Breeders focus too much on milk production: researcher

Cattle chromosomes Study of Angus and Holstein breeds shows that lack of genetic diversity could lower bulls’ fertility

LINDELL BEACH, B.C. — Recent research has found that the Y chromosome of bulls contains more active genes than the Y chromosome of other mammals, such as primates.


This discovery may help biologists more fully understand the evolution of cattle and other species and help breeders and farmers more effectively maintain genetic diversity and increase their livestock’s fertility. Wansheng Liu, associate professor of animal genomics at Pennsylvania State University, said there has been too much genetic selection for milk yield in the dairy industry at the expense of fertility.


“In the past 60 years, we have put too much attention on the milk side,” said Liu, whose study was published in the journal Proceedings of the National Academy of Sciences. 


“There is a negative association. You have a group of genes involved with reproduction at the same time as milk production.”


Consistently selecting for milk production may eventually lower bulls’ fertility, said Liu, who stressed the need for the industry to pay more attention to the problem to protect the future breeding potential of cattle. 


Male reproduction is concentrated on the mammalian Y chromosome (MSY), which contains clusters of genes for that specific function. 


The Y chromosome sequence is available for the human, chimpanzee and macaque, but little is known about the transcriptome, or genetic makeup, of the MSY in other mammals


Liu’s study looked at the MSY in cattle by direct testis cDNA selection and RNA sequence approaches.


Liu’s team studied Angus and Holstein bulls at ages of three days, three weeks, three months, six months, seven to eight months, two years and older. 


As the cattle aged, the researchers analyzed the expression of the Y-linked genes and the complementary DNA from the bulls’ testis. 


Complementary DNA is a form of DNA synthesized from a messenger RNA.


According to a news release from Penn State, the team identified 1,274 genes in the male-specific region on the bovine Y chromosome, which take part in coding proteins. The number compares to the 31 to 78 genes associated in the Y chromosome of the chimpanzee and macaque.


They also found that the genes in the bovine Y chromosome were much more “transcriptionally active.”


Transcription is the means by which cells read and regulate the genetic instruction in their genes. 


The team also identified 375 novel noncoding gene families on the bovine Y chromosome, which are mostly expressed in different stages of the testis.


“Y-linked genes have a number of functions, such as male sex determination, maleness and spermatogenesis,” Liu said. 


“Our recent data show that very limited numbers of Y-lineages (paternal lineages) are present in today’s Holstein population that may have a significant impact on the male reproduction or even the survival of this breed. So, there is a need to keep more Y-lineages in the breed by using more sires to enhance the genetic diversity.”


The problem, as Liu sees it, is that cattle breeders have little knowledge of the role the Y chromosome plays in fertility and reproduction. 


As a result, they may base their bull selection on physical characteristics, size, and meat-milk yield, all to the detriment of protecting vibrant, diversified male fertility. That genetic inheritance is passed exclusively through the male line.


“In today’s dairy breeds such as Holsteins, they have a very limited genetic pool because of the negative effects of the accumulation of inbreeding and the loss of genetic diversity,” said Liu. 


“Also, the fertility traits are genetically negative-related to milk production. Extensive selection for milk production and the heavy use of top sires in AI may lead to the decline of fertility.”


The discovery of the 1,274 transcriptionally active genes and 375 noncoding RNAs challenges the traditional notion that the MSY is gene poor and inactive. 


Liu hopes that understanding genetic diversity will help producers improve their herds through outbreeding for male fertility.


He said the lineage of most of the bulls in current Holstein herds can be traced back to just a few bulls over a couple of hundred years. 


“The bovine genome sequence was published in 2009,” he said. 


“As that genome sequence was from a female, the findings of the bovine Y chromosome study is a significant contribution to the completion of the bovine — male and female — genome project.”

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