International Journal of Aquaculture, 2025, Vol.15, No.3, 135-148 http://www.aquapublisher.com/index.php/ija 140 Streptococcus resistance in tilapia (Gao et al., 2019). Especially in terms of Tilapia Lake virus (TiLV) resistance, a 2021 study identified a region on chromosome 22 in tilapia that had a significant impact on disease-resistant survival: the mortality rate of individuals carrying resistance alleles was only 11%, while the mortality rate of homozygous susceptible alleles was as high as 43%. This finding means that relevant molecular markers can be used to select disease-resistant individuals for breeding at the seedling stage. Currently, WorldFish and other institutions are integrating TiLV resistance markers into the tilapia breeding program to cultivate tilapia strains that are highly resistant to the virus. In addition, using RNA sequencing and proteomics technology, people have deeply analyzed the immune response mechanism of tilapia after infection with pathogens, and found that the polymorphisms of immune genes such as MHC and TLR are closely related to disease resistance. These results provide clues for screening candidate genes for disease-resistant breeding. 4.3 Gender control technology and single-sex population cultivation Tilapia has the characteristics of early reproduction and high reproduction frequency. It is prone to overpropagation in a breeding environment, resulting in individual growth stagnation and irregular specifications. Therefore, achieving gender control to produce single-sex groups (usually all-male groups) is an important need in tilapia farming. All male tilapia grows faster and avoids reproductive energy consumption. Generally, the yield can be increased by more than 30% compared with mixed breeding groups. At present, the main preparation techniques for single-sex seedlings of tilapia include hormone hermaphrodite induction and YY supermale breeding. The Chinese scientific research team has achieved remarkable results in this field. For example, the "Yuemin No. 1" tilapia cultivated by the Pearl River Aquatic Products Research Institute is a new strain cultivated by Olia tilapia hybridization and backcrossing through multiple generations, combined with YY superhero technology. The father of "Yuemin No. 1" is a supermale Nile tilapia with chromosome type YY, and the mother is an excellent Nile tilapia female. The male rate of their offspring can reach more than 98%, and the growth rate is about 23.8% higher than that of the GIFT tilapia bred during the same period (Chen et al., 2018). The promotion of "Yuemin No. 1" has made large-scale hormone-free all-male seedling production possible, which is a milestone in China's tilapia industry. In addition to YY technology, modern molecular biology also provides new tools for gender control. Knockout of key genes on the gender-determining pathway such as DMRT1 or Foxl2 by gene editing can lead to gender reversal. Japanese studies have found that CRISPR knocking out the foxl2 gene of XX tilapia will transform its gonads from ovaries to semen, achieving genetic maleization. 5 Germplasm Resources and Genetic Diversity of Tilapia 5.1 Analysis of the genetic lineage of major breeding lines in the world The extensive introduction and breeding of tilapia around the world has formed several representative breeding varieties. The origin and kinship of these lines can be revealed through genetic lineage and molecular marker analysis. GIFT tilapia (GIFT) is a multi-source group breeding group. Its gene bank is fused with wild Nile tilapia blood from Egypt, Kenya, Senegal, Ghana and other places. Microsatellite and single nucleotide polymorphism (SNP) analysis showed that the genetic diversity level of the Jifu strain was high, with an average heterogeneity of about 0.62, indicating that although breeding was performed, there were still abundant alleles (Yan et al., 2014). In contrast, Taiwan’s red tilapia is a red variant produced by interspecies hybridization of Nile tilapia and Mozambique tilapia. Its genetic component contains about 50% of the background of Mozambique tilapia, so it can be distinguished from purebred Nile tilapia strains on some molecular marker seats. The Chitralada strain in Thailand originated from a few tilapia individuals introduced from Japan in the 1960s. They were bred for many years by a royal Thai farm. Because the foundation group is small and the long-term atresia breeding is low, the genetic variation is low. Some studies have found that the average number of alleles is significantly less than that of the Jifu strain. In addition, localized strains in various countries often have a certain degree of genetic confusion. Taking China as an example, the early introduction of "Taiwanese tilapia" and "Thai tilapia" often hybridize during folk breeding, which makes many breeding groups today be hybrids of multiple strains. DNA band analysis confirmed that the characteristic alleles of the Jifu and Thai lines existed in the tilapia breeding population in South China.
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