CONSERVATION IN RELATION TO MATING SYSTEM IN NESOHEDYOTIS-ARBOREA (RUBIACEAE), A RARE ENDEMIC TREE FROM ST-HELENA

A field survey of Nesohedyotis arborea reveals that there ave 132 adul t trees in the wild, making it one of the commoner endemic plants of S t Helena. The trees exhibit leaky dioecy:females are entirely male-ste rile, but 'males' do set a small amount of seed. The floral difference s between the two morphs are marked: the female is long-styled with la rge, smooth stigmatic papillae and vestigial anthers, while the male i s 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 pal tial lo ss of female function in males is associated with the production of sm aller 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 great er allocation of resources in vegetative survival by males. Pollinatio n 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 p ollen transport rather than seed dispersal. Although the population ha s been fragmented by human land-use patterns, rather few individuals a re completely reproductively isolated. An assessment is given of the i mplications 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 out weighed by the effective outbreeding that it promotes. However, subpop ulations should be monitored to assure co-sexuality, with, if appropri ate, supplementary planting of opposite sex individuals. (C) 1997 Else vier Science Ltd.