Wild Mice As Bountiful Resources of Novel Genetic Variants for Quantitative Traits

Most traits of biological importance, including traits for human complex diseases (e.g., obesity and diabetes), are continuously distributed. These complex or quantitative traits are controlled by multiple genetic loci called QTLs (quantitative trait loci), environments and their interactions. The laboratory mouse has long been used as a pilot animal model for understanding the genetic architecture of quantitative traits. Next-generation sequencing analyses and genome-wide SNP (single nucleotide polymorphism) analyses of mouse genomes have revealed that classical inbred strains commonly used throughout the world are derived from a few fancy mice with limited and non-randomly distributed genetic diversity that occurs in nature and also indicated that their genomes are predominantly Mus musculus domesticus in origin. Many QTLs for a huge variety of traits have so far been discovered from a very limited gene pool of classical inbred strains. However, wild M. musculus mice consisting of five subspecies widely inhabit areas all over the world, and hence a number of novel QTLs may still lie undiscovered in gene pools of the wild mice. Some of the QTLs are expected to improve our understanding of human complex diseases. Using wild M. musculus subspecies in Asia as examples, this review illustrates that wild mice are untapped natural resources for valuable QTL discovery.     

Wild mouse open field behavior is embedded within the multidimensional data space spanned by laboratory inbred strains

The vast majority of studies on mouse behavior are performed on laboratory mouse strains (Mus laboratorius), while studies of wild-mouse behavior are relatively rare. An interesting question is the relationship between the phenotypes of M. laboratorius and the phenotypes of their wild ancestors. It is commonly believed, often in the absence of hard evidence, that the behavior of wild mice exceeds by far, in terms of repertoire richness, magnitude of variables and variability of behavioral measures, the behavior of the classical inbred strains. Having phenotyped the open field behavior (OF) of eight of the commonly used laboratory inbred strains, two wild-derived strains and a group of first-generation-in-captivity local wild mice (Mus musculus domesticus), we show that contrary to common belief, wild-mouse OF behavior is moderate, both in terms of end-point values and in terms of their variability, being embedded within the multidimensional data space spanned by laboratory inbred strains. The implication could be that whereas natural selection favors moderate locomotor behavior in wild mice, the inbreeding process tends to generate in mice, in some of the features, extreme and more variable behavior.     

Polymorphism of Unique Noncoding DNA Sequences in Wild and Laboratory Mice

Two DNA probes, D17Tu1 and D17Tu2, were isolated from a genomic DNA library containing only two mouse chromosomes, one of which is chromosome 17, carrying the major histocompatibility complex (H-2), as well as the t complex genes. The D17Tu1 probe was mapped to the centromeric region of chromosome 17 and the D17Tu2 probe to the S region of the H-2 complex. Neither of the two probes appeared to detect any genes, but both contained unique, nonrepetitive sequences. Typing of DNA obtained from a large panel of mice revealed the presence of four D17Tu1 patterns in inbred mouse strains, one very common, one less common, and two present in one strain each. The two common patterns could not be detected in appreciable frequencies in the European wild mice tested (one of the two patterns was, however, found in Australian wild mice). Conversely, the patterns found frequently in European wild mice are absent in the laboratory mice. We therefore conclude that wild mice from the sampled regions of Europe could not have provided the ancestral stocks from which inbred strains were derived. Only one D17Tu1 pattern was found in all the populations of Mus musculus tested, while eight patterns were found in Mus domesticus, with virtually all the populations being polymorphic. We suggest that this difference reflects different modes in which the two species colonized Europe. The distribution of the D17Tu2 patterns in inbred strains correlates with the distribution of H-2 haplotypes.     

Nucleotide variation in wild and inbred mice

The house mouse is a well-established model organism, particularly for studying the genetics of complex traits. However, most studies of mice use classical inbred strains, whose genomes derive from multiple species. Relatively little is known about the distribution of genetic variation among these species or how variation among strains relates to variation in the wild. We sequenced intronic regions of five X-linked loci in large samples of wild Mus domesticus and M. musculus, and we found low levels of nucleotide diversity in both species. We compared these data to published data from short portions of six X-linked and 18 autosomal loci in wild mice. We estimate that M. domesticus and M. musculus diverged <500,000 years ago. Consistent with this recent divergence, some gene genealogies were reciprocally monophyletic between these species, while others were paraphyletic or polyphyletic. In general, the X chromosome was more differentiated than the autosomes. We resequenced classical inbred strains for all 29 loci and found that inbred strains contain only a small amount of the genetic variation seen in wild mice. Notably, the X chromosome contains proportionately less variation among inbred strains than do the autosomes. Moreover, variation among inbred strains derives from differences between species as well as from differences within species, and these proportions differ in different genomic regions. Wild mice thus provide a reservoir of additional genetic variation that may be useful for mapping studies. Together these results suggest that wild mice will be a valuable complement to laboratory strains for studying the genetics of complex traits.     

