This week’s BEACON Researchers at Work blog post is by University of Texas at Austin graduate student Emily Jane McTavish.
Emily Jane McTavish with Prairie Dancer at the Double Helix Ranch
In the ranch lands of Texas it can feel like cattle have always been a part of the western landscape. However, neither domesticated cattle (Bos taurus) nor their wild ancestor, the auroch (Bos primigenius), are native to the Americas – there were no cattle prior to 1493. The first cattle were brought by Christopher Columbus on his second voyage across the Atlantic to the Caribbean island of Hispaniola, what is today Haiti and the Dominican Republic. The cattle he brought were likely from the Canary Islands, off the western coast of Africa, then a Portuguese colony, which were in turn probably descended from animals of Portuguese origin. The imported cattle reproduced rapidly, and by 1512 importation of cattle by ship was no longer necessary. Caribbean cattle were introduced into Mexico in 1521, and had reached north into what is now Texas and south into Colombia and Venezuela within a few decades (Barragy 2003).
The settlers relied on these cattle for meat, but largely allowed them free range in the unfenced wilderness. The artificial selection was imposed mostly by the choice of which individuals to castrate for steers, and which to leave as bulls. Otherwise, natural selection drove the evolution of this group for the next 400 years, roughly 100 generations. These cattle are the ancestors of the present day new world breeds including Corriente cattle from Mexico, Texas Longhorns, and Romosinuano cattle from Colombia. This long period of selection has left these groups better adapted to these landscapes than breeds of more recent European import. Texas Longhorns are known to be immune to Tick Fever, a disease caused by bacteria in the genus Babesia, which can be fatal. They have also been described to have far greater drought resistance than other European breeds.
Neat history, sure, but how does the history of domestic cattle in the United States tie into BEACON’s mission of studying “evolution in action”?
I am a graduate student in my fourth year in David Hillis’s lab at the University of Texas at Austin, a BEACON partner institution. I am interested in the role of gene flow and hybridization in how populations diverge and adapt to their environments. My interests lie not in how divergence occurs when populations are completely isolated, but rather how differentiation between populations can occur in the face of continuous gene flow or interbreeding. In addition, within the patterns of divergence between populations is a signature of past processes, which can allow us to reconstruct the geographic history of a species. Gene flow between two groups that have been evolutionarily separated for a long period of time injects new adaptive variation into both groups, which can then be acted on by natural selection. In my research, I am using a genome wide single nucleotide polymorphism (SNP) data set to assess the population structure of cattle breeds world-wide, and specifically determine the role of hybridization two subspecies of cattle. Domesticated cattle actually consists of two subspecies (or species, depending on whom you ask), which are derived from independent domestications of the same progenitor species, the auroch; Bos taurus taurus, likely domesticated in the middle east or Europe, and Bos taurus indicus, domesticated on the Indian sub-continent. Although these domestication events likely occurred only 7,000-10,000 years ago, due to pre-existing spatial genetic structure in the auroch population these two subspecies share a most recent common ancestor 200,000 or more years ago. The clearest phenotypic difference between these groups is that indicine cattle have a noticeable hump at the withers. Brahman cattle is the most common B. t. indicus breed in the United States. Generally, B. t. indicus are more feed-stress and water-stress tolerant than taurine breeds, and more tropically adapted.
This will allow me to both assess the possible influence of indicus-derived genes on these cattle’s adaptation to the Americas, but also to track the history of hybridization, and determine whether it has occurred before or after their introduction.
I have found that New World cattle are of B. t. taurus origin, as expected from their European ancestry, but show significant introgression (approximately 10-20%) of B. t. indicus alleles into their genome. Preliminary results suggest that southern European breeds of cattle similarly show elevated levels of B. t. indicus introgression, suggesting that the hybridization may predate the introduction of cattle to the Americas. This hybridization may have given these conquistador cattle the some of genetic variation they have used to adapt to their novel environment. I am currently working on mapping what portions of the genome of breeds that show introgression are derived from each subspecies. This information will also allow me to determine whether introgression occurred in independently in several breeds, or ancestrally. One historically plausible explanation is that indicine cattle from North Africa were transported across the Mediterranean, and crossed with cattle from the Iberian Peninsula. Alternatively, hybridization may have happened recently, after the introduction of Brahman cattle to the United States.
The figure demonstrates how gene flow may have occurred, based on historic records and my preliminary data. I am continuing work on this project, and look forward to clarifying the history of this group.
I am very excited about applying genomic data to question of dispersal and gene flow, and am at this moment on a plane to Okinawa to participate in a three week genomics workshop with a focus on linkage and recombination, at the Okinawa Institute for Science and Technology (OIST), which will provide me with many more analytical tools to bring to bear on this question as well as in future projects. In particular this will be useful for a project I am beginning in the fall in collaboration with fellow BEACON members Jack Sullivan and James Foster at the University of Idaho. We will be examining genomic patterns of differentiation under divergence with gene flow. My role will be using spatial explicit simulations (DIM SUM) to parameterize expectations for patterns of genetic divergence between populations, across selected and neutral regions of the genome, under a range of dispersal distributions.
Being involved in BEACON has been a great opportunity for me to meet and collaborate with fascinating scientists, research exciting questions, and tie my work into a broader context.
References:
Barragy, JT. 2003. Gathering Texas Gold. Caye del Grullo Press.
Brown, J. K. Savidge, and E. J. McTavish, 2011. DIM SUM: Demography and Individual Migration Simulated Using a Markov chain. Molecular Ecology Resources, 11: 358–363. doi: 10.1111/j.1755-0998.2010.02925.x
For more information, contact Emily Jane at emily dot mctavish at mail dot utexas dot edu.
