International Journal of Molecular Veterinary Research, 2025, Vol.15, No.1, 43-50 http://animalscipublisher.com/index.php/ijmvr 44 2 Genetic Diversity of Tilapia 2.1 Diversity of tilapia species and germplasm resources Tilapia is a wide range of species, from which Nile tilapia (Oreochromis niloticus), blue tilapia (O. aureus), and Mozambique tilapia (O. mossambicus) are the most widely farmed. These species and their hybrids possess high genetic resources for aquaculture. Cultured populations have high genetic diversity within and among strains, which is essential for disease resistance breeding and growth performance (Chen et al., 2021; Zhu et al., 2023). 2.2 Application of molecular marker technologies in tilapia genetic diversity studies Molecular marker technologies like microsatellites, single nucleotide polymorphism (SNPs), and genome-wide association studies (GWAS) have also been utilized on a wide scale for the estimation of genetic diversity and selective breeding of tilapia. Microsatellite markers linked to immune genes were utilized in the detection of resistance against disease, which can be used in marker-assisted selection without challenge by the pathogen (Chen et al., 2021; Taukhid et al., 2024). GWAS and the SNP-based approach have identified QTLs for resistance to major pathogens such as Streptococcus agalactiae and Tilapia Lake Virus (TiLV), which has rekindled the use of genomic selection in breeding schemes (Barría et al., 2021; Vela-Avitúa et al., 2023) (Figure 1). Figure 1 Clinical signs in tilapia infected with tilapia lake virus (TiLV) (Adopted from Taukhid et al., 2024) Image caption: A = dropsy, swelling of internal organs; B = cataract and/or endophthalmus; C = dropsy, exo/endothalmus, darkening of body colour; D = exophthalmus and loss of golden ring around the eyes (uvea); E = uvea begins to disappear, necrosis in the operculum; and F = cataract and/or endophthalmus, ulcers on the body surface (Adapted from Taukhid et al., 2024) 2.3 Comparison of genetic structures and disease resistance potential among different tilapia populations Comparative studies show that there is significant genetic structure and disease resistance heterogeneity in different populations and tilapia stocks. Hybrid tilapia (O. niloticus × O. aureus), for example, have increased resistance to Streptococcus agalactiae compared to pure blue or Nile tilapia due to certain immunity mechanisms (Zhu et al., 2023). Disease resistance characters of Nile tilapia are moderately to highly heritable, hence there are good opportunities for improvement via selective breeding (Barría et al., 2020; Barría et al., 2021; Shoemaker et al., 2022). Genetic correlations between disease resistance and growth are mostly low, hence multivariate selection will have to be used in an effort to maximize both productivity and resilience (LaFrentz et al., 2020). 3 Genetic Basis of Disease Resistance in Tilapia 3.1 Innate immunity-related genes in tilapia The innate immune system is the primary defense against pathogen infection in tilapia, especially in the high-density aquatic environment with pathogens. Transcriptome and proteome analyses have demonstrated that infection with pathogens such as Streptococcus agalactiae triggers pathogen recognition, immune activation,
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