.O. Box 9000, Asheville, North Carolina 28815 USA; 3 University of St. Thomas, 2115 Summit Ave., St. Paul, Minnesota, USA 5105; and 4 University of California, Santa Barbara, Santa Barbara, California 93106 USA • Premise of the study: One explanation for the evolution of selfi ng, the drought escape hypothesis, proposes that self-fertilization may evolve under conditions of intensifying seasonal drought as part of a suite of traits that enable plants to accelerate the completion of their life cycle, thereby escaping late-season drought. Here, we test two fundamental assumptions of this hypoth- esis in Clarkia xantiana : (1) that a seasonal decline in precipitation causes an increase in drought stress and (2) that this results in changes in physiological performance, refl ecting these deteriorating conditions. • Methods: We examined seasonal and interannual variation in abiotic environmental conditions (estimated by ambient tempera- ture, relative humidity, predawn leaf water potentials, and carbon isotope ratios) and physiological traits (photosynthesis, conductance, transpiration, instantaneous water-use effi ciency, ascorbate peroxidase and glutathione reductase activities, quan- tum yield of photosystem II, PSII potential effi ciency) in fi eld populations of C. xantiana in 2009 and 2010. • Key results: In both years, plants experienced intensifying drought across the growing season. Gas exchange rates decreased over the growing season and were lower in 2009 (a relatively dry year) than in 2010, suggesting that the temporal changes from early to late spring were directly linked to the deteriorating environmental conditions. • Conclusions: Seasonal declines in transpiration rate may have increased survival by protecting plants from desiccation. Con- comitant declines in photosynthetic rate likely reduced the availability of resources for seed production late in the season. Thus, the physiological patterns observed are consistent with the conditions required for the drought escape hypothesis. Key words: California, Clarkia ; drought; gas exchange physiology; mating system evolution; Onagraceae; outcrossing; selfi ng. Evolutionary transitions in mating system within and be- tween angiosperm taxa are often associated with phenotypic evolution in phenological, life history, and fl oral traits, and the divergence between selfi ng and outcrossing sister taxa has been particularly well documented. Relative to their outcrossing rel- atives, predominantly selfi ng taxa often exhibit smaller fl owers, shorter styles, lower pollen production and pollen to ovule ra- tios, lower petal brightness and chroma, reduced herkogamy (i.e., greater proximity between anthers and stigmas), reduced dichogamy (i.e., more synchronous maturity of stigmas and an- thers), more rapid fl oral development, greater synchrony of fl ower production within individuals, earlier fl owering, shorter individual fl oral lifespan, or earlier senescence ( Cruden, 1977 ; 1 Manuscript received 23 December 2014; revision accepted 5 May 2015. The authors thank the National Science Foundation for support (grant OIS-0718227 to S.J.M. and L.S.D. and grant OIS-0718253 to S.K.E. and A.S.V.), and numerous UCSB undergraduates assisted in processing fi eld- collected samples. 5 Author for correspondence (e-mail: ldudley@warren-wilson.edu) doi:10.3732/ajb.1400557 Cruden and Lyon, 1985 ; Brunet, 1992 ; Eckhart and Geber, 1999 ; Dudley et al., 2007 ; Delesalle et al., 2008 ; Busch and Urban, 2011 ; Andersson, 2012 ; Button et al., 2012 ). The mechanisms that may contribute to the joint evolution of phenological, life history, and fl oral traits in selfi ng taxa relative to their outcrossing sister taxa have been described in detail elsewhere ( Rollins, 1963 ; Lloyd, 1979 ; Guerrant, 1989 ; Runions and Geber, 2000 ; Mazer et al., 2004 ; Goodwillie et al., 2006 ; Barrett et al., 2014 ). One proposed mechanism is that selfi ng evolves under conditions where selection favors geno- types with accelerated life cycles ( Lloyd, 1965 ; Moore and Lewis, 1965 ; Vasek and Sauer, 1971 ; Arroyo, 1973 ; Solbrig and Rollins, 1977 ; Guerrant, 1989 ; Aarssen, 2000 ; Mazer et al., 2004 ; Snell and Aarssen, 2005 ). Selfi ng can promote a more compressed life cycle because autogamous fl owers often have shorter fl oral lifespans and set seed more rapidly than the fl ow- ers of their outcrossing relatives do ( Armbruster et al., 2002 ; Eckhart et al., 2004 ; Dudley et al., 2007 ; Goodwillie et al., 2010 ; Runions and Geber, 2000 ; Mazer et al., 2004 ; Theiss et al., 2010 ; Wu et al., 2010; but see Hill et al., 1992 ). In addition, at the whole plant level, recent studies have found that fl owers are produced in more rapid succession in autogamous taxa than American Journal of Botany 102 ( 6 ): 1 – 11 , 2015 ; http://www.amjbot.org/ © 2015 Botanical Society of America Copyright 2015 by the Botanical Society of America

