Greater prairie chickens have a compact MHC-B with a single class IA locus.

The major histocompatibility complex (MHC) plays a central role in innate and adaptive immunity, but relatively little is known about the evolution of the number and arrangement of MHC genes in birds. Insights into the evolution of the MHC in birds can be gained by comparing the genetic architecture of the MHC between closely related species. We used a fosmid DNA library to sequence a 60.9-kb region of the MHC of the greater prairie chicken (Tympanuchus cupido), one of five species of Galliformes with a physically mapped MHC. Greater prairie chickens have the smallest core MHC yet observed in any bird species, and major changes are observed in the number and arrangement of MHC loci. In particular, the greater prairie chicken differs from other Galliformes in the deletion of an important class I antigen binding gene. Analysis of the remaining class IA gene in a population of greater prairie chickens in Wisconsin, USA revealed little evidence for selection at the region responsible for antigen binding.