SHARED DNA-SEQUENCES BETWEEN THE X-CHROMOSOME AND Y-CHROMOSOME IN THETAMMAR WALLABY - EVIDENCE FOR INDEPENDENT ADDITIONS TO EUTHERIAN AND MARSUPIAL SEX-CHROMOSOMES
R. Toder et al., SHARED DNA-SEQUENCES BETWEEN THE X-CHROMOSOME AND Y-CHROMOSOME IN THETAMMAR WALLABY - EVIDENCE FOR INDEPENDENT ADDITIONS TO EUTHERIAN AND MARSUPIAL SEX-CHROMOSOMES, Chromosoma, 106(2), 1997, pp. 94-98
Marsupial sex chromosomes are smaller than their eutherian counterpart
s and are thought to reflect an ancestral mammalian X and Y. The gene
content of this original X is represented largely by the long arm of t
he human X chromosome. Genes on the short arm of the human X are autos
omal in marsupials and monotremes, and represent a recent addition to
the eutherian X and Y. The marsupial X and Y apparently lack a pseudoa
utosomal region and show only end-to-end pairing at meiosis. However,
the sex chromosomes of macropodid marsupials (kangaroos and wallabies)
are larger than the sex chromosomes of other groups, and a nucleolus
organizer is present on the X and occasionally the Y. Chromosome paint
ing using DNA from sorted and microdissected wallaby X and Y chromosom
es reveals homologous sequences on the tammar X and Y chromosomes, con
centrated on the long arm of the Y chromosome and short arm of the X.
Ribosomal DNA sequences were detected by fluorescence in situ hybridiz
ation on the wallaby Xp but not the Y. Since no chiasmata have been ob
served in marsupial sex chromosomes, it is unlikely that these shared
sequences act as a pseudoautosomal region within which crossing over m
ay occur, but they may be required for end-to-end associations. The sh
ared region of wallaby X and Y chromosomes bears no homology with the
recently added region of the eutherian sex chromosomes, so we conclude
that independent additions occurred to both sex chromosomes in a euth
erian and macropodid ancestor, as predicted by the addition-attrition
hypothesis of sex chromosome evolution.