Pedigree analysis of Thoroughbred horses in Slovakia

© Slovak University of Agriculture in Nitra Faculty of Agrobiology and Food Resources, 2014 http://www.fapz.uniag.sk/ DOI: 10.15414/afz.2014.17.04.122–126


Introduction
The Thoroughbred (TB) is typical representative of modern type of race horse.Achievement of maximal speed and earliness in races is main criterion in Thoroughbred selection.TB horse breed was established in England in the early 1700s based on crosses between stallions of Arabian origin and poorly defined, possibly indigenous, groups of mares.The founder population was small; all contemporary males trace back to one of three stallions (the Godolphin Arabian, the Byerley Turk and the Darley Arabian), whilst on the female side about seventy foundation mares have been identified (Binns et al., 2010;Willett 1970).TB performance testing is provided by racing.Flat races have the highest selection level, while the selection level of hurdle races is negligible.The very first race on the teritory of today´s Slovakia took place on 22 nd May 1814 in Urmín (Mojmírovce) organized by earl Jozef Hunyady (Gressner, 1995).Nowadays, TB breeding and racing in Slovakia is controlled by Turf Direktorium, s.e.Bratislava, which is responsible for Stud Book keeping as well.The quality of Slovak races is on very high level in central European region.However Slovak TB breeding passes through the stagnation and decline in recent years, as evidenced by decreased number of born and registered foals as well as by performance quality of horses born in Slovakia.There have been some positive results of Slovak TB breeding in recent years which proved that right system of mating and rearing of young horses can lead to production of good class horses in Central European region.Except the genetic predisposition, the horse performance is significantly influenced by many environmental factors.The success of a TB racehorse is determined by a multitude of factors such as age, sex, nutrition (Gálik et al., 2011;Gálik et al., 2012) of which many are impossible to evaluate.The aim of our work was to assess breeding population (stallions and mares) of Thoroughbred in Slovakia by the methods of pedigree analysis.

Material and methods
The input information was obtained from the Slovak Stud Book for Thoroughbred (SSB), volume IV. (2006)(2007)(2008)(2009) and from the 2 nd supplement of SSB issued in 2011 by Turf Direktorium Bratislava.Breeding stallions having mating licence in 2012 and breeding mares presented in the 2 nd supplement of SSB issued in 2011, were involved in the analysis.Mares, which died or were euthanized or exported abroad in 2011, were excluded from the analysis.There were 10 breeding stallions and 123 breeding mares included in the analysis.The paternal lineage structure and structure according to sires and dam-sires in investigated population were evaluated.
In further analysis, we focussed on evaluation of genetic diversity measures in given population by the methods of pedigree analysis.The 133 individuals (10 stallions, 123 mares) set up the reference population.
These animals represented whole active breeding population of TB in Slovakia.The program ENDOG v.4.8 (Gutiérrez and Goyache, 2005) was used for computation of diversity parameters.Index of pedigree completeness (PEC) according to McCluer et al. (1983) was calculated as well.
Parameters based on probability of identity-bydescent were used as follows: y Individual inbreeding coefficent (F i ), defined as the probability that two individuals recieved two identical alleles by descent (Gutiérrez and Goyache, 2005).y Individual increase in inbreeding (∆F i ), calculated as the difference of inbreeding coefficient of individual i and its parents (Kaerney et al., 2004).y Average relatedness coefficient (AR), defined as the probability that an allele randomly chosen from the whole population belongs to a given animal (Gutiérrez and Goyache, 2005).
Parameters based on probability of gene origin were used as follows: y Number of founders (f).Founder is defined as animal with unknown genetic connections to other animals in pedigree except its own progeny (Lacy, 1989).y Effective number of founders (f e ), defined as the number of equally contributing founders that will produce the same genetic diversity as in the assessed population (Boichard et al., 1997).y Effective number of ancestors (f a ), defined as the minimal number of ancestors necessary to explain the genetic diversity in the reference population (Boichard et al., 1997).y Number of ancestors explaining 50 % of the genetic diversity.
In the final part, marginal individual contributions (IC) of the most important ancestors were calculated in given breeding population of TB in Slovakia.The marginal contribution of an individual quantifies its contribution to the reference population, which has not previously been explained by greater contributing individuals (McParland et al., 2007).

