U. Kentucky DNA Report |
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The origins of the horses on Abaco Island in the Bahamas is not clear. However, it is certain that in recent years the population size of the herd has been extremely small ( . . . as low as 3 in the 1960’s). Currently the herd numbers less than 20. Small population size inevitably results in a loss of genetic variation which usually results in a loss of fitness of the population due to the increased probability of deleterious recessive genes being paired (and therefore expressed) by in breeding. This is not always the case because these deleterious genes can, in rare cases, be removed by natural selection (especially if the population manages to survive for several generations). Inbreeding also will result in other characteristics becoming more common or fixed. In the case of the Abaco horses, there is a high incidence of the splashed white overo coat color pattern in the herd. This is a relatively uncommon coat color in domestic horse, which I will discuss at the end of this report.
Over the past two years, samples from a total of eleven Abaco horses were received by the Equine Parentage Testing and Research Laboratory at the University of Kentucky for genetic testing. All samples were tested for variation at 12 horse DNA microsatellite genetic marker systems. Two of the samples were tested for seven blood groups and 10 biochemical genetic systems as well. A variety of measures of genetic variation were calculated from the microsatellite data. I will discuss four here. Observed heterozygosity (Ho), expecteded Heterozygosity (He), total number of variants (TNV) and effective number of variants (Ae). The data from the Abaco horses was compared to the same type of data from 55 other populations of horses (both domestic breeds and other feral herds). The results are given in the following table:
The Abaco data was used in the calculation of the mean values but not for the minimum values shown in the table, for all four measures the Abaco measures were the lowest observed in horse populations. The variability measures clearly show extremely low variation in the Abaco horses consistent with the small population size in recent generations. With only two samples for testing blood group and biochremical genetic variation, no estimates of variability were calculated. He, TNV and Ae are strongly correlated with sample size and two is an insufficient number for meaningful estimates. Ho is not correlated with sample size. Ho for both horses samples was 0.30 which is considerably below the mean for horses which is 0.37. This number is consistent with predicted blood type Ho based upon mcirosatellite Ho. For the Abaco horses the predicted value was 0.317. Genetic marker analysis can give indications of relationship that could have provided information on the origins of the Abaco horse. However, due to the observed reduced genetic variation, comparison of the Abaco horses to 30 other breeds representing all major types of horses showed the Abaco horses to be the most divergent with no clear relationship to any of the breeds. The blood type results did suggest the possibility that the Abaco horses were derived from horses that originated from the Iberian Peninsula. However, this is simply based upon the presence of two genetic markers which is insufficient for making significant deductions. (M. Rehor notes: If we accept his two indicators, and the conclusion from UCLA, not only do we have Spanish horses, but they are so ‘pristine’ that they probably are the closest thing to the original horse brought to the New World that you will find ever find on this planet. We await the third opinion). The combination of extremely low genetic variation and the high frequency of the splashed white overo color type do make the Abaco herd potentially important for genetic research. Few herds have faced the severe population reduction that has occurred in the Abaco herd and it could be worthwhile following the herd over time to see how it responds to such a bottleneck. The results could have implications for the genetic conservation of rare breeds of livestock. The genetic basis of the splashed white overo color is not known. The high frequency of this color in the Abaco herd could make these horses a valuable study population for determination of the inheritance of the splashed white and also for identification of the gene responsible for the pattern. If the molecular genetic basis of splashed white could be determined it could be developed into a diagnostic test to aid breeders interested in producing horses with this color pattern. Paint type horses are very popular currently and a color pattern such as that of the Abaco horse is likely to be highly desirable if there were a better understanding of how the pattern was controlled. (M. Rehor notes: Not only are our pintos beautifully colored,(and some of our bay mares have produced pinto foals) several of them are naturally gaited, which would effectively put them in the stratosphere of desirability, should we carry the color gene too.Their proven hardiness, gentleness and intelligence just add to their irreplaceable combination of qualities. I have arranged to have the genetic search begin as soon as we begin our hoof care program and can draw blood without trauma from each remain horse. Once again, I must emphasize that we have something of inestimable value here, and it simply cannot be permitted to go extinct because of the lack of some relatively simple solutions like a preserve, and decent and timely veterinary support. Money is not the issue. Land and government concern are the issues. We can do this, we can do it now. But it has to happen as fast as possible, we’re down to 17.) E. Gus Cothran |
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