Steinemann and Steinemann 1998

Steinemann M, Steinemann S. 1998. Enigma of Y chromosome degeneration: Neo-Y and Neo-X chromosomes of Drosophila miranda a model for sex chromosome evolution. Genetica 102/103: 409-420.

These authors examined the Neo-Y chromosome and its meiotic-pairing partner X2 in Drosophila miranda, a species in the Drosophila psuedoobscura species subgroup, about 25 million years removed from D. melanogaster.

These sex chromosomes are unique to D. miranda, and represent an early stage in the evolution of degenerate Y chromosomes. Two major changes distinguish degenerate Y chromosomes from X chromosomes: loss of functional genes by psuedogenization and conversion of euchromatin to heterochromatin.

Muller’s ratchet and the accumulation of point mutations on a non-recombining Y chromosome can account for the first difference, but does not explain the heterochromatinization of the Neo-Y chromosome. A block of repeats homologous to telomere sequences was found in the interior region of the Neo-Y, suggesting an end-to-end chromosome fusion event as the origin of the Neo-Y. These authors examined a region of the Neo-Y associated with a clustered family of genes. On the Neo-Y but not on the X2 they found a “massive accumulation of transposable elements” in this region, associated with the silencing of two of the genes in the examined family.

Finally, the authors suggest that transposable elements are directly involved in the formation of heterochromatin, though the mechanism by which this occurs is not described. The high level of TE insertion in the Neo-Y of D. miranda is strong evidence for chromosome degeneration as in Y chromosomes driven at least partly by the activities of TEs.