Abstract
Kryptolebias marmoratus, a small killifish that lives in mangrove habitat from southern Florida to Brazil, is one of the planet's only known self-fertilizing hermaphroditic vertebrates. Generation after generation, hermaphroditic individuals simultaneously produce sperm and eggs and internally self-fertilize to produce what are, in effect, highly inbred clones of themselves. Although populations are composed primarily of hermaphrodites, they also contain some true males. The frequency of males in a population varies geographically, from <2% in Florida to as high as 25% in Belize. Males are known to mate occasionally with hermaphrodites, thereby releasing genetic variation that has profound consequences for population genetic structure. However, it is unknown whether hermaphrodites can or do sporadically mate with each other also. Here, we test whether hermaphroditic individuals of the killifish Kryptolebias marmoratus are capable of crossing with one another, in addition to their much more common habits of self-fertilization and occasional outcrossing with pure males. We employ an experimental design in which replicate hermaphrodite pairs were housed together and allowed to reproduce naturally. Among 173 embryos screened at diagnostic microsatellite loci, all were found to result from selfing (i.e., no embryos were the product of a hermaphrodite cross). We thus conclude that hermaphrodite pairs are unlikely to cross, or do so exceedingly rarely.
Highlights
The mangrove killifish, Kryptolebias marmoratus and its congener K. ocellatus, are unique among vertebrates in consisting primarily of hermaphroditic individuals that are capable of internal self-fertilization (Costa et al 2010; Harrington 1961)
In K. marmoratus occasional outcrossing between hermaphrodites and true males has been documented in the laboratory (Harrington and Kallman 1968; Mackiewicz et al 2006a), when unfertilized eggs are released by a hermaphrodite and fertilized externally by a male
If we assume that occasional outcrossing provides some fitness advantage, presumably it should not matter whether the source of sperm is from a true male or another hermaphrodite
Summary
The mangrove killifish, Kryptolebias marmoratus and its congener K. ocellatus, are unique among vertebrates in consisting primarily of hermaphroditic individuals that are capable of internal self-fertilization (Costa et al 2010; Harrington 1961). The outcross events that produce genetically diverse progeny are thought to result from hermaphrodites mating with true males This is because outcrosses between males and hermaphrodites have been documented in the laboratory (Harrington and Kallman 1968; Mackiewicz et al 2006a), and in the wild there is a positive correlation between the frequency of males in a population and the inferred frequency of outcrossing (based on levels of genetic variation/homozygosity) (Tatarenkov et al 2015). In K. marmoratus occasional outcrossing between hermaphrodites and true males has been documented in the laboratory (Harrington and Kallman 1968; Mackiewicz et al 2006a), when unfertilized eggs are released by a hermaphrodite and fertilized externally by a male. We provide a critical test of this possibility by genotyping many progeny at diagnostic microsatellite loci
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