Variation in sex organ dimensions across individuals, morphs and species: consequences for reciprocity, herkogamy, and reproductive success in species with style length polymorphism

Abstract

Style length polymorphism is a floral polymorphism characterised by the presence of two or three floral morphs in a population which differ in style length. The positioning of anther and stigma within and between flowers of these morphs is one of the important drivers of inter-morph pollen transfer and helps in avoiding self-pollination. Despite this, general patterns of relative sex organ positions in these species are not understood. Moreover, the extent of intra-population variation in sex organ positions, which is known to reduce inter-morph pollen transfer is not understood completely.

To understand the relative sex-organ position and intra-population variation in sex organ dimensions in naturally occurring populations, I quantified: a) herkogamy, the separation between anthers and stigmas within a flower which reduces self-pollination; and, b) reciprocity, the match in height of complementary anthers and stigma between morphs, which promotes inter-morph pollination, from data extracted that included 296 species, from 25 families. I also examined the relationship between herkogamy and reciprocity, the fundamental features of species with style length polymorphism. I found that the morphs differ significantly in herkogamy and a large range of intra-population variation is present in sex organ dimensions in these species. Both positive and negative relationships were observed between herkogamy and reciprocity. This relationship has not been reported before and implies a trade-off between avoidance of illegitimate self-pollen deposition, and legitimate inter-morph pollen deposition. This trade-off is important for understanding the evolution of style length polymorphisms in the context of pollen limitation, reproductive assurance, inbreeding depression and self-incompatibility. To better understand the effect of intra-population variation on reciprocity and consequently pollen transfer in these species, I developed an index to quantify reciprocity, which appropriately incorporates intra-population variation in sex organ dimensions. Contrary to the current understanding, using this newly developed index and simulated as well as data from naturally occurring populations, I demonstrated that greater intra-population variation can result in greater reciprocity and help promote pollen transfer between morphs. Finally, I examined the relative importance of herkogamy and reciprocity in promoting inter-morph pollen transfer in Jasminum malabaricum Wight, a species with style length polymorphism that exhibits an imperfect reciprocity. The results suggest that herkogamy is more important than reciprocity in increasing legitimate pollen transfer and reproductive success. These results can help us explain the stabilization of character states that have imperfect reciprocal arrangement of sex organs and are evolving towards perfect heterostyly.