Temporal sexual maturation and incremental staminal movement encourages mixed mating in Withania somnifera - An insurance for reproductive success

摘要

Withania somnifera is a multipurpose plant of immense therapeutic value and wide geographic distribution. It exhibits extensive phenotypic and chemical variability. Here we describe a sexual mechanism deployed by the species to make the maximum out of sexual polymorphism. Our results suggest that the species practices mixed mating as a consequence of partial temporal dichogamy of protogynous type under which the receptive stigma remains exserted beyond the undehisced staminal cone to receive cross pollen through insect vectors. In a probable situation of non-receipt of pollen through insect pollinators, autonomous fertilization is guaranteed by the upward staminal increase to form a cone connivent about the receptive stigma. Floral pheno-events suggest that crossing and autonomous selfing are mutually exclusive as the self-pollen arrives late during floral ontogeny. Seed-set efficiency and fruiting success are not influenced by pollen genotypes (self/cross) under different pollination treatments (autogamy, geitonogamy and xenogamy). Preemergent reproductive success averages 37% with open pollination. DNA content (chromatin length 47.51 mu m) is distributed asymmetrically among 24 chromosomes (2n = 48). Meiotic system reveals high recombination index (71.2). In accordance with the theoretical predictions for mixed mating, intermediate levels of heterozygosity are maintained. This is illustrated by the proportion of polymorphic loci calculated as Shannon index employing RAPD. High pollen-ovule ratio (631.77), pronounced protogyny, copious nectar production and insect visitation provide for strong outcrossing. However, small flower size, high seed-to-ovule ratio (0.76) and moderate fruit-to-flower ratio (0.49) are in conformity with the predicts of 'reproductive assurance hypothesis'. Mixed mating and the efficient meiotic recombination system provide a reliable strategy that guarantees reproductive success and genetic polymorphism 'with' you or 'without' you pollinator environment.