Sex ratio distortion in bovine sperm correlates to recombination in the pseudoautosomal region

A total of 2122 single sperm from 35 bulls belonging to six different pater nal half-sib groups were analysed with respect to two markers in the bovine pseudoautosomal region (PAR) and sex-specific loci on the X and Y chromoso mes, respectively. A segregation ratio significantly different from 1:1 was observed in a test over all families, with a higher proportion of X-bearin g gametes (53.5 %). The analysis of recombination conducted separately for X- and Y-bearing sperm showed that X-bearing sperm cells possess highly sig nificant individual and between-family variability in recombination rate, w hereas Y-bearing sperm show linkage homogeneity. To test whether the two ph enomena are related, different logistic regression models were fitted to th e data. The results show that sex ratio significantly correlates with chang es in recombination rate among X-bearing but not among Y-bearing sperm. Dif ferent hypotheses to explain these observations are discussed.