A lot of studies have been done on growth and development, physiology, culture and toxicity of Mozambique Tilapia in Pakistan but the genetic issue remained unattempted. The appraisal fish; Oreochromis mossambicus is an important freshwater fish but now considered as a threatened species due to hybridization, pollution and habitat loss. The data on genetic diversity of O. mossambicus is limited only to the studies conducted on Southern Mozambique and South Africa. To assess the factors which are mainly responsible for the lack of genetic diversity of O. mossambicus populations, this is the first study being conducted in Pakistan from River Jhelum, Punjab, Pakistan.
In the present study, moderate level of genetic diversity was revealed by the studied microsatellite loci. All the microsatellite loci were polymorphic according to the 0.95% allele frequency criteria. The results are in conformity with Simbine et al. (2014) who used five microsatellite loci (UNH104, UNH129, UNH142, UNH222 and UNH222) to examine the genetic diversity of O. mossambicus collected from four rivers in southern Mozambique. High level of polymorphism among all the loci was observed.
Average Na and Ar per locus ranged from 2.400 to 3.600, which are lower than reported by Rutten et al. (2004) who observed the average Na per locus varying from 5.0 to 7.5. In all populations, moderate level of heterozygosity was observed. The values of Ho and He ranged from 0.550 to 0.693 and 0.542 to 0.676, respectively. He was observed higher as compared to Ho which suggests homozygosity at a locus or a population experiencing a bottleneck. Parallel findings to ours were given by Sultana et al. (2015) but contrary results were reported by Kariuki et al. (2021) who detected the very similar levels of expected and observed heterozygosity.
Except PDK population which exhibited negative average value, increasing values of FIS demonstrated heterozygote deficiency. Findings are quite parallel to Ukenye et al. (2016) whose results showed that inbreeding coefficient (FIS) was positive across seven loci reflecting excess of homozygotes. Out of 25 tests, a total of 3 tests deviated from HWE significantly after applying multiple test correction. Similar conclusions were reported by Bbole et al. (2020) in the case of Oreochromis andersonii and O. macrochir, two Southern African cichlid species. These findings demonstrated a significant divergence from HWE, but it was more apparent in O. andersonii. Deficits of the heterozygotes could be the cause of deviation from the HWE. In natural populations, inbreeding, fishing pressure, non-random sampling, genetic drift, intra-population structure could be the reason of the heterozygote deficit (Abbas et al. 2010). In the second half of last century, due to large scale hydrological alterations, there has been a decline in the diversity of fish populations and this might be the reason of heterozygote deficiency in the riverine populations of Pakistan.
F ST is one of the most used methods for measuring genetic differentiation within and between populations, and it can offer crucial information on the evolutionary procedures that lead to genetic differentiation between populations (Holsinger and Weir 2009). The FST estimates indicated that all the populations were non-homogenous. The highest differentiation level was found between the population pair of THW-JB that specified dissimilar genetic origin. Lowest differentiation level was between the population of RB-PDK that might be due to physical connectivity and hence, gene flow among two riverine populations. Diyie et al. (2021) revealed low genetic differentiation in some population pairs of O. niloticus that could be due on-going gene flow in population subdivisions. Compared finding to ours are also reported by Ahmed and Abbas (2018), Nazish et al. (2018) and Diyei et al. (2021).
Analysis of molecular variance (AMOVA) was performed to determine genetic variability within and among populations, which specified that majority of variations (87.050%) were attributed to “within individuals” in wild populations. These results are in agreement with Nazish et al. (2018) in populations of Hypophthalmichthys molitrix inferred by microsatellite markers.
Based on Nei’s genetic distance, phylogenetic tree was constructed which yielded two clusters; PDK, RB, MD and JB populations were in one cluster while THW population alone was in second cluster. Although populations in one cluster are geographically separate, but share genetic similarity to similar origin. Genetic relatedness was further explored by a structural grouping technique using STRUCTURE HARVESTER. Maximum yielded log probability mean value and delta k value was k = 3. However, Evanno et al. (2005) asserted that estimation of the precise number of groups could be accomplished by calculating the delta k with as few as five microsatellite markers.
In O. mossambicus populations from numerous sites of Jhelum River, a general assessment of microsatellite DNA-based genetic diversity revealed a moderate level of genetic diversity and population differentiation. Meanwhile, the IUCN Red List has listed the Mozambique tilapia, O. mossambicus, as "near threatened" due to hybridization with the commonly introduced O. niloticus. Sustainable resource usage and the preservation of significant genetic variations are necessary for the sustainable management of freshwater fish species. The best strategy is to routinely monitor fish stocks in natural habitats, prevent overfishing, and enhance breeding and feeding grounds by optimizing riverine water flow.