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Putting the brake on drive: meiotic drive of t haplotypes in natural populations of mice
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Putting the brake on drive: meiotic drive of t haplotypes in natural populations of mice
M ost genetic analyses assume that segregation is mendelian, that is, the two alleles of a heterozygous individual will be equally represented in the gametes. Indeed, unbiased segregation is central to our understanding of the evolutionary process, where natural selection is typically a much weaker force in fixing mutations than most forms of non-mendelian transmission. Yet a growing number of genetic elements have been described, particularly on the sex chromosomes, that violate Mendel’s law, and actively bias segregation in their favour1–3. These are collectively referred to as meiotic drive systems, although in several cases the advantage one allele gains over another might occur post-meiotically. Because of their considerable segregation advantage, these elements are expected to be driven rapidly to fixation and, through hitchhiking, could carry adjacent genetic regions to fixation, irrespective of fitness. Studies of segregation ratios in interspecific hybrids of Drosophila species, however, have failed to find evidence that meiotic drive elements are fixed between species4, and indeed, all drive systems have been identified because they remain unfixed in the populations in which they occur. Thus, considerable attention has focused on what prevents them from reaching fixation. The t haplotypes of the house mouse are one of the best studied meiotic drive systems, where attempts to determine the forces maintaining t haplotypes at low frequencies have been ongoing for 40 years. This review focuses on recent research that has begun to shed some light on this question.
t haplotypes: the low-frequency paradox t haplotypes were first recognized in 1927 (Ref. 5) and are now known to occur worldwide in all subspecies of the house mouse, Mus musculus. They are genetically complex, comprising a 20 cM (30–40 Mbp) region of the proximal third of chromosome 17 (Ref. 6; Fig. 1). Multiple independent loci, the T complex distorters (Tcds) and a T complex responder (Tcr), interact to cause the drive phenotype (Fig. 1). These are usually inherited as a single unit, owing to the presence of four nonoverlapping inversions that suppress recombination across the region in +/t heterozygous individuals. High levels of segregation distortion (known as transmission ratio distortion, TRD, and measured as the percent departure from 50:50 segregation), occur in +/t heterozygous male mice only6–8. Although +/t males actually produce both gamete types in equal ratios, the sperm carrying the wild type (+) chromosomes become functionally inactivated, probably due to motility defects9, such that 90% or more of the offspring of that male end up inheriting the t chromosomes. A
KRISTIN G. ARDLIE ([email protected]) Mouse t haplotypes are a ‘selfish’ form of chromosome 17 that show non-mendelian transmission from heterozygous +/t males. The considerable transmission bias in favour of t haplotypes should result in very high frequencies of these chromosomes in natural populations, but they seldom occur at the high frequencies expected. Recent research on this and other meiotic drive systems has shown how a variety of mechanisms have evolved to suppress drive, and to re-establish mendelian segregation.
candidate gene for Tcr, Tcp10b, was isolated and cloned (Fig. 1). Targeted mutagenesis of Tcp10b in t haplotypes failed to eliminate TRD, however10, and the mechanism by which TRD occurs remains mysterious9,10. An obvious consequence of the high TRD of t haplotypes is that they should become fixed in natural
Wild type Tcp10a 48 119
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* H2 **
* **
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in(17)1
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FIGURE 1. t-Haplotype and wild-type forms of mouse chromosome 17. Shaded boxes represent the four t-associated inversions, in(17)1 through in(17)4. The second inversion, in(17)2, is believed to have arisen on the wild-type lineage, where it breaks up the Tcp10 gene family, and not within the t-haplotype lineage11,12. Several distorter loci, Tcd1 through Tcd3 (and possibly Tcd4 and Tcd5), interact with the responder locus, Tcr, to cause the high transmission ratio distortion of the t haplotype. DNA markers used to identify t haplotypes are shown in their relative positions on the wild-type chromosome. The markers D17Leh48, D17Leh119, D17Leh122, D17Leh54 and D17Leh89 are represented by 48, 119, 122, 54 and 89 respectively. The locations of the t-associated lethal mutations are indicated by stars. Phylogenetic analyses of the DNA sequences of two t-associated loci [an intron of the Tcp1 gene in in(17)2 (Ref. 13), and the Hba-ps4 pseudogene in the fourth inversion, in(17)4 (Ref. 14)], suggest that the in(17)2 polymorphism is much older (~3 million years) than the in(17)4 polymorphism (~1.5 million years), and was probably the first inversion to have arisen. Despite this ancient age, sequence comparisons among independent t haplotypes show extremely reduced levels of nucleotide polymorphism in contrast to the high levels obtained in comparisons of independent wild-type chromosomes. This suggests that all contemporary t haplotypes might share a more-recent common ancestor, which must have spread rapidly, perhaps because of drive, into all subspecies of Mus musculus in which t haplotypes are now found6,12–15. TIG MAY 1998 VOL. 14 NO. 5
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0168-9525/98/$19.00 PII: S0168-9525(98)01455-3
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+/t frequency
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Migration rate FIGURE 2. Theoretical relationship between the frequency of a lethal t haplotype and several factors that can affect t frequency, including transmission ratio distortion (TRD), the fitness of +/t heterozygotes, and the migration rate between family groups (demes) (modified from Ref. 23). The results shown here are for a subdivided population with a deme size of six (one male and five females). Migration rate is the average number of migrants per deme per generation. +/t Frequencies typically reported from natural populations range from 0.106 to ~0.25 (Refs 20–22). For a TRD of 0.95, with high migration rates, which approximates a freely breeding population, t haplotypes reach a high frequency of ~77%, as demonstrated by Bruck19. Reduced migration rates can result in a lower t haplotype frequency, near empirically observed frequencies, although this is relatively unstable23: small fluctuations in the migration rate around 0.1 can result in either the extinction of the t haplotype, or in very high frequencies. Even a small reduction in TRD, to 0.7, can have a large impact on reducing t haplotype frequency. In fact, transmission ratios between 0.5–0.7 typically result in the extinction of the t haplotype (see Fig. 4 in Ref. 24). A reduction in the fitness of +/t heterozygotes relative to +/+ mice can also produce lower frequencies of t haplotypes, but generally only if the fitness of +/t mice is much less than that of +/+ mice. The relative fitness of +/t animals shown here is 45% less than that of +/+ animals.
