ASSIGNMENT OF RFP-Y TO THE CHICKEN MAJOR HISTOCOMPATIBILITY COMPLEX NOR MICROCHROMOSOME AND EVIDENCE FOR HIGH-FREQUENCY RECOMBINATION ASSOCIATED WITH THE NUCLEOLAR ORGANIZER REGION

Rfp-Y is a second region in the genome of the chicken containing major histocompatibility complex (MHC) class I and II genes. Haplotypes of Rfp-Y assort independently from haplotypes of the B system, a region k nown to function as a MHC and to be located on chromosome 16 (a microc hromosome) with the single nucleolar organizer region (NOR) in the chi cken genome. Linkage mapping with reference populations failed to reve al the location of Rfp-Y, leaving Rfp-Y unlinked in a map containing > 400 markers. A possible location of Rfp-Y became apparent in studies o f chickens trisomic for chromosome 16 when it was noted that the inten sity of restriction fragments associated with Rfp-Y increased with inc reasing copy number of chromosome 16. Further evidence that Rfp-Y migh t be located on chromosome 16 was obtained when individuals trisomic f or chromosome 16 were found to transmit three Rfp-Y haplotypes. Finall y, mapping of cosmid cluster LII of the molecular map of chicken R-IHC genes (containing a MHC class II gene and two rRNA genes) to Rfp-Y va lidated the assignment of Rfp-Y to the MHC/NOR microchromosome. A gene tic map can now be drawn for a portion of chicken chromosome 16 with R fp-Y, encompassing two MHC class I and three MHC class II genes, separ ated from the B system by a region containing the NOR and exhibiting h ighly frequent recombination.