Chapter 2 Genetic characterisation of selected bread wheat (Triticum
2.4 Discussion
2.4.2 Genic distance and genetic diversity
Across populations, the mean number of realised alleles was 6.78 and values ranged from 5.9 to 7.6 (Table 2.4). The mean number of effective alleles was 4.94 and values ranged from 4.31 to 5.59. Highest and lowest genetic differentiation values were 0.02 and 0.07 among the sub-populations, in that order. Negative genetic differentiation values indicated an excess of heterozygotes detected for the loci WMS153, WMS30, WMS179 and XGWM132 with values of -0.22; -0.13; -0.12 and -0.07, in that order (Table 2.3). Standardised values of the fixation index, as reported by Wright (1978), are noted as negligible for values ranging from 0–0.005, moderate for values ranging from 0.05–0.15, great for values 0.15–0.25 and values exceeding 0.25 are considered to express populations with large genetic differentiations.
Morjan and Rieseberg (2004) described how values of gene flow may indicate the degree to which genetic material is divergent between populations. Values of gene flow which are below a unit are considered low, values equivalent to a unit are considered moderate gene flow, whereas values exceeding a unit are excessively high. The recorded gene flow, in the current study, ranged from a low of 4.7 to a high of 12.3. This indicated an excessive exchange of genes between the different wheat populations.
The mean Shannon's information index was 1.62 among the different wheat populations (Table 2.4). Populations expressing high number of private alleles reveal higher genetic diversity of the population among 47 wheat genotypes. According to population distribution, the highest mean number of private alleles expressed was 10 for Population III. Similarly a mean value of 10 private alleles were realised in a genetic structure study of 172 landraces and 20 modern cultivars of durum wheat genotyped by 44 SSR markers (Soriano et al., 2016).
According to Nielsen et al. (2014), factors such as the density of markers per chromosome, marker clustering and the presence and distribution of private alleles per locus can have an effect on the allelic richness. The presence of private alleles in the current study further reveals the large degree of heterozygous loci (Andrews, 2010). Population III consisted of the highest proportion of private alleles which makes this sub-group genetically dissimilar. These can be individuals which may have undergone rare mutations to develop distinct alleles. These private alleles distinguish the mutant individuals from pool of genotypes in the meta-population.
Correspondingly, Soriano et al. (2016) ascribed the genic diversity to the presence of private alleles at the different loci in a study of durum wheat.
The genetic distances between the four populations ranged from 0.01 to 0.31. The values reported by Desta et al. (2014) were markedly higher ranging from 0.01 to 0.89 and having a mean value of 0.66. This reveals a fairly low genetic diversity among the different wheat populations used in this study. The population stratification can be brought about by geographical isolation of a group of individuals, artificial and natural selection as well as genetic drift (Nielsen et al., 2014). Linkage disequilibrium can result as a consequence of the uneven frequency of alleles within the different groups of genotypes as noted by Soriano et al. (2016). There is need for a further investigation of linkage disequilibrium.
The genic identity between the different wheat genotypes had minimum and maximum values of 0.74 and 0.99 indicating high similarity between the different populations (Table 2.5). The commonly shared parents among the different genotypes which were Pastor, Altar 84, Aegilops squarrosa and Pifed may have also contributed to limited genetic variability between the sub-populations. The populations which were most genetically dissimilar were Populations II and III. Populations II and III expressed the highest genetic distance and the lowest genetic identity. Thus, Populations II and III were considered to be potentially distinct. These can house potential candidates to utilise as crossing parents. On the contrary, the populations which expressed a higher degree of exchange of genetic material were the Populations III and IV. Populations III and IV expressed the lowest genetic distances and the highest value for the genetic identity.
2.4.3 Loci polymorphism and effectiveness for genotype discrimination
The highest PIC detected was for the marker XGWM132 with a value of 0.93 and mean value of 0.80 (Table 2.3). In a genetic diversity study of bottle gourd, a highly diverse horticultural crop, the mean PIC was reported to be above 0.5 for the 9 molecular markers in the study conducted by Mashilo et al. (2016). On the other hand, the mean PIC value reported by Desta et al. (2014) in Eritrean wheat accessions was a high of 0.63. The value obtained in the current study was markedly higher than the latter study. This suggests the proficiency of the selected molecular markers to uniquely distinguish the different genotypes. Although the value obtained in the current study was comparatively lower than that reported for the molecular markers WMC262 located on chromosome 4A, WMC44, situated on chromosome 1B and GWM174 which is located on chromosome 5D had PIC values of 0.96; 0.954 and 0.948, respectively (Tascioglu et al., 2016). On the contrary, the PIC values reported by Nielsen et al. (2014) were comparatively lower than that of the current study. Values reported by Nielsen et al. (2014)
ranged from 0.16 and 0.38 for the genotyping of modern cultivars and landraces, with a mean value of 0.30.