Subfertile females may have misplaced male DNA

Apr 6, 2012

The genomes of cows who have difficulty reproducing often contain fragments of the male Y chromosome, according to a new study titled “Y are you not pregnant: Identification of Y chromosome segments in female cattle with decreased reproductive efficiency.” 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 facilities 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 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 at least some of these cows were freemartins. A freemartin is a female who was a twin in utero with a male 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.

Tara McDaneld, a researcher at the Roman L. Hruska U.S. Meat Animal Research Center and one of the authors of the study, said that twinning was not particularly common in cattle. The absorption of the male twin is similarly uncommon.

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 reasearchers plan to test the possibility further.

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.

Downs Syndrome is an example of a chromosomal translocation in humans.

“[Chromosomal translocation] is something that can randomly occur during recombination and events like that. It’s not very common. A lot of time it can be detrimental and the fetus can be lost right after conception,” said McDaneld of the phenomenon in cattle.

“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,” Mc- Daneld continued.

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 who 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.”

WLJ asked McDaneld if affected heifers might point to a less than desirable sire or dam who should be culled.

“That’s one thing we have looked at to see if there’s one sire who contributes to these heifers.

We looked in our own herd, but we haven’t been able to find a specific sire who is responsible.”

She continued, saying they were looking into the issue. “We’ve been karyotyping to look at the individual cells of affected animals to see if it was a mutation or an inherited translocation.”

Sex chromosome abnormalities that affect reproduction have been found in other species of animals. Gene translocation 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. — Kerry Halladay, WLJ Editor