Chapter 7 - Mangrove Rivulus

Abstract

This inch-long killifish (Cyprinodontidae) is special in several regards. Among vertebrate species, it is the world’s only animal known to self-fertilize routinely. Inside each hermaphrodite is an ovotestis organ that simultaneously produces eggs and sperm, which unite within the fish’s body to produce zygotes that are expelled to the outside world. Also intriguing is the fact that pure males exist in this species too, reaching substantial frequencies in some androdioecious populations. Furthermore, these males sometimes mediate outcross events with the hermaphrodites, thereby unleashing extreme recombinational potential among the resulting progeny. The outcross events apparently transpire when a hermaphrodite sheds a few unfertilized ova that then are fertilized externally by a male’s sperm. Thus, the Mangrove Rivulus actually displays a “mixed-mating” system of occasional outcrossing against a backdrop of predominant selfing. Continued self-fertilization leads to a quasi-clonal population structure that occasional outcrossing then tends to dismantle. All of these facts, and others, have emerged from molecular genetic investigations on this peculiar little fish.