Chromosome Research

, Volume 19, Issue 5, pp 635-644

First online:

Additional copies of CBX2 in the genomes of males of mammals lacking SRY, the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis)

  • Asato KuroiwaAffiliated withGraduate School of Life Science, Hokkaido UniversityLaboratory of Animal Cytogenetics, Faculty of Science, Hokkaido University Email author 
  • , Sanae HandaAffiliated withGraduate School of Life Science, Hokkaido University
  • , Chigusa NishiyamaAffiliated withGraduate School of Life Science, Hokkaido University
  • , Eriko ChibaAffiliated withGraduate School of Life Science, Hokkaido University
  • , Fumio YamadaAffiliated withForestry and Forest Products Research Institute
  • , Shintaro AbeAffiliated withNaha Nature Conservation Office, Ministry of the Environment
  • , Yoichi MatsudaAffiliated withLaboratory of Animal Genetics, Department of Applied Molecular Biosciences, Graduate School of Bioagricultural Sciences, Nagoya University

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Tokudaia osimensis (the Amami spiny rat) and Tokudaia tokunoshimensis (the Tokunoshima spiny rat) have a sex chromosome composition of XO/XO, no Y chromosome. The mammalian sex-determining gene, SRY, is also absent in these species, which indicates that these spiny rats exhibit a novel sex-determining mechanism that is independent of SRY. To identify a candidate gene that controls this mechanism, the copy numbers and chromosomal locations of 10 genes with important functions in gonadal differentiation were determined: ATRX, CBX2 (M33), DMRT1, FGF9, NR0B1 (DAX1), NR5A1 (Ad4BP/SF1), RSPO1, SOX9, WNT4, and WT1. Multiple bands were detected for NR0B1 in Southern blot analysis, which suggested the presence of multiple copies of the gene in the genomes of these two species. CBX2 was localized to two loci in both sexes of the two species by fluorescence in situ hybridization mapping: 3q24 and 6p11.2 in T. osimensis and 10q25–q26 and 14q12–q13.1 in T. tokunoshimensis. Quantification of copy numbers in the two species by quantitative real-time PCR indicated that there were two or three more copies of CBX2 per haploid genome in males (T. osimensis, n = 3; T. tokunoshimensis, n = 2) than in females (T. osimensis, n = 4; T. tokunoshimensis, n = 2), whereas NR0B1 was present as a single copy in both. The results suggest that additional copies of CBX2 in males might be involved in a novel sex-determining mechanism in species that lack SRY.


sex determination sex differentiation XO Y chromosome