Pig sex chromosomes are getting their DNA read for the first time, gene by gene. Scientists already know the full genetic sequence of human and mouse sex chromosomes, but pigs—a major livestock species and a key model for human biology—have been left out. That gap matters because the X and Y chromosomes evolve rapidly and differently in each mammal lineage, carrying genes that control male fertility, sperm production, and even the ratio of male to female offspring. Without a pig reference, researchers cannot tell which sex chromosome features are universal and which are unique to rodents or primates. This project will map the pig Y chromosome’s gene content, identify amplified gene families shared between the X and Y, and look for DNA sequences that might influence litter sex ratios. It will also compare Y chromosomes across wild and domestic pig species to link genetic variation with differences in boar fertility. The work is fundamental science—it will not produce a new drug or breeding tool tomorrow. But similar foundational sequencing of human and mouse sex chromosomes has already revealed how Y-linked genes drive sperm production and led to insights about male infertility. A pig sex chromosome map could eventually help breeders select for more fertile boars and give reproductive biologists a better comparative framework for understanding why some mammals produce skewed sex ratios.
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The X and Y chromosomes in mammals have evolved from a common ancestral pair of identical chromosomes. As they have diverged from one another the Y has acquired a specific function in determining male sex. At the same time, the X and Y chromosomes have accumulated genes that confer advantage to the male, including many genes involved in male fertility and behaviour. Particular variants of these genes are postulated to give advantage to some males over others in the competition for reproductive success. The genes that have evolved on the sex chromosomes that are involved in these functions may differ between different mammalian species. To date, the DNA sequence and gene content of the mouse and human sex chromosomes has been the most intensively studied. This has defined X and Y genes that appear to be shared between most mammals, but has also revealed the presence of lineage specific sex chromosome gene content. These studies have demonstrated a number of what appear to be emerging themes that define sex chromosomes; namely (1) gene amplification on the X and Y, (2) palindromic organisation of amplified sequence, (3) expression of genes specifically in the testis and (4) from studies in the mouse, apparent competition between X-Y homologous amplified gene families that influence the number of males to females in the offspring (sex ratio). As only two well defined mammalian lineages have been sequenced and annotated extensively (with fragmentary data on number of primates and other mammals) it is important to extend studies of the sex chromosomes to encompass all branches of mammalian evolution in order to determine the universality of these themes associated with the X and Y. This project will investigate the porcine sex chromosomes in order (1) to understand their gene content and to determine if the pig harbours testis expressed genes not shared by other mammals, (2) to determine the patterns of X-Y homology, (3) to determine what sequences have become co-amplified on the X and Y that may have a role in influencing sex ratio, (4) to understand how these sequences are organised and (5) to understand the variation between Y chromosomes carried by suids and how this relates to variation in boar fertility.
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