Discovery of male sterile plants and their contrasting occurrence between self-compatible and self-incompatible populations of the hermaphroditic perennial Trillium camschatcense

The sex type of Trillium camschatcense has been considered to be strictly monomorphic, composed of only hermaphroditic individuals. However, field observations have detected anther-degenerated (AD) plants coexisting with hermaphrodites. The present study aimed to elucidate whether AD plants could be regarded as females and, if so, how their loss of male function is compensated. Because T. camschatcense retains both self-compatible (SC) and self-incompatible (SI) populations, the frequencies of AD plants in multiple populations (22 SC and eight SI) were examined to predict the fate of these individuals under contrasting breeding systems. Morphological and genetic analyses in a SC population demonstrated that AD plants were completely male sterile, but female fertile. Although the quantity of seed produced was similar, hermaphrodites produced seeds predominantly via selfing, whereas seeds of AD plants were entirely outcrossed. Because inbreeding depression was severe, AD plants achieved a fitness advantage through inbreeding avoidance. However, the frequency of AD plants varied among SC populations (0-42%), suggesting that environmental variance can modify the relative fitness. Conversely, the frequency was suppressed among SI populations (0-2%). Because SI hermaphrodites are free from inbreeding, AD plants would not benefit from inbreeding avoidance and would find it difficult to invade a SI population.