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.