BREEDING SYSTEMS AND THE EVOLUTION OF DIOECY IN NEW ZEALAND APIOID UMBELLIFERAE.

Abstract

Darwin (1877) restricted Linnaeus' term polygamous to species with hermaphrodite, male, and female plants or hermaphrodite, male, and female flowers on a single plant, and termed as gynodioecious species which have two forms of plant, hermaphrodites and females. Recent work on those angiosperm breeding systems involving two, sometimes variable, sex forms emphasizes the value of a functional rather than a morphological approach to their definition (Vaarama and Jiiiiskelainen, 1967; Connor, 1974; Arroyo and Raven, 1975; Lloyd, 1976). The distinction between gynodioecy and dioecy, while supposedly clear-cut in terms of morphological flower type (female and hermaphrodite plants vs. female and male ones), is much less so in functional terms, for the so-called hermaphrodites have many functionally male flowers and have been shown in many gynodioecious species to contribute many more genes via pollen than ovules. These plants are more appropriately considered as inconstant males (see Lloyd, 1974a) and are treated as such herein. Lewis (1942) viewed gynodioecy and dioecy as independent outbreeding systems and suggested that gynodioecy was unlikely to be a step towards dioecy. However, an opposite view, that gynodi-oecy is one of the pathways from hermaphroditism to dioecy, has been expressed in general terms by Darwin (1877), and explicitly postulated for particular genera by subsequent authors (e.g., Correns, 1916; Love, 1943). In spite of a large theoretical literature on the maintenance and evolution of gynodioecious breeding systems (e.g., Lewis, 1941; Ho and Ross, 1973; Valdeyron et al., 1973; Lloyd, 1974a, 1974b, 1975, 1976; Ross and Weir 1976; Charlesworth and Charlesworth, 1978) there are few detailed studies of groups of related gynodioecious species. Consequently, conclusive evidence of the evolution of dioecy from hermaphroditism via gynodioecy in any one taxonomic group is lacking. A promising source of such evidence is provided by a group of apioid Umbelliferae endemic to or centered in New Zealand. These genera show a range of sexual dimorphism (Dawson, 1967; Lloyd, 1973). The genera considered are Aciphylla (ca. 40 spp. N.Z., 2 spp. Australia); Anisotome (15 spp. N.Z., 1 sp. Australia); Gingidia (5 spp. N.Z., 2 spp. Australia, 1 sp. Australia and N.Z.); Scandia (2 spp. N.Z.); and Lignocarpa (2 spp. N. Z.). Webb (1976) described differences in flowering periods between males and females, and Lloyd and Webb (1977) discussed secondary sex characters in these genera. The maintenance of females in populations, sex ratios, and other aspects of the breeding system will be considered in detail elsewhere. This paper assesses the evolution and nature of the breeding systems in the five genera.