BREEDING SYSTEM AND SEX RATIO VARIATION IN MULBERRIES (MORUS, MORACEAE)

摘要

Flowering plants exhibit a diverse array of sex expression patterns that become of particular interest when considered in the context of coexisting native-invasive congeners. This study presents findings on sex expression and sex ratio variation in two congeneric tree species, the native Moras rubra and the exotic M. alba, in the Flint Hills region of the Great Plains in the United States. Both species exhibited a subdioe-cious breeding system (with male, female and hermaphrodite individuals co-occurring within populations), and significantly male-biased sex ratios (i.e., males are more numerous than females). Cumulative sex ratio deviation was higher in the native M. rubra populations than in the exotic M. alba. Within-species sex ratio did not vary significantly among populations. In one focus study area surveyed over three years, most individuals were consistent in sex expression, but approximately 10% switched their sex at least once; the vast majority of these changes were between unisexual and hermaphrodite morphs. There was no size dependence of sex expression based on current sampling. Our results document subdioecy as the breeding system in these Morus species, and advance the group as an interesting study system for its reproductive biology.