Autogamously self-fertilizing taxa have evolved from outcrossing progenitors at least 12 times in the annual wildflower genus, Clarkia (Onagraceae). In C. xantiana, individuals of the selfing subspecies (ssp. parviflora) flower at an earlier age, produce successive flowers more rapidly, and produce flowers that complete their development more rapidly than their outcrossing counterparts (ssp. xantiana). Two hypotheses have been proposed to explain the joint evolution of these whole-plant and individual floral traits. The accelerated life cycle hypothesis proposes that selection favoring a short life cycle in environments with short growing seasons (such as those typically occupied by parviflora) has independently favored genotypes with early reproduction, synchronous flower production, and rapidly developing, self-fertilizing flowers. The correlated response to selection hypothesis similarly proposes that selection in environments with short growing seasons favors early reproduction, but that rapid floral development and increased selfing evolve as correlated responses to selection due to genetic linkage (or pleiotropy) affecting both whole-plant and floral development. We conducted a greenhouse experiment using maternal families from two field populations of each subspecies to examine covariation between floral and whole-plant traits within and among populations to seek support for either of these hypotheses. Our results are consistent with the accelerated life cycle hypothesis but not with the correlated response to selection hypothesis.

The relative allocation of resources to male and female functions may vary among flowers within and among individual plants for many reasons. Several theoretical models of sex allocation in plants predict a positive correlation between the resource status of a flower or individual and the proportion of reproductive resources allocated to female function. These models assume that, independent of resource status, a negative correlation exists between male and female investment. Focusing on the allocation of resources within flowers, we tested these theoretical predictions and this assumption using the annual Clarkia unguiculata(Onagraceae). We also sought preliminary evidence for a genetic component to these relationships. From 116 greenhouse-cultivated plants representing 30 fieldcollected maternal families, multiple flowers and fruits per plant were sampled for gamete production, pollen : ovule ratio, seed number, ovule abortion, seed biomass/fruit, mean individual seed mass, and petal area. If sex allocation changes as predicted, then (1) assuming that flowers produced early have access to more resources than those produced later, basal flowers should exhibit a higher absolute and proportional investment in female function than distal flowers and (2) plants of high resource status (large plants) should produce flowers with a higher proportional investment in female function than those of low resource status. Within plants, variation in floral traits conformed to the first prediction. Among plants and families, no significant effects of plant size (dry stem biomass) on intrafloral proportional sex allocation were observed. We detected no evidence for a negative genetic correlation between male and female investment per flower, even when controlling for plant size.

Many species exhibit reduced siring success of self-relative to outcross-pollen donors. This can be attributed either to postfertilization abortion of selfed ovules or to cryptic self-incompatibility (CSI). CSI is a form of self-incompatibility whereby the advantage to outcross pollen is expressed only following pollinations where there is gametophytic competition between self and outcross pollen. Under the definition of CSI, this differential success is due to the superior prefertilization performance (pollen germination rate and pollen tube growth rate) of outcross pollen relative to self pollen. Although CSI has been demonstrated in several plant species, no studies have assessed among-population variation in the expression of CSI. We conducted a greenhouse study on Clarkia unguiculata (an annual species with a mixed-mating system) to detect CSI, and we compare our observations to previous reports of CSI in C. gracilis and another population of C. unguiculata. In contrast to these previous studies of CSI in Clarkia, we used genetic rather than phenotypic markers to measure the relative performance of selfed vs. outcross pollen. In this study, we measured the intensity of CSI in C. unguiculata from a large population in southern California and we determined whether the magnitude of pollen competition (manipulated by controlling the number of pollen grains deposited on a stigma) influenced the outcome of competition between self and outcross pollen. In contrast to previous investigations of Clarkia, we found no evidence for CSI. The mean number of seeds sired per fruit did not differ between self and outcross pollen following either single-donor or mixed pollinations. In addition, the relative success of selfed vs. outcross pollen was independent of the magnitude of pollen competition. These results suggest that: (1) one of the few nonheterostylous species previously thought to be cryptically self-incompatible is completely self-compatible (at least in the population studied here) or (2) phenotypic markers may be problematic for the detection of CSI.

