Biased sex ratios in laboratory strains of guppies, Poecilia reticulata

Abstract

SUMMARYOlder laboratory strains of guppies, Poecilia reticulata, exhibit female-biased sex ratios. The results of interstrain crosses and crosses involving exceptional YY males showed that the sex ratio phenomenon is determined primarily by V-linked genes which result either in decreased production of Y-bearing sperm, or in lowered ability of V-bearing sperm to compete with X-bearing sperm for ova. Inbreeding and local mate competition cannot account for the evolution of the sex ratio deviations. It is suggested that the sex ratio effect is caused either by pleiotropic effects of Y-linked genes increasing the ability of males to compete for females, or by a genetic deterioration of the Y-chromosomes through accumulation of deleterious alleles. The latter suggestion is supported by the theoretical models of Nei (1970) and Charlesworth (1978) on the evolution of inert V-chromosomes.

Female choice for good genes and sex‐biased broods in guppies

“…Regardless of the lack of predators in the experiments described here, low male survival is not an adequate explanation for the female-biased sex ratios since, even if all the young that died before sexing were counted as male, the sex ratio was still strongly biased in favour of females. Female-biased sex ratios have been reported in laboratory strains of guppies (Farr, 1981;Brooks, 2000). This phenomenon appears to be related to the age of the strain and may be caused by the accumulation of deleterious alleles on the Y chromosome (Farr, 1981).…”

“…Adult guppy sex ratios have been found to be female-biased in the field (Rodd & Reznick, 1997) and it has been suggested that, since sex ratio is even at birth (Farr, 1981), this is due to the lower survival rate of males compared with females because of their brighter colours and aspects of their behaviour that make them more vulnerable to predation (Rodd & Reznick, 1997). Regardless of the lack of predators in the experiments described here, low male survival is not an adequate explanation for the female-biased sex ratios since, even if all the young that died before sexing were counted as male, the sex ratio was still strongly biased in favour of females.…”

“…Female-biased sex ratios have been reported in laboratory strains of guppies (Farr, 1981;Brooks, 2000). This phenomenon appears to be related to the age of the strain and may be caused by the accumulation of deleterious alleles on the Y chromosome (Farr, 1981). Alternatively, it may indicate that guppies have the ability to manipulate the sex-ratio of their offspring, which may be adaptive under natural conditions.…”

GENETIC AND ENVIRONMENTAL BASIS OF VARIABLE SEX RATIOS IN LABORATORY STRAINS OF POECILIOPSIS LUCIDA

“…Recently though, temperature-dependent sex determination has been discovered in a number of oviparous vertebrates (Bull and Vogt, 1979;Ferguson and Joanen, 1982;Conover and Kynard, 1981;Yntema, 1979), yielding sex ratios that depart from unity. Poeciliid fishes exhibit a wide variety ofsex-determining mechanisms, and primary sex ratios other than 1:1 are not unknown and can be the result of inbreeding (Farr, 1981), polygenic sex determination (Kosswig, 1964), multiple sex chromosomes (Kallman, 1965), autosomal regulatory genes or epistatic factors (Kallman, 1984). Among naturally occuring adult populations of poeciliids, sex ratios other than 1:1 are common, and females usually predominate (Thibault, 1974;Snelson and Wetherington, 1980).…”

“…Poeciliid fishes exhibit a wide variety ofsex-determining mechanisms, and primary sex ratios other than 1:1 are not unknown and can be the result of inbreeding (Farr, 1981), polygenic sex determination (Kosswig, 1964), multiple sex chromosomes (Kallman, 1965), autosomal regulatory genes or epistatic factors (Kallman, 1984). Among naturally occuring adult populations of poeciliids, sex ratios other than 1:1 are common, and females usually predominate (Thibault, 1974;Snelson and Wetherington, 1980).…”