TAL1-mediated regulation of hemocyte proliferation influences red blood phenotype in the blood clam Tegillarca granosa

Red blood phenotype, indicating the degree of red color in blood or hemolymph, is a significant economic trait in Tegillarca granosa, closely associated with its health condition and nutritive value. However, the molecular regulatory processes that account for the variation in the blood clams' red blood phenotype remain largely unexplored. In our previous studies, weighted gene co-expression network analysis (WGCNA) showed that TAL1 exhibits high intramodular connectivity in red blood-related module. Furthermore, TAL1 serves as a critical transcription factor in the survival and quiescence of hematopoietic stem cells, along with being essential in the terminal maturation processes in vertebrates. Consequently, we investigated the function of the TAL1 in blood clams by conducting a comparative analysis between high total hemocyte count (HTHC: THC ≥ 14 × 104 cells/μL) and low total hemocyte count (LTHC: THC ≤ 2.5 × 104 cells/μL) groups, as well as between negative control and Tg-TAL1 RNA interference groups of blood clams. The results revealed that, compared to LTHC group, there was a significant upregulation of TAL1 mRNA and protein expression levels in the HTHC group, which may be a crucial factor contributing to the alterations observed in THC. RNA interference of the TAL1 in blood clams resulted in a significant decline in THC, causing the blood color to become lighter. EdU staining experiments further confirmed a diminished cell proliferative vitality within T. granosa gills after TAL1 knockdown. Moreover, immunohistochemical analysis and immunofluorescence assays revealed a gradual escalation in TAL1 protein expression and cytoplasmic/nuclear ratio from gill filament chamber endothelial cells to gill filament chamber cells in gills, potentially representing different cell types in the maturation process of erythrocytes in blood clams. It suggesting that the gills might act as one of the hematopoietic tissues in blood clams, with hemocytes potentially originating from the gill filament chamber endothelium of the gills through division and differentiation. This research improves comprehension of the molecular mechanisms behind the variation in T. granosa's red blood phenotype, potentially assisting in the selective breeding of high THC T. granosa populations.