Reproductive isolation can rise either as a consequence of genomic divergence in allopatry or as a byproduct of divergent selection in parapatry. To determine whether reproductive isolation in gynodioecious Silene nutans results from allopatric divergence or from ecological adaptation following secondary contact, we investigated the pattern of postzygotic reproductive isolation and hybridization in natural populations using two phylogeographic lineages, western (W1) and eastern (E1). Experimental crosses between the lineages identified strong, asymmetric postzygotic isolation between the W1 and the E1 lineages, independent of geographic overlap. The proportion of ovules fertilized, seeds aborted, and seeds germinated revealed relatively little effect on the fitness of hybrids. In contrast, hybrid mortality was high and asymmetric: while half of the hybrid seedlings with western lineage mothers died, nearly all hybrid seedlings with E1 mothers died. This asymmetric mortality mirrored the proportion of chlorotic seedlings, and is congruent with cytonuclear incompatibility. We found no evidence of hybridization between the lineages in regions of co-occurrence using nuclear and plastid markers. Together, our results are consistent with the hypothesis that strong postzygotic reproductive isolation involving cytonuclear incompatibilities arose in allopatry. We argue that the dynamics of cytonuclear gynodioecy could facilitate the evolution of reproductive isolation.

Gynodioecy involves the stable co-occurrence of females and hermaphrodites. Its maintenance theoretically depends on differences in female and male reproductive success among gender morphs. Although many gynodioecious species also include gynomonoecious individuals that carry a mixture of female and perfect flowers, little is known about the male and female fitness of this third morph. Here, we present the first study of the reproductive system of Silene nutans, including females, gynomonoecious plants, and hermaphrodites. By measuring 10 floral traits in controlled conditions, we showed that females bear smaller and lighter flowers than hermaphrodites, with female and perfect flowers of gynomonoecious plants being intermediates. By measuring pollen quantity and quality, we showed that gynomonoecious plants had a lower potential male fitness than hermaphrodites at the level of both flowers and individuals. In addition, gynomonoecious plants were shown to widely vary their proportion of female flowers (0.03-0.9) and their floral traits, suggesting a quantitative restoration of male fertility. Finally, controlled pollinations showed evidence for inbreeding depression (d ¼ 0:3) in progeny of hermaphrodites and gynomonoecious individuals, affecting both pre-and postdispersal traits; this could provide a selective advantage for females.