Harsh environments favour gender diomorphism in the 'Wurmbea biglandulosa' (Colchicaceae)

In flowering plants, the evolution of dimorphic sexual systems (e.g. gynodioecy and dioecy) from monomorphic ancestors is often associated with aridity, or otherwise harsh environmental conditions. One hypothesis to explain this pattern is that harsh conditions adversely affect seed fertility of hermaphrodites more than females, resulting in greater relative female fitness in such environments. 'Wurmbea biglandulosa' exhibits interpopulation variation in sexual system and in female frequency in dimorphic populations. We investigate how environmental quality and hermaphrodite seed fertility differ between monomorphic (hermaphrodites only) and dimorphic (females and hermaphrodites) populations and whether relative female fitness and female frequency are higher in harsher sites. Dimorphic populations were characterised by hotter and drier conditions with lower soil moisture and less vegetation cover than monomorphic populations. Consistent with these environmental differences, hermaphrodites had lower seed fertility in dimorphic than monomorphic populations. In dimorphic populations, relative hermaphrodite fitness was correlated with site quality and the frequency of females. By contrast, female frequency was not correlated with site quality. These results confirm that harsh environmental conditions play a role in the evolution of gender dimorphism in 'W. biglandulosa' by differently affecting the seed fertility of the sexual morphs.