populations, but they remain polymorphic because of two counterbalancing forces. First, all individual males that carry two t haplotypes (t/t) are completely sterile. It seems likely that the same distorter (Tcd) genes involved in drive also act in a recessive manner to produce sterility9,16. Thus, the mechanism of drive in t haplotypes might always have precluded their ultimate fixation (Fig. 1). Additionally, most t haplotypes carry recessive lethal mutations17. To date, 16 independent complementing lethal loci have been identified, predominantly from t haplotypes isolated in Europe18. Mice that are homozygous for the same lethal t haplotype die early in gestation, while individuals carrying two t haplotypes with different, complementing lethals (e.g. tx/ty) are viable, but male-sterile. Typically, all t haplotypes that do not carry lethal mutations still result in embryonic lethality in a small proportion of homozygous offspring, and are hence referred to as ‘semilethal’ t haplotypes. This antagonism between meiotic drive increasing the frequency of t haplotypes and natural selection acting against them does not, however, account for the observed frequencies of t haplotypes in natural populations19. In the first mathematical model of this system, Bruck showed that even for a lethal t haplotype, a TRD of 95% would result in a high equilibrium frequency of t haplotype ‘alleles’ of 0.385. In other words, roughly 77% of
wild mice should be heterozygous for a t haplotype (Fig. 2). Empirical studies, in contrast, indicate that considerably fewer mice, perhaps 10–25% (Refs 20–22), actually carry t haplotypes. Moreover, this finding of much lower than expected frequencies also holds true for other polymorphic drive systems, such as segregation distorter (SD) in Drosophila melanogaster, and sex ratio (SR) in D. pseudoobscura1,25, suggesting that there could be strong selection acting against drive systems in general. Several forces have been proposed that might maintain a low frequency equilibrium of t haplotypes: a reduction in TRD in wild populations; lowered fitness of +/t heterozygotes relative to +/+ mice; mate choice; selection in deme-structured populations and/or systematic inbreeding. But historically, the study of t haplotypes in natural populations has been stymied by the lack of observable phenotypic markers for +/t heterozygote and t/t homozygous mice. Traditionally, they could be identified only in conjunction with a null mutation at the Brachyury (T ) locus (Fig. 1), through a developmental effect on tail length: T/+ animals have short tails, while T/t animals are tailless. Thus, empirical studies, which entailed laborious genotyping of mice by breeding, lagged far behind theoretical research until DNA markers became available26. These allowed larger and more accurate surveys of t haplotypes to be conducted, and the contributions of each of the above forces to be investigated in natural populations.
Do modifiers of meiotic drive reduce transmission bias? Theoretical studies show that strong selection to reduce the transmission bias of drive chromosomes will favour the spread of genes that suppress meiotic drive27. This kind of genetic suppression of drive has been described both for SD in D. melanogaster 28, and more recently for an X-linked meiotic drive system in D. simulans29. Driving X chromosomes were found in D. simulans populations worldwide, although they never resulted in an excess of females within populations because of the systematic co-occurrence of resistance factors, including autosomal suppressors and insensitive Y chromosomes, which prevent the expression of drive29. Only when females were crossed with males from different geographic origins did the resulting male offspring show very high levels of sex-ratio drive. Similarly, all theoretical models of t haplotypes show that absolute levels of TRD strongly affect the expected frequency of t haplotypes. Even moderate reductions in TRD can result in considerably reduced t frequencies (Fig. 2). Nevertheless, evidence for modifiers of drive is mixed. Two studies have found reduced transmission of t haplotypes, owing to a general effect of genetic background and to an effect of the homologous chromosome, but both studies were in mice that were maintained in the laboratory for over 20 years30,31. By contrast, wild mice bred in the laboratory typically show very high TRDs7. Moreover, a study of TRD in litters from pregnant wild female mice and in matings among wild mice derived from different geographic origins, found no evidence for reduced TRDs, suggesting that modifiers are not prevalent in natural populations8. The transmission ratio of t haplotypes might be reduced through other non-genetic means, however. Two recent studies of field-inseminated litters have found
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REVIEWS that, while litters sired by +/t males typically have very high TRDs, a small number have low TRDs of 20% or less (Ref. 8 and A. Baker, pers. commun.). In at least one study these litters were found to have resulted from the multiple mating of +/+ females with both +/t and +/+ males8. Thus, TRD could be much lower in populations with a high frequency of multiple matings, although the significance of these findings is unclear because we do not know how much multiple mating occurs in the wild. Why are modifiers of drive not prevalent in this system, when they have evolved to counteract drive in many other systems? One possibility is that suppressors are at a low frequency in, or absent from, the populations examined so far, which are predominantly from the US, because of stochastic events related to the recent spread of the species. Thus, modifiers might be more common in older European populations of Mus musculus, as has been found for older African populations of D. simulans29. Another possibility is that selection might be acting on other components of fitness instead. This is the case in at least one other drive system, the Xlinked SR system of D. pseudoobscura, where genetic modifiers of segregation distortion are also absent25.
different from their own. However, the expression of these preferences can be altered by rearing pups with adults whose genotypes differ from their own, and the dominance status of males appears to have a more important effect on mating decisions than does t haplotype genotype34. Additionally, olfactory preferences based on t haplotypes might be confounded by the presence of the major histocompatibility complex, or H2, within the 4th inversion of the t haplotype (Fig. 1). Several independent studies on inbred mice have demonstrated that mice can distinguish among H2 haplotypes using odour cues in urine. Further, mice have been show to display mating preferences for individuals whose H2 haplotype differs from their own when genetic background is controlled for35. t Haplotypes carry a restricted range of H2 haplotypes, and many of these are not represented on wild type chromosomes7. The apparent t-based odour preferences could, therefore, be a bi-product of H2-based odour preferences. What impact this has in natural populations is unclear, but because it is assumed that such H2 selection operates on a negative frequency-dependent basis, it is possible that t haplotypes might be at an advantage when rare in a population.
