Emergence and divergence of blood cell lineages in the history of animal evolution.

Humans possess various blood cell types, such as T cells, B cells, monocytes/macrophages, neutrophils, dendritic cells, erythrocytes, and platelets/thrombocytes, and these lineages are shared by nearly all vertebrates. Although invertebrates such as tunicates, sea urchins, flies, and sponges possess different blood cell lineages, some homology exists between vertebrate and invertebrate blood cells. Comparing cell lineages between different species can help estimate the evolution of blood cell lineages and immune systems across more than 600 million years of animal history. Studies on blood cells in vertebrate and invertebrate species have put some mile stones in the roads of blood cell evolution, but a comprehensive map has not been drawn. Recent advances in sequencing and bioinformatic technologies and the accumulation of genomic information for various animal species enabled us to develop an analytical method to compare cell lineages between far different species. Our recent work suggested one probable evolutionary history of blood cell lineages, in which T cells, NK cells, ILC, mast cells, erythrocytes, and thrombocytes evolved from prototypic mast cells. In addition, a vestige of this history was detected in our body as a linage restriction process in blood cell differentiation. Thus, studying evolution can help clarify current biological systems, and vice versa.