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

Area of Science:

• * Evolutionary biology
• * Reproductive strategies in vertebrates
• * Population genetics

Background:

• * Rivulus marmoratus is unique as the only known vertebrate with obligate, synchronous hermaphroditic fertilization.
• * While males are rare in most populations, they are relatively abundant in Twin Cays, Belize, where outcrossing has been observed.
• * This contrasts with the species' typical automictic cloning reproductive strategy.

Purpose of the Study:

• * To investigate the phylogenetic position of the Twin Cays population of Rivulus marmoratus.
• * To determine if the observed outcrossing in Belize represents an ancestral trait.
• * To explore the evolutionary basis of outcrossing in this hermaphroditic species.

Main Methods:

• * Phylogenetic analysis using restriction site and sequence data.
• * Comparison of genetic divergence among Florida, Belize, Brazil, and Bahamas populations.
• * Analysis of reproductive strategies and population structures.

Main Results:

• * Phylogenetic analysis showed western Caribbean (Florida, Belize) and eastern populations (Brazil, Bahamas) are distinct lineages.
• * The Twin Cays population is phyletically indistinguishable from other western Caribbean populations.
• * Western lineages share a more recent common ancestor than other populations, indicating the Twin Cays population is not ancestral.

Conclusions:

• * The Twin Cays population is not a remnant of an ancestral outcrossing lineage.
• * Outcrossing in Rivulus marmoratus is likely a phenotypically plastic trait.
• * The expression of outcrossing may be dormant and triggered by uncharacterized ecological factors.