Selection against +/t heterozygotes
Is genetic drift common in mice?
Lower frequencies of t haplotypes in natural populations can also result if the fitness of +/t mice is considerably less than that of +/+ mice (Fig. 2). While both advantages and disadvantages of +/t mice have been reported for various components of fitness, the most consistent evidence emerging, of selection acting against +/t heterozygotes, comes from studies of litter size: 26% fewer pups were produced over a fixed period by +/t females relative to +/+ females in one study32, and an 18% reduction in the size of the first litter sired by +/t males relative to +/+ males was also reported33. This is further supported by a large study of wild mice where mean litter size was found to be 20% less for litters produced by either +/t males or +/t females relative to litters from two wild type (+/+) parents (Ref. 8 and K. Ardlie, and L. Silver, unpublished). This effect was also found in litters from the congenic inbred strains 129 and 129-tw5, where litters produced by either a 129-tw5 male or female parent were significantly smaller in size than those from two 129 (+/+) parents. Because a reduction in litter size is reported from both sexes, across several studies, it is unlikely to be due to the t haplotype effect on sperm function in +/t males. Rather, it probably represents a reduced viability of +/t embryos in utero. Reduced fitness of +/t heterozygotes cannot be the only force counterbalancing drive, because the ~20% reported here is considerably less than that required by most models (Fig. 2). However, it is likely to be one contributing factor among several that might also include a reduction in TDR through multiple mating.
The role of population subdivision, in the genetics of mouse populations in general and in the t haplotype polymorphism specifically, has long been controversial. Lewontin and Dunn36 were the first to demonstrate that if mouse populations are not panmictic and randomly mating, as assumed by Bruck19, but are subdivided into small family groups, or ‘demes’, with limited betweendeme migration, then genetic drift will result in fixation of the wild-type chromosome in many demes, thus lowering the overall population frequency of a lethal t haplotype. Numerous simulation models since have confirmed this basic finding, and it is clear that drift and/or inbreeding can indeed lower t haplotype frequency for certain values of deme size and migration rate (Fig. 2). The question has always been, however, whether these values are realistic; that is, whether mouse populations are indeed as rigidly subdivided as models indicate they must be to result in the low observed t frequency. While most populations of mice are commensal and associated with humans, they can be found in a wide range of environments, and population subdivision seems to occur at several levels. Significant genetic subdivision has been found for both M. m. domesticus and M. m. musculus populations principally at the level of different farms or villages, where gene flow is limited and occurs predominantly among neighbouring subpopulations37. Estimates of the effective population sizes of these subpopulations are on the order of several thousand, suggesting a low but persistent rate of migration between subpopulations when considered over many generations. Estimates of effective population sizes from temporal samples over shorter time scales, however, are several orders of magnitude less, and this could be important for the establishment of chromosomal variants38, and the various lethal mutations associated with t haplotypes18. Further, in at least a few instances, there is good evidence that large commensal populations can also be subdivided into social, territorial breeding units which
Mating preferences There has been a long history of interest in the possibility that mice might be able to distinguish t-bearing individuals using odour cues and so avoid mating with them (reviewed in Ref. 24). Individuals of both sexes have been shown to prefer odours of +/+ to +/t individuals, and to prefer t haplotypes in complementation groups
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FIGURE 3. (a) The distribution of mouse population sizes taken from several trapping studies20–22,39. In at least two of these studies (222 of the total 307 samples), sample sizes are inferred to be good estimates of population sizes22,39. Most commensal populations of mice tend to have relatively few individuals, while large populations are rare. These samples were primarily trapped in the US, however, and the distribution of sample sizes might differ in other countries, if farming practices, food, or the availability of shelter differ. (b) The frequencies of +/t mice derived from three of the studies in Fig. 3a20–22. Mean +/t frequencies (± standard error) are calculated for a reduced number of sample-size categories because of the small numbers of mice tested in many samples. The range in +/t frequency is shown in parentheses. t-Haplotype frequency is generally higher, and more variable, in the small to medium-sized samples, and much lower in the larger populations sampled, where t haplotypes are often absent. t Haplotypes were present in only about 40% of all populations sampled.
show isolation by distance over a scale of metres (Ref. 39; and K. Ardlie and L. Silver, unpublished). The effect of such population structure on t haplotype frequency remains largely unknown. Nevertheless, a broad pattern emerging from recent empirical studies suggests some relationship between t haplotypes and inferred population size and structure (Fig. 3). Trapping studies show that mouse populations vary in size. Most, at least in the USA, appear to be relatively small and might represent single demes only (Figs 3 and 4). They undergo frequent extinctions and re-founding, and t haplotype frequency in these populations is highly variable and
often quite high. By contrast, in the few larger populations examined so far, t haplotype frequency is generally much lower and often remains so, or decreases, over repeated temporal samples22. Population structure has been demonstrated within some large populations (see above) but the role of genetic drift or inbreeding in lowering t frequency remains speculative, as deme sizes are difficult to quantify and could be large if mice choose mates differing from themselves35. Yet even a small reduction in TRD (from multiple matings), combined with a 20% reduction in heterozygote fitness, and moderate levels of population subdivision can be shown to lower t frequencies in simulation studies (D. Durand, pers. commun.). Further empirical work is clearly needed. The notion that t frequency might vary depending on population size and structure (Fig. 3), nevertheless receives some support from a theoretical study, which concluded that a stable low level t haplotype frequency might not exist (Ref. 23 and Fig. 2). Rather, populations might be in one of two stable states, with t haplotypes extinct or at high frequency, or in transition between them, such that any overall frequency of t is low but variable.
