High values for genetic identity means low values for genetic distance and vice versa. The Dendrogram is based on Nei’s (1972) using Genetic distance: Method = UPGMA (computer software), modified from NEIGHBOR procedure of PHYLIP Version 3.5 was used to draw the phylogeny tree between the three breeds understudy. The dendrogram showed that the Butana and Kenana are within one cluster while Fuga is in another cluster, (Fig 1).
Discussion
The sustainability of species and populations in the future is affected by the genetic diversity which shaped the past populations process (Soule, 1987). Maintaining of genetic diversity is a key to the long-term survival of most species including cattle (Hall and Bradley 1995). Many studies proved that farm animal genetic diversity is needed to meet current production requirements to allow sustained genetic improvement and to facilitate the rapid adaptation to changing breeding goals (Hall and Bradley 1995; Kumar et al. 2006). Diversity can be defined as the genetic variation between and within different breeds, so it is essential to characterize a breed for its conservation. Microsatellites markers are the best genetic marker have been used successfully to define genetic structures and genetic relationships among different breeds. Microsatellites usually show higher numbers of alleles and subsequently polymorphism. Consequently, they enable population differentiation to be found more efficiently. Microsatellites markers especially autosomal had been the most used genetic markers to estimate genetic diversity and to investigate different breed relationships moreover to define conservation priorities (Lenstra et al., 2012).
Neutral genetic diversity preservation is expected to contribute to maintaining specific breed traits due to natural and manmade selection.
Indeed, some microsatellites can be present in genes associated with important quantitative traits loci (QTLs) including adaptation (Hall et al., 2012).
Previous studies have been performed concerning genetic diversity and relationship between three local cattle populations (Gangatiri, Shahabadi and Purnea) and two established cattle breeds (Bachaur and Siri) of eastern India by using 21 FAO and ISAG recommended microsatellite markers (Sharma et al., 2013). In a study conducted by Rehman and Khan (2009) for identification the genetic diversity of Hariana and Hissar Pakistani cattle breeds using 30 bovine microsatellite markers suggested by a joint committee of the Food and Agriculture Organization and the International Society for Animal Genetics. However, no information is available on gene differentiation among different cattle breeds raised in Sudan. In the present study, genetic variation within and between three Sudanese cattle breeds named: Fuga, Butana, and Kenana were estimated using genotypic data of 9 microsatellite markers recommended by ISAG (2012) for such studies. The total numbers of animals genotyped were 75 animals, 25 animals from each breed. Out of the 9 microsatellite loci, 74 loci amplified successfully and produced definite banding patterns. Since it is observed a large numbers of alleles for these microsatellite markers, these markers could be fruitfully used in further studies on quantitative trait loci (QTL) detection and subsequently marker assisted selection (MAS). However, the allele sharing results did not show any obvious unique or specific alleles for specific breed. This is may come from the lack of breeding programs or in another words the absence of selection for genetic improvement.
The average of observed allele number was 8 alleles; this number lies within the range of 6-9 alleles, which was reported in many cattle breeds from Europe (MacHugh et al., 1997, 1998), Africa (MacHugh et al., 1997; Ibeagha-Awemu et al., 2004); Brazil (Egito et al., 2007). From another side, the observed average allele number is less than that reported for Indian zebu cattle, which is ranging from 4-16 (Mukesh et al., 2004; Chaudhari et al., 2009, Sodhi et al., 2011). This may be due to the large number of cattle breeds raised in India and the microsatellite used in the study, some microsatellites can produce more allele than others. The observed number of alleles demonstrated that almost all the microsatellite loci utilized in the present study were sufficiently polymorphic. All breeds showed that by the increase of number of alleles at different loci, there was an increase in mean genetic diversity in population and supported by Moioli et al. (2001). This is an indication for the high ratio of heterozygosity which arises from the absence or weak selection or organized breeding programs for the Sudanese cattle. The effective number of alleles (Ne) can be identified as an estimate for the number of alleles with equal frequencies corresponding to a particular PIC value. Fuga cattle have the highest mean effective number of alleles (3.963) when compared with the Butana (3.307) and Kenana (3.123) breeds. The observed mean (Ho) and expected (He) heterozygosity were 0.778 and 0.725 in Fuga vs. 0.737 and 0.695 in Butana and 0.693 and 0.651 in Kenana cattle, respectively. In all the three breeds studied and for all the markers used, there were few individuals carrying homozygous alleles. Accordingly the values of the expected heterozygosity were very high for all the markers and populations under study. The values of observed heterozygosity were higher than the expected heterozygosity indicates much of variability.
