Summary 2: "Introgression & Purity Assessment in Dexters" by Sheppy

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ARTICLE SUMMARY 2
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"Introgression and Purity Assessment in Dexter Cattle"
by Andrew Sheppy
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Paper presented to the Second World Congress on Dexter Cattle held in Australia in 2002 (9 pages long)
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Andrew Sheppy is a lecturer, researcher and author on rare farm livestock breeds, especially poultry but also cattle and sheep. He is the founder of the Cobthorn Trust and of the British Rare Poultry Society. He has a longstanding involvement with the UK Rare Breeds Survival Trust, and has served as its Vice-President. He has a keen interest in Dexters, operating the Cobthorn stud. For the Cobthorn Trust website, see www.datazenith.com/cobthorn.
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~~ Start of Article Summary ~~
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Introduction
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In the UK the Dexter has traditionally been described as a small horned breed usually black or red. Since 1970, greatly increased variation has been noted including colour variants such as dun, brindle, black & white, and red & white, as well as many shades of red. Early UK herdbooks showed only very dark red Dexters. In addition, polled Dexters have appeared, unknown in the herdbooks before 1970. "While there is a possibility that all the new variants arose from mutation, it would seem more likely that much of the variation owes its origin to outcrossing, particularly as all the variants can be seen in other breeds, in fact breeds which have been implicated in introgression already identified" (p.1). Sheppy points out that genetic and biochemical studies have shown divergence between the Dexter and the Kerry which are regarded historically as nearest relatives. Alderson's paper to the First World Dexter Congress concluded that the present Dexter population is not closely related to the Kerry, owing a closer relationship to a mixed group of breeds.
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Recorded Introgression
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The foundation stock of the Dexter breed is taken to be those animals registered in Ireland through the Royal Dublin Society from 1890 to 1918 and those registered initially in England through the Kerry and Dexter Cattle Society there. Prior to 1945, there is no official record of crossbreeding in the UK herdbooks. In 1945 , the UK Dexter Cattle Society (DCS) introduced an Appendix Register to the herdbook, allowing the entry of crossbred animals in an upgrading scheme. Appendix A cows were of crossbred or unknown origin. Appendix B cows were the offspring of a fully registered pedigree Dexter bull and an Appendix A cow. Appendix C cows were the offspring of such a bull and Appendix B cows. The daughters of Appendix C cows were pedigree. Bulls were allowed to be registered in the herdbook if both parents had full pedigree. A wide range of breeds came into the herdbook through the Appendix, especially Aberdeen Angus and Jersey, but including most British breeds and some non-British.
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In the 1960s, in order to try to breed out the bulldog calf defect, an Experimental Register was established by the DCS and Aberdeen Angus and Jersey breeds were deliberately introduced. The first Experimental bull was Atlantic Anchor out of an Aberdeen Angus/Jersey cross cow by a pedigree Dexter bull, His son, Atlantic Finbar, born in 1966, was made available by AI. Sheppy contends that another important AI sire, Templeton Michaelmouse Squeak, was also descended from an upgraded cow line with a Jersey foundation.
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Unrecorded Introgression
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According to Sheppy, "that there has been unrecorded crossbreeding in the ancestry of some animals recorded in the DCS herdbook is undisputed" (p.3). This is clear from progeny showing atypical characteristics and from admission s from herd owners. In the case of the Parndon bloodline, blood from the Guernsey breed has been passed on. Colour variation is an important indicator of introgression, as are horn characteristics (including polledness).
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New Research
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Modern DNA research techniques enable an analysis of the mixture of breeds in any population. "A research programme has been established to investigate the likely derivation of the various bloodlines in the current UK population of Dexter cattle" (p.4). Sheppy proposes that a benchmark DNA profile be developed from Dexters believed to be free of introgression, and then be compared with other Dexters.
