Evolutionary correlation of heterodichogamy and floral longevity

Abstract

Heterodichogamy, the coexistence of morphs differing in the time when the shift between male and female phases occurs, is a rare sexual dimorphism derived from synchronous dichogamy. In dichogamous populations, plants need to adjust the temporal sex role to deal with the negative frequency dependent selection imposed by population sex ratios. However, such temporal adjustment may be constrained in dichogamous species with bisexual flowers and short floral longevities (<2 days) and therefore the reciprocal floral strategy can easily invade the population, ultimately leading to heterodichogamy. We predict that heterodichogamy has evolved in species with short floral longevities. To test this prediction, we compiled data on floral longevity for 377 dichogamous and heterodichogamous species with bisexual flowers and ran a phylogenetic logistic regression between floral strategy (dichogamy/heterodichogamy) and floral longevity. The results showed that heterodichogamous species had significantly shorter floral longevities than dichogamous species. This result together with the fact that evolutionary transitions always occurred from dichogamy to heterodichogamy supports the conclusion of the existence of a significant and negative evolutionary correlation between the floral longevity and the probability to evolve heterodichogamy from dichogamy in bisexual species. We discuss that the rarity of heterodichogamy in nature may be the result of other selective pressures towards long floral longevity in dichogamous species.