Genetic sex determination and sexual instability in Amborella trichopoda

Amborella trichopoda, the sister species of all other extant angiosperms, is typically dioecious, producing male and female flowers on separate individuals. Anger et al. map sex phenotypes onto a phylogeny of basally diverging angiosperms using maximum parsimony. Sex-ratios and phenotypes were studied in parallel over two consecutive flowering seasons for an ex situ population of A. trichopoda. The sex-ratio of an in situ population was also assessed.

The results show that the primary male-female ratio in A. trichopoda is 1:1, suggesting a genetic mechanism of sex determination, however, male-biased sex ratios were evident in natural populations. Rare sex-change events were observed in A. trichopoda, with bisexual flowers manifesting on otherwise male plants. The authors propose a method to date the origin of dioecy in A. trichopoda and deduce that sexual instability in A. trichopoda suggests the evolution of dioecy via a gynodioecious intermediate.

This paper is available with FREE access as an Editor’s Choice from the March issue of Annals of Botany.