Understanding Chromosome Pairing and Crossovers in Beefalo Spermatocytes

With the demand of an increasing population and changing climate, there is a need to produce efficient and adaptable livestock. In an effort to achieve this, hybrid mating of Bos taurus to Bos indicus and Bos taurus to Bison bison have been used. However, producing viable and fertile offspring from hybrid crosses is a major concern. To better understand how chromosomes pair and crossover in hybrid animals, 700 spermatocytes from B. taurus x B. bison (beefalo) were examined and compared to 500 spermatocytes from B. taurus (cattle). Immunofluorescent staining was used to identify chromosome synapsis (SYCP3) and crossovers (MLH1) in pachytene stage spermatocytes. Despite both species having 30 chromosomes, beefalo exhibit on average ~15% fewer crossovers per cell when compared to cattle (41 MLH1, 49 MLH1), respectively. Importantly, one crossover per chromosome arm is required to ensure proper chromosome segregation and ~12% of beefalo spermatocytes had at least one pair of chromosomes fail to crossover. Furthermore, ~12% of cells exhibited the structural defect of a circular/fused chromosome. The consequence of improper chromosome segregation in spermatocytes is ultimately cell death; as defective spermatocytes will not develop into mature spermatozoa. Hybrid animals may provide specific attributes and adaptability to ensure sustainability and profitability in a changing industry. However, the effective use of hybrid animals requires a greater understanding of chromosomal compatibility to improve fertility and offspring viability. This research will provide valuable insight relating to chromosome pairing, synapsis, and crossovers in spermatocytes of beefalo, an important strategy towards progress of livestock production.