DEFINING THE ANATOMY OF THE RANGIFER TARANDUS SEX-CHROMOSOMES

A comprehensive cytogenetic characterization of the unusually large re indeer (Rangifer tarandus) sex chromosomes is presented for the purpos e of studying the evolution of these atypical gonosomes. Sex chromosom e idiograms were constructed from G-banded and C-banded chromosomes to illustrate the relative amounts and locations of euchromatin and hete rochromatin. Hybridization with a Mazama gouazoubira X whole-chromosom e paint revealed that essentially all reindeer X-linked euchromatin an d most reindeer Y-linked euchromatin is conserved interspecifically. S ubsequently, painting probes were generated from flow-sorted reindeer X chromosomes, flow-sorted reindeer Y chromosomes, and from microdisse ctions of specific gonosomal regions to establish specific segment-to- segment homologies between these gonosomes. In particular, one microdi ssection-generated paint demonstrated that certain constituent repetit ive DNAs, found in C-band region Xq31, were also present in essentiall y all heterochromatin blocks of the Y chromosome. Microdissection-gene rated paints from other X-linked heterochromatin blocks revealed the p resence of DNA sequences that lacked homologous sequences on the Y chr omosomes and were more specific for their region of origin. These char acteristics of the reindeer sex chromosomes are consistent with the no tion that mammalian sex chromosomes were derived from homologous proge nitor chromosome pairs and provide insights into the evolution of thes e atypical mammalian gonosomes.