Several goals of evolutionary biologists can be approached by comparisons of species that occupy different habitats or that have evolved distinct suites of life history characters. One is to determine the ecological conditions favoring particular morphological or life history traits, and another is to detect adaptive combinations of traits that have ‘‘coevolved.’’ Recently, due to the development of statistical tools that use hypothesized phylogenetic relationships to detect the joint evolution of life history and ecological attributes among taxa, the comparative analysis of plant life history traits has enjoyed increased attention. Silvertown, Franco, and Harper responded to this interest by convening in 1996 a Royal Society Symposium aimed in part to highlight alternative approaches to the analysis of comparative data. This book represents the outcome of this meeting and includes 15 chapters, most of which use comparative data to interpret patterns involving the distribution of plant life history, mating system, or reproductive traits across taxa and environmental conditions. In addition, one chapter (by Michael Donoghue and David Ackerly) explores the effects of uncertainty in phylogenetic reconstruction on the outcome of one statistical test that makes use of phylogenetic information. Three chapters provide reviews of mathematical models: one concerning the allocation and packaging of resources among flowers and fruits (by Lawrence Venable); a second evaluating the relative importance of the automatic selfing vs. reproductive assurance hypotheses for the evolution of self-pollination and the use of coalescent theory to detect evolutionary events associated with the evolution of self-fertilization from outcrossing ancestral taxa (by Daniel Schoen, Martin Morgan, and Thomas Bataillon); and the third exploring the evolution of life history traits in heterogeneous environments (by Richard Sibly). It is tough to identify the best audience for this book. The chapters are neither review articles nor rigorous primary research articles. Most summarize new data analyses, but in few cases do they provide sufficient statistical and methodological detail to allow the reader to evaluate fully their appropriateness or interpretation. Consequently, I would not recommend this book as the main focus, say, of a graduate seminar, unless it were to be accom

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.

Multiple field populations of two pairs of diploid sister taxa with contrasting mating systems in the genus Clarkia (Onagraceae) were surveyed to test predictions concerning the effects of resource status, estimated as plant size, on pollen and ovule production and on the pollen:ovule (P:O) ratio of flowers. Most theoretical models of size-dependent sex allocation predict that, in outcrossing populations, larger plants should allocate more resources to female function. Lower P:O ratios in larger plants compared to smaller plants have been interpreted as supporting this prediction. In contrast, we predicted that P:O ratio should not vary with plant size in predominantly selfing plants, in which each flower contributes to reproductive success equally through male and female function. We found that, in all four taxa, both ovule and pollen production per flower usually increased significantly with plant size and that the shape of this relationship was decelerating. However, ovule production either decelerated more rapidly than or at the same rate as pollen production with plant size. Consequently,the P:O ratio increased or had no relationship with plant size. This relationship was population-specific (not taxon-specific) and independent of the mating system. Possible explanations for the increasing maleness with plant size are discussed.

The study of plant responses to touch, particularly the responses of leaves, stems, and roots, has a long history. By contrast, floral responses are relatively unexplored. Stigma closure is common in the Bignoniaceae, but the factors influencing it are not well understood. We investigated factors influencing stigma closure and reopening and its effects on pollen receipt in seven Oroxylum indicum trees near Hat Yai, Thailand. The effects of pressure, conspecific and heterospecific pollen, and pollen load (the amount of pollen deposited) on stigma behavior were examined in 270 flowers (of the total 430 flowers evaluated in the entire study). Pressure alone resulted in faster closure than did conspecific pollination and faster reopening than did heterospecific pollination. Stigmas never reopened after conspecific pollination. Pollen load had no effect on stigma behavior. Stigmas discriminated between conspecific and foreign pollen; they reopened only after pollination with the latter. A manipulative experiment revealed that stigma closure did not affect the number of conspecific pollen grains received. We also counted pollen tubes in styles that were either hand-supplemented with outcross conspecific pollen or open-pollinated. Pollen tube numbers were highest after light pollination (∼900 grains), indicating that interference among pollen grains may occur after pollination with very heavy loads (>6000 grains). Possible fitness consequences of these responses are discussed.