Gastrointestinal microbiota of wild and inbred individuals of two house mouse subspecies assessed using high‐throughput parallel pyrosequencing

The effects of gastrointestinal tract microbiota (GTM) on host physiology and health have been the subject of considerable interest in recent years. While a variety of captive bred species have been used in experiments, the extent to which GTM of captive and/or inbred individuals resembles natural composition and variation in wild populations is poorly understood. Using 454 pyrosequencing, we performed 16S rDNA GTM barcoding for 30 wild house mice (Mus musculus) and wild‐derived inbred strain mice belonging to two subspecies (M. m. musculus and M. m. domesticus). Sequenced individuals were selected according to a 2 × 2 experimental design: wild (14) vs. inbred origin (16) and M. m. musculus (15) vs. M. m. domesticus (15). We compared alpha diversity (i.e. number of operational taxonomic units – OTUs), beta diversity (i.e. interindividual variability) and microbiota composition across the four groups. We found no difference between M. m. musculus and M. m. domesticus subspecies, suggesting low effect of genetic differentiation between these two subspecies on GTM structure. Both inbred and wild populations showed the same level of microbial alpha and beta diversity; however, we found strong differentiation in microbiota composition between wild and inbred populations. Relative abundance of ~ 16% of OTUs differed significantly between wild and inbred individuals. As laboratory mice represent the most abundant model for studying the effects of gut microbiota on host metabolism, immunity and neurology, we suggest that the distinctness of laboratory‐kept mouse microbiota, which differs from wild mouse microbiota, needs to be considered in future biomedical research.     

Mouse phenome research: implications of genetic background

Now that sequencing of the mouse genome has been completed, the function of each gene remains to be elucidated through phenotypic analysis. The "genetic background" (in which each gene functions) is defined as the genotype of all other related genes that may interact with the gene of interest, and therefore potentially influences the specific phenotype. To understand the nature and importance of genetic background on phenotypic expression of specific genes, it is necessary to know the origin and evolutionary history of the laboratory mouse genome. Molecular analysis has indicated that the fancy mice of Japan and Europe contributed significantly to the origin of today's laboratory mice. The genetic background of present-day laboratory mice varies by mouse strain, but is mainly derived from the European domesticus subspecies group and to a lesser degree from Asian mice, probably Japanese fancy mice, which belong to the musculus subspecies group. Inbred laboratory mouse strains are genetically uniform due to extensive inbreeding, and they have greatly contributed to the genetic analysis of many Mendelian traits. Meanwhile, for a variety of practical reasons, many transgenic and targeted mutant mice have been created in mice of mixed genetic backgrounds to elucidate the function of the genes, although efforts have been made to create inbred transgenic mice and targeted mutant mice with coisogenic embryonic stem cell lines. Inbred mouse strains have provided uniform genetic background for accurate evaluation of specific genes phenotypes, thus eliminating the phenotypic variations caused by mixed genetic backgrounds. However, the process of inbreeding and selection of various inbred strain characteristics has resulted in inadvertent selection of other undesirable genetic characteristics and mutations that may influence the genotype and preclude effective phenotypic analysis. Because many of the common inbred mouse stains have been established from relatively small gene pools, common inbred strains have limitations in their genetic polymorphisms and phenotypic variations. Wild-derived mouse strains can complement deficiencies of common inbred mouse strains, providing novel allelic variants and phenotypes. Although wild-derived strains are not as tame as the common laboratory strains, their genetic characteristics are attractive for the future study of gene function.     

Physical performance and soleus muscle fiber composition in wild-derived and laboratory inbred mouse strains.

