PHENOLOGY OF SEX EXPRESSION AND GENDER VARIATION IN A HETERODICHOGAMOUS MAPLE, ACER JAPONICUM

Abstract

Heterodichogamy, that is, dimorphic sex expression of protogynous (stigmas are receptive before pollen is shed) and protandrous (pollen is shed before stigmas are receptive) mating types provides a possible evolutionary pathway to dioecy (dimorphism of males and females) in plant species. I investigated whether the gender of protogynous or protandrous morphs is specialized in male or female function in a population of a heterodichogamous maple, Acer japonicum (Aceraceae), by examining the phenology of sex expression and phenotypic gender in two morphs. Most of the trees in the study (>90%) were bisexual and from either the protogynous (PG) or the protandrous (PA) type. The remaining trees (<10%) produced only male flowers over three successive years (M) or produced only male flowers one year and both male and female flowers in other years (M/PA). Potential mating opportunity based on the phenology of sex expression was higher between PG and PA type than within these types. Both floral sex allocation and total sex allocation, including investment to fruit development, showed a wider range of variation in PA than in the PG type. But the differences of the means of phenotypic and functional gender between PG and PA were not evident. Floral sex allocation in one year was positively correlated with that in the next year in both PG and PA types. Plant size did not account for the variation in phenotypic gender, which suggests that the size-advantage hypothesis is not applicable to this maple. There were no significant differences between PG and PA in fruit abortion or in seed and pollen germination rates. Although gender specialization was not evident between PG and PA types, large gender variation among individuals in the protandrous type provides a high probability that male plants have derived from protandrous morph.