Conclusions FIGURE 4. Model of the mouse populations from Fig. 3a22,39. Most populations (shown on the left) are small to medium in size. These are prone to frequent local extinction and recolonization; unfilled circles represent vacant habitat patches. If t haplotypes are present in the founders of these populations they might reach a high frequency rapidly, because of drive, in the few generations the population is present. A few populations, shown on the right, are larger, support higher densities of mice, and persist for many generations. There is some evidence that these populations might be further subdivided into territorial units or ‘demes’.
Unlike many of the other well-described drive systems, the frequency of t haplotypes appears not to be influenced by modifiers of transmission ratio, but rather by a set of complex interacting factors. These include direct effects of the drive chromosome itself on the fitness of +/t individuals, and indirect, or behaviourally mediated, effects of forces, such as multiple mating and population size and structure. Many questions remain regarding this polymorphism, including what are the relative contributions of the factors that have now been identified, and are they equally important in all subspecies of
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REVIEWS Mus musculus in which t haplotypes are found? t Haplotypes continue to provide a model system for ecological genetics. That we now have an appreciation of the extraordinary complexity of how drive is counterbalanced in this system is testimony to the advances made possible by the availability of molecular markers. Hopefully, with more highly resolved tools for investigating the structure of natural populations, t haplotypes will continue to yield insights about the evolutionary process, well into the future.
Acknowledgements I thank M. Polz, A. Berry, S. Alberts, N. Hussain, D. Haig, D. Petrov, S. Pilder, L. Robertson and L. Silver for their comments, ideas and discussion.
References 1 Lyttle, T.W. (1993) Trends Genet. 9, 205–210 2 Hurst, L. and Pomiankowski, A. (1991) Genetics 128, 841–858 3 Jaenike, J. (1996) Am. Nat. 148, 237–254 4 Coyne, J.A. and Orr, H.A. (1993) Evolution 47, 865–687 5 Dobrovolskaia-Zavadskaia, N. and Kobozieff, N. (1927) C. R. Séances Soc. Biol. Ses Fil. 97, 116–119 6 Silver, L.M. (1993) Trends Genet. 9, 250–254 7 Silver, L.M. (1985) Annu. Rev. Genet. 19, 179–208 8 Ardlie, K.G. and Silver, L.M. (1996) Genetics 144, 1787–1797 9 Olds-Clarke, P. (1997) Rev. Reprod. 2, 157–164 10 Ewulonu, U.K. et al. (1996) Genetics 144, 785–792 11 Hammer, M.F., Schimenti, J. and Silver, L.M. (1989) Proc. Natl. Acad. Sci. U. S. A. 86, 3261–3265 12 Schimenti, J., Vold, L., Socolow, D. and Silver, L.M. (1987) J. Mol. Biol. 194, 583–594 13 Morita, T. et al. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 6851–6855 14 Hammer, M.F. and Silver, L.M. (1993) Mol. Biol. Evol. 10, 971–1001 15 Willison, K.R., Dudley, K. and Potter, J. (1986) Cell 44, 727–738
16 Lyon, M.F. (1986) Cell 44, 357–363 17 Bennett, D. (1975) Cell 6, 441–454 18 Klein, J., Sipos, P. and Figueroa, F. (1984) Genet. Res. 44, 39–46 19 Bruck, D. (1957) Proc. Natl. Acad. Sci. U. S. A. 43, 152–158 20 Lenington, S, Franks, P. and Williams, J. (1988) J. Mammal. 69, 489–499 21 Ruvinsky, A. et al. (1991) Genetics 127, 161–168 22 Ardlie, K.G. and Silver, L.M. Evolution (in press) 23 Durand, D. et al. (1997) Genetics 145, 1093–1108 24 Lenington, S. (1991) Adv. Study Behav. 20, 51–86 25 Beckenbach, A.T. (1996) Evolution 50, 787–794 26 Fox, H.S. et al. (1985) Cell 40, 63–69 27 Charlesworth, B. and Hartl, D. (1978) Genetics 89, 171–192 28 Hiraizumi, Y. and Thomas, A.M. (1984) Genetics 95, 693–706 29 Atlan, A., Mercot, H., Landre, C. and Montchamp-Moreau, C. (1995) Evolution 51, 1886–1895 30 Bennett, D., Alton, A.K. and Artzt, K. (1983) Genet. Res. 41, 29–45 31 Gummere, G.R., McCormick, P.J. and Bennett, D. (1986) Genetics 114, 235–245 32 Johnson, P.G. and Brown, G.H. (1969) Am. Nat. 103, 5–21 33 Lenington, S., Coopersmith, C.B. and Erhart, M. (1994) Am. Nat. 143, 766–784 34 Lenington, S., Coopersmith, C. and Williams, J. (1992) Am. Zool. 32, 40–47 35 Potts, W.K. and Wakeland, E.K. (1991) Trends Genet. 9, 408–412 36 Lewontin, R.C. and Dunn, L.C. (1960) Genetics 45, 705–722 37 Dallas, J.F. et al. (1995) Mol. Ecol. 4, 311–320 38 Nachman, M.W. and Searle, J.B. (1995) Trends Ecol. Evol. 10, 397–402 39 Selander, R.K. (1970) Am. Zool. 10, 53–66 K.G. Ardlie is at the Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA.
