Pollen production per flower increases with floral display size across animal-pollinated flowering plants.

PREMISE: The number of open flowers on a plant (i.e., floral display size) can influence plant fitness by increasing pollinator attraction. However, diminishing marginal fitness returns with increasing floral display are expected as pollinators tend to visit more flowers per plant consecutively. An extended flower visitation sequence increases the fraction of ovules disabled by self-pollination (ovule discounting) and reduces the fraction of a plant's own pollen that is exported to sire seeds in other plants (pollen discounting). Hermaphroditic species with a genetic system that prevents self-fertilization (self-incompatibility) would avoid ovule discounting and its fitness cost, whereas species without such a genetically based barrier would not. Contrarily, pollen discounting would be an unavoidable consequence of a large floral display irrespective of selfing barriers. Nevertheless, the increasing fitness costs of ovule and pollen discounting could be offset by respectively increasing ovule and pollen production per flower. METHODS: We compiled data on floral display size and pollen and ovule production per flower for 1241 animal-pollinated, hermaphroditic angiosperm species, including data on the compatibility system for 779 species. We used phylogenetic general linear mixed models to assess the relations of pollen and ovule production to floral display size. RESULTS: Our findings provide evidence of increasing pollen production, but not of ovule production, with increasing display size irrespective of compatibility system and even after accounting for potentially confounding effects like flower size and growth form. CONCLUSIONS: Our comparative study supports the pollen-discount expectation of an adaptive link between per-flower pollen production and floral display across animal-pollinated angiosperms.

Genetic variation and clonal diversity in floating aquatic plants: Comparative genomic analysis of water hyacinth species in their native range.

Many eukaryotic organisms reproduce by sexual and asexual reproduction. Genetic diversity in populations can be strongly dependent on the relative importance of these two reproductive modes. Here, we compare the amounts and patterns of genetic diversity in related water hyacinths that differ in their propensity for clonal propagation - highly clonal Eichhornia crassipes and moderately clonal E. azurea (Pontederiaceae). Our comparisons involved genotype-by-sequencing (GBS) of 137 E. crassipes ramets from 60 locations (193,495 nucleotide sites) and 118 E. azurea ramets from 53 locations (198,343 nucleotide sites) among six hydrological basins in central South America, the native range of both species. We predicted that because of more prolific clonal propagation, E. crassipes would exhibit lower clonal diversity than E. azurea. This prediction was supported by all measures of clonal diversity that we examined. Eichhornia crassipes also had a larger excess of heterozygotes at variant sites, another signature of clonality. However, genome-wide heterozygosity was not significantly different between the species. Eichhornia crassipes had weaker spatial genetic structure and lower levels of differentiation among hydrological basins than E. azurea, probably because of higher clonality and more extensive dispersal of its free-floating life form. Our findings for E. crassipes contrast with earlier studies from the invasive range which have reported very low levels of clonal diversity and extensive geographic areas of genetic uniformity.

Architectural constraints, male fertility variation and biased floral morph ratios in tristylous populations.

Tristyly is a genetic polymorphism in which populations are comprised of three floral morphs (mating types) differing reciprocally in sex-organ height. Intermorph (disassortative) mating governed by a trimorphic incompatibility system should result in 1:1:1 morph ratios at equilibrium, but both deterministic and stochastic processes can cause skewed morph ratios in tristylous populations. Here, we investigate mechanisms causing morph-ratio bias in Pontederia parviflora, an emergent aquatic native to tropical America. We compared reproductive traits among morphs and surveyed 71 populations to determine patterns of morph-ratio bias. We then used simulation models of morph-frequency dynamics to test the hypothesis that morph-specific differences in pollen production and their influence on male fertility can explain patterns of morph-ratio bias. Ninety-seven percent of populations that we sampled were tristylous, but with a significant excess of the short-styled morph and a deficiency of the long-styled morph. Atypically for a tristylous species, mid-level anthers of the short-styled morph produced over twice as much pollen compared with the corresponding anthers of the long-styled morph. Our computer models incorporating this difference in male fertility resulted in morph ratios not significantly different from the average frequencies from our survey suggesting that the short-styled morph is more successful than the long-styled morph in siring ovules of the mid-styled morph. We propose that the difference in male fertility between morphs may be a non-adaptive consequence of a developmental constraint caused by the architecture of tristyly in Pontederiaceae.