The Next generation sequencing has become the technology of choice for Genome-wide identification and development of SSR markers [7]. In O. bimaculatus, majority of repeats were dinucleotide (73.4%), followed by tetranucleotide repeats (14.06%) and trinucleotide repeats (11.72%) with very low frequency of pentanucleotide and hexanucleotide repeats. Similar pattern of distribution of the repeat motifs was observed in three neotropical catfishes by Restrepo-Escobar [24]. In this study, PIC values of thirty novel polymorphic SSR loci indicated that all thirty loci are highly informative, indicating the usefulness for genetic stock structure analysis [25]. OBL1181 locus with PIC value of 0.927 implied highly informative loci. With these 30 polymorphic loci, all four riverine populations revealed relatively lower genetic diversity within population. It was reported that fishes with large population size that migrate during breeding season have high genetic diversity [26]. In contrast, O. bimaculatus is considered to be a local migrant and travels short distance for feeding or locating suitable breeding grounds in new water bodies, to avoid stress conditions of existing habitat [27], this might be the reason for relatively lower genetic diversity within population.
The number (Na) of allele per locus ranged from 5 (OBL92038) to 21 (OBL1913), with an average allele per locus 10.93, indicating higher genetic variability than the reported average allele number per loci (9.1) for fresh water species [28]. High Na was also observed in other catfish with high genetic diversity [25]. It was interesting to find highest number of alleles (21) to be located in the intron of KCNK2 gene. It may be possible that the polymorphism present due to this intronic microsatellite is not presenting any functional constraint, however it needs to be further investigated.
The departure from Hardy-Weinberg equilibrium was noticed in seventeen loci, which might be due to homozygote excess (as indicated by positive FIS values). However, presence of significant level of null alleles were only in two loci (OBL894 and OBL1957). It may be due to small number of samples studied in the present study, and thus Hardy-Weinberg may be violated [29]. The significant value of genetic differentiation (overall FST) agreed with that of the results obtained through molecular variance. Positive FIS values in loci may be due to reduction in heterozygotes, which may be the result of reduction in population size by overexploitation, adverse ecological changes, localised isolated population and non-migratory nature [27].
Under current study, analysis of five gene-associated SSR markers (Type II) showed lower overall pairwise FST value of 0.1574, as compared to that of neutral loci. It has been reported that in contrast to neutral markers (microsatellites in noncoding regions), gene-associated microsatellites marker might be more susceptible to selection pressure and therefore, showed low values of gene diversity which is concurrent to current study [25].
Present study revealed the effects of divergent and balancing selection on eight loci (four each), which also included two gene-associated loci. It was interesting to find both the associated genes under selection pressure have been reported to respond to environmental changes. One marker is associated with Insulin Like Growth Factor 2 Receptor (IGF2R), which is a cell-surface receptor and known to have a binding site for Insulin Like Growth Factor 2 (IGF2), where it endocytoses the growth factor and regulates the concentration of IGF2 within tissues in mammals [30]. It was reported that growth related diseases have been found to be associated with deregulation of expression of IGF2R in human. However, in teleost, role of the IGF2 is not clear in growth [31], and was found to be divergent selective pressures for adaptation in sticklebacks [32]. Another gene, under negative selectin is Epsin-2, encodes a protein, involved in clathrin-mediated endocytosis [33], which plays important role in nutrient uptake and cell to cell communications [34]. However, little information on its function in fishes is available. Nevertheless, both the genes found to be under selection pressure belong to the same pathway: Endocytosis, important for regulation of cell signalling and antigen presentation (keg.jp/entry/ko4144). It has also been reported to have an adaptive response in plasma membrane remodelling in the face of environmental changes [34].
With all 30 loci, the major genetic variation, present among individuals within populations with overall moderate genetic differentiation was as per the threshold values suggested by Wright [35]. Above all, the level of variation remained at the same level, when the type I and type II markers were analysed separately. However, the markers under balancing (positive) selection revealed lower genetic variation within individuals and higher differentiation. As the balancing selection, exerted by the adaptive forces, resulting in the maintenance of genetic variation and increased diversity at the linked sites [36]. In the present data set, at the balancing loci (regime), population differentiation is increased due to the maintenance of different sets of alleles in four populations analysed and it was proposed as the difference in fitness of the genotypes in populations, under the different environmental conditions [37]. Thus, in addition to the fact that the species under study is a non-migratory nature of species, the selection pressure working on the loci under study may contribute to the existence of high genetic differentiation within the same river system. A similar pattern of high genetic differentiation was observed in other catfishes in India, i.e., Indian Catfish, Clarias magur (FST: 0 .01383 to 0.62069) [38], giant river-catfish Sperata seenghala (0.135–0.173) [25], however selection pressure has not been examined in these cases.
Genetic diversity analysis showed significant differences between different riverine populations across India, i.e., Kaveri river (South India), Brahmaputra (North east India, Assam), Bichiya a tributary of Ganga river (Central India, Madhya Pradesh) and Gomti, a tributary of Ganga river (North India, Uttar Pradesh). In general, the population of same riverine system are expected to have low genetic variability as a result of genetic decline, genetic drift, selection and inbreeding [39]. However, in this study high genetic variability in populations Bichiya and Gomti river (p < 0.01) could be considered from different perspectives: (1) Bichiya river had originated from plateau of central India and before commencing into Ganga river system it has to go through the barrage and dams in the river resulting in isolated population; (2) Kaveri river is originated in Western Ghats and do not have any connectivity with Ganga and Brahmaputra river (3) Brahmaputra and Ganga river is fragmented by Farakka barrage (4) Non-migratory nature of the species (5) recent genetic bottlenecks in the wild population.
Some studies on fish have reported bottlenecks in natural populations, particularly due to habitat loss and fragmentation of river [40]. Fragmented populations exhibit higher genetic variation, especially in freshwater fishes [41]. In the case of Bichiya River, the fragmentation of the river was seen due to barrage in Maraila, Madhya Pradesh (24°79'00.13"N, 81°31'00.60"E); hydroelectric project Rewa, Madhya Pradesh, (24°48 '46.76"N, 81°29 '62.43"E); Jhinna, Madhya Pradesh (24°22 '25.36"N, 81°27 '49.03"E). There are considerable number of hydropower dams and barrage in the river, which can affect migration, egg and larvae drift of fish [42] and subsequent high genetic differentiation of Bichiya population from Gomti and other populations. Alteration of the migratory flow, leads to a reduction or interruption of gene flow, consequently reducing the population size and making it more susceptible to the effects of genetic drift [43], which results in genetic structure in fishes [44]. Furthermore, the higher genetic diversity could be expected as rivers belong to distant geographical locations and it is likely that the populations investigated could have evolved in isolation after fragmentation.
The cross-species amplification efficiency is inversely proportional to the phylogenetic distance between the two species [45]. Based on the mitogenome sequences analysis, O. pabda showed the highest similarity to O. bimaculatus in the genus with 86% identity [46]. Cross-amplification of 30 polymorphic loci of O. bimaculatus successfully amplified for 29 polymorphic loci in Ompok pabda. The allele size pattern of O. pabda was similar to that of O. bimaculatus. Cross-species amplification can be successfully used in genotyping and genetic stock characterization in O. pabda which will help in saving resources, effort and time. Cross-species amplification has been successful in other catfishes i.e., four related species of Silonia silondia [47], endangered catfish Rita rita [48].