Included in the June issue of Trends in Genetics: Combinatorial control in ubiquitin-dependent proteolysis by E.E Patton, A.R. Willems and M. Tyers How many homeobox genes does it take to make a pituitary gland? by D.E. Watkins-Chow and S.A. Camper The retinoblastoma gene family: cousins with overlapping interests by G. Mulligan and T. Jacks Limbs are moving: where are they going? by J.W.R. Schwabe, C. Rodriguez-Esteban and J.C. Izpisúa Belmonte Conservation of OTX2 function by A.C. Sharman and M. Brand Wasted by an elongation factor by M. Hafezparast and E. Fisher A simple mechanism for the avoidance of entanglement during chromosome replication by J.E. Hearst, L. Kauffman and W.M. McClain
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Putting the brake on drive: meiotic drive of t haplotypes in natural populations of mice
M ost genetic analyses assume that segregation is mendelian, that is, the two alleles of a heterozygous individual will be equally represented in the gametes. Indeed, unbiased segregation is central to our understanding of the evolutionary process, where natural selection is typically a much weaker force in fixing mutations than most forms of non-mendelian transmission. Yet a growing number of genetic elements have been described, particularly on the sex chromosomes, that violate Mendel’s law, and actively bias segregation in their favour1–3. These are collectively referred to as meiotic drive systems, although in several cases the advantage one allele gains over another might occur post-meiotically. Because of their considerable segregation advantage, these elements are expected to be driven rapidly to fixation and, through hitchhiking, could carry adjacent genetic regions to fixation, irrespective of fitness. Studies of segregation ratios in interspecific hybrids of Drosophila species, however, have failed to find evidence that meiotic drive elements are fixed between species4, and indeed, all drive systems have been identified because they remain unfixed in the populations in which they occur. Thus, considerable attention has focused on what prevents them from reaching fixation. The t haplotypes of the house mouse are one of the best studied meiotic drive systems, where attempts to determine the forces maintaining t haplotypes at low frequencies have been ongoing for 40 years. This review focuses on recent research that has begun to shed some light on this question.
t haplotypes: the low-frequency paradox t haplotypes were first recognized in 1927 (Ref. 5) and are now known to occur worldwide in all subspecies of the house mouse, Mus musculus. They are genetically complex, comprising a 20 cM (30–40 Mbp) region of the proximal third of chromosome 17 (Ref. 6; Fig. 1). Multiple independent loci, the T complex distorters (Tcds) and a T complex responder (Tcr), interact to cause the drive phenotype (Fig. 1). These are usually inherited as a single unit, owing to the presence of four nonoverlapping inversions that suppress recombination across the region in +/t heterozygous individuals. High levels of segregation distortion (known as transmission ratio distortion, TRD, and measured as the percent departure from 50:50 segregation), occur in +/t heterozygous male mice only6–8. Although +/t males actually produce both gamete types in equal ratios, the sperm carrying the wild type (+) chromosomes become functionally inactivated, probably due to motility defects9, such that 90% or more of the offspring of that male end up inheriting the t chromosomes. A
KRISTIN G. ARDLIE ([email protected]) Mouse t haplotypes are a ‘selfish’ form of chromosome 17 that show non-mendelian transmission from heterozygous +/t males. The considerable transmission bias in favour of t haplotypes should result in very high frequencies of these chromosomes in natural populations, but they seldom occur at the high frequencies expected. Recent research on this and other meiotic drive systems has shown how a variety of mechanisms have evolved to suppress drive, and to re-establish mendelian segregation.
candidate gene for Tcr, Tcp10b, was isolated and cloned (Fig. 1). Targeted mutagenesis of Tcp10b in t haplotypes failed to eliminate TRD, however10, and the mechanism by which TRD occurs remains mysterious9,10. An obvious consequence of the high TRD of t haplotypes is that they should become fixed in natural
Wild type Tcp10a 48 119
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* **
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FIGURE 1. t-Haplotype and wild-type forms of mouse chromosome 17. Shaded boxes represent the four t-associated inversions, in(17)1 through in(17)4. The second inversion, in(17)2, is believed to have arisen on the wild-type lineage, where it breaks up the Tcp10 gene family, and not within the t-haplotype lineage11,12. Several distorter loci, Tcd1 through Tcd3 (and possibly Tcd4 and Tcd5), interact with the responder locus, Tcr, to cause the high transmission ratio distortion of the t haplotype. DNA markers used to identify t haplotypes are shown in their relative positions on the wild-type chromosome. The markers D17Leh48, D17Leh119, D17Leh122, D17Leh54 and D17Leh89 are represented by 48, 119, 122, 54 and 89 respectively. The locations of the t-associated lethal mutations are indicated by stars. Phylogenetic analyses of the DNA sequences of two t-associated loci [an intron of the Tcp1 gene in in(17)2 (Ref. 13), and the Hba-ps4 pseudogene in the fourth inversion, in(17)4 (Ref. 14)], suggest that the in(17)2 polymorphism is much older (~3 million years) than the in(17)4 polymorphism (~1.5 million years), and was probably the first inversion to have arisen. Despite this ancient age, sequence comparisons among independent t haplotypes show extremely reduced levels of nucleotide polymorphism in contrast to the high levels obtained in comparisons of independent wild-type chromosomes. This suggests that all contemporary t haplotypes might share a more-recent common ancestor, which must have spread rapidly, perhaps because of drive, into all subspecies of Mus musculus in which t haplotypes are now found6,12–15. TIG MAY 1998 VOL. 14 NO. 5
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Migration rate FIGURE 2. Theoretical relationship between the frequency of a lethal t haplotype and several factors that can affect t frequency, including transmission ratio distortion (TRD), the fitness of +/t heterozygotes, and the migration rate between family groups (demes) (modified from Ref. 23). The results shown here are for a subdivided population with a deme size of six (one male and five females). Migration rate is the average number of migrants per deme per generation. +/t Frequencies typically reported from natural populations range from 0.106 to ~0.25 (Refs 20–22). For a TRD of 0.95, with high migration rates, which approximates a freely breeding population, t haplotypes reach a high frequency of ~77%, as demonstrated by Bruck19. Reduced migration rates can result in a lower t haplotype frequency, near empirically observed frequencies, although this is relatively unstable23: small fluctuations in the migration rate around 0.1 can result in either the extinction of the t haplotype, or in very high frequencies. Even a small reduction in TRD, to 0.7, can have a large impact on reducing t haplotype frequency. In fact, transmission ratios between 0.5–0.7 typically result in the extinction of the t haplotype (see Fig. 4 in Ref. 24). A reduction in the fitness of +/t heterozygotes relative to +/+ mice can also produce lower frequencies of t haplotypes, but generally only if the fitness of +/t mice is much less than that of +/+ mice. The relative fitness of +/t animals shown here is 45% less than that of +/+ animals.
