I don't know if there's a
phylogenetic pattern in Y chromosome reduction
I thought that, while shrinking, the "human" (primate? placental?
mammalian?) Y chromosome has attracted sex-specific chromosomes from
elsewhere?
And that [the _s_ex-determining _r_egion on the _Y_
chromosome] can transpose - rarely, but it has happened
at least once and perhaps twice in _Ellobius_ (a rodent).