THE KARYOTYPE OF ALLIGATOR-MISSISSIPPIENSIS, AND CHROMOSOMAL MAPPING OF THE ZFY X HOMOLOG, ZFC/

Comparative mapping studies of X-linked genes in mammals have provided insights into the evolution of the X chromosome. Many reptiles includ ing the American alligator, Alligator mississippiensis, do not appear to possess heteromorphic sex chromosomes, and sex is determined by the incubation temperature of the egg during embryonic development. Mappi ng of homologues of mammalian X-linked genes in reptiles could lead to a greater understanding of the evolution of vertebrate sex chromosome s. One of the genes used in the mammalian mapping studies was ZFX, an X-linked copy of the human ZFY gene which was originally isolated as a candidate for the mammalian testis-determining factor (TDF). ZFX is X -linked in eutherians, but maps to two autosomal locations in marsupia ls and monotremes, close to other genes associated with the eutherian X. The alligator homologue of the ZFY/ZFX genes, Zfc, has been isolate d and described previously. A detailed karyotype of A. mississippiensi s is presented, together with chromosomal in situ hybridisation data l ocalising the Zfc gene to chromosome 3. Further chromosomal mapping st udies using eutherian X-linked genes may reveal conserved chromosomal regions in the alligator that have become part of the eutherian X chro mosome during evolution.