Genetic Pastorale

A QUANTUM OF SCIENCE

Are cows responsible for the spread of a gene allowing humans to digest their milk?

Lactase is an enzyme that breaks down lactose, the disaccharide found in cow milk, into glucose and galactose.



This is important because lactose itself cannot serve as a source of energy for the human body, but glucose and galactose can. The gene encoding lactase can have either one or two copies and still produce enough lactase for carriers to process milk – humans lacking a copy cannot digest cow milk and are called lactose intolerant. Another term, "lactase persistence," refers to the strange stubbornness of the lactase gene, which is inherited in some human populations with far more frequency and fidelity than in other human populations.

Now a new publication shows statistical attempts to model the spread of lactase persistence in European populations as a correlate with how common dairying – raising cows for their milk – is in that region over the last, say, ten thousand years.

While it might seem intuitively obvious that intensive dairying would provide a strong selective pressure for a gene that makes cow milk digestible, this model shows exactly how strong that pressure is. Think about it this way: dairy cows were first domesticated around 9,000 years ago in the early Neolithic age. Since then, their presence has managed to exert a strong influence on human genetics, making those who raise them and care for them much more able to benefit from their milk. Before cattle were domesticated for milk (as opposed to for meat) there was little advantage for humans to possess even a single copy of the lactase gene; today, there is a sharp genetic profile between human populations associated with dairying and those that are not. This map, taken from the statistical modeling publication, shows how sharp that genetic profile is.



Researchers suggest, based on this model, that lactase persistence originated somewhere in the red area on the map around 7,500 years ago. This matches archeological evidence as well, showing the value of the model for predicting genetic drift over time and distance.

As much as human civilization has impacted the genetic destiny of the cow, the reverse is true as well. This begs the question: how many other creatures in our environment are shaping us, even now? It is easy to believe that Homo sapiens is the apex of the evolutionary ladder, but even the humble cow has some say in our genetic path.

For more information:

The Origins of Lactase Persistence in Europe (Itan et al)

Lactase (Wikipedia article)

Lactose intolerance (Wikipedia article)



© AQOS / P. Smalley (2009)
Reproduction with attribution is appreciation

How now, brown cow?

A QUANTUM OF SCIENCE

What do cows now have in common with dogs, guinea pigs, armadillos, lemurs, the platypus, slime molds and wild mustard?

Bessie is the latest member in the club of organisms with sequenced genomes.

At present this club consists of approximately twenty single-celled organisms, twenty-two plants and thirty-six animals (not to mention 360 bacterial species). The club was founded in 1995 with the sequencing of the bacterium Haemophilus influenzae. Humans joined in 2000, fashionably late. The bovine genome has taken six years of work by 300 scientists at a cost of a quite modest $53 million (compared to the Human Genome Project at a bit over $3 billion). The project was spearheaded by researchers at Baylor College of Medicine in Houston, Texas and the results are provoking a level of interest far in excess of other organisms recently sequenced. The last "big" genome to be sequenced was the mouse, whose importance as a research tool can hardly be overstated. Almost all pharmaceuticals and therapies used on humans are tested on mice, so understanding their genetic variability was a huge milestone. So why is the cow genome making such a stir?

Unsurprisingly, it has to do with money. The cow is the first animal to be sequenced that has a significant commercial value associated with it. And ranchers and dairy farmers are already starting to queue up to have their herds genotyped, hoping to find out which ones carry genes associated with increased milk production, better tissue-building properties (read: faster meat) and improved resistance to pathogens. From a scientific basis this latter trait is among the most fascinating: even though cows diverged from the evolutionary tree before mice and well before humans, they share more genes in common with humans than they do with mice. In part this may be due to the far faster reproduction of mice giving them more generations to evolve, but there is some suggestion that humans and cows evolved more along similar tracks because of their symbiotic relationship over the last 10,000 years – in other words, domestication. 

There were some definite surprises when the full map of the bovine DNA record was analyzed. Of particular note is the degree of repetition in their genome. Genes coding for immune defenses exist in myriad copies in the bovine genome compared to humans, perhaps due to the significantly greater exposure they have to microbes associated with digestion of cellulose. Curiously, the same thing may have caused cows to lose the genes for certain digestive enzymes from their genome that humans retained – since microbes were doing the digesting for them, cows did not need to retain those genes. 

What good can come from this project? The significant benefit of knowing the blueprint for cows lies primarily in breeding. Currently most cattle breeding centers on bulls, and the cost to bring a single bull to an age where it can be bred is between $25,000 to $50,000 – and there are no guarantees a particular bull will make good breeding stock. Now, breeders can test bulls shortly after birth and determine which ones will make the best stock for particular desirable traits, saving tremendous amounts of money and making the process far less fraught with uncertainty and error. Best of all, breeding programs can now begin to reduce the dairy industry’s reliance on additives like rBST, the recombinant bovine somatotropin (better known as bovine growth hormone), a product used since 1993 as a means of increasing milk production but which has caused considerable controversy regarding the health of the animals in which it was used. 

As a result of the sequencing of the bovine genome, the beef and dairy industries are poised for a potential rebirth as modern genetic techniques become available and better decisions can be made to improve the health of the industry, the consumer, and the cows.



For more information:

http://sciencenow.sciencemag.org/cgi/content/full/2009/423/2

http://en.wikipedia.org/wiki/List_of_sequenced_eukaryotic_genomes


© AQOS, Peter Smalley (2009)
Distribution with attribution is appreciation