4.3 Consequences of shifts to homostyly on mating system and population genetics
Changes in floral morphology can have profound effects on mating system (Opedal, 2018)⁠. Specifically, the reduction of anther and stigma separation and loss of self-incompatibility of homostylous morphs should increase selfing when compared to heterostyles. Evidence of this mating system transition in homostyles has been reported in Eichhornia (Husband & Barrett 1993), Turnera(Belaoussoff & Shore, 1995)⁠, Amsinckia(Schoen et al., 1997)⁠ and some Primula species (Yuan et al., 2017; Zhou et al., 2017; Zhong et al., 2019). In the case of P. vulgaris , previous work provided varying estimates of the mating system of homostyles (Bodmer, 1984)⁠. On the one hand, analyses of floral morph segregation in the progeny of open-pollinated plants concluded that outcrossing rates of homostyles were as high as 0.80 (Bodmer 1958). Conversely, similar analyses under both controlled and natural field conditions (Crosby 1958), as well as allozyme analyses of progeny arrays (Piper et al., 1986)⁠, reported outcrossing rates for homostyles ranging from 0.05 to 0.10. Our results from progeny arrays indicate that outcrossing rates in homostyles are significantly lower than in heterostyles (tm = 0.14 vs. 1.0, respectively; Figure 5), corresponding to the definition of selfers for homostyles (i.e.,tm ≤ 0.2; sensu Schemske & Lande 1985; Goodwillie et al., 2005). Additionally, the outcrossing rates of homostyles are more variable than those of heterostyles (Figure 5), suggesting that occasional outcrossing can occur in the former. This is in accordance with previous results in P. halleri , a homostylous species with variable herkogamy which also displayed variable outcrossing rates (de Vos, Keller, Zhang, Nowak, & Conti, 2018)⁠. Furthermore, the influence of homostyly on mating system was supported by a positive correlation between the frequency of homostyles within populations and population-level estimates of selfing rates (Figure 6A). Overall, our findings clarify previous conflicting results about the effects of homostyly on mating system in P. vulgaris , confirming that transitions to homostyly increase selfing.
Mating systems have important effects on how genetic diversity is partitioned both within and among populations (Barrett, 2010b; Wright et al., 2013)⁠. The higher selfing rate of P. vulgarishomostyles (Figures 5 and 6A) should increase homozygosity within populations, leading to an increment of inbreeding and genetic differentiation among populations (Hamrick & Godt, 1996). In accordance with these expectations, we found that inbreeding coefficients are positively correlated with the frequency of homostyles within populations (Figure 6B). Moreover, pairwise genetic differentiation among populations is correlated with pairwise increase in the frequency of homostyles (Mantel test, P = 0.001). These results are in accordance with previous comparisons among homostylous and heterostylous taxa in Primula section Obconicolisteri (Zhong et al., 2019) and populations in P.chungensis (Zhou et al., 2017), P. oreodoxa (Yuan et al., 2017), and Eichhornia paniculata (Husband & Barrett, 1993; Ness et al., 2010). Therefore, our findings confirm the population genetic consequences predicted for the transition to homostyly as a transition to increased selfing.