Sexual Traits Are Not Markedly Vestigialized in Asexual Populations of Decodon verticillatus (Lythraceae)

Abstract

Premise of research. Transitions from sexual to asexual reproduction are predicted to be accompanied by the reduction and developmental disintegration of sexual traits that no longer maintain fitness. While such transitions are common in plants, the evolutionary fate of sexual traits in derived asexual populations remains largely unknown, especially in species where the loss of sex is not confounded with a change in ploidy. In the tristylous wetland plant Decodon verticillatus, populations polymorphic for style length are sexual, while monomorphic populations are typically asexual under field conditions.Methodology. We compared ploidy, per-ramet flower production, flower size, and floral developmental stability between 22 monomorphic asexual populations and 26 polymorphic sexual populations distributed across the northern half of the species range in the Great Lakes region of North America.Pivotal results. Flow cytometry revealed that all populations were diploid—except for one monomorphic population that exhibited nuclear DNA content consistent with triploidy, which is known to cause sexual sterility. After accounting for the potential effects of temperature during flowering on floral development, we found that ramets in asexual populations produced ~50% (although not quite significantly) fewer flowers than those in sexual populations. Flowers varied widely in size among populations but were not smaller in asexual populations. Developmental stability of flowers was not lower in asexual populations than in sexual populations, although among monomorphic populations stability was lowest in populations for which confidence in exclusive asexuality was highest.Conclusions. Sex has been lost repeatedly across the northern range edge of D. verticillatus via at least two genetic pathways. Yet investment in and developmental stability of sexual traits is not consistently, and rarely significantly, reduced. The observation that no asexual genotype of D. verticillatus is widespread suggests that asexual populations do not persist long—perhaps only long enough to exhibit the very first stages of sexual vestigialization.