We compared four inbred mouse strains in their physical performance, measured as a maximal treadmill running time, characteristics of soleus muscle, anatomic character, and growth. The strains used were Mus musculus domesticus [C57BL/6 (B6) and BALB/c], Mus musculus molossinus (MSM/Ms), and Mus spretus. Maximal running time was significantly different among these four mouse strains. Running time until exhaustion was highest in MSM/Ms and lowest in M. spretus. Maximal times for the laboratory mouse strains were nearly identical. Soleus muscle fiber type and cross-sectional area also differed significantly among the species. In particular, M. spretus was significantly different from the other inbred mouse strains. Growth in the wild-derived inbred mice appeared to be complete earlier than in the laboratory mice, and the body size of the wild strains was about half that of the laboratory strains. From these results, we propose that wild-derived inbred mouse strains are useful models for enhancing phenotypic variation in physical performance and adaptability.     

Large-scale genomic correlations in Arabidopsis thaliana relate to chromosomal structure

Background

The genome of classical laboratory strains of mice is an artificial mosaic of genomes originated from several mouse subspecies with predominant representation (>90%) of the Mus m. domesticus component. Mice of another subspecies, East European/Asian Mus m. musculus, can interbreed with the classical laboratory strains to generate hybrids with unprecedented phenotypic and genotypic variations. To study these variations in depth we prepared the first genomic large insert BAC library from an inbred strain derived purely from the Mus m. musculus-subspecies. The library will be used to seek and characterize genomic sequences controlling specific monogenic and polygenic complex traits, including modifiers of dominant and recessive mutations.

Results

A representative mouse genomic BAC library was derived from a female mouse of the PWD/Ph inbred strain of Mus m. musculus subspecies. The library consists of 144 768 primary clones from which 97% contain an insert of 120 kb average size. The library represents an equivalent of 6.7 × mouse haploid genome, as estimated from the total number of clones carrying genomic DNA inserts and from the average insert size. The clones were arrayed in duplicates onto eight high-density membranes that were screened with seven single-copy gene probes. The individual probes identified four to eleven positive clones, corresponding to 6.9-fold coverage of the mouse genome. Eighty-seven BAC-ends of PWD/Ph clones were sequenced, edited, and aligned with mouse C57BL/6J (B6) genome. Seventy-three BAC-ends displayed unique hits on B6 genome and their alignment revealed 0.92 single nucleotide polymorphisms (SNPs) per 100 bp. Insertions and deletions represented 0.3% of the BAC end sequences.

Conclusion

Analysis of the novel genomic library for the PWD/Ph inbred strain demonstrated coverage of almost seven mouse genome equivalents and a capability to recover clones for specific regions of PWD/Ph genome. The single nucleotide polymorphism between the strains PWD/Ph and C57BL/6J was 0.92/100 bp, a value significantly higher than between classical laboratory strains. The library will serve as a resource for dissecting the phenotypic and genotypic variations between mice of the Mus m. musculus subspecies and classical laboratory mouse strains.     

Multiple trait measurements in 43 inbred mouse strains capture the phenotypic diversity characteristic of human populations.

The breadth of genetic and phenotypic variation among inbred strains is often underappreciated because assessments include only a limited number of strains. Evaluation of a larger collection of inbred strains provides not only a greater understanding of this variation but collectively mimics much of the variation observed in human populations. We used a high-throughput phenotyping protocol to measure females and males of 43 inbred strains for body composition (weight, fat, lean tissue mass, and bone mineral density), plasma triglycerides, high-density lipoprotein and total cholesterol, glucose, insulin, and leptin levels while mice consumed a high-fat, high-cholesterol diet. Mice were fed a chow diet until they were 6-8 wk old and then fed the high-fat diet for an additional 18 wk. As expected, broad phenotypic diversity was observed among these strains. Significant variation between the sexes was also observed for most traits measured. Additionally, the response to the high-fat diet differed considerably among many strains. By the testing of such a large set of inbred strains for many traits, multiple phenotypes can be considered simultaneously and thereby aid in the selection of certain inbred strains as models for complex human diseases. These data are publicly available in the web-accessible Mouse Phenome Database (http://www.jax.org/phenome), an effort established to promote systematic characterization of biochemical and behavioral phenotypes of commonly used and genetically diverse inbred mouse strains. Data generated by this effort builds on the value of inbred mouse strains as a powerful tool for biomedical research.     