Phenotypic and genetic variation and correlations among floral traits within and among four Primula species were measured to seek evidence for potential constraints on the independent evolution of floral characters, to examine the relationship between mating system, ploidy level, and sex allocation, and to determine whether some traits are more conservative than others within and across these congeners. We measured mean flower diameter, corolla depth, pollen production, modal pollen grain volume, ovule number per flower, and pollen: ovule ratios for 64 field-collected genotypes from northern Europe. These represented one heterostylous (P. farinosa: 2n = 18) and three homostylous (P. scotica: 2n = 54, P. scandinavica: 2n = 74, and P. stricta: 2n 126) species. All traits differed significantly among species and among the six taxon/morph categories identified (including three morphs of P. farinosa: pin, thrum, and homostylous). Pollen production per flower was significantly higher (and individual pollen grain volume lower) in the outcrossing P.farinosa than in any ofthe homostylous species; also, pin morphs produced significantly more pollen per flower than thrums in P. farinosa. Among the homostylous species, there were significant differences in all traits except modal pollen grain volume. Ovule number per flower and flower size were less variable among taxa than pollen production and pollen volume. Within species, there were several strong negative correlations among genets between pairs of traits, but each species exhibited a unique set of inverse relationships. We detected only one significant positive genetic correlation; in P. stricta, ovule number and pollen production per flower were positively correlated among genets. Among species means, two pairs of traits were negatively correlated: mean ovule number per flower vs. flower diameter (but P = 0.0587), and mean pollen production per flower vs. modal pollen grain volume. These negative correlations within and among taxa suggest that there may be intrinsic genetic constraints on the independent evolution of these floral characters, but that these constraints differ among species.

To determine the effects of density, genotype, and their interaction on individual seed mass in Raphanus sativus L., we replicated maternal and paternal families of seed across two planting densities in an experimental garden. Seeds were produced by a nested breeding design performed in the greenhouse. Among garden-raised plants, density had a strong negative effect on the mass of seeds produced. At low density, the identity of the greenhouse-grown maternal plants had a strong effect on F2 seed mass, while in high-density plots, there were no significant parental effects on mean seed mass. Significant parental genotype x density interactions contributed to variation in F2 seed mass. Norms of reaction for each of the 15 paternal sibships illustrate paternal family x density interactions. Three sibships exhibited significant declines in mean seed mass with increasing density; 12 sibships showed no change. Maternal family x density interaction effects on seed mass were also detected; among maternal sibships, mean seed mass at low density was negatively correlated with mean seed mass at high density. These results demonstrate: a) planting density has a strong effect on mean individual seed mass produced by adults; b) density influences the magnitude of maternal effects on progeny phenotype; and c) genotype x density interactions influence seed mass, potentially contributing to the maintenance of maternal genetic variation in seed mass in natural populations of wild radish.

Seed weight varies significantly within and among fruits of wild radish (Raphanus raphanistrum). To determine sources of this variation, we studied fertilization and seed development following controlled pollinations. Within fruits, central ovules were fertilized prior to distal ovules and attained greater seed size. Ninety-seven percent of the variation in mean seed wt per fruit was explained by an analysis of variance incorporating parental effects, pollination date, and the number of seeds per fruit. We document strong maternal effects on the number of ovules per ovary, the number of fertilized ovules per ovary, the number of seeds per fruit, and mean individual seed wt per fruit. Across females, pollen donor had a slight but significant effect on seed wt; no paternal effects on fertilization rate, zygote number, or seed number per fruit were detected. Within females, with one exception, pollen donor had no significant effect on these components of seed development. Stronger maternal main effects may be due to donor x recipient interactions, cytoplasmic factors, the genetic inequity within triploid endosperm, and/ or strict maternal control over resource allocation. The large maternal effects relative to paternal effects may limit the rate at which natural selection acts on paternal traits expressed prior to seed maturation.