Conservation in relation to mating system in Nesohedyotis arborea (Rubiaceae), a rare endemic tree from St Helena

Abstract

A field survey of Nesohedyotis arborea reveals that there are 132 adult trees in the wild, making it one of the commoner endemic plants of St Helena. The trees exhibit leaky dioecy: females are entirely male-sterile, but ‘males’ do set a small amount of seed. The floral differences between the two morphs are marked: the female is long-styled with large, smooth stigmatic papillae and vestigial anthers, while the male is short-styled with small, rugose stigmatic papillae and large anthers overarching the style. The dioecy probably evolved autochthonously on the island from an ancestral heterostylous condition. The partial loss of female function in males is associated with the production of smaller ovules: approximately a third of the volume of the female ovules. The sex ratio of adult trees is 3:2 in favour of males, due to greater allocation of resources in vegetative survival by males. Pollination is by small syrphid flies and is highly efficient up to distances of 50 m, and most of the gene flow in the population is contributed by pollen transport rather than seed dispersal. Although the population has been fragmented by human land-use patterns, rather few individuals are completely reproductively isolated. An assessment is given of the implications for conservation of the patterns of gene flow and spatial distribution of individuals. The mating system of Nesohedyotis reduces the effective population size, but this is likely to be more than outweighed by the effective outbreeding that it promotes. However, subpopulations should be monitored to assure co-sexuality, with, if appropriate, supplementary planting of opposite sex individuals.