Technical information - General

Tackling delayed puberty in Brahmans


From Beef Bulletin Q1 - 2012 BEEF CRC PUBLICATION

Research shows genes associated with late-onset puberty in heifers are also the same as those associated with late-onset puberty in bulls. This means that selecting either bulls or heifers for earlier puberty will also reduce age at puberty in their half-sibs

There are major differences in cattle reproductive rates in northern and southern Australia, in part because the hardy Bos indicus breeds that dominate beef production in the tropics reproduce more slowly than the temperate Bos taurus breeds, particularly when they are suckling a calf.

In the harsh tropical environments where these breeds evolved, this was a desirable survival mechanism to ensure survival of the breed. However under current production systems in northern Australia, where management practices such as weaning of calves and supplementary feeding can be implemented, the poorer reproductive rates now represent a major inefficiency for the beef industry.

Beef CRC and CSIRO PhD researcher Marina Fortes says the major beef breed in Australia's north, the Brahman, is much better adapted than the taurine breeds to high temperatures and humidity, parasites such as ticks, worms and buffalo flies and seasonally poor pasture quality during the dry seasons.

Under these conditions Brahman bull calves typically do not reach puberty before 18 to 24 months of age, much later than the taurine breeds. Tropical composite breeds like Belmont Red, Charbray, Santa Gertrudis, Brahman crossbreds and Senepol, comprising various admixtures of indicus and taurine genes and grazed at pasture in the tropics also reach puberty later than British bulls reared in temperate environments.

Ms Fortes studied veterinary science as an undergraduate at the University of Sao Paulo in Brazil, and came to Australia in 2008 with husband Laercio Porto Neto, who had won a PhD scholarship to work with Dr Bill Barendse at the Beef CRC.

Initially she worked for a year as a research assistant in the Animal Genetics Laboratory (AGL) at the University of Queensland, before successfully applying to also undertake PhD studies with the Beef CRC, supported by Meat and Livestock Australia.

Delayed puberty in Brahman and Tropical Composite breeds, in combination with the harsh production conditions, means most Brahman cows do not produce their first calves until they are at least three years old.

Ms Fortes is passionate about her research, because of its potential to boost the productivity of the beef industry in northern Australia, and in other tropical areas throughout the world.

Her research has confirmed that some of the genes associated with late-onset puberty in heifers are also the same as those associated with late-onset puberty in bulls. This means that selecting either bulls or heifers for earlier puberty will also reduce age at puberty in their half-sibs.

Marina says reducing the age of puberty in Bos indicus and crossbred bulls to less than 12 months has potential to improve productivity, reduce production costs, and, by shortening the generation interval, accelerate rates of genetic gain for other desirable traits. But achieving rapid gains requires a radically new breeding strategy, based on highprecision genomics.

"As part of my PhD project, I used data from two herds of cows bred by the Beef CRC, one comprising 843 Brahmans and the other 866 Tropical Composite cows," Marina said. "CRC researchers

from the Animal Genetics and Breeding Unit (AGBU) had previously found there was a large variation in the age of puberty, particularly in Brahman heifers, indicating strong potential for genetic improvement They had also found that age at puberty was highly heritable and related to other traits such as weight gain and body composition traits."

"In addition to this field data, the CRC used the Illumina 50K SNP chip to genotype each animal."

More recently, Beef CRC researchers used the Illumina 800K SNP chip to screen selected Brahman and Tropical Composite cows, as well as some bull calves born to the Brahman cows. Ms Fortes then cross-matched their SNP data with 22 measured traits recorded as part of the AGBU analyses to identify candidate genes for early puberty.

She found several SNP markers on chromosome 14 that were associated with the concentration of growth-promoting Insulin-like Growth Factor-1 (IGF1) in blood samples of both the experimental bulls and heifers.

Cattle chromosome 14 turned out to be highly prospective: it carried several candidate genes for early puberty, known rather cryptically as PLAG1, XKR4, MOS and PENK.

PLAG1 is a transcription factor gene – a master regulator gene – that may control serum levels of IGF1. The PENK gene codes for an opioid receptor in the brain, known to be associated with fertility, while mice lacking a functional MOS gene are infertile.

Ms Fortes says the genes cluster within a 2-megabase (2 million DNA "letters") region of chromosome 14, where the SNPs used for genotyping are too sparse to indicate which of the genes is the key player in early puberty. So the researchers are re-analysing the region with the high-resolution 800K SNP chip to see if they can pinpoint the exact gene causing the differences between animals in age at puberty.

The X chromosome is another hotspot for earlypuberty genes, especially genes that influence sperm quality and scrotum circumference, another predictor of early puberty. "It's funny to think that the female chromosome is associated with male reproductive traits, but this is what we found," Ms Fortes said.

Each key gene will contain at least one unique variation (a marker) in its DNA sequence that distinguishes it from all other variants of the same gene: i.e. the mutation that originally caused the functional change contributing to early puberty. A gene-testing kit to detect these markers will allow breeders to screen their breeding line for animals carrying the gene variants once they are identified.

Ms Fortes says the impact on herd reproduction rates from these advances in genomics will not be immediate. But beef producers in northern Australia will benefit as superior herd bulls selected for early puberty and high fertility progressively increase the calf output in their herds.

Using the 800K chip

Genomics, the study of the organisation and function of a species' full DNA sequence, allows molecular geneticists to pinpoint the very source of the differences between individuals of the same species, or breed.

Most genetic variation between individuals of the same species, or breed, traces to tiny differences in DNA sequence which subtly alter the function of the protein encoded by a gene, or change the way the gene functions. Most of these variations involve single nucleotide polymorphisms (SNPs – pronounced "snips"): single-letter substitutions that accumulate over evolutionary timescales through mutation.

Genomics technology company Illumina's BovineSNP50 chip carries an array of 50,000 short DNA sequences from the spectrum of known SNPs from individual cattle. Beef CRC researchers have used it to screen the DNA of more than 10,000 animals in a technique called a genome-wide association study, or GWAS.

Whenever a DNA sequence on the chip detects a matching sequence in the DNA of one of the cows or bulls in the sample herd, the tiny patch of the chip carrying that SNP changes colour, confirming the cow has inherited that particular variation in the DNA sequence.

The chip reveals which variants or "alleles" of the DNA sequence every animal has inherited in 50,000 spots (SNPs). This reflects the extent to which the animals have inherited the same gene variants.

Checking shared SNP alleles against measurements of the full range of different carcase and meat quality, feed efficiency and reproduction traits in the test herds reveals associations between a particular trait, and particular SNPs that occur at high frequency in animals exhibiting the desirable aspect of the trait.

The location of every SNP on the chip is known accurately, from a chromosomal map of the cattle genome. When several SNP alleles from the same region of a particular chromosome consistently appear in animals that for example, reach puberty early but not in animals with delayed puberty, geneticists know an anonymous gene involved in early puberty lies nearby.

The closer the SNP to the desirable gene variant, the more likely they are to be inherited together. Rarely, a SNP marker will actually lie within the gene of interest, so its presence guarantees the animal carries the "gold standard" allele of the gene. It is ready-made for inclusion in a commercial gene-testing kit to rapidly identify animals with the potential to produce early-breeding progeny.