Sex expression variation in response to shade in gynodioecious-gynomonoecious species: Silene littorea decreases flower production and increases female flower proportion

Abstract

Sexual expression can vary at different levels in diclinous sexual systems. To measure total phenotypic variation, it is necessary to consider the genetic component, the effect of environment, and interaction (i.e. the heritable component of plasticity). Silene littorea has a gynodioecious-gynomonoecious sexual system, with a high proportion of gynomonoecious individuals in populations, making this species a good system for studying how male and female sexual functions vary within and among individuals in response to a depletion of resources. In the current study, we experimentally assigned sibs from different maternal genotypes to sun vs. shade treatments for three populations and measured their effect on sex expression, onset of flowering, total flower production, and aboveground and belowground biomass. We found significant differences between genotypes and light environments in total flower number, with shaded plants producing fewer total flowers and a higher proportion of female flowers. Throughout the season, shaded plants produced a higher ratio of female to hermaphroditic flowers, with an increase at the end of the flowering period. Shaded plants produced less significant aboveground and belowground biomass as well as fewer flowers per plant, and flowering started later. Sexes of S. littorea differed in their first flowering day, with male-sterile plants starting to flower later than male-fertile plants. We found among population differences in relative distance plasticity index (RDPI) for total number of flowers and aboveground biomass. The plasticity in sexual expression found in S. littorea agrees with the bet-hedging strategy proposed for gynomonoecy: ginomonoecious plants may adjust for the resources available, producing a variable amount of small and less costly female flowers, which in addition could have lower selfing rates.