Partial self-incompatibility and inbreeding depression in a native tree species of La Reunion (Indian Ocean)

We investigated the reproductive system of the threatened taxon Dombeya acu tangula ssp. acutangula Cav. (Sterculiaceae), an endemic tree of the Mascar ene archipelago (Indian Ocean). A controlled crossing experiment was perfor med in two natural populations located in the remnants of the low-elevation dry forest on the island of La Reunion. Active pollination, probably mainl y by insects, was necessary for reproduction in this species. Individuals v aried in their degree of self-sterility from 0 to 100%. Outcrossing between nearby individuals produced lower seed set than did crosses between more d istant individuals within one of the two tested populations. The variation in reproductive success on selfing and in the different types of crosses co uld result from inbreeding depression causing embryo death, and we provide evidence that progenies from selfing have lower seed size and quality. Howe ver, for inbreeding depression to account for the dramatic variation in see d set found in our crossing experiment, the distribution of genetic load an d number of lethal factors required appear unrealistic. We favour an altern ative interpretation, that D. acutangula possesses an incompatibility syste m similar to that found in other Sterculiaceae species such as Theobroma ca cao L. Such an incompatibility system allows a certain amount of selfing, a nd different individuals vary in their degree of self-incompatibility. The low success of crosses among close neighbours in one population suggests th at there was spatial structure for incompatibility alleles in that populati on. This could partly explain the decline of the species in fragmented and disturbed habitats, since relatedness at incompatibility loci may increase in small or isolated population and thus reduce mate availability.