Genetic Mapping in Xenopus Laevis: Eight Linkage Groups Established

Inheritance of alleles at 29 electrophoretically detected protein loci and one pigment locus (albinism) was analyzed in Xenopus laevis by backcrossing multiply heterozygous individuals generated by intersubspecies hybridization. Pairwise linkage tests revealed eight classical linkage groups. These groups have been provisionally numbered from 1 to 8 in an arbitrarily chosen order. Linkage group 1 includes ALB-2 (albumin), ADH-1 (alcohol dehydrogenase), NP (nucleoside phosphorylase), and ap (periodic albinism). Linkage group 2 contains ALB-1 and ADH-2, and probably is homeologous to group 1. Linkage group 3 comprises PEP-B (peptidase B), MPI-1 (mannosephosphate isomerase), SORD (sorbitol dehydrogenase), and mIDH-2 (mitochondrial isocitrate dehydrogenase). Linkage group 4 contains GPI-1 (glucosephosphate isomerase) and EST-4 (esterase 4). Linkage group 5 contains GPI-2 and PEP-D (peptidase D). Linkage group 6 comprises ACP-3 (acid phosphatase), sME (cytosolic malic enzyme), and GLO-2 (glyoxalase). Linkage group 7 consists of sSOD-1 (cytosolic superoxide dismutase), GPD-2 (glycerol-3-phosphate dehydrogenase), mME (mitochondrial malic enzyme), and the sex determining locus. Linkage group 8 includes FH (fumarate hydratase) and TRF (transferrin). Recombination frequencies between linked loci showed differences related to the genomic constitution (parental subspecies) and to the sex of the heterozygous parent. Independent assortment was observed between the duplicate ALB loci. This is true for the duplicate ADH, GLO, and MPI loci as well, supporting the view that these genes have been duplicated as part of a genome duplication that occurred in the evolutionary history of X. laevis. Comparative analysis of genetic maps reveals a possible conservation of several linkages from the Xenopus genome to the human genome.