Sex Ratio Variation in Iberian Pigs

Abstract

Within the area of sex allocation, one of the topics that has attracted a lot of attention is the sex ratio problem. Fisher (1930) proposed that equal numbers of males and females have been promoted by natural selection and it has an adaptive significance. But the empirical success of Fisher's theory remains doubtful because a sex ratio of 0.50 is also expected from the chromosomal mechanism of sex determination. Another way of approaching the subject is to consider that Fisher's argument relies on the underlying assumption that offspring inherit their parent's tendency in biased sex ratio and therefore that genetic variance for this trait exists. Here, we analyzed sex ratio data of 56,807 piglets coming from 550 boars and 1893 dams. In addition to classical analysis of heterogeneity we performed analyses fitting linear and threshold animal models in a Bayesian framework using Gibbs sampling techniques. The marginal posterior mean of heritability was 2.63 3 10 À4 under the sire linear model and 9.17 3 10 À4 under the sire threshold model. The probability of the hypothesis p(h 2 ¼ 0) fitting the last model was 0.996. Also, we did not detect any trend in sex ratio related to maternal age. From an evolutionary point of view, the chromosomal sex determination acts as a constraint that precludes control of offspring sex ratio in vertebrates and it should be included in the general theory of sex allocation. From a practical view that means that the sex ratio in domestic species is hardly susceptible to modification by artificial selection.T HE theory of sex allocation predicts, given that an organism reproduces sexually, how it should allocate resources to male and female functions and constitutes one of the most popular topics in evolutionary biology. It involves many related questions such as parental investment or mating systems and there is a vast array of theoretical and empirical literature on these predictions (Charnov 1982;Frank 1990). Here we focus on one of the topics that has attracted a lot of attention within the area of sex allocation: how individuals adjust the proportion of their offspring that are male (the sex ratio problem). Fisher (1930) proposed that equal numbers of males and females have been promoted by natural selection and it has an adaptive significance. His argument is that in a population with a biased sex ratio, parents with a genetic tendency to produce more progenies of the rarer sex attain a higher average number of grandchildren, and therefore the biased sex ratio will disappear.In mammals, males are the heterogametic (XY) and females are the homogametic (XX) sex. Therefore when male and female gametes unite randomly, half of the resulting zygotes will be XY and differentiate as males and the other half will be XX and differentiate as females. As a consequence the expected primary sex ratio, defined as the proportion of males at conception, is also expected to be 0.50 although with a substantial variance corresponding to the binomial distribution. Because the same sex ratio (0...

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