Discussion

The Pimelea species at Cass, and others in New Zealand are gynodioecious both in structure and function, although some variations in sex expression are extant, as they are in many plant and animal populations. Some genetic experiment is needed to establish the basis for control of sex determination in Pimelea and to discover reasons for variation between different populations in numbers of the sexes. In wild populations the presence of relative proportions of one sex to another is interpreted by Heslop-Harrison (1957) as being due to the interaction of environment with the hormone system of the plants. In Pimelea maintenance of a high proportion of females benefits the species by ensuring outcrossing. This is enabled by activity of animal pollinating agents and the dimorphism of the sexes. Differences in pollinating species in different areas may contribute to variations in proportions of females to hermaphrodites. Ford (1957) in discussion of “intersexes” and the heterostyly of Primula shows how the control of these conditions is genetic and changes are induced by selective influences.

Gynodioecy is in the main an outbreeding system. The higher the proportion of females to hermaphrodites, the greater the amount of outcrossing which is possible. Thus P. “short tussock” and P. “snow tussock” each with 50% of their populations females, at Cass, potentially are able to exchange genes most readily within or between species. The consequences of this, especially with regard to the plasticity of and hybridization between the Pimelea species are most interesting. The probable selective action of flower structure and insect activity in maintaining this situation are supplemented by the low fruit set in hermaphrodite plants as compared with females. However, a little inbreeding can take place, and this could also be of benefit. In P. prostrata and P. traversii the proportion of females is not as high but must also have an important bearing on their variation and ability to hybridize. It is clear, however, that the amount of hybridization between them and other species is more limited than between P. “snow tussock” and other species. They are also less variable throughout their range than P. “snow tussock”. The outbreeding of the latter thus contributes to its plasticity and to the amount of hybridization occurring between it and other species, including P. aridula and P. sericeo-villosa at Cass and elsewhere. Some of this hybridization (observed in terms of variations in morphology and supported by experimental work) is very far-reaching, and probably of the type described by Anderson (1953) and others, as introgression. Widespread hybridization in the genus Pimelea may be attributed in a large measure to gynodioecy in many of the species.