Results and discussion
The pedigree completeness has been evaluated at the beginning of our analysis.McCluer´s index of pedigree completeness was 100 % (Figure 1) in the first five generation, hence presented results have high reliability.
Higher pedigree completeness has been confirmed by Cunningham et al. (2001), Pjontek et al. (2012) andDruml et al. (2009) in several horse breeds.The overview of the lineage structure in TB population is presented in Table 1.There were 6 paternal lineages found in the stallions.Three stallions linked to Nasrullah line (30 %) and 2 stallions linked to Northern Dancer (20 %) and Native Dancer lines (20 %).Lineages Blenheim, Ribot and War Relic were represented by only 1 stallion.Over a half of mares were represented by Northern Dancer line (50.41 %).This paternal lineage is considered as the most important in recent years.Further important paternal lineages were Nasrullah (11.38 %), Birkhahn (6.5 %), Turn-To (5.69 %) and Native Dancer (4.88 %) in mares´ population.
The lineage structure of populations is mainly used in horse breeding.In our study, Northern Dancer was the most influential lineage in Slovak TB population.Moreaux et al. (1996) state, that line variability in TB is dropping off and stresses the increasing tendention of Northern Dancer line within global TB population, mainly in the European countries.Vlček (2006) claims that the main reason of monopolization of lines Northern Dancer, Nasrullah, Native Dancer and Turn-To is the effort of the breeders to react the increasing demand on international racing horse markets.Cunnigham et al. (2001) presented while other stallions were responsible for paternal lineages early in the history of Thoroughbred, by the mid-1800s only three remained: the Byerley Turk, the Darley Arabian and the Godolphin Arabian.The percentage of paternal lineages attributable to the Darley Arabian line has been increasing for nearly 175 years, and is now responsible for 95 % of paternal lineages in the modern population.Each of these three foundation stallions is linked to the population in paternal lineage through contributions of only one of their male-line descendants.The influance of the Byerley Turk is carried on through his great great grandson Herod (1758), the Darley Arabian through his great great grandson Eclipse (1764) and Godolphin Arabian through his grandson Matchem (1748).
Each stallion originated from different sire (Allaged, Alrayed, Dictator´s Song, Green Desert, High Estate, King´s Best, Local Suitor, Raut, Sadler´s Wells a Stimul).Two of them had the same dam-sire (Mill Reef ).There were 93 sires identified in mares´ popoulation.Sebastian had the largest number of daughters (7 mares; 5.69 %), Solarstern and Najoom had five daughters (4.07 %), Muscatite and Platini had four daughters (3.25 %) and Czas had three daughters (2.44 %).The rest of sires had two or only one daughter.There were 113 dam-sires found in mares pedigrees as well.The 3 mares (2.44 %) had Nebos as dam-sire, the rest of dam-sires had two or one granddaughter.
The TB is the breed created by using of inbreeding.The International Agreement on Breeding, Racing and Wagening prohibits using of artificial insemination in TB breeding what positively influences the number of offsprings per superior animals especially stallions.This fact helps to maintain inbreeding on acceptable level.The genetic diversity parameters based on probability of identity-by-descent are presented in Table 2.
The number of founders in investigated population was 949.The effective number of founders was 202, while the effective number of ancestors was 67.The difference between these values indicates unbalanced using of small number of ancestors, especially breeding stallions.Given results show the bottleneck effect as well.In recent years, the total TB breeding population has decreased and many former important breeders disappeared (Motešice, Šamorín).These results point out unbalanced contributing of founders and ancestors in reference populations as well as the bottleneck effect occurance.The overview of parameters based on probability of gene origin is presented in Table 3.
In the final part of our work, marginal individual contributions (IC) of the most important ancestors were computed.The list of most important ancestors according to their IC is presented in Table 4. Individual contributions express the gene proportion of given ancestor in the population.The stallion Northern Dancer born in Canada was the most important ancestor (IC 7.88 %).This stallion is considered as the most important ancestor in global TB populations.The second most important ancestor was Nasrullah (IC 3.25 %) which is relative with Northern Dancer in paternal side of pedigree.The 10 most important ancestors explained 25.6 % of diversity.

Conclusions
The Thoroughbred is unique horse breed spread all over the world.Slovak TB population is small.Slovak breeding population can not provide enough horses to make Slovak horse racing self sufficient.The breeding population consisted of 133 horses of those 10 stallions and 123 mares.The paternal lineage structure is satisfying; modern lineages are represented sufficiently.The state of genetic diversity within the population is satisfying.It points out unbalanced using of small number of ancestors.The most important ancestor is the stallion Northern Dancer which represents the most popular paternal lineage world wide.FAO (2007) states the critical value of ∆F = 1 %, whereby in our case it was not higher than 0.5 %.

References
Figure 1Pedigree completeness index in evaluated population BINNS, M. M., BOEHLER, D. A. and LAMBERT, D. H. (2010) Identification of the myostatin locus (MSTN) as having a major

Table 1
Paternal lineages structure of mares and stallions

Table 3
Genetic diversity parameters based on probability of gene origin

Table 4
The most important ancestors in the reference population IC -individual contributions, CC -cummulative contributions, 1 -male, DI -Dosage index, CD -Center of distribution