Divergent T-cell receptor delta chains from marsupials

Complementary DNAs (cDNAs) encoding T-cell receptor delta (TRD) chains from the northern brown bandicoot, Isoodon macrourus, were identified while sequencing expressed sequence tags (ESTs) from a thymus cDNA library. Surprisingly, the I. macrourus TRD sequences were not orthologous to previously published TRD sequences from another Australian marsupial, the tammar wallaby, Macropus eugenii. Identification of TRD genes in the recently completed whole genome sequence of the South American opossum, Monodelphis domestica, revealed the presence of two highly divergent TRD loci. To determine whether the presence of multiple TRD loci accounts for the lack of orthology between the I. macrourus and M. eugenii cDNAs, additional TRD sequences were obtained from both species of marsupials. The results of this analysis revealed that, unlike eutherian mammals, all three species of marsupials have multiple, highly divergent TRD loci. One group of marsupial TRD sequences was closely related to TR sequences from eutherian mammals. A second group of TRD sequences formed a unique marsupial-specific clade, separate from TR sequences from eutherians. An interesting expression pattern of TRD variable (TRDV) and constant (TRDC) segments was evident in cDNAs from I. macrourus and M. eugenii. TRDV and TRDC sequences that were closely related to TRD genes from eutherian mammals were only found in association with each other in cDNAs from both marsupial species. A similar pattern was seen between TRDV and TRDC sequences that were most closely related to other marsupial TRD genes.