While the genetics of coat colour of domestic animals are comparably well-studied, the genetic background of other aspects, such as the horn shape and size of cattle, remains nebulous. Only the genes for the polled and scurred conditions are resolved, while the genes that determinate the shape of the horns as well as the size are unresolved.
Horn size is a quantitative trait. That means it is influenced by a larger quantity of genes and shows a continuum. The identity and influence of those individual genes is probably largely unknown, but crossbreeding results can provide a clue for speculations.
The idea for this post came to my mind when I saw photos of the Taurus cow “Lippe”, which is an F2 (Sayaguesa x Heck) individual.
F2 Heck x Sayaguesa cow Lippe in the Lippeaue © Matthias Scharf
It is notable that the cow has very small horns like a Chianina. Yet it has no Chianina in its ancestry, only Heck and Sayaguesa. Two breeds that have horns that can be described as at least medium sized – the horns of the Heck cattle used in the Lippeaue are actually comparably large. So it is possible to breed individuals with tiny horns from two breeds that have at least medium-sized horns within only two generations.
How is that possible? One possibility is that the gene or genes for this tiny horn size is or are recessive, and that the F2 carries two of the recessive alleles and is thus homozygous. However, in this case we would also see tiny-horned Sayaguesa and Heck cattle on occasion. The other explanation, which is much more plausible to me, is that this tiny horn size is the result of a cumulative effect. Sayaguesa may have alleles for small horn size one the one locus, and Heck cattle on another locus. In the F1 generation these loci would be heterozygous, thus the horn size would still be medium-sized. But in the F2 generation, coincidentally, the cow might be homozygous for the alleles causing small horn size on both loci, resulting in the very small horns not seen in the parental breeds or F1 animals. It might involve even more loci, two would be the minimum.
The horn size we see in Chianina might be caused by different loci or alleles, we cannot know without resolving the alleles and testing it. Crossbreeds suggest that at least some of the alleles causing the small horn size in Chianina are recessive. The crossbred Taurus bull 01 856 was the son of the bull Laokoon and the cow Larissa, two individuals with medium-sized horns. However, both parents were part Chianina (25% respectively 62,5%). 01 856 happened to have rather small horns, not larger than in Chianina. This suggests that at least some of the alleles causing the horn size in Chianina are recessive. It would also explain why many half-Chianina individuals had horns of medium size (such as the bull Luca or the cow Larissa).
If the small horn size of Chianina is indeed recessive, this is bad news. Recessive alleles are difficult to purge effectively from the population.
The bull 01 856
If it is possible to breed horns smaller than in both parental breeds within two generations, as the cow Lippe demonstrates, it might also be possible to breed large horns out of two breeds with medium-sized horns in few generations. This would depend on which alleles the parental breeds have.
I wonder if focusing on particular genes is the best idea. We do not inherit one gene alone but a complete chromosome with thousands of genes.
ReplyDeleteAs the auroch dna is sequenced, is it possible to identify the least derived versions of chromosoms in european breeds ? If possible, I imagin that you just have to breed until your cattle has as much "primitive" chromosoms as possible, without caring about the phenotype.
I don't knom if it's realistic.
Well, for starters, the aurochs genome doesn't tell us anything about their chromosomes. For another, only having the one genome means we don't actually know the full genetic variation of the aurochs, and consequently we don't know exactly which cattle genes were or were not also found in aurochs. Lastly, to try and recreate an aurochs genotype using selective breeding alone would be practically impossible, since you'd be selecting on hundreds of thousands of loci, the vast majority of which wouldn't even be important for an aurochs phenotype. You could try to determine which genes are important first, but again you'd probably need some more genomes and you'd have to initiate a hugely expensive and long-term project to test the function of each allele, and if you had that knowledge and resources you would probably use genetic editing techniques instead of selective breeding. Overall it's kind of a non-starter.
DeleteIf you have the complete genome of an auroch (ATCG etc..) and the DNA sequence from a cattle chromosom you cannot compare them ?
ReplyDeleteI agree on the fact that for now, we only have the british auroch, but maybe we will have new genomes in the future. And I agree on the fact that selecting on particular loci seems impossible. But if you select chromosoms, catlle have only 30 pairs.
According to this map , Highland Dexter and Kerry have something like 10/60 auroch chromosoms. Would be intresting to know how many from the british auroch are still in modern breeds.
https://genomebiology.biomedcentral.com/articles/10.1186/s13059-015-0790-2/figures/5
You can certainly compare two genomes, but preserved genomes do not reflect chromosome composition.
DeleteHaving followed your blog for a few years, I'm wondering whether development speed is a factor. You have rather convincingly argued that much of the difference between body size and leg proportions between aurochs and cattle is down do cattle having been selected for rapid maturity. Could the same be the case for horns? Would a more slowly growing individual (let's say one that spent 10 rather that 8 years to reach maturity) also grow a more impressive set of horns?
ReplyDeleteI think it's very likely that development influences horn length and curvature, yes. I covered that in several articles, like this one: https://breedingback.blogspot.com/2017/08/the-genetic-and-developmental.html or that one: https://breedingback.blogspot.com/2020/08/a-watussi-bull-with-perfectly-aurochs.html
Delete