Overview of the proposed independent evolution of the X1X2Y sex chromosomes in HMA and EER based on karyotypic features and cross-FISH results with sex chromosome wcp. (1) The HMA X1X2Y system originated by tandem fusions between the poorly differentiated Y chromosome of HMA karyomorph C and an autosome, which created the large neo-Y chromosome in HMA karyomorph D. The unpainted extension of the neo-Y corresponds to the ancestral homolog of the X2 chromosome, which lacks homology with the X1 chromosome. (2) The same sex system in EER derived from a centric fusion between a morphologically undifferentiated Y chromosome in EER karyomorph A and an autosome, which gave rise to the large neo-Y chromosome in EER karyomorph D. Although the XY chromosomes have not been cytogenetically identified in EER karyomorph A, they are presumed to be one of the two acrocentric pairs that wholly hybridized to the Ee-Y probe. The chromosomes painted with the Hm-X1 and Ee-Y probes are indicated in green and red, respectively. Note that although the HMA and EER chromosomes share sequence homology, the two sex chromosome systems have evolved independently.
Cioffi et al. BMC Evolutionary Biology 2011 11:186 doi:10.1186/1471-2148-11-186