Models Of Microsatellite Evolution Research Articles

Genetic diversity evaluation of Sojat goat population of India

Sojat goat forms the part and parcel of the lives of the farmers in the Barmer region of Rajasthan. These are large-sized goats, white in colour, and have dual utility. These goats fetch premium price during the Eid festival. Assessment of diversity is essential for germplasm characterization and management. Genomic microsatellite markers being a valuable tool for estimating genetic diversity were selected for exploring existing genetic variability in the Sojat goat population. The standard metrics of genomic diversity detected moderate variability with a total of 162 alleles across 22 loci in this lesser-known population. The expected number of alleles had a mean value of 3.40±0.39. Similarly, a moderate magnitude of diversity was recorded in the Sojat population as the mean observed heterozygosity was 0.54±0.05. Expected heterozygosity was higher than the observed (0.60±0.06), indicating a deviation from Hardy-Weinberg Equilibrium (HWE) and the possibility of inbreeding due to the non-random mating in the population. Accordingly, significant heterozygote deficiency was noticed (F=0.08±0.03). The population did not suffer a reduction in effective population size in the last few generations. Mutation drift equilibrium did not reveal significant heterozygosity excess under different models of microsatellite evolution and no shift was recorded in the frequency distribution of alleles. To conclude, the results provided the first insights into the genetic diversity of Sojat goats. A moderate genetic variability with heterozygote deficiency within the population warrants immediate attention for scientific management of this unique goat population to conserve the existing genetic variation and to avoid any escalation of inbreeding.

Microsatellite-based Genetic Diversity and Mutation-Drift Equilibrium in Dharwadi Buffalo (Bubalus bubalis) of India

Genetic diversity was assessed in a buffalo population (Dharwadi) of Karnataka by utilizing 25 heterologous bovine microsatellite markers. Genotypic status of individuals at each locus was identified by automated DNA sequencer, and allelic data were analyzed for genetic diversity parameters. Observed number of alleles varied between 5 and 19 with 13.12 ± 0.80 alleles per locus. High level of heterozygosity (0.63) indicated plenty of existing genetic diversity in this buffalo population. Within-population inbreeding estimate (FIS) was significantly positive (0.186) and 88% of the investigated loci contributed toward the homozygote excess. Accordingly, significant deviation from Hardy–Weinberg equilibrium was observed at 24 of 25 analyzed loci. The population did not suffer reduction in effective population size in the last few generations. Assessment of Dharwadi buffalo population for mutation-drift equilibrium did not reveal significant heterozygosity excess under all the models of microsatellite evolution as per Sign test. No shift was recorded in the frequency distribution of alleles, and a normal L-shaped curve indicated absence of any recent genetic bottleneck. The study highlighted sufficient genetic variability within Dharwadi buffalo population that can be utilized in combination with phenotype and production information for registration as a distinct breed of Indian buffalo.

Genetic architecture and bottleneck analyses of Salem Black goat breed based on microsatellite markers

