4.4 Does homostyly spread among populations and why has it not become fixed in Primula vulgaris?
Gene flow plays a central role in the spreading of mutations (Morjan & Rieseberg, 2004; Ralph & Coop, 2010),⁠ potentially contributing to the migration of homostylous alleles among populations. In P. vulgaris, previous hypotheses proposed that homostyly could have migrated from initial places of origin to neighboring populations through transport of pollen or seed, the former being more likely (Crosby, 1949, 1960). Estimates of gene flow through pollen and seed in this species indicated that dispersal is restricted to a maximum of a few hundred meters from the parental plant (Cahalan & Gliddon, 1985), but occasional pollen flow over 1–3 kilometers has also been reported (Van Geert, Van Rossum, & Triest, 2010)⁠. Our population genetic analyses support a model of moderate levels of gene flow among populations (Nm = 1.96; Figure S3), while rejecting the hypothesis of a recent fragmentation of an ancestral, widespread population. Accordingly, our results do not support strong genetic differentiation among populations (Figures 7A and B) or isolation by distance (Mantel test, P = 0.457), further favoring the hypothesis of gene flow among populations. Moreover, the geographic distribution of CYPᵀ alleles in our study indicates that 12 homostylous individuals from five populations (D*11, T03, T04, T07 and T10) separated by 3-18 Km shared the sameCYPᵀ allele (CYPᵀ -2; Figure 4). This suggests that homostylous alleles could have migrated between neighboring populations, as the independent origin of exactly the same mutation should be unlikely. Altogether, these results favor the conclusion that gene flow might have facilitated the migration of homostylous alleles among geographically close populations of P. vulgaris , confirming the importance of gene flow for the spread of potentially advantageous mutations.
A crucial question remains as to why homostyly has not become fixed inP. vulgaris despite automatic selfing advantage (Fisher, 1941, 1949)⁠ and reproductive assurance (Crosby, 1949)⁠. In fact, the opposite has been found, since previous studies recorded the loss or decrease of homostyles in revisited populations (Curtis & Curtis 1985; Boyd et al., 1990). A possible explanation for the failure of homostyly to completely replace heterostyly is that the negative effects of inbreeding depression (Richards 1984; Goodwillie et al., 2005) can outweigh the advantages of homostyly (Piper et al., 1984; Boyd et al., 1990). Two recent studies investigated the relationship between inbreeding depression and the spread of self-compatibility, finding contrasting results. In experimental populations of Linaria cavanillesi , self-compatible individuals with no inbreeding depression displaced self-incompatible individuals in just three generations (Voillemot & Pannell, 2017; Voillemot, Encinas-Viso, & Pannell, 2019)⁠. In contrast, inbreeding depression values of 0.54 prevented the spread of SC plants within experimental patches of SI plants inLaevenworthia alabamica (Layman, Fernando, Herlihy, & Busch, 2017) ⁠. Our estimate of inbreeding depression (0.58) in P. vulgaris is closer to the latter study, suggesting that inbreeding depression might be sufficiently high to prevent the fixation of homostyly within populations. While theory proposes that increased selfing should purge inbreeding depression over time by eliminating recessive deleterious alleles from populations (Schemske & Lande, 1985)⁠, transitions to homostyly in P. vulgaris may be too recent for the purging process to have occurred. Moreover, recessive deleterious alleles could have been re-introduced into populations via gene flow, thus slowing the decrease of inbreeding depression. Our study estimated moderate levels of gene flow among populations of P. vulgaris (Nm = 1.96; Supplementary Figure S3), implying that re-introduction of genetic load is feasible. Overall, our findings suggest that inbreeding depression plays a key role in maintaining heterostylous morphs within populations despite the reproductive advantages of homostyly.