Identification, comparison of genetic diversity, heat tolerance, and growth performance among Micropterus salmoides salmoides, Micropterus salmoides floridanus, and their reciprocal hybrids

Largemouth bass (Micropterus salmoides) is an important freshwater fish species in China. However, it exhibits reduced growth performance and declined disease resistance under high temperatures. In this study, crosses between the rapid growing "Youlu No.3" (M. salmoides salmoides, NB) and the heat-tolerant but slower-growing M. s. floridanus (FB) were conducted. Based on previously developed InDel markers, a new pair of mitochondrial DNA primers were developed to distinguish the reciprocal hybrids for the first time. The hybrid offspring of NB♀× FB♂ (NF) exhibited mitochondrial sequences consistent with NB, while hybrid offspring of FB♀× NB♂ (FN) matched with FB. Genetic diversity analysis by seven fluorescence-labeled microsatellite markers revealed that both hybrid offsprings (NF and FN) showed higher values in terms of number of alleles (Na), effective number of alleles (Ne), Shannon's diversity index (I*), observed heterozygosity (Ho), and expected heterozygosity (He) compared to the NB population. Through acute heat stress ranging from 35 ℃ to 41 ℃, it was found that the median lethal temperatures (LT₅₀) for NB, NF, FN, and FB populations were 37.25 ℃, 38.45℃, 39.02℃, and 39.15 ℃, respectively. Under chronic heat stress for seven weeks, the FN hybrids demonstrated significantly higher survival and average growth rates at 34 ℃ than the NF hybrids. Specifically, the performance of the FN hybrids at 34 ℃ surpassed that of the NB population at 32.3 ℃ by 13.44% in survival rate and by 30.14% in growth rate. In pond culture, the NB population grew the fastest. By 9 and 10 months of age, the growth performances of FN and NF hybrids were reduced by approximately 10% and 36-70%, respectively, relative to the NB population. These results indicated that hybridization can effectively enhance heat tolerance and genetic diversity in largemouth bass. The cross of FB♀× NB♂ proved to be the better favorable combination. This study provides useful information for the development of varieties with enhanced heat tolerance in the future.