Differences and similarities of sex ratios between dioecious angiosperms and dioicous bryophytes

Abstract

Observation: For plant species with separate sexes, the average population sex ratio is female biased in mosses and liverworts, male biased in trees and shrubs, and unbiased in short-lived herbs. Key assumption: When fertilization occurs, female investment in reproduction greatly outweighs male investment. Questions: Can we explain the general pattern in population sex ratios from primary sex ratios in seeds or spores? What does genomic conflict theory predict for sex ratios of angiosperms and bryophytes? Are there other explanatory factors? Data studied: Literature review of variation in sex ratio found in seeds and spores and of male and female performance. Conclusion: Primary sex ratios were often variable, suggesting genomic conflict involving both cytoplasmic factors and sex-linked drive. Even though theory predicts some differences, primary sex ratios of angiosperms and bryophytes were remarkably similar. Males sometimes perform worse than females in the pre-reproductive phase and this may result in a female bias before reproduction occurs. In the reproductive stage, females invest more than males and may suffer greater mortality. The sex ratio of a cohort may then slowly change from female to male bias. This might explain the different population sex ratios of short-lived (herbs) and long-lived angiosperms (trees). In bryophytes, low fertilization rates, reducing female allocation to reproduction, could shift the balance towards consistent female bias in their populations.