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In vertebrates, the development of lymphocytes from undifferentiated haematopoietic precursors takes place in so-called primary lymphoid organs, such as the thymus. Therein, lymphocytes undergo a complex differentiation and selection process that culminates in the generation of a pool of mature T cells that collectively express a self-tolerant repertoire of somatically diversified antigen receptors. Throughout this entire process, the microenvironment of the thymus in large parts dictates the sequence and outcome of the lymphopoietic activity. In vertebrates, direct genetic evidence in some species and circumstantial evidence in others suggest that the formation of a functional thymic microenvironment is controlled by members of the Foxn1/4 family of transcription factors. In teleost fishes, both Foxn1 and Foxn4 contribute to thymopoietic activity, whereas Foxn1 is both necessary and sufficient in the mammalian thymus. The evolutionary history of Foxn1/4 genes suggests that an ancient Foxn4 gene lineage gave rise to the Foxn1 genes in early vertebrates, raising the question of the thymopoietic capacity of the ancestor common to all vertebrates. Recent attempts to reconstruct the early events in the evolution of thymopoietic tissues by replacement of the mouse Foxn1 gene by Foxn1-like genes isolated from various chordate species suggest a plausible scenario. It appears that the primordial thymus was a bi-potent lymphoid organ, supporting both B cell and T cell development; however, during the course of vertebrate, evolution B cell development was gradually diminished converting the thymus into a site specialized in T cell development.

Summary: DNA methylation is a universal epigenetic mechanism involved in regulation of gene expression and genome stability. The DNA maintenance methylase DNMT1 ensures that DNA methylation patterns are faithfully transmitted to daughter cells during cell division. Because loss of DNMT1 is lethal, a pan-organismic analysis of DNMT1 function is lacking. We identified new recessive dnmt1 alleles in medaka and zebrafish and, guided by the structures of mutant proteins, generated a recessive variant of mouse Dnmt1. Each of the three missense mutations studied here distorts the catalytic pocket and reduces enzymatic activity. Because all three DNMT1 mutant animals are viable, it was possible to examine their phenotypes throughout life. The consequences of genome-wide hypomethylation of DNA of somatic tissues in the Dnmt1 mutants are surprisingly mild but consistently affect the development of the lymphoid lineage. Our findings indicate that developing lymphocytes in vertebrates are sensitive to perturbations of DNA maintenance methylation.