We plan to sequence and annotate the complete genome of Ellobius species. The subterranean rodent E.lutescens has an odd diploid 17,X karyotype, in both females and males. Closely related species have XX females and XY males (E.fuscocapillus) or XX females and males (E.talpinus and E.tancrei). In the present study, we focus on male E.lutescens and male E.fuscocapillus, addressing the following questions: 1) The single X chromosome of E.lutescens cannot recombine during meiosis. Does this result in degeneration of the X of E.lutescens, compared to X from E.fuscocapillus? 2) E.lutescens embryos with an 18,XX constitution are not viable. Has E.lutescens lost genes controlling X chromosome inactivation (XCI)? 3) In placental mammals, meiotic sex chromosome inactivation (MSCI) has driven the evolution of X-to-autosomal retrogenes (functional retrotransposed genes). In addition, autosomal-to-autosomal retrotransposition in E.lutescens may have generated novel retrogenes involved in sex differentiation and spermatogenesis. Have Ellobius species gained or lost specific retrogenes? 4) In E.lutescens, genes from the original Y chromosome which are crucial for spermatogenesis must have been translocated or replaced by autosomal genes or genes on the X, before the Y chromosome was lost. Which essential genes are maintained somewhere in the genome? Also, E.lutescens does not have an Sry gene, on any chromosome. Can we identify a novel primary male sex-determining gene? All these questions are highly relevant also to our understanding of the impact of the evolution of the heterologous sex chromosomes on human biology, with possible applications in medical genetics.