POPULATION DIFFERENTIATION AND DENETIC DIVERSITY OF PARATRICHODINA AFRICANA PARASITIC ON GILLS OF TILAPIA

This study focused on Paratrichodina africana El-Tantawy & Kazubski, 1986 from gills of tilapia, aiming to investigate population differentiation and genetic diversity based on the morphometric quantization of adhesive disc and molecular evidence of SSU rDNA in Chongqing and Guangdong populations. Morphometric analysis of the adhesive disc revealed significant differences ( P<0.05) in denticle morphology, including denticle span, denticle length, blade length, and denticle ratio, while no significant differences were observed in other aspects. Principal component analysis (PCA) indicated that the scatter plot distribution of Guangdong population was nearly contained within that of Chongqing population, suggesting a high degree of morphological similarity between the two populations. Genetic diversity analysis identified 10haplotypes among 28 SSU rDNA samples, including seven unique haplotypes and three shared haplotypes. Hap3 was the largest haplotype, present in both Chongqing and Guangdong population. The genetic diversity of Guangdong population (high H_d, high P_i) was higher than that of Chongqing population (high H_d, low P_i), implying that water temperature and geographic distribution may influence the genetic diversity of P. africana. Molecular phylogenetic analysis placed Hap5 from Guangdong at the basal position of phylogenetic tree, with Guangdong samples distributed across all branches. The genetic differentiation index (F_st) and gene flow (N_m) indicated moderate genetic differentiation with limited gene exchange between the two populations. The above results suggested that the Chongqing population probably originated from the Guangdong population. Analysis of molecular variance (AMOVA) study further confirmed that the majority of genetic variation (92.18%) originated within populations. Neutrality tests (Tajima’s D and Fu’s F_s) and nucleotide mismatch analysis indicated that the whole population of P. africana (CQ+GD) might have undergone a population expansion event in the early stage but not recently, however the different geographic populations (Guangdong and Chongqing populations) had not undergone any population expansion events. The present study has laid a foundation for the future research on population genetics and phylogeography of trichodinids, which could provide reference data for the prevention and control of trichodinasis in aquaculture.