Evidence for positive selection of taurine genes within a QTL region on chromosome X associated with testicular size in Australian Brahman cattle

Abstract

Background: Previous genome-wide association studies have identified significant regions of the X chromosome associated with reproductive traits in two Bos indicus-influenced breeds: Brahman cattle and Tropical Composites. Two QTL regions on this chromosome were identified in both breeds as strongly associated with scrotal circumference measurements, a reproductive trait previously shown to be useful for selection of young bulls. Scrotal circumference is genetically correlated with early age at puberty in both male and female offspring. These QTL were located at positions 69-77 and 81-92 Mb respectively, large areas each to which a significant number of potential candidate genes were mapped.

Results: To further characterise these regions, a bioinformatic approach was undertaken to identify novel non-synonymous SNP within the QTL regions of interest in Brahman cattle. After SNP discovery, we used conventional molecular assay technologies to perform studies of two candidate genes in both breeds. Non-synonymous SNP mapped to Testis-expressed gene 11 (Tex11) were associated (P < 0.001) with scrotal circumference in both breeds, and associations with percentage of normal sperm cells were also observed (P < 0.05). Evidence for recent selection was found as Tex11 SNP form a haplotype segment of Bos taurus origin that is retained within Brahman and Tropical Composite cattle with greatest reproductive potential.

Conclusions: Association of non-synonymous SNP presented here are a first step to functional genetic studies. Bovine species may serve as a model for studying the role of Tex11 in male fertility, warranting further in-depth molecular characterisation.

Figure 1

Analysis of the 5′UTR region of AR. Alignment of three haplotypes identified in the 5′UTR of the AR gene in the Brahman bull population. * highlights sites of variation across haplotypes.

Figure 2

Distribution of the proportion of B. taurus alleles across the X chromosome in the Brahman bull population based upon grouping based upon candidate marker alleles.

Figure 3

Flow of research procedures from genome-wide association discovery of associated genomic regions to test of novel single nucleotide polymorphisms located in candidate genes.