engleski

Sex difference in the inbreeding level in the Pseudo-Autosomal Region

While the X chromosome (or Z chromosome in birds) behaves similarly to autosomes in homogametic sex, it recombines only in the pseudoautosomal region (PAR) in heterogametic sex, resulting in a higher recombination rate on PAR than on the rest of the X chromosome, but also in most species than elsewhere in the genome. From an evolutionary perspective, why is PAR the only region on the sex chromosome that still recombines in both sexes? Many authors point out that PAR is simply required for proper segregation of sex chromosomes during meiosis. However, if this were the only reason, then the size of PAR would be very small compared to the rest of the sex chromosomes (e.g., in mice). On the other hand, if PAR is relatively large (e.g., in dogs and sheep), this could be the result of classical forces that favour recombination, such as avoidance of autozygosity and balancing selection. Therefore, in two case studies (metapopulation of East Adriatic sheep ; 201 individuals, Illumina OvineHD BeadChip and the Labrador Retriever breed ; 273 individuals, Illumina CanineHD BeadChip), ROH-based inbreeding was estimated at PAR and compared with values on the remaining X chromosome and on autosomes. While the average inbreeding in females (FROH_PAR = 0.057 in sheep and 0.159 in dogs) was similar to that on autosomes, males on PAR showed extremely low inbreeding (FROH_PAR = 0.023 in sheep and 0.091 in dogs). The obtained results suggest that the higher recombination rate on PAR appears significantly only in heterogametic sex, in species where PAR is relatively large. This could be due to the avoidance of autozygosity, but we also note a high observed heterozygosity (HO) at/near the end of PAR in males, suggesting the presence of inversion and/or balancing selection, which we will investigate further.

Pseudo-Autosomal Region ; inbreeding ; high recombination rate ; males ; females

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