EVIDENCE THAT AN OUTCROSSING POPULATION IS A DERIVED LINEAGE IN A HERMAPHRODITIC FISH (RIVULUS MARMORATUS).

Rivulus marmoratus is the only known vertebrate with obligate, synchronous hermaphroditic fertilization. Males can be experimentally induced in the laboratory and are rare or absent in most populations, but at the isolated Twin Cays, Belize, locality, males are relatively abundant. At this locality, evidence of outcrossing has been documented in this otherwise automictic cloning species. Phylogenetic analysis of restriction sites and sequence characters revealed that all Florida and Belize western Caribbean populations (including Twin Cays) are phyletically indistinguishable yet divergent from eastern populations in Brazil and the Bahamas. Further, these western lineages shared a common ancestor more recently than all other populations. Therefore, the Twin Cays population is not a remnant ancestral outcrossing population. Outcrossing is suspected to have evolved as a phenotypically plastic character, and its expression in R. marmoratus may be dormant unless triggered by some ecological factor that is not well understood.