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Putting the brake on drive: meiotic drive of t haplotypes in natural populations of mice.

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.

Animals↗

Maternal transmission ratio distortion at the mouse Om locus results from meiotic drive at the second meiotic division.

We have observed maternal transmission ratio distortion (TRD) in favor of DDK alleles at the Ovum mutant (Om) locus on mouse chromosome 11 among the offspring of (C57BL/6 x DDK) F(1) females and C57BL/6 males. Although significant lethality occurs in this backcross ( approximately 50%), differences in the level of TRD found in recombinant vs. nonrecombinant chromosomes among offspring argue that TRD is due to nonrandom segregation of chromatids at the second meiotic division, i.e., true meiotic drive. We tested this hypothesis directly, by determining the centromere and Om genotypes of individual chromatids in zygote stage embryos. We found similar levels of TRD in favor of DDK alleles at Om in the female pronucleus and TRD in favor of C57BL/6 alleles at Om in the second polar body. In those embryos for which complete dyads have been reconstructed, TRD was present only in those inheriting heteromorphic dyads. These results demonstrate that meiotic drive occurs at MII and that preferential death of one genotypic class of embryo does not play a large role in the TRD.

Animals↗

Meiotic drive of chromosomal knobs reshaped the maize genome.

Meiotic drive is the subversion of meiosis so that particular genes are preferentially transmitted to the progeny. Meiotic drive generally causes the preferential segregation of small regions of the genome; however, in maize we propose that meiotic drive is responsible for the evolution of large repetitive DNA arrays on all chromosomes. A maize meiotic drive locus found on an uncommon form of chromosome 10 [abnormal 10 (Ab10)] may be largely responsible for the evolution of heterochromatic chromosomal knobs, which can confer meiotic drive potential to every maize chromosome. Simulations were used to illustrate the dynamics of this meiotic drive model and suggest knobs might be deleterious in the absence of Ab10. Chromosomal knob data from maize's wild relatives (Zea mays ssp. parviglumis and mexicana) and phylogenetic comparisons demonstrated that the evolution of knob size, frequency, and chromosomal position agreed with the meiotic drive hypothesis. Knob chromosomal position was incompatible with the hypothesis that knob repetitive DNA is neutral or slightly deleterious to the genome. We also show that environmental factors and transposition may play a role in the evolution of knobs. Because knobs occur at multiple locations on all maize chromosomes, the combined effects of meiotic drive and genetic linkage may have reshaped genetic diversity throughout the maize genome in response to the presence of Ab10. Meiotic drive may be a major force of genome evolution, allowing revolutionary changes in genome structure and diversity over short evolutionary periods.

Chromosome Aberrations↗

Four loci on abnormal chromosome 10 contribute to meiotic drive in maize.

We provide a genetic analysis of the meiotic drive system on maize abnormal chromosome 10 (Ab10) that causes preferential segregation of specific chromosomal regions to the reproductive megaspore. The data indicate that at least four chromosomal regions contribute to meiotic drive, each providing distinct functions that can be differentiated from each other genetically and/or phenotypically. Previous reports established that meiotic drive requires neocentromere activity at specific tandem repeat arrays (knobs) and that two regions on Ab10 are involved in trans-activating neocentromeres. Here we confirm and extend data suggesting that only one of the neocentromere-activating regions is sufficient to move many knobs. We also confirm the localization of a locus/loci on Ab10, thought to be a prerequisite for meiotic drive, which promotes recombination in structural heterozygotes. In addition, we identified two new and independent functions required for meiotic drive. One was identified through the characterization of a deletion derivative of Ab10 [Df(L)] and another as a newly identified meiotic drive mutation (suppressor of meiotic drive 3). In the absence of either function, meiotic drive is abolished but neocentromere activity and the recombination effect typical of Ab10 are unaffected. These results demonstrate that neocentromere activity and increased recombination are not the only events required for meiotic drive.

Centromere↗

Sex chromosome meiotic drive in stalk-eyed flies.

Meiotically driven sex chromosomes can quickly spread to fixation and cause population extinction unless balanced by selection or suppressed by genetic modifiers. We report results of genetic analyses that demonstrate that extreme female-biased sex ratios in two sister species of stalk-eyed flies, Cyrtodiopsis dalmanni and C. whitei, are due to a meiotic drive element on the X chromosome (Xd). Relatively high frequencies of Xd in C. dalmanni and C. whitei (13-17% and 29%, respectively) cause female-biased sex ratios in natural populations of both species. Sex ratio distortion is associated with spermatid degeneration in male carriers of Xd. Variation in sex ratios is caused by Y-linked and autosomal factors that decrease the intensity of meiotic drive. Y-linked polymorphism for resistance to drive exists in C. dalmanni in which a resistant Y chromosome reduces the intensity and reverses the direction of meiotic drive. When paired with Xd, modifying Y chromosomes (Ym) cause the transmission of predominantly Y-bearing sperm, and on average, production of 63% male progeny. The absence of sex ratio distortion in closely related monomorphic outgroup species suggests that this meiotic drive system may predate the origin of C. whitei and C. dalmanni. We discuss factors likely to be involved in the persistence of these sex-linked polymorphisms and consider the impact of Xd on the operational sex ratio and the intensity of sexual selection in these extremely sexually dimorphic flies.

