The evolution and maintenance of gender dimorphism in New Zealand Hebe (Scrophulariaceae).

The evolution and maintenance of gender dimorphism was investigated in the New Zealand genus Hebe (Scrophulariaceae). Hebe originated within New Zealand and gender dimorphism is postulated to have evolved separately four times within the genus. Gender was found to be controlled at least partially by nuclear genes for one species of Hebe. Most species are self-compatible, and H. subalpina was shown to exhibit inbreeding depression. The frequency of females was found to be negatively correlated with the amount of fruit set on the polleniferous (or male) morpho Dimorphism is correlated with altitude, and it is hypothesized, based on the pollinators present at different altitudes and degree to which they self-fertilize flowers, that separate sexes evolved in response to the increased level of self pollination that occurs at the higher altitudes. Female frequency varied greatly among the populations of H. strictissima, and was found to be negatively correlated with the relative seed fitness of the males. Fruit set on the males varied greatly in response to plant vigour within plants, among plants within populations, and among populations. Fruit set on females was not correlated with plant vigour, and varied less between populations. Female plants were more prevalent in relatively poor sites where male seed fitness was low. Gender estimates for H. subalpina, based on fruit set, sex ratio, seed production, inbreeding depression, and the rate of self-fertilization, showed that males received less than four per cent of their fitness through ovules. H. subalpina is, therefore, near the dioecy end of gynodioecy. Fruit set on males varies much more than that on females, and is dependent on both the status of individual shoots and the genotype of the plant. Resource allocation patterns differ according to sex, and reflect the way in which the sexes receive their fitness