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The mangrove rivulus Kryptolebias marmoratus and a closely related species are the world's only vertebrates that routinely self-fertilize. Such uniqueness presents a model for understanding why this reproductive mode, common in plants and invertebrates, is so rare in vertebrates. A survey of 32 highly polymorphic loci in >200 specimens of mangrove rivulus from multiple locales in the Florida Keys, USA, revealed extensive population-genetic structure on microspatial and micro-temporal scales. Observed heterozygosities were severely constrained, as expected for a hermaphroditic species with a mixed-mating system and low rates of outcrossing. Despite the pronounced population structure and the implied restrictions on effective gene flow, isogenicity (genetic identity across individuals) within and among local inbred populations was surprisingly low even after factoring out probable de novo mutations. Results indicate that neither frequent bottlenecks nor directional genetic adaptation to local environmental conditions were the primary driving forces impacting multilocus population-genetic architecture in this self-fertilizing vertebrate species. On the other hand, a high diversity of isogenic lineages within relatively small and isolated local populations is consistent with the action of diversifying selection driven by the extreme spatio-temporal environmental variability that is characteristic of mangrove habitats.

We analyzed the effect of periodic drying in the Florida Everglades on spatiotemporal population genetic structure of eastern mosquitofish (Gambusia holbrooki). Severe periodic drying events force individuals from disparate sources to mix in dry season relatively deep-water refuges. In 1996 (a wet year) and 1999 (a dry year), we sampled mosquitofish at 20 dry-season refuges distributed in 3 water management regions and characterized genetic variation for 10 allozyme and 3 microsatellite loci. In 1996, most of the ecosystem did not dry, whereas in 1999, many of our sampling locations were isolated by expanses of dried marsh surface. In 1996, most spatial genetic variation was attributed to heterogeneity within regions. In 1999, spatial genetic variation within regions was not significant. In both years, a small but significant amount of variation (less than 1% of the total variation) was partitioned among regions. Variance was consistently greater than zero among long-hydroperiod sites within a region, but not among short-hydroperiod sites within a region, where hydroperiod was measured as time since last marsh surface dry-down forcing fishes into local refuges. In 1996, all sites were in Hardy-Weinberg equilibrium. In 1999, we observed fewer heterozygotes than expected for most loci and sites suggesting a Wahlund effect arising from fish leaving areas that dried and mixing in deep-water refuges.

Effluent from a paper mill discharging into the Fenholloway River, Taylor County, Florida, USA, contains chemicals that masculinize females of the resident population of eastern mosquitofish (Gambusia holbrooki), as evidenced in females by elongated anal fins, which is normally a male-specific trait. To identify androgenic components in the effluent, water collected from the Fenholloway River and a control tributary was fractionated using solid-phase extraction and reverse-phase high-performance-liquid chromatography. Two Fenholloway River fractions induced androgen receptor-dependent transcriptional activity in transient transfection cell culture assays. Of these, androstenedione was confirmed by liquid chromatography-mass spectrometry with multiple reaction monitoring.