Wild mice: an ever-increasing contribution to a popular mammalian model

Classical laboratory inbred strains of mice have been extremely helpful for research in immunology and oncology, and more generally, for the analysis of complex traits. Unfortunately, because they all derive from a relatively small pool of ancestors, their genetic polymorphism is rather limited. However, recently strains belonging to different species of Mus have been established from wild progenitors. These are an interesting addition to the arsenal of mouse geneticists, because they can be crossed with classical laboratory strains to produce viable and fertile offspring with a large number of polymorphisms of natural origin. These strains are helpful for making genome annotations because they permit highly refined genotype-phenotype correlations. They also allow the interpretation of molecular variation within a clear evolutionary framework. In this article, we provide examples with the aim of promoting the use of these new strains.     

Possible Derivation of the Laboratory Mouse Genome from Multiple Wild MUS Species

Laboratory strains of mice are thought to be derived from wild populations of Mus domesticus. Many instances of non-domesticus genetic information fixed in these strains have been described, however, and the amount of strain-to-strain genetic variation exceeds that found in wild domesticus populations. In order to estimate the extent of the non-domesticus contribution to laboratory mouse genomes, and to determine whether it could account for observed variation, we have used computer simulations to investigate the properties of genetically marked chromosomal segments and the distribution of residual allogenicity at various times during inbreeding. A locus or chromosomal segment is allogenic if it is unfixed within a lineage at a given time. The odds of fixation of a foreign chromosome segment are predicted to be an exponentially decreasing function of its length. The median segment length is predicted to be 17 centimorgans. Available data for markers of chromosomes 1, 9 and 12 in recombinant inbred strain sets conform to these predictions. Together, the results suggest that introgression of non-domesticus chromosomes and segregation of residual allogenicity are sufficient to account for the genetic diversity observed among inbred mouse strains and substrains.     

Analysis of wild-derived mice for Fv-1 and Fv-2 murine leukemia virus restriction loci: a novel wild mouse Fv-1 allele responsible for lack of host range restriction.

Wild-derived mice originally obtained from Asia, Africa, North America, and Europe were typed for in vitro sensitivity to ecotropic murine leukemia viruses and for susceptibility to Friend virus-induced disease. Cell cultures established from some wild mouse populations were generally less sensitive to exogenous virus than were cell cultures from laboratory mice. Wild mice also differed from inbred strains in their in vitro sensitivity to the host range subgroups defined by restriction at the Fv-1 locus. None of the wild mice showed the Fv-1n or Fv-1b restriction patterns characteristic of most inbred strains, several mice resembled the few inbred strains carrying Fv-1nr, and most differed from laboratory mice in that they did not restrict either N- or B-tropic murine leukemia viruses. Analysis of genetic crosses of Mus spretus and Mus musculus praetextus demonstrated that the nonrestrictive phenotype is controlled by a novel allele at the Fv-1 locus, designated Fv-10. The wild mice were also tested for sensitivity to Friend virus complex-induced erythroblastosis to type for Fv-2. Only M. spretus was resistant to virus-induced splenomegaly and did not restrict replication of Friend virus helper murine leukemia virus. Genetic studies confirmed that this mouse carries the resistance allele at Fv-2.     

Heritability and coefficient of genetic variation analyses of phenotypic traits provide strong basis for high-resolution QTL mapping in the Collaborative Cross mouse genetic reference population

Most biological traits of human importance are complex in nature; their manifestation controlled by the cumulative effect of many genetic factors interacting with one another and with the individual’s life history. Because of this, mouse genetic reference populations (GRPs) consisting of collections of inbred lines or recombinant inbred lines (RIL) derived from crosses between inbred lines are of particular value in analysis of complex traits, since massive amounts of data can be accumulated on the individual lines. However, existing mouse GRPs are derived from inbred lines that share a common history, resulting in limited genetic diversity, and reduced mapping precision due to long-range gametic disequilibrium. To overcome these limitations, the Collaborative Cross (CC) a genetically highly diverse collection of mouse RIL was established. The CC, now in advanced stages of development, will eventually consist of about 500 RIL derived from reciprocal crosses of eight divergent founder strains of mice, including three wild subspecies. Previous studies have shown that the CC indeed contains enormous diversity at the DNA level, that founder haplotypes are inherited in expected frequency, and that long-range gametic disequilibrium is not present. We here present data, primarily from our own laboratory, documenting extensive genetic variation among CC lines as expressed in broad-sense heritability (H²) and by the well-known “coefficient of genetic variation,” demonstrating the ability of the CC resource to provide unprecedented mapping precision leading to identification of strong candidate genes.     

