OUTCROSSING IN A NATURAL-POPULATION OF A SELF-FERTILIZING HERMAPHRODITIC FISH

Outcrossing has been documented in a natural population of the self-fe rtilizing hermaphroditic killifish, Rivulus marmoratus. All of the 24 hermaphrodites collected in 1991 on Twin Gays, a small island adjacent to the Belize barrier reef, proved, by direct assay of their progeny, to be multiply heterozygous for mini- and microsatellite loci detecte d by DNA fingerprinting. The results are strikingly different from tho se obtained previously with this species, for ail other populations st udied have consisted of arrays of homozygous clones. The outcrossing i n the population presumably stems from male x hermaphrodite matings. M ales of the species are usually rare in nature, but were relatively co mmon on Twin Gays, possibly produced by temperature extremes on the is land. Outcrossing in the Twin Gays populations may therefore be the di rect result of the environmental induction of males. If true, this wou ld be an example of phenotypic plasticity of almost unprecedented impa ct. However, there is evidence that social factors, as yet unresolved, may also be important in both the requisites of outcrossing: the indu ction of males and the reduction of internal self-fertilization in her maphrodites.