The genomes of cows that have difficulty reproducing often contain fragments of the male Y chromosome, according to a new study in press in the Journal of Animal Science. The researchers say this finding could help cattle producers identify subfertile females before investing time and resources on breeding attempts.
In the study, cows from several facilties were assigned to groups based on reproductive efficiency, and their DNA was pooled for genotyping. The researchers used a technique called an SNP marker assay, in which a specifically designed genetic probe fluoresces when it attaches to certain markers.
Interestingly, the researchers found that genetic probes designed for the Y chromosome, which is the male sex chromosome, were fluorescing in the population of subfertile female cattle.
The authors of the study considered two possibilities for why the Y chromosome genes were in some females. One potential explanation was that at least some of these cows were freemartins. A freemartin is a female cow that was a twin in utero with a male cow and received Y chromosomes through blood transfer from the male twin. Though breeders usually identify freemartins prior to mating, freemartins can sometimes remain undetected if the male twin is lost early in pregnancy.
The other possibility is that fragments of the Y chromosome may have crossed over to the father's X chromosome, which the female offspring then inherits. This crossover, or "translocation," would result in female offspring having fragments of the Y chromosome in their genome.
The scientists concluded that freemartins in the DNA pool they tested were rare and probably did not contribute heavily to the amount of male DNA present. However, the researchers plan to test the possibility further.
"By karyotyping [evaluating the appearance of the chromosome] these females, we will be able to determine if the Y anomaly is due to the presence of the whole Y chromosome, as seen in freemartins, or is due to a translocation," said Tara McDaneld, a researcher at the Roman L. Hruska US Meat Animal Research Center and one of the authors of the study.
The discovery that subfertile females have fragments of Y chromosomal DNA could result in a new tool for identifying subfertile heifers.
"From the results of the Y marker panel, producers will have a tool to identify replacement heifers that have no potential to breed and direct these females to a different management strategy," said McDaneld.
The study estimates that genotyping tests for Y fragments could save $5 to $10 per female.
"This will decrease cost by removing the heifers that have no potential to breed prior to incurring heifer development expenses," McDaneld said. "It will also increase the percentage of females in the breeding herd that are pregnant after breeding."
Sex chromosome abnormalities that affect reproduction have been found in other species of animals. Abnormalities in humans and laboratory mice are of interest to the medical community. These abnormalities can be caused by mutations on the sex chromosome or by crossover of pieces of the Y chromosome crossing over to the X chromosome. This leads to a discrepancy between the physical sex characteristics of the animal and the sexual genotype.
The study was titled "Y are you not pregnant: Identification of Y chromosome segments in female cattle with decreased reproductive efficiency." It can be read in full at jas.fass.org.