A complex genetic basis to X-linked hybrid male sterility between two species of house mice

Abstract

The X chromosome plays a central role in the evolution of reproductive isolation, but few studies have examined the genetic basis of X-linked incompatibilities during the early stages of speciation. We report the results of a large experiment focused on the reciprocal introgression of the X chromosome between two species of house mice, Mus musculus and M. domesticus. Introgression of the M. musculus X chromosome into a wild-derived M. domesticus genetic background produced male-limited sterility, qualitatively consistent with previous experiments using classic inbred strains to represent M. domesticus. The genetic basis of sterility involved a minimum of four X-linked factors. The phenotypic effects of major sterility QTL were largely additive and resulted in complete sterility when combined. No sterility factors were uncovered on the M. domesticus X chromosome. Overall, these results revealed a complex and asymmetric genetic basis to X-linked hybrid male sterility during the early stages of speciation in mice. Combined with data from previous studies, we identify one relatively narrow interval on the M. musculus X chromosome involved in hybrid male sterility. Only a handful of spermatogenic genes are within this region, including one of the most rapidly evolving genes on the mouse X chromosome.

Figure 1.—

Experimental design for the reciprocal introgression of the X chromosome. (A) Three targeted X chromosome congenic genotypes (proximal, center, and distal) for experiment 1 and experiment 2. For each male genotype, the genetic composition of three autosomes, the X chromosome, and the Y chromosome is given. White chromosomes represent domesticus^LEWES and black chromosomes represent musculus^PWK. For both experiments the mitochondrial DNA derives from musculus^PWK. (B) Example of congenic strain construction for introgression of the distal musculus^PWK X chromosome over 10 generations of breeding. Expected levels of heterozygosity for each generation are given in parentheses; at the N₁₀ generation the genomic background will be ∼99.8% homozygous for domesticus^LEWES alleles.

Figure 2.—

Negative correlation between length of musculus^PWK X chromosome introgression and relative testis weight (n = 202 N_6–8 males, r² = 0.45, P < 0.0001). All males are from the N₆, N₇, or N₈ generations of experiment 1. The abscissa depicts the length of introgressed regions regardless of their location on the X chromosome.

Figure 3.—

Reduction in relative testis weight and sperm count for four different musculus^PWK X chromosome introgressions. All males are from the N₆, N₇, or N₈ generations of experiment 1. Bars indicate average values for relative testis weight (shaded) and sperm count (open), error bars indicate ±1 SE. Different genotypic classes are represented on the left margin with open chromosomal regions deriving from domesticus^LEWES and solid chromosomes deriving from musculus^PWK. The control genotype (all open) represents backcross males with a domesticus^LEWES X chromosome. Sample sizes are given on the left margin. The locations of screened markers are indicated with arrowheads along the bottom. Shading for each genotype indicates uncertainty in recombination breakpoints between surveyed markers. Significance was based on Wilcoxon pairwise comparisons vs. the control backcross males.

Figure 4.—

Influence of the musculus^PWK X chromosome on abnormal sperm head morphology in experiment 1. (A) Examples of three types of sperm head morphologies observed in N_6–8 males from experiment 1. (B) Sperm head morphologies observed for males with progressively large introgressions of musculus^PWK X chromosome (solid chromosomes). The first genotypic class represents control backcross males with a domesticus^LEWES X chromosome (all open). Sample sizes are given on the left margin and the locations of screened markers are indicated with arrowheads along the bottom. Shading for each genotype indicates uncertainty in recombination breakpoints between surveyed markers. Sperm head morphology is partitioned into four phenotypic classes proceeding from normal to abnormal: (1) normal, (2) flattened head, shortened hook, (3) round head, short or absent hook, and (4) severely abnormal. Severely abnormal head morphologies were grouped together and took on a variety of dramatically altered shapes.

Figure 5.—

Results for QTL mapping of hybrid male sterility phenotypes in experiment 1. LOD scores are based on CIM analysis of testis weight, sperm count, and the proportion of abnormal sperm in males from the N₆, N₇, and N₈ generations. The physical location, identity, and genetic position (cM) of screened markers are given along the left margin. The significance threshold (α = 0.05) for each trait is shaded and is based on 1000 permutations. Arrows indicate the position of individual QTL that remained significant in MIM models.

Figure 6.—

Results for QTL mapping of differences in testis weight between domesticus^LEWES and musculus^PWK in experiment 2. LOD scores are based on CIM analysis of testis weight in males from the N₄, N₆, and N₈ generations. The physical location, identity, and genetic position (cM) of screened markers are given along the left margin. The significance threshold (α = 0.05; LOD = 2.25) is indicated with shading and is based on 1000 permutations. Arrows indicate the position of individual QTL that remained significant in MIM models.