Evolution of a Balanced Sex Ratio by Frequency-Dependent Selection in a Fish

Abstract

Balanced (1 to 1) sex ratios are thought to evolve by a process known as frequency- dependent selection of the minority sex. Five populations of a fish with genetically based variation in temperature-dependent sex determination were maintained for 5 to 6 years in artificial constant-temperature environments that initially caused the sex ratio to be highly skewed. Increases in the proportion of the minority sex occurred in subsequent generations until a balanced sex ratio was established, thus confirming a central premise underlying the theory of sex-ratio evolution.

Evolution of Autosomal Suppression of the Sex-Ratio Trait in Drosophila

“…This work demsors, expected to run to fixation unless they are deleterionstrated that sexual proportion actually responds to ous. These conclusions are relevant for the understandnatural selection as postulated by Fisher (see also Coning of naturally occurring sex-ratio polymorphisms in over and Van Voorhees 1990;Basolo 1994). Drosophila.…”

Analysis of reproductive patterns of fishes from three Large Marine Ecosystems

“…With respect to elasmobranchs, the problem is more complex due to the large number of individuals discarded (Beerkircher et al, 2002). Conover and Voorhees (1990) pointed that differences of sex ratio in a given fish population tend to a balanced sex ratio through generations. These authors also indicated that high temperature observed during the late breeding season may cause most offspring produced to become males (observed for Menidia menidia under controlled experiments).…”

Temperature-Dependent Sex Determination in Vertebrates

“…Mortality during this period is too low to account for the magnitude of sex ratio changes with temperature (Conover and Kynard 1981). Four lines of evidence demonstrate that strong genetic effects also influence sex determination in Menidia: (1) full-sib and half-sib family sex ratios differ at the same temperature and in their temperature sensitivity; (2) local populations differ in their temperature sensitivity (Conover and Heins 1987a); (3) extreme temperatures do not produce monosex progeny, and therefore at least some temperature-insensitive genotypes are present in all populations; and (4) the sex ratio and its response to temperature evolve in response to frequency-dependent selection (Conover and Van Voorhees 1990;Conover et al 1992). The genetic component of sex determination clearly involves the segregation of a major sex factor: family sex ratios tend to cluster around Mendelian proportions (1:0, 0:1, 3:1, or 1:1), at least in northern populations (Lagomarsino and Conover 1993).…”

“…That variation in sex ratio response to temperature is capable of responding to selection was proven experimentally in Menidia (Conover and Van Voorhees 1990;Conover et al 1992). During the larval period, captive populations of Menidia were subjected each generation to either extreme high or extreme low temperatures that skewed the sex ratio in opposite directions.…”