Widespread Over-Expression of the X Chromosome in Sterile F1 Hybrid Mice

Abstract

The X chromosome often plays a central role in hybrid male sterility between species, but it is unclear if this reflects underlying regulatory incompatibilities. Here we combine phenotypic data with genome-wide expression data to directly associate aberrant expression patterns with hybrid male sterility between two species of mice. We used a reciprocal cross in which F1 males are sterile in one direction and fertile in the other direction, allowing us to associate expression differences with sterility rather than with other hybrid phenotypes. We found evidence of extensive over-expression of the X chromosome during spermatogenesis in sterile but not in fertile F1 hybrid males. Over-expression was most pronounced in genes that are normally expressed after meiosis, consistent with an X chromosome-wide disruption of expression during the later stages of spermatogenesis. This pattern was not a simple consequence of faster evolutionary divergence on the X chromosome, because X-linked expression was highly conserved between the two species. Thus, transcriptional regulation of the X chromosome during spermatogenesis appears particularly sensitive to evolutionary divergence between species. Overall, these data provide evidence for an underlying regulatory basis to reproductive isolation in house mice and underscore the importance of transcriptional regulation of the X chromosome to the evolution of hybrid male sterility.

Meiotic Sex Chromosome Inactivation Is Disrupted in Sterile Hybrid Male House Mice

“…Strikingly, a large number of genes on the musculus PWK X chromosome are overexpressed in the testes of sterile F 1 males (Good et al 2010). However, because whole testes comprise germ cells from all stages of spermatogenesis, together with somatic cells, we could not determine whether overexpression is specific to germ cells in which the X is normally silenced or repressed (Good et al 2010).Here, we test the hypothesis that the timing of X chromosome overexpression in sterile hybrid male house mice is consistent with disrupted MSCI. We used fluorescence-activated cell sorting (FACS) to isolate discrete germ cell populations from reciprocal F 1 males and quantified the expression of X-linked genes using real-time reversetranscription PCR (qRT-PCR).…”

“…We hypothesized that disrupted MSCI in hybrid males results from local mismatch between loci in the domesticus LEWES autosomal genome and cis-acting sequences distributed along the musculus PWK X. To test this, we selected genes along the length of the X that were previously shown to be overexpressed in sterile F 1 testes (Good et al 2010) and measured whole-testis expression in F 1 males with recombinant X chromosomes (X introgression F 1 s). If MSCI is disrupted in cis, only musculus PWK alleles should be overexpressed.…”

“…Although genotype means for this population were not significantly different (RS, F (62,1) = 3.50, P = 0.07), there was a clear trend toward overexpression in the sterile F 1 ( Figure 1A). We found previously that a large number of X-linked genes are overexpressed in sterile F 1 male testes (Good et al 2010). However, because progressive cell loss throughout spermatogenesis in sterile F 1 males could bias the cellular composition of the testis toward early spermatogenic cell types we excluded those genes normally expressed in mitotic/early meiotic cells and focused on postmeiotic genes only (i.e., genes that escape PSCR).…”

“…The fact that failed silencing of several Y-linked genes does not cause meiotic arrest in transgenic lab mice suggests that misexpression of some genes can be tolerated (Royo et al 2010). However, a large number of X-linked genes are overexpressed in sterile F 1 testes (Good et al 2010). Given that all protein-coding genes on the X are subject to MSCI, it is likely that even moderate overexpression of many X-linked genes would be toxic to primary spermatocytes.…”

“…At the start of pachytene, the X and Y undergo global chromatin remodeling and transcriptional silencing of all protein-coding genes, and are sequestered from the autosomes in a heterochromatin domain known as the sex body (reviewed in Handel 2004;Turner 2007). The sex body is disassembled at the end of prophase I but the sex chromosomes remain We previously studied gene expression in sterile male hybrids between the house mouse subspecies, Mus musculus musculus and M. m. domesticus (Good et al 2010). In the cross between wild-derived inbred strains musculus PWK and domesticus LEWES , hybrid male sterility is asymmetric and strongly X linked: both F 1 and late backcross males with a musculus PWK X chromosome are sterile or nearly so whereas males with a domesticus LEWES X are normal (Good et al 2008a,b).…”