populations, but they remain polymorphic because of two counterbalancing forces. First, all individual males that carry two t haplotypes (t/t) are completely sterile. It seems likely that the same distorter (Tcd) genes involved in drive also act in a recessive manner to produce sterility9,16. Thus, the mechanism of drive in t haplotypes might always have precluded their ultimate fixation (Fig. 1). Additionally, most t haplotypes carry recessive lethal mutations17. To date, 16 independent complementing lethal loci have been identified, predominantly from t haplotypes isolated in Europe18. Mice that are homozygous for the same lethal t haplotype die early in gestation, while individuals carrying two t haplotypes with different, complementing lethals (e.g. tx/ty) are viable, but male-sterile. Typically, all t haplotypes that do not carry lethal mutations still result in embryonic lethality in a small proportion of homozygous offspring, and are hence referred to as ‘semilethal’ t haplotypes. This antagonism between meiotic drive increasing the frequency of t haplotypes and natural selection acting against them does not, however, account for the observed frequencies of t haplotypes in natural populations19. In the first mathematical model of this system, Bruck showed that even for a lethal t haplotype, a TRD of 95% would result in a high equilibrium frequency of t haplotype ‘alleles’ of 0.385. In other words, roughly 77% of
wild mice should be heterozygous for a t haplotype (Fig. 2). Empirical studies, in contrast, indicate that considerably fewer mice, perhaps 10–25% (Refs 20–22), actually carry t haplotypes. Moreover, this finding of much lower than expected frequencies also holds true for other polymorphic drive systems, such as segregation distorter (SD) in Drosophila melanogaster, and sex ratio (SR) in D. pseudoobscura1,25, suggesting that there could be strong selection acting against drive systems in general. Several forces have been proposed that might maintain a low frequency equilibrium of t haplotypes: a reduction in TRD in wild populations; lowered fitness of +/t heterozygotes relative to +/+ mice; mate choice; selection in deme-structured populations and/or systematic inbreeding. But historically, the study of t haplotypes in natural populations has been stymied by the lack of observable phenotypic markers for +/t heterozygote and t/t homozygous mice. Traditionally, they could be identified only in conjunction with a null mutation at the Brachyury (T ) locus (Fig. 1), through a developmental effect on tail length: T/+ animals have short tails, while T/t animals are tailless. Thus, empirical studies, which entailed laborious genotyping of mice by breeding, lagged far behind theoretical research until DNA markers became available26. These allowed larger and more accurate surveys of t haplotypes to be conducted, and the contributions of each of the above forces to be investigated in natural populations.
Do modifiers of meiotic drive reduce transmission bias? Theoretical studies show that strong selection to reduce the transmission bias of drive chromosomes will favour the spread of genes that suppress meiotic drive27. This kind of genetic suppression of drive has been described both for SD in D. melanogaster 28, and more recently for an X-linked meiotic drive system in D. simulans29. Driving X chromosomes were found in D. simulans populations worldwide, although they never resulted in an excess of females within populations because of the systematic co-occurrence of resistance factors, including autosomal suppressors and insensitive Y chromosomes, which prevent the expression of drive29. Only when females were crossed with males from different geographic origins did the resulting male offspring show very high levels of sex-ratio drive. Similarly, all theoretical models of t haplotypes show that absolute levels of TRD strongly affect the expected frequency of t haplotypes. Even moderate reductions in TRD can result in considerably reduced t frequencies (Fig. 2). Nevertheless, evidence for modifiers of drive is mixed. Two studies have found reduced transmission of t haplotypes, owing to a general effect of genetic background and to an effect of the homologous chromosome, but both studies were in mice that were maintained in the laboratory for over 20 years30,31. By contrast, wild mice bred in the laboratory typically show very high TRDs7. Moreover, a study of TRD in litters from pregnant wild female mice and in matings among wild mice derived from different geographic origins, found no evidence for reduced TRDs, suggesting that modifiers are not prevalent in natural populations8. The transmission ratio of t haplotypes might be reduced through other non-genetic means, however. Two recent studies of field-inseminated litters have found
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REVIEWS that, while litters sired by +/t males typically have very high TRDs, a small number have low TRDs of 20% or less (Ref. 8 and A. Baker, pers. commun.). In at least one study these litters were found to have resulted from the multiple mating of +/+ females with both +/t and +/+ males8. Thus, TRD could be much lower in populations with a high frequency of multiple matings, although the significance of these findings is unclear because we do not know how much multiple mating occurs in the wild. Why are modifiers of drive not prevalent in this system, when they have evolved to counteract drive in many other systems? One possibility is that suppressors are at a low frequency in, or absent from, the populations examined so far, which are predominantly from the US, because of stochastic events related to the recent spread of the species. Thus, modifiers might be more common in older European populations of Mus musculus, as has been found for older African populations of D. simulans29. Another possibility is that selection might be acting on other components of fitness instead. This is the case in at least one other drive system, the Xlinked SR system of D. pseudoobscura, where genetic modifiers of segregation distortion are also absent25.
different from their own. However, the expression of these preferences can be altered by rearing pups with adults whose genotypes differ from their own, and the dominance status of males appears to have a more important effect on mating decisions than does t haplotype genotype34. Additionally, olfactory preferences based on t haplotypes might be confounded by the presence of the major histocompatibility complex, or H2, within the 4th inversion of the t haplotype (Fig. 1). Several independent studies on inbred mice have demonstrated that mice can distinguish among H2 haplotypes using odour cues in urine. Further, mice have been show to display mating preferences for individuals whose H2 haplotype differs from their own when genetic background is controlled for35. t Haplotypes carry a restricted range of H2 haplotypes, and many of these are not represented on wild type chromosomes7. The apparent t-based odour preferences could, therefore, be a bi-product of H2-based odour preferences. What impact this has in natural populations is unclear, but because it is assumed that such H2 selection operates on a negative frequency-dependent basis, it is possible that t haplotypes might be at an advantage when rare in a population.
Selection against +/t heterozygotes
Is genetic drift common in mice?
