We have previously reported the molecular cloning of the mammalian maj
or histocompatibility complex (MHC) class III gene, complement factor
B (Bf) from Xenopus laevis, and linkage of the gene to the frog MHC. H
ere, we estimated the copy number of the Xenopus Bf gene by genomic So
uthern blotting analysis and demonstrated that Xenopus laevis has two
copies of the Bf gene. Both genes co-segregated with the MHC-linked HS
P70 genes among 19 offspring of an f/r x f/r cross, indicating a close
linkage of the two Bf genes to the frog MHC. Both genes are transcrib
ed and contain open reading frames. When compared with the previously
determined cDNA sequence (Xenopus Bf A), the predicted amino acid sequ
ence of the second cDNA species (Xenopus Bf B) shows 82% overall ident
ity. Polymerase chain reaction analysis indicated that all of the part
ially inbred frogs with the f, r, g, and j MHC haplotypes, as well as
12 outbred frogs tested have both Bf genes, suggesting that the duplic
ated Bf genes are stable genetic traits in Xenopus laevis.