Polymorphism and conservation of the genes encoding Qa1 molecules

To evaluate the polymorphism and conservation of the major histocompatibility complex class Ib molecule Qa1 in wild mouse populations, we determined the nucleotide sequence of exons 1–3 of Qa1 of eight mouse haplotypes derived from wild mice, including Mus musculus domesticus, M. m. castaneus, M. m. bactrianus, and M. spretus, as well as two t haplotypes. Our data identify eight new alleles of Qa1. Taken together with previously published data on Qa1 among the common laboratory inbred strains, and in agreement with cytotoxic T-lymphocyte, serological, and biochemical data, these results further confirm the existence of two families of Qa1 molecules, Qa1^a-like and Qa1^b-like, and illuminate the extreme conservation of the peptide-binding region of these molecules, even across species.The wild mouse Qa1 nucleotide sequences are available from GenBank at accession numbers AF100695–703     

A protocol for high-throughput phenotyping, suitable for quantitative trait analysis in mice

Whole-genome genetic association studies in outbred mouse populations represent a novel approach to identifying the molecular basis of naturally occurring genetic variants, the major source of quantitative variation between inbred strains of mice. Measuring multiple phenotypes in parallel on each mouse would make the approach cost effective, but protocols for phenotyping on a large enough scale have not been developed. In this article we describe the development and deployment of a protocol to collect measures on three models of human disease (anxiety, type II diabetes, and asthma) as well as measures of mouse blood biochemistry, immunology, and hematology. We report that the protocol delivers highly significant differences among the eight inbred strains (A/J, AKR/J, BALBc/J, CBA/J, C3H/HeJ, C57BL/6 J, DBA/2 J, and LP/J), the progenitors of a genetically heterogeneous stock (HS) of mice. We report the successful collection of multiple phenotypes from 2000 outbred HS animals. The phenotypes measured in the protocol form the basis of a large-scale investigation into the genetic basis of complex traits in mice designed to examine interactions between genes and between genes and environment, as well as the main effects of genetic variants on phenotypes.     

Distribution of the molossinus allele of Sry, the testis-determining gene, in wild mice

When the Y chromosome of the laboratory inbred mouse strain C57BL/6 (B6) is replaced by the Y of certain strains of Mus musculus domesticus, testis determination fails and all XY fetuses develop either as hermaphrodites or XY females (XY sex reversal). This suggests the presence of at least two alleles of Sry, the male-determining gene on the Y:M. m. domesticus and B6. The B6 Y chromosome is derived from the Japanese house mouse, M. m. molossinus and therefore carries a molossinus Sry allele. As a first step to determine how the molossinus Sry allele evolved, its distribution pattern was determined in wild mice. The cumulative data of 96 M. musculus samples obtained from 58 geographical locations in Europe, North Africa, and Asia show the molossinus Sry allele is restricted to Japan and the neighboring Asian mainland and confirm that Japanese M. m. molossinus mice were derived in part from a race of M. m. musculus from Korea or Manchuria. Sry polymorphisms, as illustrated by the molossinus Sry allele, can serve as molecular markers for studies on the evolution of wild M. musculus populations and can help determine the role sex determination plays in speciation.      

Use of a dense single nucleotide polymorphism map for in silico mapping in the mouse

Rapid expansion of available data, both phenotypic and genotypic, for multiple strains of mice has enabled the development of new methods to interrogate the mouse genome for functional genetic perturbations. In silico mapping provides an expedient way to associate the natural diversity of phenotypic traits with ancestrally inherited polymorphisms for the purpose of dissecting genetic traits. In mouse, the current single nucleotide polymorphism (SNP) data have lacked the density across the genome and coverage of enough strains to properly achieve this goal. To remedy this, 470,407 allele calls were produced for 10,990 evenly spaced SNP loci across 48 inbred mouse strains. Use of the SNP set with statistical models that considered unique patterns within blocks of three SNPs as an inferred haplotype could successfully map known single gene traits and a cloned quantitative trait gene. Application of this method to high-density lipoprotein and gallstone phenotypes reproduced previously characterized quantitative trait loci (QTL). The inferred haplotype data also facilitates the refinement of QTL regions such that candidate genes can be more easily identified and characterized as shown for adenylate cyclase 7.     