Lower frequencies of t haplotypes in natural populations can also result if the fitness of +/t mice is considerably less than that of +/+ mice (Fig. 2). While both advantages and disadvantages of +/t mice have been reported for various components of fitness, the most consistent evidence emerging, of selection acting against +/t heterozygotes, comes from studies of litter size: 26% fewer pups were produced over a fixed period by +/t females relative to +/+ females in one study32, and an 18% reduction in the size of the first litter sired by +/t males relative to +/+ males was also reported33. This is further supported by a large study of wild mice where mean litter size was found to be 20% less for litters produced by either +/t males or +/t females relative to litters from two wild type (+/+) parents (Ref. 8 and K. Ardlie, and L. Silver, unpublished). This effect was also found in litters from the congenic inbred strains 129 and 129-tw5, where litters produced by either a 129-tw5 male or female parent were significantly smaller in size than those from two 129 (+/+) parents. Because a reduction in litter size is reported from both sexes, across several studies, it is unlikely to be due to the t haplotype effect on sperm function in +/t males. Rather, it probably represents a reduced viability of +/t embryos in utero. Reduced fitness of +/t heterozygotes cannot be the only force counterbalancing drive, because the ~20% reported here is considerably less than that required by most models (Fig. 2). However, it is likely to be one contributing factor among several that might also include a reduction in TDR through multiple mating.
The role of population subdivision, in the genetics of mouse populations in general and in the t haplotype polymorphism specifically, has long been controversial. Lewontin and Dunn36 were the first to demonstrate that if mouse populations are not panmictic and randomly mating, as assumed by Bruck19, but are subdivided into small family groups, or ‘demes’, with limited betweendeme migration, then genetic drift will result in fixation of the wild-type chromosome in many demes, thus lowering the overall population frequency of a lethal t haplotype. Numerous simulation models since have confirmed this basic finding, and it is clear that drift and/or inbreeding can indeed lower t haplotype frequency for certain values of deme size and migration rate (Fig. 2). The question has always been, however, whether these values are realistic; that is, whether mouse populations are indeed as rigidly subdivided as models indicate they must be to result in the low observed t frequency. While most populations of mice are commensal and associated with humans, they can be found in a wide range of environments, and population subdivision seems to occur at several levels. Significant genetic subdivision has been found for both M. m. domesticus and M. m. musculus populations principally at the level of different farms or villages, where gene flow is limited and occurs predominantly among neighbouring subpopulations37. Estimates of the effective population sizes of these subpopulations are on the order of several thousand, suggesting a low but persistent rate of migration between subpopulations when considered over many generations. Estimates of effective population sizes from temporal samples over shorter time scales, however, are several orders of magnitude less, and this could be important for the establishment of chromosomal variants38, and the various lethal mutations associated with t haplotypes18. Further, in at least a few instances, there is good evidence that large commensal populations can also be subdivided into social, territorial breeding units which
Mating preferences There has been a long history of interest in the possibility that mice might be able to distinguish t-bearing individuals using odour cues and so avoid mating with them (reviewed in Ref. 24). Individuals of both sexes have been shown to prefer odours of +/+ to +/t individuals, and to prefer t haplotypes in complementation groups
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(a)
(b) 0.5
0.25
(0–1) (0–0.6)
0.2
Mean +/t frequency
Frequency of population
0.4
(0–0.62)
0.3
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(0–0.23) (0–0.11)
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0
0 + 200
1–1 0 11– 20 21– 30 31– 40 41– 50 51– 60 61– 70 71– 80 81– 90 91– 100 101 –12 121 0 –14 141 0 –16 161 0 –18 181 0 –20 0
(0–0) 1–20
21–40
41–60
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Sample size
Sample size
FIGURE 3. (a) The distribution of mouse population sizes taken from several trapping studies20–22,39. In at least two of these studies (222 of the total 307 samples), sample sizes are inferred to be good estimates of population sizes22,39. Most commensal populations of mice tend to have relatively few individuals, while large populations are rare. These samples were primarily trapped in the US, however, and the distribution of sample sizes might differ in other countries, if farming practices, food, or the availability of shelter differ. (b) The frequencies of +/t mice derived from three of the studies in Fig. 3a20–22. Mean +/t frequencies (± standard error) are calculated for a reduced number of sample-size categories because of the small numbers of mice tested in many samples. The range in +/t frequency is shown in parentheses. t-Haplotype frequency is generally higher, and more variable, in the small to medium-sized samples, and much lower in the larger populations sampled, where t haplotypes are often absent. t Haplotypes were present in only about 40% of all populations sampled.
show isolation by distance over a scale of metres (Ref. 39; and K. Ardlie and L. Silver, unpublished). The effect of such population structure on t haplotype frequency remains largely unknown. Nevertheless, a broad pattern emerging from recent empirical studies suggests some relationship between t haplotypes and inferred population size and structure (Fig. 3). Trapping studies show that mouse populations vary in size. Most, at least in the USA, appear to be relatively small and might represent single demes only (Figs 3 and 4). They undergo frequent extinctions and re-founding, and t haplotype frequency in these populations is highly variable and
often quite high. By contrast, in the few larger populations examined so far, t haplotype frequency is generally much lower and often remains so, or decreases, over repeated temporal samples22. Population structure has been demonstrated within some large populations (see above) but the role of genetic drift or inbreeding in lowering t frequency remains speculative, as deme sizes are difficult to quantify and could be large if mice choose mates differing from themselves35. Yet even a small reduction in TRD (from multiple matings), combined with a 20% reduction in heterozygote fitness, and moderate levels of population subdivision can be shown to lower t frequencies in simulation studies (D. Durand, pers. commun.). Further empirical work is clearly needed. The notion that t frequency might vary depending on population size and structure (Fig. 3), nevertheless receives some support from a theoretical study, which concluded that a stable low level t haplotype frequency might not exist (Ref. 23 and Fig. 2). Rather, populations might be in one of two stable states, with t haplotypes extinct or at high frequency, or in transition between them, such that any overall frequency of t is low but variable.
