Assessing diversity losses due to selection for coat colour in the endangered bay-Asturcón pony using microsatellites
Introduction
The Asturcón pony is a ‘Celtic type’ breed which is considered to be one of the oldest representatives of native Spanish horses (García-Dory, 1980, Royo et al., 2005a). The breed received its name from the description of the Asturian horses made by the Roman chroniclers during the Cantabrian Wars 80 B.C. (Álvarez Sevilla, 1995). The Asturcón population was on the brink of extinction during the second third of the 20th century (Álvarez Llana, 1995, Álvarez Sevilla, 1995). The recovery of the Asturcón pony breed started during the 1970s with 21 black-coated reproductive individuals. (García-Dory, 1980, Álvarez Llana, 1995). This seminal initiative for conservation created controversy. Historical reports showed that the majority of the Asturcón individuals in the first third of the 20th century were bay-coated, with black-coated individuals in moderate to low frequency (García-Dory, 1980, Álvarez Llana, 1995).
Recently, the breeders' association (ACPRA) included in its breeding programme bay-coated Asturcón individuals (Royo et al., 2007). The ideas underlying this second initiative for conserving the Asturcón pony were to preserve a genetic resource which was still present in semi-feral conditions in Western Asturias (the bay-coated Asturcón) and also the production of black-coated Asturcón individuals from heterozygous bay-Asturcón parents in order to contribute to the control of inbreeding in the black-coated Asturcón population (Royo et al., 2007). Only 3 out of 7 founder stallion lines (unbroken descent through male animals only from an ancestor to a descendant) remain at present in the black-coated Asturcón population (Royo et al., 2007). However, registered offspring in the bay-coated Asturcón include 20 different founder stallion lines. Up to present, no crosses are allowed between individuals deriving from the initial ACPRA conservation initiative (A20 population) and those belonging to the most recent Asturcón registering (A21 population). Common breeding policies for the two Asturcón pony populations are still to be decided.
The recovery strategy applied for the A21 population was different to that carried out for the A20 Asturcón. Although only individuals with full accordance with breed standard were used, ACPRA decided to enlarge as much as possible the number of founders of the A21 population. The aim was to implement a strong selection for type traits on a relatively large founder population to ensure type quality of the offspring. The main selective pressure is done on coat colour (see Rieder, 2009, Thiruvenkadan et al., 2008, for reviews on the genetic determination of coat colour in horses). The chestnut coat is not accepted in the A21 population register studbook. Also, A20 breeders usually reject for reproduction those individuals carrying this recessive trait. Therefore, selective pressure against chestnut carrier individuals is particularly intense. Carriers of recessive black are not rejected in the A21 population studbook. In fact, a few black-coated individuals were included as founders of this Asturcón population. However, the possible importance of heterozygous black-carrier A21 individuals on the genetic viability of the A20 population is yet to be ascertained.
There is an increasing interest in the conservation of genetic variability in horse populations (Avdi and Banos, 2008, Azor et al., 2007, Cervantes et al., 2008, Cervantes et al., 2009). It has recently been shown in horses that preserving coat colour variation can lead to the conservation of the genetic variability in a genetic stock (Druml et al., 2009, Bartolomé et al., 2010). The aim of the present analysis is to assess, using microsatellites, to what extent the genetic variability of the A21 population may be affected by selection strategies aimed at decreasing the frequency of the chestnut allele. Also, the genetic impact on the A20 population of the inclusion into its breeding programme of the black-coated offspring of A21 individuals will be assessed. Consequences for the conservation programme of the Asturcón pony breed are discussed.
Section snippets
Sampling and genotyping
From the start of its recovery to July 2008, a total of 319 A21 individuals were registered in the ACPRA studbook. From them, no parents were known from 195 individuals that were considered founders. A total of 297 blood samples were obtained from these individuals. From them, 180 were founders and will be hereafter referred as “A21 founder subpopulation”. Samples from non-founder individuals will be referred as “A21 offspring subpopulation”. Total DNA was isolated following standard procedures
Results
Table 1 gives the allelic and genotypic frequencies for the MC1r and ASIP genes in the analysed (sub)populations. The wild alleles E and A were the most frequent (77.8% and 59.4%) in the whole A21 population and are more frequent in the offspring subpopulation than in the founder subpopulation. Both the e and ea recessive chestnut alleles were identified although the allele ea was in low frequency (1.0% in the whole population. Both the e and ea alleles were in heterozygosity with the wild
Discussion
Here we assess the losses of genetic variability of a small horse population in a very early moment after the implementation of its conservation programme. Monitoring of genetic variability in small populations usually include genealogical information (Goyache et al., 2003, Gutiérrez et al., 2005b). However, pedigrees in the A21 Asturcón population are very shallow. In such scenarios, molecular polymorphism information has been shown to be the best option to assess the effect of selection or
Acknowledgments
This research was partially funded by a grant MICIN-INIA RZ2008-00010. The authors are indebted to the Asturcón pony breeders' association (ACPRA; http://www.asturcones.com/) for its kind collaboration.
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