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Current Status of Bloodlines
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Sheppy argues that his analysis of pedigrees from the UK herdbook has demonstrated "the dangerously low population of animals thought possibly free of any introgression. The position is significantly worsened by the fact that one line (Woodmagic) is hugely dominant in that group" (p.5). [Later, Sheppy argues that "no documentary evidence of introgression has been found in respect of this line, but it remains distinctly atypical of the traditional type" - p.6 - at least partly because of the dun colour in it.] If the assumption that the Woodmagic line is not pure is made, then only 137 cows and 9 bulls in the 1982 DCS herdbook are thought to be clear of introgression. Furthermore, the number of registrations of Dexters in the UK since then believed to be pure and not containing Woodmagic ancestry has fallen dramatically. In 1982, there were 137 cows and nine bulls registered with these characteristics. By 1987, this had fallen to seven cows and two bulls; by 1992, it was seven cows and no bulls, and between 1995 and 1999 (inclusive) no cows and no bulls were registered with believed pure registrations not containing Woodmagic ancestry. As a consequence, when animals were being identified for the DNA benchmark, "there were only 30 living, but mostly elderly females and no males, which were thought possibly pure Dexter and which did not have Woodmagic ancestry" (p.5), although stored semen from 10 purebred bulls was also available.
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Sheppy then reviews the status of the eight dominant bloodlines he identified in an earlier paper (summary also on this website "Bloodlines, Breed Structure, and the Influence of Artificial Insemination in Dexter Cattle"):
Line 1, based on Atlantic Finbar, is an experimental line based on the introduction of Aberdeen Angus and Jersey ancestry.
Line 2, Canwell Buster, base of the Summerdale line, is considered pure (free from introgression).
Line 3, Shadwell Robert, is viewed by Sheppy as of dubious purity and he comments that "further investigation of this bull's provenance has now revealed that his maternal granddam was not in fact a pure Dexter" (p.6). This accounts for the inconsistencies in the red animals of this line.
Line 4, Sylvan Ebony, base of the Weatheroak line, is considered pure.
Line 5, Templeton Michaelmas Squeak of the Doesmead line is known to descend from an Appendix A cow with a Jersey foundation.
Line 6, Woodmagic, is considered by Sheppy to be of dubious purity as "it remains distinctly atypical of the traditional type" though "no documentary evidence of introgression has been found" (p.6).
Line 7, based on Marsh Warrior, is regarded as pure although it is almost extinct as a separate line.
Line 8, the Parndon line based on Melbry Duster, poses problems. "For the purposes of the current research programme, it has been decided to give the benefit of the doubt to offspring of Parndon Buster Finch who was registered as a son of Parndon Charley Pudding, but could in fact have been pure" (p.7) (see "Bloodlines, Breed Structure, and the Influence of Artificial Insemination in Dexter Cattle" for a discussion of Parndon Charley Pudding).
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Sheppy notes that there are lines in the USA which are purebred according to the registered records and directly originate from purebred foundation stock imported from either the UK or Ireland early in the twentieth century. "This stock could well represent an important reservoir of pure Dexter genetics" (p.7). The same may be true of other national stocks, with South Africa being the next most likely candidate as a host of purebred animals.
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Maintenance of Purebred Lines
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"Much has been done to enhance and promote the commercial and production qualities of the breed, but a corresponding ensuring of the breed's purity has not taken place" (p.7). Breeding programmes urgently need to be established to make sure that the remaining purebred lines survive. The DCS in the UK has encouraged such programmes in order to produce new bulls that could be used for Artificial Insemination and to produce purebred heifers to replace the elderly purebred cows remaining.
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The effects of introgression can be reduced through pure breeding. "In general terms, the working basis of pedigree assessment is five generations of ancestry...Beyond the fifth generation it is extremely unlikely that all ancestors have even contributed to the genetic constitution of any individual animal" (p.8). Each generation away from any introgression halves the likelihood of the introgressed genes being passed on.
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