Conclusions FIGURE 4. Model of the mouse populations from Fig. 3a22,39. Most populations (shown on the left) are small to medium in size. These are prone to frequent local extinction and recolonization; unfilled circles represent vacant habitat patches. If t haplotypes are present in the founders of these populations they might reach a high frequency rapidly, because of drive, in the few generations the population is present. A few populations, shown on the right, are larger, support higher densities of mice, and persist for many generations. There is some evidence that these populations might be further subdivided into territorial units or ‘demes’.
Unlike many of the other well-described drive systems, the frequency of t haplotypes appears not to be influenced by modifiers of transmission ratio, but rather by a set of complex interacting factors. These include direct effects of the drive chromosome itself on the fitness of +/t individuals, and indirect, or behaviourally mediated, effects of forces, such as multiple mating and population size and structure. Many questions remain regarding this polymorphism, including what are the relative contributions of the factors that have now been identified, and are they equally important in all subspecies of
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REVIEWS Mus musculus in which t haplotypes are found? t Haplotypes continue to provide a model system for ecological genetics. That we now have an appreciation of the extraordinary complexity of how drive is counterbalanced in this system is testimony to the advances made possible by the availability of molecular markers. Hopefully, with more highly resolved tools for investigating the structure of natural populations, t haplotypes will continue to yield insights about the evolutionary process, well into the future.
Acknowledgements I thank M. Polz, A. Berry, S. Alberts, N. Hussain, D. Haig, D. Petrov, S. Pilder, L. Robertson and L. Silver for their comments, ideas and discussion.
References 1 Lyttle, T.W. (1993) Trends Genet. 9, 205–210 2 Hurst, L. and Pomiankowski, A. (1991) Genetics 128, 841–858 3 Jaenike, J. (1996) Am. Nat. 148, 237–254 4 Coyne, J.A. and Orr, H.A. (1993) Evolution 47, 865–687 5 Dobrovolskaia-Zavadskaia, N. and Kobozieff, N. (1927) C. R. Séances Soc. Biol. Ses Fil. 97, 116–119 6 Silver, L.M. (1993) Trends Genet. 9, 250–254 7 Silver, L.M. (1985) Annu. Rev. Genet. 19, 179–208 8 Ardlie, K.G. and Silver, L.M. (1996) Genetics 144, 1787–1797 9 Olds-Clarke, P. (1997) Rev. Reprod. 2, 157–164 10 Ewulonu, U.K. et al. (1996) Genetics 144, 785–792 11 Hammer, M.F., Schimenti, J. and Silver, L.M. (1989) Proc. Natl. Acad. Sci. U. S. A. 86, 3261–3265 12 Schimenti, J., Vold, L., Socolow, D. and Silver, L.M. (1987) J. Mol. Biol. 194, 583–594 13 Morita, T. et al. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 6851–6855 14 Hammer, M.F. and Silver, L.M. (1993) Mol. Biol. Evol. 10, 971–1001 15 Willison, K.R., Dudley, K. and Potter, J. (1986) Cell 44, 727–738
16 Lyon, M.F. (1986) Cell 44, 357–363 17 Bennett, D. (1975) Cell 6, 441–454 18 Klein, J., Sipos, P. and Figueroa, F. (1984) Genet. Res. 44, 39–46 19 Bruck, D. (1957) Proc. Natl. Acad. Sci. U. S. A. 43, 152–158 20 Lenington, S, Franks, P. and Williams, J. (1988) J. Mammal. 69, 489–499 21 Ruvinsky, A. et al. (1991) Genetics 127, 161–168 22 Ardlie, K.G. and Silver, L.M. Evolution (in press) 23 Durand, D. et al. (1997) Genetics 145, 1093–1108 24 Lenington, S. (1991) Adv. Study Behav. 20, 51–86 25 Beckenbach, A.T. (1996) Evolution 50, 787–794 26 Fox, H.S. et al. (1985) Cell 40, 63–69 27 Charlesworth, B. and Hartl, D. (1978) Genetics 89, 171–192 28 Hiraizumi, Y. and Thomas, A.M. (1984) Genetics 95, 693–706 29 Atlan, A., Mercot, H., Landre, C. and Montchamp-Moreau, C. (1995) Evolution 51, 1886–1895 30 Bennett, D., Alton, A.K. and Artzt, K. (1983) Genet. Res. 41, 29–45 31 Gummere, G.R., McCormick, P.J. and Bennett, D. (1986) Genetics 114, 235–245 32 Johnson, P.G. and Brown, G.H. (1969) Am. Nat. 103, 5–21 33 Lenington, S., Coopersmith, C.B. and Erhart, M. (1994) Am. Nat. 143, 766–784 34 Lenington, S., Coopersmith, C. and Williams, J. (1992) Am. Zool. 32, 40–47 35 Potts, W.K. and Wakeland, E.K. (1991) Trends Genet. 9, 408–412 36 Lewontin, R.C. and Dunn, L.C. (1960) Genetics 45, 705–722 37 Dallas, J.F. et al. (1995) Mol. Ecol. 4, 311–320 38 Nachman, M.W. and Searle, J.B. (1995) Trends Ecol. Evol. 10, 397–402 39 Selander, R.K. (1970) Am. Zool. 10, 53–66 K.G. Ardlie is at the Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA.
Included in the June issue of Trends in Genetics: Combinatorial control in ubiquitin-dependent proteolysis by E.E Patton, A.R. Willems and M. Tyers How many homeobox genes does it take to make a pituitary gland? by D.E. Watkins-Chow and S.A. Camper The retinoblastoma gene family: cousins with overlapping interests by G. Mulligan and T. Jacks Limbs are moving: where are they going? by J.W.R. Schwabe, C. Rodriguez-Esteban and J.C. Izpisúa Belmonte Conservation of OTX2 function by A.C. Sharman and M. Brand Wasted by an elongation factor by M. Hafezparast and E. Fisher A simple mechanism for the avoidance of entanglement during chromosome replication by J.E. Hearst, L. Kauffman and W.M. McClain
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