Low Temperature Induction of Males and Other Developmental Anomalies in a Self-Fertilizing Hermaphroditic Fish Species
Fisher, Michael T. IV
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Rivulus marmoratus (Cyprinodont; Aplochelidae) is the only vertebrate known to reproduce by obligate internal self-fertilization. Selfing is the ultimate form of inbreeding and leads to virtual homozygosity in approximately ten generations, so that this mode of reproduction is effectively clonal. Populations of this species consist of arrays of homozygous clones, with high clonal diversity and low representation of each clone. Males occur at low frequency (<1%) in most populations, with a few notable exceptions. Females have never been recorded.High frequencies of males have been reported on two occasions; high numbers of males were reported on the island of Curacao in the 1970's, and males comprised up to 24% of the populations sampled on several Belize Cays during 1990-91. Hermaphrodites collected from one island, Twin Cays, proved to be heterozygous at mini-satellite loci when progeny tested and DNA fingerprinted. This was evidence that recent outcrossing had occurred.Earlier experiments with Floridian clonal lines had suggested that sex in this species was thermolabile; incubation of embryos at low temperatures (19Â° C)) produced up to 100% males. It was suggested that this might be part of an environmental sex determination (ESD) system in this species.If the induction of males is indeed part of and ESD in this species, then it may be that the temperature at which males are induced in a particular clone will be related to the temperature regime that clone encounters in nature. Therefore, it was hypothesized that the temperature at which males are induced would be related to the geographic origin of each clone, so that clones from the cooler extremes of the range would produce males at a lower threshold temperature than clones from the more equatorial center of the range. This study used laboratory reared descendants of clones from Vero Beach, Florida, Rio de Janiero, Brazil (the extreme north and south of the range), and from several Belize barrier islands and the Belize mainland (near the center of the range) in an effort to detect differences in the production of males between clonal lines based on their geographic origin. Eggs were collected from these hermaphrodites and incubated at 26Â° C)C, 22.5Â° C), or 19 Â° C), representing normal, and below average temperatures for this species. These embryos were hatched and reared to sexual maturity at which time their sex was evaluated.Incubation at 26Â° C) resulted in 2-17% males, incubation at 22.5Â° C) resulted in 6-53% males, and incubation at 19Â° C) resulted in 47-74% males. However, the variation in male induction between clonal lines did not correspond to a geographic hypothesis.Also, significant numbers of deformed individuals were noted, particularly among individuals incubated at the lower two temperatures. Incubation at 26Â° C) resulted in 0-24% deformed, incubation at 22.5Â° C) resulted in 6-66% deformed, and incubation at 19Â° C) resulted in 36-87% deformed. It is clear that development at these lower temperatures is difficult for this species, and it may be that low temperature developmental stress is not a general challenge to this species.
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