Solanum, a genus of ~1500 global species, is one of the more interesting plant groups in which to study reproductive biology and ecology. Overwhelmingly, species in this group express full cosexuality, where individual plants have flowers containing both fully-functioning “male” (staminate) and “female” (carpellate) organs. However, there have been multiple and widespread evolutionary transitions within the genus to non-normative variations on this ancestral condition. Australian bush tomatoes (ca. 40 species) are especially diverse in this regard, with uncommon variation and combinations of unisexuality and cosexuality -- including, most notably, two sexual systems known as dioecy (unisexual male or female flowers on separate plants) and andromonoecy (combinations of male and cosexual flowers on every plant). This commentary summarizes 40+ years of study and highlights some of the more intriguing observations/findings that make the bush tomatoes an ideal model system for examining plant sexual expression – including functionally “female” flowers (in which “male” organs are formed and pollen is produced, but that pollen serves only as a reward to pollinators and plays no other role in sexual reproduction), leaky “male” plants (in which “male” flowers sometimes express “female” function via functional pistils and thus become cosexual), the preponderance of vestigial opposite-sex organs in unisexual flowers, and species (such as the recently-described S. plastisexum) where variation in sexual expression is the norm rather than the exception.
Integrative and Comparative Biology
Martine, C.T. 2023. Fluidity and inconstancy: Australian bush tomatoes as an exemplar of non-normative sex expression. Integrative and Comparative Biology. https://doi.org/10.1093/icb/icad051
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