T lymphocytes or T cells are key components of the vertebrate response to pathogens and cancer. There are two T cell classes based on their TCRs, αβ T cells and γδ T cells, and each plays a critical role in immune responses. The squamate reptiles may be unique among the vertebrate lineages by lacking an entire class of T cells, the γδ T cells. In this study, we investigated the basis of the loss of the γδ T cells in squamates. The genome and transcriptome of a sleepy lizard, the skink Tiliqua rugosa, were compared with those of tuatara, Sphenodon punctatus, the last living member of the Rhynchocephalian reptiles. We demonstrate that the lack of TCRγ and TCRδ transcripts in the skink are due to large deletions in the T. rugosa genome. We also show that tuataras are on a growing list of species, including sharks, frogs, birds, alligators, and platypus, that can use an atypical TCRδ that appears to be a chimera of a TCR chain with an Ab-like Ag-binding domain. Tuatara represents the nearest living relative to squamates that retain γδ T cells. The loss of γδTCR in the skink is due to genomic deletions that appear to be conserved in other squamates. The genes encoding the αβTCR chains in the skink do not appear to have increased in complexity to compensate for the loss of γδ T cells.

Marsupials, such as kangaroos and opossums, are members of a lineage of mammals that last shared a common ancestor with humans and mice ~160 million years ago. Although they share many of the same components of the adaptive immune system of humans and mice, they also have some significant differences. For example, in addition to conventional αβ and γδ T cells is the presence of a third T cell lineage, the γμ T cell. Like humans and mice, marsupials have homologues of the classical MHC class I and II molecules and many of the non-conventional MHCs such as CD1 and MR1, but in addition, opossums have another MHC class I family called the UT family. Like humans and mice, opossums also have FcRN. However, the development of B cells may also differ significantly. B cell development in the opossum is entirely postnatal. For example, opossums lack the VpreB1 and 2 and λ5 surrogate light chains; however, like birds, they have VpreB3. B cell development also occurs at a slower pace in the opossum and the sites of B cell development in marsupials remain largely unknown. To address these differences and unknowns, we will present our results regarding timing and location of B cell development, and the establishment of immuno-competence in a model marsupial, the laboratory opossum Monodelphis domestica. Supported by NSF award IOS-2103367