New Y chromosomes and early stages of sex chromosome differentiation: sex determination in Megaselia

Abstract

The phorid fly Megaselia scalaris is a laboratory model for the turnover and early differentiation of sex chromosomes. Isolates from the field have an XY sex-determining mechanism with chromosome pair 2 acting as X and Y chromosomes. The sex chromosomes are homomorphic but display early signs of sex chromosome differentiation: a low level of molecular differences between X and Y. The male-determining function (M), maps to the distal part of the Y chromosome's short arm. In laboratory cultures, new Y chromosomes with no signs of a molecular differentiation arise at a low rate, probably by transposition of M to these chromosomes. Downstream of the primary signal, the homologue of the Drosophila doublesex (dsx) is part of the sex-determining pathway while Sex-lethal (Sxl), though structurally conserved, is not.

Sex chromosome evolution in moths and butterflies

“…Quite recently, the common house fly, Musca domestica, replaced its old XY system in many populations worldwide for new ones based on former autosomes (Franco et al 1982). An extreme case is the fly Megaselia scalaris where replacement of the sex chromosome system by non-homologous new ones can be observed even under laboratory conditions (Traut 2010).…”

Reversal of an ancient sex chromosome to an autosome in Drosophila

“…4). Novel sex determining genes occurring on different chromosomes or translocations of M-factors onto autosomes have been observed in houseflies 21 or humpbacked flies 22 . The fixation of the new male-determining gene on element A would lead to reversal of the ancestral X (Muller F) to an autosome, and the ancestral Y would be completely lost.…”

Sex determination strategies in 2012: towards a common regulatory model?

“…Their sex determination pattern typically involves X chromosome activation and a "counting process" that estimates the X: autosome chromosome ratio within the whole organism (fruitflies) [75,76] or any given cell autonomously (e.g. worms) [77].…”