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Correns's 1903 (Berichte der Deutschen Botanischen Gesellschaft 21:133-147) crosses between a monoecious and a dioecious species of Bryonia revealed the simple Mendelian inheritance of dioecy and provided the first instance of an XY sex determination system in any organism. Bryonia ranges from the Canary Islands to Central Asia and comprises seven dioecious and three monoecious species; its closest relative, Ecballium elaterium, has dioecious and monoecious populations. We used chloroplast (cp) and nuclear (nr) gene phylogenies to infer sexual system evolution in Bryonia. We also tested for associations between sexual system and ploidy level, based on published and original chromosome counts. Conflicts between cp and nr topologies imply that the dioecious hexaploid B. cretica arose from hybridization(s), probably involving the dioecious diploids B. dioica, B. syriaca, and/or B. multiflora. A tetraploid dioecious endemic on Corsica and Sardinia probably originated from B. dioica via autopolyploidy. While the cp phylogeny resolves few species relationships, the nr tree implies at least two evolutionary changes in sexual system. There is no correlation between sexual system and ploidy level. Molecular clocks suggest that the deepest divergence, between a species on the Canary Islands and the ancestor of all remaining species, occurred ca. 10 million years ago. Key words: Bryonia; chloroplast DNA haplotypes; chromosome counts; Cucurbitaceae; dioecy; hybridization; monoecy; nuclear LEAFY intron; polyploidy.

Bryonia alba (Cucurbitaceae) is a Eurasian herbaceous vine that spreads vegetatively through the production of many stems from a large, tuberous root. The only known U.S. populations of this aggressive apomict are in Idaho, Montana, Utah, and Washington and likely stem from deliberate (and subsequent accidental) introductions. To assess levels and patterns of genetic diversity of B. alba across its introduced range, 23 populations were analyzed for allozyme variation using 12 enzyme systems. On average, 14.9% of loci are polymorphic per population (1.19 alleles per locus) - low values compared to other vascular plant taxa. Mean percent polymorphic loci differed among regions, with the highest values in the Washington and northern Idaho region (20.2%) and Montana (19.0%) and lower values in populations from the Utah and southern Idaho region (7.4%). Observed heterozygosity exceeded that expected at Hardy-Weinberg equilibrium in six of 23 populations, and a statistically significant excess of heterozygosity was detected at Pgm-1 in 18 of 23 populations. The level of population differentiation is high ([G.sub.ST] = 0.544); however, the level of differentiation among populations within regions is much lower. These results are consistent with the genetic variation and structure expected for an apomict. Based on the level of genetic differentiation among populations, the current disjunct distribution of B. alba in its new range results from two, and possibly three, separate introductions in the western United States. These introductions may stem, in part, from the vine's 19th century use as a medicinal and ornamental plant.

Ibervillea lindheimeri is a dioecious member of the Cucurbitaceae whose male flowers possess a number of sex-specific characteristics including glandular trichomes on the anther connective. A 2-year study (1989 and 1990) was undertaken to assess the potential for sexual selection of anther trichome number as a male floral trait that enhances pollen removal (a male fitness component). Correlation analysis of ten reproductive and floral traits showed that only the mean number of anther trichomes per flower was significantly associated with mean pollen removal per flower. Anther trichomes are two-celled, consisting of a small, caplike, apical cell and a large, bulbous basal cell. The basal cell is under tension and produces a sticky polysaccharide that is forcefully ejected upward when the apical cell is broken off by visiting bees. The sticky polysaccharide accumulates on the bees and aids in adherence of pollen. We propose that there is directional selection on individual males for number of anther trichomes per flower. The more anther trichomes per flower, the more pollen exported, and the greater the likelihood of siring seeds. Males differed significantly in mean number of anther trichomes per flower and were relatively consistent in their rank order over a 2-year period; both are prerequisites for sexual selection to operate. Selection coefficients and differentials for 1989 and 1990 were 0.005 and 0.007, and 0.22 and 0.32, respectively, and suggest moderate to strong directional selection on anther trichome number.

The pollen competition hypothesis predicts that when the number of pollen grains deposited onto stigmas exceeds the number of ovules, selection can operate in the time frame between deposition and fertilization. Moreover, because of the overlap in gene expression between the two phases of the life cycle, selection on microgametophytes may alter the resulting sporophytic generation. The extent to which pollen competition occurs in nature has been unclear, because tests of the predictions of the pollen competition hypothesis have used cultivars and/or artificial growth conditions and hand-pollination techniques. In this study we used a wild species, Cucurbita foetidissima, in its natural habitat (southern New Mexico) to determine the amount and timing of the arrival of pollen onto stigmas, the relationship between pollen deposition and seed number, and the effects of the intensity of pollen competition on progeny vigor. We found that ~900 pollen grains are necessary for full seed set and that a single visit by a pollinator results in the deposition of 653.0 [+ or -] 101.8 pollen grains. About 29% of the flowers receiving a single pollinator visit had 900 or more pollen grains on its stigma. Moreover, within 2 h of anthesis, [is greater than] 4000 pollen grains were deposited onto a typical stigma, indicating that multiple pollinator visits must have occurred. Fruits produced by multiple visits had greater seed numbers (206 vs. 147) than fruits produced by a single visit. Finally, the progeny produced by multiple pollinator visits were more vigorous than those produced by single visits with respect to five measures of vegetative growth (MANCOVA, Wilks' lambda = 0.96, [F.sub.6,370] = 2.54, P [is less than] 0.02. These data demonstrate that conditions for pollen competition exist in nature and support the prediction that pollen competition enhances offspring vigor. Key words: Cucurbita foetidissima; Cucurbitaceae; La Jornada Long-Term Ecological Research Site; pollen; pollen competition.