X chromosomes of American marsupials contain minimal amounts of euchromatin.
The karyotypes of four South American didelphid marsupials, representing diploid numbers of 2n = 14 and 18, have been analyzed by a variety of banding techniques. The 2n = 14 karyotypes display a high degree of homoeology, but there also exist distinct similarities between the 2n = 14 and 2n = 18 karyotypes. The interspecific differences found are due to centric fissions, pericentric inversions, and variations in the amount and composition of the constitutive heterochromatin. Contrary to the evolutionary conservation of the banding patterns in all autosomal arms, there are multiple differences in the number and chromosomal location of the nucleolus organizer regions. In species with X-linked nucleolus organizers, the 18S + 28S ribosomal RNA genes escape inactivation in female cells. Measurements on the X chromosomes of Marmosa fuscata and Micoureus demerarae unexpectedly reveal the lowest quantities of euchromatin so far determined in the X chromosomes of mammals: 1.5% and 1.8%, respectively, of their haploid female genomes. This is significantly less than the amount of euchromatin in the basic X chromosomes of other marsupials (3%) or eutherians (5%).
Schmid
M
M
Department of Human Genetics, Biocenter, University of Würzburg, Würzburg, Germany. m.schmid@biozentrum.uni-wuerzburg.de
Feichtinger
W
W
Steinlein
C
C
Haaf
T
T
Visbal García
R
R
Fernández Badillo
A
A
D000818:Animals; D002871:Chromosome Banding; D022041:Euchromatin / Q000235:genetics*; D005260:Female; D007621:Karyotyping; D008297:Male; D008394:Marsupialia / Q000235:genetics*; D009893:Opossums / Q000235:genetics; D013045:Species Specificity; D014960:X Chromosome / Q000235:genetics*
D022041:Euchromatin