Genetic Architecture of Skewed X Inactivation in the Laboratory Mouse

X chromosome inactivation (XCI) is the mammalian mechanism of dosage compensation that balances X-linked gene expression between the sexes. Early during female development, each cell of the embryo proper independently inactivates one of its two parental X-chromosomes. In mice, the choice of which X chromosome is inactivated is affected by the genotype of a cis-acting locus, the X-chromosome controlling element (Xce). Xce has been localized to a 1.9 Mb interval within the X-inactivation center (Xic), yet its molecular identity and mechanism of action remain unknown. We combined genotype and sequence data for mouse stocks with detailed phenotyping of ten inbred strains and with the development of a statistical model that incorporates phenotyping data from multiple sources to disentangle sources of XCI phenotypic variance in natural female populations on X inactivation. We have reduced the Xce candidate 10-fold to a 176 kb region located approximately 500 kb proximal to Xist. We propose that structural variation in this interval explains the presence of multiple functional Xce alleles in the genus Mus. We have identified a new allele, Xce^e present in Mus musculus and a possible sixth functional allele in Mus spicilegus. We have also confirmed a parent-of-origin effect on X inactivation choice and provide evidence that maternal inheritance magnifies the skewing associated with strong Xce alleles. Based on the phylogenetic analysis of 155 laboratory strains and wild mice we conclude that Xce^a is either a derived allele that arose concurrently with the domestication of fancy mice but prior the derivation of most classical inbred strains or a rare allele in the wild. Furthermore, we have found that despite the presence of multiple haplotypes in the wild Mus musculus domesticus has only one functional Xce allele, Xce^b. Lastly, we conclude that each mouse taxa examined has a different functional Xce allele.     

Diverse wild mouse origins of xenotropic, mink cell focus-forming, and two types of ecotropic proviral genes

We analyzed wild mouse DNAs for the number and type of proviral genes related to the env sequences of various murine leukemia viruses (MuLVs). Only Mus species closely related to laboratory mice carried these retroviral sequences, and the different subclasses of viral env genes tended to be restricted to specific taxonomic groups. Only Mus musculus molossinus carried proviral genes which cross-reacted with the inbred mouse ecotropic MuLV env gene. The ecotropic viral env sequence associated with the Fv-4 resistance gene was found in the Asian mice M. musculus molossinus and Mus musculus castaneus and in California mice from Lake Casitas (LC). Both M. musculus castaneus and LC mice carried many additional Fv-4 env-related proviruses, two of which are common to both mouse populations, which suggests that these mice share a recent common ancestry. Xenotropic and mink cell focus-forming (MCF) virus env sequences were more widely dispersed in wild mice than the ecotropic viral env genes, which suggests that nonecotropic MuLVs were integrated into the Mus germ line at an earlier date. Xenotropic MuLVs represented the major component of MuLV env-reactive genes in Asian and eastern European mice classified as M. musculus molossinus, M. musculus castaneus, and Mus musculus musculus, whereas Mus musculus domesticus from western Europe, the Mediterranean, and North America contained almost exclusively MCF virus env copies. M. musculus musculus mice from central Europe trapped near the M. musculus domesticus/M. musculus musculus hybrid zone carried multiple copies of both types of env genes. LC mice also carried both xenotropic and MCF viral env genes, which is consistent with the above conclusion that they represent natural hybrids of M. musculus domesticus and M. musculus castaneus.     

Sperm Morphology in Two House Mouse Subspecies: Do Wild-Derived Strains and Wild Mice Tell the Same Story?

Being subject to intense post-copulatory selection, sperm size is a principal determining component of male fitness. Although previous studies have presented comparative sperm size data at higher taxonomic levels, information on the evolution of sperm size within species is generally lacking. Here, we studied two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus, which undergo incipient speciation. We measured four sperm dimensions from cauda epididymis smears of 28 wild-caught mice of both subspecies. As inbred mouse strains are frequently used as proxies for exploring evolutionary processes, we further studied four wild-derived inbred strains from each subspecies. The subspecies differed significantly in terms of sperm head length and midpiece length, and these differences were consistent for wild mice and wild-derived strains pooled over genomes. When the inbred strains were analyzed individually, however, their strain-specific values were in some cases significantly shifted from subspecies-specific values derived from wild mice. We conclude that: (1) the size of sperm components differ in the two house mouse subspecies studied, and that (2) wild-derived strains reflect this natural polymorphism, serving as a potential tool to identify the genetic variation driving these evolutionary processes. Nevertheless, we suggest that more strains should be used in future experiments to account for natural variation and to avoid confounding results due to reduced variability and/or founder effect in the individual strains.