Iintrodution

A Knowledge of the breeding system of a plant species is necessary in order to understand the patterns of group variation. On the breeding behaviour depends the genetic structure and ultimately the kind and degree of genetic, heritable variation. Darwin (1884) defines gynodioecy as the state where there is a group of plants having female and hermaphrodite individuals. The term gynodioecious is restricted to species of plants which maintain in their populations a high proportion of female plants, contributing significantly to the type of genetic structure of the population. If the selfed hermaphrodites set seed the recombination benefits of outcrossing are conferred while the certainty of inbreeding is retained. In many populations of hermaphrodit species, there is a small propotion of male sterility, resulting from abortion of pollen, which is not significant in consideration of the breeding system of a population (see Frankel, 1940). This is not to be regarded as gynodioecy.

Lewis (1941) showed that where male sterility is due to a dominant or recessive gene the females cannot exist in wild populations unless more than twice as fertile as hermaphrodites (on the female side). This arises from the fact that hermaphrodites contribute potentially three times the amount of genic material that females do. Lewis, in this paper, explains the reason why, at that time (1941), cytoplasmic inheritance was the only explanation for the existence of gynodioecy in the natural populations of plants then investigated. In the case of cytoplasmic male sterility, only a slight advantage of the females is necessary to maintain them in the field, as opposed to the large advantage required when the process is controlled by nuclear genes. Although Correns (1928, ex Lewis 1941), Lewis (1941) and many other writers (mentioned by the latter) gave the gynodioecious breeding system a cytoplasmic.

basis, Lewis and Crowe (1956) have re-examined the field in terms of orthodox genetics since then. Some of the data published by Correns is found, in the light of work by East (1934, ex Lewis 1956), to be explicable on an orthodox genetic basis. C. & H. Yampolsky (1922) list only 97 species of 5 genera as being gynodioecious. They mention Pimelea as having 70–80 species, hermaphrodite polygamous or dioecious. Baker (1948) gave more than 70 species in 16 families in which functional female flowers were smaller than functional hermaphrodites. Many of these were gynodioecious. The Thymelaeaceae (to which Pimelea belongs) was not listed by him. Godley (1955 and 1957) described gynodioecy in Fuchsia, Cyathodes and Leucopogon in New Zealand.