Unusually large sex chromosomes: new methods of measuring and description of karyotypes of six rodents (Myomorpha and Hystricomorpha) and one lagomorph (Ochotonidae)

Chromosome studies of six rodents, of which three have unusually large X chromosomes, and one lagomorph have been made. Two new methods of measuring unusually large sex chromosomes are presented. Compound sex elements are more often discovered as more species are studied, but the phenomenon in species without an X-to-autosome translocation is thus far restricted to the Artiodactyla, Rodentia, and Marsupialia, a point discussed in the text. Eutherian mammals known to have X chromosomes of more than 6 % of the haploid complement and all members of the Hystricomorpha that have been studied karyologically have been summarized.