Genetic Constraints on Floral Evolution in a Sexually Dimorphic Plant Revealed by Artificial Selection

Abstract

Sexual dimorphism is one of the most widespread and recognizable patterns of phenotypic variation in the biotic world. Sexual dimorphism in floral display is striking in the dioecious plant Silene latifolia, with males making many, small flowers compared to females. We investigated this dimorphism via artificial selection on two populations to determine whether genetic variation exists within populations for flower size and the extent of the between-sex correlation, whether a flower size and number trade-off exists within each sex, and whether pollen and ovule production vary with flower size. We selected for decreased flower size (calyx width) in females and increased flower size in males and measured the response to selection in size and correlated responses in flower dry mass, flower number, and pollen or ovule number per flower. Four bouts of selection in each of two selection programs were performed, for a total of three selection lines to decrease size, three to increase it, and two control lines. Flower size always significantly responded to selection and we always found a significant correlated response in the sex not under selection. Selection decreased but did not eliminate the sexual dimorphism in flower dry mass and number. A negative relationship between flower size and number within each sex was revealed. Whereas ovule number showed a significant correlated response to selection on flower size, pollen number did not. Our results indicate that although substantial additive genetic variation for flower size exists, the high between-sex genetic correlation would likely constrain flower size from becoming more sexually dimorphic. Furthermore, floral display within each sex is constrained by a flower size and number trade-off. Given this trade-off and lack of variation in pollen production with flower size, we suggest that sexual dimorphism evolved via sexual selection to increase flower number in males but not females.

The effects of inbreeding, genetic dissimilarity and phenotype on male reproductive success in a dioecious plant

“…In particular, flower production increased male reproductive success, which may be a consequence of either greater pollen production [41] or pollinator attraction. Indeed, males with larger floral displays receive more pollinator visits [40].…”

“…Indeed, males with larger floral displays receive more pollinator visits [40]. The fitness increase associated with the higher production of flowers in males has important implications for the evolution of sexual dimorphism [41,65,66], which is thought to be driven by selection for increased flower number in males [41,42], the most extremely dimorphic trait [67]. For selection to occur, genetic variation should translate into differences in fitness.…”

“…Males are likely to produce many flowers as a result of sexual selection to attract more pollinators [40], whereas fecundity selection may favour females with larger and hence fewer flowers because of a negative genetic correlation between flower size and number, and a positive genetic correlation between flower size and ovule number [39,41,42]. Positive selection on the number of flowers in males has been suggested, but never, to our knowledge, tested with direct methods.…”

No amicable divorce? Challenging the notion that sexual antagonism drives sex chromosome evolution

“…In this and many other species, males and females differ in morphology and behaviour and so natural selection upon traits associated with survival and reproduction is expected to favour different optima in the two sexes. However, responses to selection are often correlated between males and females, with the result that opposing selection pressures reduce the average fitness of both sexes [24,25]. One cause of these deleterious intersexual genetic correlations is sexually antagonistic pleiotropy [26][27][28].…”

“…including sexually antagonistic traits [25,59]. The position of these genes in the PAR shows that they accumulated prior to suppression of recombination.…”