The role of sex ratio as a context for selection in Silene vulgaris

Abstract

When the sex ratio varies among populations, it is likely that the fitness consequences of sexual phenotypes will also differ among those populations. Sex ratio can also vary at a fine scale within populations, and so there may be multiple, potentially overlapping scales over which sex ratio can affect the evolutionary process. I explored the fitness consequences of sex ratio variation in Silene vulgaris, a perennial herb that maintains a sexual polymorphism between plants that produce both pollen and seed, and plants that are only capable of producing seed. My results demonstrate that there is variance in the local sex ratio individuals experience within wild populations, which creates an opportunity for selection to drive differences in fitness among individuals at a fine scale. Further, I show that nocturnal insects, which are the effective pollinators of this species, do not exhibit preferences for the sex of individuals or the sex ratio of groups. This lack of preference in pollinator behavior means that pollinator service should be uniform among individuals regardless of sex or sex ratio, and so fine-scale variation in pollen availability is likely to generate heterogeneous fitness consequences for sexual phenotypes. Finally, I use experimental populations to show that both sexes have the highest fitness when they are rare, and that the direction and magnitude of selection on floral traits is substantially altered by sex ratio. Thus, the sex ratio an individual experiences is a critical context in which selection on sexual phenotypes occurs, and should have dramatic effects on the evolution of those phenotypes at multiple scales within and among populations.iii ACKNOWLEDGEMENTS

Pollination context alters female advantage in gynodioecious Silene vulgaris

“…It must be noted that the interactions we observed are not representative of all gynodioecious systems, of all populations of S. vulgaris , or even across all years at the same site. Similar experiments on S. vulgaris at the same site but in different years failed to find significant differences between nocturnal and diurnal pollinators (Sanderson, ), indicating the potential for temporal variation in pollinator services to influence the maintenance of gynodioecy. Whereas we recorded no seed predation at our site, nocturnal pollinators in Silene's European range, particularly from the genus Hadena , are known to act as seed predators that may shift the pollination interaction from mutualistic towards parasitic (Pettersson, ; Giménez‐Benavides et al ., ).…”

“…Alonso (2004) and Barthelmess et al (2006) demonstrated the practicality of experimentally manipulating pollinator guilds to study the effects of their component groups in gynodioecy-rich lineages. Direct evidence of pollinators' influence on female advantage by measuring both sexes' seed production in multiple pollination contexts, however, remains limited (Ashman, 2006;Dufa€ y & Billard, 2012;Sanderson, 2016). Alonso (2004) reported variation in female and hermaphrodite fruit set associated with pollinator exclusion in Daphne laurelola, but found no significant sex-by-pollination treatment effects.…”