Animals↗

The metabolic costs of meiotic drive.

Selfish genetic elements, such as meiotic drive genes, disrupt Mendel's law of equal segregation by biasing their own transmission, often at a detriment to the rest of the genome. Metabolic costs of the X-linked sex ratio (SR) meiotic drive were investigated in stalk-eyed flies (Teleopsis dalmanni). The experiments demonstrate that individuals with SR have reduced capacity for ATP synthesis. The disruption in mitochondrial function leads to compensation exhibited in increased basal metabolic rate and greater food consumption across a range of diets. The range of metabolic costs of drive was evident in males and females at a similar magnitude. The likely cause lies in the accumulation of deleterious mutations within the series of large inversions on the drive X chromosome, subject to low recombination and weak natural selection. In females, the drive chromosome had a dominant effect, with a single copy causing substantial metabolic compromise. There was little evidence of male-specific metabolic costs, nor evidence of greater effects of drive chromosomes on female metabolism. This suggests that direct metabolic costs from meiotic drive on spermatogenesis and from sexually antagonistic selection are relatively weak. Our results underscore the broad physiological impacts that selfish genetic elements have on host metabolism and fitness.

Animals↗

Meiotic drive alters sperm competitive ability in stalk-eyed flies.

Meiotic drive results when sperm carrying a driving chromosome preferentially survive development. Meiotic drive should therefore influence sperm competition because drive males produce fewer sperm than non-drive males. Whether meiotic drive also influences the competitive ability of sperm after ejaculation is unknown. Here we report the results from reciprocal crosses that are designed for estimating the sperm precedence of male stalk-eyed flies (Cyrtodiopsis whitei) with or without X-linked meiotic drive. We find that nearly half of all sex-ratio males, as compared with 14% of non-sex-ratio males, fail to produce young in a reciprocal cross. Furthermore, the proportion of progeny sired by a sex-ratio male in a female jointly inseminated by a non-sex-ratio male was less than expected from the number of sperm transferred. These effects are not due to differential sperm storage by females because, after a single mating with a sex-ratio male, all females stored sperm and because two sex-ratio males share paternity after jointly mating with a female. In addition to demonstrating a new mechanism of sperm competition, these results provide insight into the maintenance of sex-ratio polymorphisms. Sex-ratio males have less than one-half the fertility of non-sex-ratio males, as is required in order for frequency-dependent selection on males to produce a stable sex-ratio polymorphism.

Analysis of Variance↗

Evolution of the segregation ratio: modification of gene conversion and meiotic drive.

We compare the evolutionary pressures that direct the modification of gene conversion and meiotic drive at loci subject to purifying and overdominant viability selection. Gene conversion differs from meiotic drive in that modifers do not affect their own segregation ratios, even when linked to the viability locus. Segregation distortion generates gametic level disequilibria between alleles at the viability locus and modifiers of gene conversion and meiotic drive: enhancers of segregation distortion become positively associated with driven alleles. Suppression of gene conversion evolves if the driven allele is marginally disadvantageous (overdominant viability selection), and higher rates evolve if the driven alleles are relatively advantageous (purifying viability selection). Gametic disequilibria permit enhancers of meiotic drive that are linked to the driven locus to promote their own segregation. We attribute the failure of genetic modifiers of gene conversion and meiotic drive to maximinize mean fitness to the generation of such associations.

Biological Evolution↗

The maize Ab10 meiotic drive system maps to supernumerary sequences in a large complex haplotype.

The meiotic drive system on maize abnormal chromosome 10 (Ab10) is contained within a terminal domain of chromatin that extends the long arm of Ab10 to approximately 1.3 times the size of normal chromosome 10L. Ab10 type I (Ab10-I) does not recombine with normal chromosome 10 (N10) over an approximately 32-cM terminal region of the long arm. Comparative RFLP mapping demonstrates that multiple independent rearrangements are responsible for the current organization of Ab10-I, including a set of nested inversions and at least one long supernumerary segment at the end of the chromosome. Four major meiotic drive functions, i.e., the recombination effect, smd3, 180-bp neocentromere activity, and the distal tip function, all map to the distal supernumerary segment. TR-1-mediated neocentromere activity (the fifth known drive function) is nonessential in the type II variant of Ab10 and maps to a central region that may include a second supernumerary insertion. Both neocentromere activity and the recombination effect behave as dominant gain-of-function mutations, consistent with the view that meiotic drive involves new or alien gene products. These and other data suggest that the Ab10 meiotic drive system was initially acquired from a related species and that a complex haplotype evolved around it.