Aim: The present study was undertaken in Salem Black goat population for genetic analysis at molecular level to exploit the breed for planning sustainable improvement, conservation and utilization, which subsequently can improve the livelihood of its stakeholders. Materials and Methods: Genomic DNA was isolated from blood samples of 50 unrelated Salem Black goats with typical phenotypic features in several villages in the breeding tract and the genetic characterization and bottleneck analysis in Salem Black goat was done using 25 microsatellite markers as recommended by the Food and Agricultural Organization, Rome, Italy. The basic measures of genetic variation were computed using bioinformatic software. To evaluate the Salem Black goats for mutation drift equilibrium, three tests were performed under three different mutation models, viz., infinite allele model (IAM), stepwise mutation model (SMM) and two-phase model (TPM) and the observed gene diversity (He) and expected equilibrium gene diversity (Heq) were estimated under different models of microsatellite evolution. Results: The study revealed that the observed number of alleles ranged from 4 (ETH10, ILSTS008) to 17 (BM64444) with a total of 213 alleles and mean of 10.14±0.83 alleles across loci. The overall observed heterozygosity, expected heterozygosity, inbreeding estimate and polymorphism information content values were 0.631±0.041, 0.820±0.024, 0.233±0.044 and 0.786±0.023 respectively indicating high genetic diversity. The average observed gene diversities (He) pooled over different markers was 0.829±0.024 and the average expected gene diversities under IAM, TPM and SMM models were 0.769±0.026, 0.808±0.024 and 0.837±0.020 respectively. The number of loci found to exhibit gene diversity excess under IAM, TPM and SMM models were 18, 17 and 12 respectively. Conclusion: All the three statistical tests, viz., sign test, standardized differences test and Wilcoxon sign rank test, revealed significant deviation of Salem Black goats from mutation-drift equilibrium under IAM and TPM models, however, nonsignificant deviation under SMM model. The qualitative test of mode shift analysis supported the results under SMM indicating the absence of the genetic bottleneck in the recent past in Salem Black goats.

Detection of occurrence of a recent genetic bottleneck event in Indian hill cattle breed Bargur using microsatellite markers

The effective number of breedable individuals is a crucial determinant for maintaining genetic variability within a population. The population of Bargur, the hill cattle of South India, has gone down drastically by more than 93 % in the past three decades, and only a few thousand animals are available at present. The present study was undertaken to evaluate Bargur cattle for mutation drift equilibrium and to detect the occurrence of recent genetic bottleneck event, if any, in this population. About 50 unrelated animals, true to the type, were sampled and genotyped at 25 microsatellite loci. The mean observed heterozygosity (0.808 ± 0.023) was higher than the mean expected heterozygosity (0.762 ± 0.008) with 15 out of 25 microsatellite loci exhibiting heterozygosity excess when assumed under Hardy-Weinberg equilibrium. To evaluate Bargur cattle for mutation drift equilibrium, three tests were performed under three different mutation models, viz., infinite allele model (IAM), stepwise mutation model (SMM) and two-phase model (TPM). The observed gene diversity (H (e)) and expected equilibrium gene diversity (H(eq)) were estimated under different models of microsatellite evolution. All the 25 loci were found to exhibit gene diversity excess under IAM and TPM, while 22 loci were having gene diversity excess under SMM. All the three statistical tests, viz., sign test, standardized differences test, and Wilcoxon sign rank test, revealed significant (P < 0.01) deviation of Bargur cattle population from mutation-drift equilibrium under all the three models of mutation. Furthermore, the qualitative test of allele frequency distribution in Bargur cattle population revealed a strong mode shift from the normal L-shaped form suggesting that the population had experienced genetic bottleneck in the recent past. The occurrence of genetic bottleneck might have led to the loss of several rare alleles in the population, which point towards the need for efforts to conserve this important cattle germplasm. The present study is the first report in demonstrating the genetic basis of demographic bottleneck in an Indian cattle population.

Evaluation of genetic architecture and mutation drift equilibrium of Marathwada buffalo population in Central India

In the present study, microsatellite data on 24 loci were generated and utilized to evaluate the genetic architecture and mutation drift equilibrium of Marathwada buffaloes, a Central Indian population maintained under low input system. Sufficient allelic diversity was observed with a total of 109 alleles across different loci. The genetic diversity analysis of Marathwada buffaloes displayed moderate level of within breed variability in terms of mean number of alleles per locus (4.48) and heterozygosity values (Ho = 0.532, He = 0.624). The studied Indian buffalo population showed considerable heterozygote deficiency ( F IS = 0.138) and deviation from HWE at many investigated loci. Three quantitative tests viz. sign test, standardized difference test and Wilcoxon sign rank test and a qualitative test for mode shift distortion of allelic frequencies were employed to evaluate mutation drift equilibrium under three different models of microsatellite evolution. The population was found to deviate significantly under IAM and TPM, while it was reverse under SMM. The qualitative test for mode shift supported the results under SMM indicating the absence of genetic bottleneck in the recent past in Marathwada buffaloes.

