Understanding the Ramification of Recombination Variation in Sheep

Meiotic recombination is an important process during gamete formation that ensures proper chromosome segregation and controls genetic variation. It is clear from previous studies that at least one recombination event per chromosome arm is necessary to avoid mis-segregation. In addition, placement of recombination is not random, showing some preferences called “hotspots.” According to previous studies using linkage maps, recombination rates can vary as much as 10% in domestic sheep breeds, and is reported to be lower in bighorn sheep. However, global recombination rates determined from linkage maps or haplotype phasing underestimate the number of global recombination events per meiotic cell due to chromosomal independent assortment. To better understand and account for recombination differences between breeds of sheep, we used a cytological approach to quantify recombination events in spermatocytes. Testicular tissue samples were taken from mature Targhee, Suffolk, and Icelandic rams, and spermatocytes were spread and fixed on slides. In total, we examined over 2,200 spermatocytes and approximately 150,000 recombination events. Immunofluorescent staining was performed to identify the synaptonemal complexes (SYCP3) and recombination events (MLH1) of pachytene stage prophase cells. The total number of recombination per meiotic cell was quantified and compared between the three breeds using an ANOVA with a subsequent Tukey-Kramer test. Interestingly, Targhee rams have significantly higher (p < 0.01) recombination than Suffolk and Icelandic rams. This research gives us the tools to better understand chromosomal inheritance that consequently improves the accuracy of genetic predictions and contributes valuable information towards the sustainability of the sheep industry.