Habitat preference and flowering-time variation contribute to reproductive isolation between diploid and autotetraploid Anacamptis pyramidalis.

Tetraploid lineages are typically reproductively isolated from their diploid ancestors by post-zygotic isolation via triploid sterility. Nevertheless, polyploids often also exhibit ecological divergence that could contribute to reproductive isolation from diploid ancestors. In this study, we disentangled the contribution of different forms of reproductive isolation between sympatric diploid and autotetraploid individuals of the food-deceptive orchid Anacamptis pyramidalis by quantifying the strength of seven reproductive barriers: three prepollination, one post-pollination prezygotic and three post-zygotic. The overall reproductive isolation between the two cytotypes was found very high, with a preponderant contribution of two prepollination barriers, that is phenological and microhabitat differences. Although the contribution of post-zygotic isolation (triploid sterility) is confirmed in our study, these results highlight that prepollination isolation, not necessarily involving pollinator preference, can represent a strong component of reproductive isolation between different cytotypes. Thus, in the context of polyploidy as quantum speciation, that generates reproductive isolation via triploid sterility, ecological divergence can strengthen the reproductive isolation between cytotypes, reducing the waste of gametes in low fitness interploidy crosses and thus favouring the initial establishment of the polyploid lineage. Under this light, speciation by polyploidy involves ecological processes and should not be strictly considered as a nonecological form of speciation.

Reproductive isolation between autotetraploids and their diploid progenitors in fireweed, Chamerion angustifolium (Onagraceae).

Polyploidy is viewed as an important mechanism of sympatric speciation, but few studies have documented the reproductive barriers between polyploids and their diploid progenitors or explored the significance of assortative mating for polyploid establishment. Here we synthesize new and existing data on five prezygotic (geographic isolation, flowering asynchrony, pollinator fidelity, self-pollination, gametic selection) and two postzygotic (selection against triploid hybrids, inbreeding depression) reproductive barriers between diploid and autotetraploid individuals of the perennial plant Chamerion angustifolium. We also present estimates of realized rates of between-ploidy mating and examine the impact of assortative mating on polyploid dynamics using computer simulation. Reproductive isolation (measured from 0 to 1) was enforced by each barrier, including: geographic separation (RI = 0.41), flowering asynchrony (0.13), pollinator fidelity (0.85), self-pollination (0.44), gametic selection (0.44) and postzygotic isolation (0.87). Total reproductive isolation was 0.997, with the largest relative contributions by geography (41%) and pollinator fidelity (44%). Prezygotic barriers accounted for 97.6% isolation overall; however, tetraploids were more assortatively mating (98%) than diploids (79%). Realized reproductive isolation between ploidy levels in sympatric populations was 87% and tetraploids produced significantly fewer triploids than did diploids. Simulations indicated that the observed prezygotic isolation will reduce the strength of minority disadvantage acting on tetraploids and increase the importance of differences in viability and fertility between cytotypes in regulating polyploidy establishment.