Evolution of chloroplast mononucleotide microsatellites in Arabidopsis thaliana

The level of variation and the mutation rate were investigated in an empirical study of 244 chloroplast microsatellites in 15 accessions of Arabidopsis thaliana. In contrast to SNP variation, microsatellite variation in the chloroplast was found to be common, although less common than microsatellite variation in the nucleus. No microsatellite variation was found in coding regions of the chloroplast. To evaluate different models of microsatellite evolution as possible explanations for the observed pattern of variation, the length distribution of microsatellites in the published DNA sequence of the A. thaliana chloroplast was subsequently used. By combining information from these two analyses we found that the mode of evolution of the chloroplast mononucleotide microsatellites was best described by a linear relation between repeat length and mutation rate, when the repeat lengths exceeded about 7 bp. This model can readily predict the variation observed in non-coding chloroplast DNA. It was found that the number of uninterrupted repeat units had a large impact on the level of chloroplast microsatellite variation. No other factors investigated--such as the position of a locus within the chromosome, or imperfect repeats--appeared to affect the variability of chloroplast microsatellites. By fitting the slippage models to the Genbank sequence of chromosome 1, we show that the difference between microsatellite variation in the nucleus and the chloroplast is largely due to differences in slippage rate.

Microsatellite Length Differences Between Humans and Chimpanzees at Autosomal Loci Are Not Found at Equivalent Haploid Y Chromosomal Loci

When homologous microsatellites are compared between species, significant differences in mean length are often noted. A dominant cause of these length differences is ascertainment bias due to selection for maximum repeat number and repeat purity when the markers are being developed. However, even after ascertainment bias has been allowed for through reciprocal comparisons, significant length differences remain, suggesting that the average microsatellite mutation rate differs between species. Two classes of mechanism have been proposed: rapid evolution of enzymes involved in the generation and repair of slippage products (enzyme evolution model) and heterozygote instability, whereby interchromosomal events at heterozygous sites offer extra opportunities for mutations to occur (heterozygote instability model). To examine which of these hypotheses is most likely, we compared ascertainment bias and species length differences between humans and chimpanzees in autosomal and Y chromosomal microsatellites. We find that levels of ascertainment bias are indistinguishable, but that interspecies length differences are significantly greater for autosomal loci compared with haploid Y chromosomal loci. Such a pattern is consistent with predictions from the heterozygote instability model and is not expected under models of microsatellite evolution that do not include interchromosomal events such as the enzyme evolution model.

Microsatellite and morphological analysis of population structure in the parthenogenetic freshwater snail Melanoides tuberculata: insights into the creation of clonal variability.

The distribution of variability was studied at various geographical scales in the tropical freshwater snail Melanoides tuberculata, in order to analyse the role of factors shaping this distribution, including the mating system and population dynamics. This parthenogenetic polyploid species reproduces mainly asexually, with males occurring at low frequency. About 800 individuals (38 sites) were sampled from Africa and the Middle East, where the species originated, and from recently colonized habitats in South and Central America, and especially the island of Martinique. We first described variation of general aspects and ornamentation of the shells. This analysis confirms the existence of discrete morphs. Second, individuals were studied at three microsatellite loci, showing that each morph is a genetic clone with some minor variation compatible with models of microsatellite evolution. The genetic analysis also showed much more variation within than between clones. However, two populations from Africa exhibited a large amount of variability, and a mixture of sexual and asexual reproduction might explain these genetic patterns. The worldwide distribution of variability is, therefore, compatible with the African origin of the species, and the introduction of a few clones in other parts of the world. These results also suggest that the distribution of variability in Martinique is influenced by flooding events, and that two morphs from Martinique can be interpreted as hybrids between two pre-existing morphs, based on morphological, genetic and geographical arguments.