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• Reinforcement is the process through which prezygotic reproductive barriers evolve in sympatry due to selection against hybridization between co-occurring, closely related species. The role of self-fertilization in reinforcement and reproductive isolation is uncertain in part because its efficiency as a barrier against heterospecific mating can depend on the timing of autonomous selfing. • To investigate whether increased autonomous selfing has evolved as a mechanism for reinforcement, we compared Phlox cuspidata populations across their native Texas range using both estimates of genetic diversity and experimental manipulation with morphological measurements. Specifically, we investigated patterns of variation in floral traits and timing of selfing between individuals from allopatric populations of P. cuspidata and from populations sympatric with the closely related species, P. drummondii. • We infer intermediate rates of selfing across field-collected individuals with no significant difference between allopatric and sympatric populations. Among greenhouse grown plants, we find no differences in timing of selfing or other floral traits including anther dehiscence timing, anther-stigma distances, autonomous selfing rate and self-seed count between allopatric and sympatric populations. However, our statistical analyses indicate that P. cuspidata individuals sympatric with P drummondii seem to have generally larger flowers compared to allopatric individuals. • Despite strong evidence of costly hybridization with P. drummondii, we find no evidence of trait divergence due to reinforcement in P. cuspidata. Although we document nearly complete autonomous self-seed set in the greenhouse, estimates of selfing rates from genetic data imply realized selfing is much lower in nature suggesting an opportunity for reinforcing selection to act on this trait.
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Limited reproductive interference despite high rates of heterospecific pollen transfer among co‐occurring bat‐pollinated Burmeistera
Abstract Premise Many tropical plants are bat‐pollinated, but these mammals often carry copious, multispecific pollen loads making bat‐pollinated plants susceptible to heterospecific pollen deposition and reproductive interference. We investigated pollen transfer between sympatric bat‐pollinated Burmeistera species and their response to heterospecific pollen deposition from each other. Methods We quantified conspecific and heterospecific pollen deposition for two populations of B. ceratocarpa , a recipient species in heterospecific pollen transfer interactions, that co‐occur with different donor relatives ( B. borjensis and B. glabrata ). We then used a cross‐pollination scheme using pollen mixtures to assess the species' responses to heterospecific pollen deposition in terms of fruit abortion and seed production. Results Burmeistera ceratocarpa received significantly more heterospecific pollen from its relatives at both sites than its own pollen was deposited on its relatives. However, heterospecific pollen deposition only affected seed production by B. borjensis and B. glabrata , but not by B. ceratocarpa , suggesting that early acting post‐pollination barriers buffer the latter against reproductive interference. Crosses between sympatric and allopatric populations suggest that the study species are fully isolated in sympatry, while isolation between allopatric populations is strong but incomplete. Conclusions We did not observe evidence of reproductive interference among our study species, because either heterospecific pollen deposition did not affect their seed production ( B. ceratocarpa ) or they receive heterospecific pollen only rarely ( B. borjensis and B. glabrata ). Frequent heterospecific pollen deposition might favor the evolution of barriers against foreign pollen (as in B. ceratocarpa ) that alleviate the competitive costs of sharing low fidelity pollinators with co‐occurring species.
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- Award ID(s):
- 1754802
- PAR ID:
- 10466165
- Date Published:
- Journal Name:
- American Journal of Botany
- Volume:
- 110
- Issue:
- 6
- ISSN:
- 0002-9122
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Understanding how pollen moves between species is critical to understanding speciation, diversification, and evolution of flowering plants. For co-flowering species that share pollinators, competition through interspecific pollen transfer (IPT) can profoundly impact floral evolution, decreasing female fitness via heterospecific pollen deposition on stigmas and male fitness via pollen misplacement during visits to heterospecific flowers. The pollination literature demonstrates that such reproductive interference frequently selects for reproductive character displacement in floral traits linked to pollinator attraction, pollen placement, and mating systems and has also revealed that IPT between given pairs of species is typically asymmetric. More recent work is starting to elucidate its importance to the speciation process, clarifying the link between IPT and current and historical patterns of hybridization, the evolution of phenotypic novelty through adaptive introgression, and the rise of reproductive isolation. Our review aims to stimulate further research on IPT as a ubiquitous mechanism that plays a central role in angiosperm diversification.more » « less
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Abstract PremiseCo‐occurring plant species that share generalist pollinators often exchange pollen. This heterospecific pollen transfer (HPT) impacts male and female reproductive success through pollen loss and reductions in seed set, respectively. The resulting fitness cost of HPT imposes selection on reproductive traits (e.g., floral color and shape), yet we currently lack strong predictors for the post‐pollination fate of heterospecific pollen, especially within community and phylogenetic contexts. MethodsWe investigated the fate of heterospecific pollen at three distinct stages of plant reproduction: (1) pollen germination on the stigma, (2) pollen tube growth in the style, and (3) fertilization of ovules. We experimentally crossed 11 naturally co‐flowering species in the subalpine meadows of the Colorado Rocky Mountains, across a spectrum of phylogenetic relatedness. Using generalized linear mixed models and generalized linear models, we evaluated the effect of parental species identity and phylogenetic relatedness on pollen tube growth at each reproductive stage. ResultsWe found that heterospecific pollen tubes can germinate and grow within pistils at each reproductive stage, even when parental species are >100 My divergent. There was no significant effect of phylogenetic distance on heterospecific pollen success, and no evidence for a mechanism that suspends heterospecific pollen germination or pollen tube growth within heterospecific stigmas or styles. ConclusionsOur results show that even in communities where HPT is common, pre‐zygotic post‐pollination mechanisms do not provide strong barriers to interspecific fertilization. HPT can result in the loss of ovules even between highly diverged plant species.more » « less
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Abstract Prezygotic isolation is often stronger between sympatric as opposed to allopatric taxa, but the underlying cause can be difficult to infer from comparative studies alone. Experimental evolution, where evolutionary responses to treatments manipulating the presence/absence of heterospecific individuals are tracked, can provide a powerful complementary approach. We used experimental evolution to investigate a naturally occurring pattern of reproductive character displacement in the mushroom-feeding fly, Drosophila subquinaria. In nature, female D. subquinaria from populations sympatric with the closely related Drosophila recens discriminate more strongly against heterospecific males than do females from allopatric populations. Starting with 16 replicate allopatric populations of D. subquinaria, we manipulated the presence/absence of D. recens during mating (experimental sympatry vs. control) and, when present, we allowed hybrids to live or kill them each generation. Across 12 generations, heterospecific offspring production from no-choice mating trials between D. subquinaria females and D. recens males declined in both experimental sympatry treatments relative to the control, suggesting increased sexual isolation. Male cuticular hydrocarbon profiles also evolved, but only in the hybrids killed treatment. Our results strongly imply that the existing reproductive character displacement in wild D. subquinaria populations was an evolutionary response to selection arising from secondary contact with D. recens.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/ajb2.16199
