Evolution of Gender Dimorphism in Hebe (Plantaginaceae)

Abstract

The evolution of gender dimorphism in New Zealand Hebe (Plannginaceae) was investigated in this thesis. Quantitative gender estimates of three hermaphroditic species and ten gender dimorphic populations illustrated a possible evolutionary pathway from cosexuality to dioecy via gynodioecy in Hebe. There was no difference among species in the extent of allocation to male and female components of fitness in Hebe' Female frequencies in Hebe appear to be negatively correlated with the fruitset on fruiting males. Any selective force that would affect the seed fitness of the fruiting males would therefore indirectly affect the population female frequencies. Flower dimorphism was evident in most gender dimorphic Hebe species and pollinator observations suggest that pollinators could be an important selection factor in the evolution of exaggerated floral traits in fruiting males and gender dimorphism. Using both allozyme markers, high levels of inbreeding depression and outcrossing rates were estimated in Hebe. Results from controlled experimental crosses showed that levels of inbreeding depression were found to differ with life history (seed production, seed germination, seedling establishment and seedling survival stage). The evolution and maintenance of gender dimorphism was postulated to be driven by outcrossing advantages, high inbreeding depression and changes in selfing rates. ITS (internal transcribed spacer of nuclear ribosomal DNA) from 95 taxa and chloroplast DNA (trnK-psbA intergenic spacer, trnL-trnF intron and trnL-trnF intergenic spacer) from 100 taxa, representing all nine informal groups of Hebe, two informal groups of Leonohebe and one species each of Parahebe and Heliohebe and H. formosa (outgroup) were sequenced. Unfortunately, reconstruction of the evolutionary history of the sexual systems in Hebe had been unsuccessful due to both the lack of resolution and incongruencies between the nuclear and chloroplast molecular phylogenies. This would suggest that the evolutionary history of this New Zealand group is more complex than previously suspected. This could be accounted for by the postulation of a reticulate evolution, such as hybridization, both ancient and recent, with the additional confounds of isolated island origins and recent radiation.