Functional heterogeneity of immune defenses in molluscan oysters Crassostrea hongkongensis revealed by high-throughput single-cell transcriptome

Oyster is the worldwide aquaculture molluscan and evolves a complex immune defense system, with hemocytes as the major immune system for its host defense. However, the functional heterogeneity of hemocyte has not been characterized, which markedly hinders our understanding of its defense role. Here, we used the single-cell transcriptome profiling (scRNA-seq), which provides a high-resolution visual insight into its dynamics, to map the hemocyte and assess its heterogeneity in a molluscan oyster Crassostrea hongkongensis. By combining with the cell type specific RNA-seq, thirteen subpopulations belonging to granulocyte, semi-granulocyte, and hyalinocyte were revealed. The granulocytes mainly participated in immune response and autophagy process. Pseudo-temporal ordering of granulocytes identified two different cell-lineages. The hematopoietic transcription factors regulated networks controlling their differentiations were also identified. We further identified one subpopulation of granulocytes in immune activate states with the cell cycle and immune responsive genes expressions, which illustrated the functional heterogeneity of the same cell type. Collectively, our scRNA-seq analysis demonstrated the hemocytes diversity of molluscans. The results are important in our understanding of the immune defense evolution and functional differentiation of hemocytes in Phylum Mollusca.