The Polymorphism Information Content (PIC) is an expected heterozygosity derived from allele frequencies in random mating populations. PIC is an indicator of how many alleles a certain marker has how much these alleles divided evenly. For example if a marker has many alleles but only one of them is frequent, the PIC will be low. The overall mean values of (PIC) obtained in the present study were 0.664 in Fuga, 0.630 in Butana and 0.596 in Kenana. While The average gene diversity over all loci were 0.684 that is almost similar to the previously reported by Loftus et al. (2002), which was 0.78 during their study concerning the identification of zebu alleles in some cattle breeds. There was a significant positive relationship between averages within population gene diversity for each locus. Kalinowski (2002) observed high values of (PIC) and attributed it to the large number of alleles or heterozygosity. The observed high number of alleles may be attributed to the absence of selection pressure used for the improvement of draught characters. These findings are in agreement with Muralidhar (2003), who used ten microsatellite markers and obtained PIC values in Indian cattle which ranged from 0.150 to 0.790 in Ongole cattle breed and from 0.13 to 0.80 in Deoni cattle breed. Moreover, Rehman and Khan (2009) demonstrated that the value of PIC was 0.749 in Hariana and 0.719 in Hissar cattle. Higher PIC values were also seen in the Brazilian and Indian zebu cattle investigated earlier using microsatellite markers (Egito et al., 2007; Pandey et al., 2006; Kale et al., 2010 and Sodhi et al., 2011).
According to Holsinger and Weir (2009), Wright’s F-statistics provide important insights into evolutionary processes that influence the structure of genetic variation within and between populations, for that they are most widely used descriptive statistics in population and evolutionary genetics. Hart and Clark (1997), measures the heterozygote deficit relative to its expectation under HWE (Fst). Regarding the interpretation of fixation index (Fst), it had been accepted that a value ranging between 0 to 0.05 indicates low genetic differentiation; a value ranging between 0.05 and 0.15, medium differentiation; a value ranging between 0.15 and 0.25 big differentiation; and a value above 0.25, very big genetic differentiation (Wright, 1978; Balloux and Lugon-Moulin, 2002). Accordingly in our study Moderate genetic differentiation (Fst) among breeds (8.4%) implies that 91.6% of the total genetic variation corresponds to differences among individuals. In addition, a very low inbreeding rates (Fit= 0.1%) between the three breeds was detected that means absence of inbreeding between the populations under study.
Genetic differentiation of similar magnitude has been reported among 12 African Bos indicus and Bos taurus cattle breeds (Ibeagha-Awemu and Erhardt, 2005). However, Figures is higher than the 7 % of the total genetic variability (mean FST=0.07) reported by Canon et al. (2001) among local European cattle breeds and much more higher than the 1.6% given by Ibeagha-Awemu and Erhardt (2006) among Red Bororo and White Fulani cattle breeds of Nigeria and Cameroon. However, the same value was found among 12 African Bos indicus and Bos taurus cattle breeds (Ibeagha-Awemu and Erhardt, 2005).
In this study Fst value may indicate the presence of gene flow between cattle breeds. The highest gene flow between breeds was found in the marker INRA023 (8.8508), while the lowest gene flow was shown in the marker SPS115 (0.814). On the other hand, the presence of gene flow between these breeds may be due to their common origin (Canon et al., 2000).
The inbreeding estimates were calculated using the FIS values (Wright’s Fixation Index). This revealed that Sudanese breeds are having wider genetic variability. It is observed that the lowest Fis value was found in Butana (-0.830) as compared with Kenana (-0.195) and Fuga (-0.317) with an overall mean of deficit of heterozygotes (Fis) is (-0.091). This negative mean value 0.091 suggests that 9.1% of heterozygous excess individuals available in the breed and the samples were collected from highly heterozygous breed. This high heterozygosity values are comparable with Umblachery cattle breed (-0.0487) (Karthickeyan et al., 2007). In contrast to our results, Metta (2004) reported in Indian Ongole cattle breed a high Fis values (0.36) and the author attribute this high value to the small sample size studied (n=17). Similar results were obtained by Sharma et al., (2006) in their study on Indian Bachaur cattle breed (Fis=0.22) and Sharma et al., (2007) in Indian Gangatiri cattle breed (Fis=0.31). The estimated time of divergence revealed that the biggest divergence time (1407 years) was between the Fuga and Butana cattle; in contrast the lowest divergence time (343 years) was between Butana and Kenana. These results are confirming the phylogeny dendrogram obtained using UPGMA method that proved that Butana and Kenana are within one cluster while Fuga is in another cluster; the three breeds are then coming from one ancestor. This result could be logic due to raising of both the Kenana and Butana cattle in near or close areas as they raised in north of Sudan while Fog were raised in the North Kordofan (Yousif and Fadl El- Moula, 2006).
In conclusion, this study reports on a comprehensive study of the genetic structure and diversity of three native zebu cattle breeds in Sudan. The genetic analysis data showed that a significant amount of genetic variation is maintained in the three studied Sudanese local zebu cattle breeds and all breeds studied could be considered as distinct genetic content. The three breeds displayed a markedly higher allelic richness most likely as a result of a combination of natural selection in diverse environmental conditions. Several authors declared that the amount and distribution of genetic diversity should be taken into account when dealing with conservation strategies of livestock species. It should be also taken into consideration that cultural, historical, and traditional aspects regarding the use of particular breeds are relevant issues. Moreover, it should be realized the fact that directional selection for genetic improvement achieved by animal breeders has shaped animal genomes in unexpected ways through choosing the good or favorite alleles or genes structures for which the surrogate neutral markers used in diversity surveys are not necessarily fully representative.
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