Physical variability and fluctuating asymmetry in heterozygous and homozygous populations ofRivulusmarmoratus

Abstract

The debate over the influence of heterozygosity and homozygosity on developmental stability is ongoing, and rarely is an animal model found that represents these two genetic states in naturally occurring populations. Rivulus marmoratus (Pisces: Aplocheilidae), a small cyprinodontid fish indigenous to mangrove marshes of the western tropical Atlantic coast, may be an appropriate model. This species is well known as the only self-fertilizing hermaphroditic vertebrate, and populations normally consist of genetically diverse groups of homozygous clones. However, male fish are known from a few populations, and outcrossing has been documented from one area (Belize in Central America) where heterozygosity is the norm. Are there differences in developmental stability, as represented by fluctuating asymmetry (FA) and physical variability, between naturally occurring homozygous and heterozygous populations? Thirty-one meristic and morphometric characters in 180 fish from 10 populations were examined to determine physical variability. In addition, FA and the presence of physical anomalies were analyzed. Belize fish (heterozygous) were compared with eight other (homozygous) populations. To control for possible environmental effects, offspring of two populations (Belize and Florida) were reared at 25°C and similarly analyzed. The FA analyses and variability comparisons revealed no differences between heterozygous and homozygous populations, and the same was true for fish reared at a constant temperature. However, some populations (e.g., Brazil) were more variable than others. These results indicate that coadapted gene complexes may be more important than heterozygosity in canalizing developmental stability in this animal.