Base Sequence↗

Meiotic drive and evolution of female choice.

As a special version of the good-genes hypothesis, it was recently proposed that females could benefit from choosing drive-resistant males in a meiotic drive system. Here, we examine with a three-locus, six-allele population genetic model whether female choice for drive resistance can evolve. An allele leading to female preference for drive-resistant males was introduced at low frequency into a population polymorphic for meiotic drive and drive resistance. Our simulations show that female choice of drive-resistant males is disadvantageous when resistance is Y-linked. This disadvantage occurs because, at equilibrium, drive-resistant males have lower reproductive success than drive-susceptible males. Thus, female choice of drive-susceptible males can evolve when resistance is Y-linked. When resistance is autosomal, selection on female choice for drive resistance is less strong and depends on the frequency of choice: female preference of resistant males is favoured when choice is rare and disadvantageous when choice is frequent, leading to a stable equilibrium at a low frequency of the choice allele. Independent of the location of drive resistance alleles, males with the non-driving allele always have above average reproductive success. Female choice is therefore beneficial when choosy females prefer males with the non-driving allele.

Animals↗

Induction of centromeric activity in maize by suppressor of meiotic drive 1.

The Abnormal chromosome 10 (Ab10) in maize causes normally-quiescent blocks of heterochromatin called knobs to function as meiotic centromeres. Under these circumstances genetic markers associated with knobs exhibit meiotic drive, i.e., they are preferentially transmitted to progeny. Here we describe a mutation called suppressor of meiotic drive (smd1) that partially suppresses meiotic drive, and demonstrate that smd1 causes a quantitative reduction in the mobility of knobs on the meiotic spindle. We conclude that Smd1 encodes a product that is necessary for the activation of ectopic centromeres, and that meiotic drive occurs as a consequence of the resulting change in chromosome movement. As a genetic system, Ab10 offers a new and powerful approach for analyzing centromere/kinetochore function.

Anaphase↗

Sperm development, age and sex chromosome meiotic drive in the stalk-eyed fly, Cyrtodiopsis whitei.

The cytological basis of X chromosome meiotic drive or sex ratio (SR) has been reported for several species of Drosophila but not for other species. Here we describe how sperm development in the stalk-eyed fly, Cyrtodiopsis whitei, influences progeny sex proportion, in order to determine if a common developmental mechanism could cause meiotic drive in these distantly related taxa. Because age has been found to affect the degree of segregation distortion in some Drosophila, we tested flies from six to 26 weeks of age. We find that spermatocyst bundles in SR males frequently contain incompletely elongated spermatid nuclei independently of male age. Older males have, however, more spermatocyst bundles in their testes than younger males. Abnormal spermatid elongation affects male fertility since SR males produce 74% as many progeny per week as ST males. The proportion of spermatocyst bundles with improperly elongated spermatid nuclei explains 71% of the variation in progeny sex proportion. After reviewing the literature on sperm development and meiotic drive, we conclude that the cytological basis of meiotic drive in diopsids closely resembles Drosophila. Across species in both groups, the production of fertile males is associated with less than half of all spermatids not elongating normally in a spermatocyst bundle. We discuss the possibility that frequency-dependent selection on male fertility could stabilize the drive polymorphism in these unusual flies.

Animals↗

X chromosome effect on maternal recombination and meiotic drive in the mouse.

We observed that maternal meiotic drive favoring the inheritance of DDK alleles at the Om locus on mouse chromosome 11 was correlated with the X chromosome inactivation phenotype of (C57BL/6-Pgk1(a) x DDK)F(1) mothers. The basis for this unexpected observation appears to lie in the well-documented effect of recombination on meiotic drive that results from nonrandom segregation of chromosomes. Our analysis of genome-wide levels of meiotic recombination in females that vary in their X-inactivation phenotype indicates that an allelic difference at an X-linked locus is responsible for modulating levels of recombination in oocytes.

Animals↗

Spore-killing meiotic drive factors in a natural population of the fungus Podospora anserina.

In fungi, meiotic drive is observed as spore killing. In the secondarily homothallic ascomycete Podospora anserina it is characterized by the abortion of two of the four spores in the ascus. We have identified seven different types of meiotic drive elements (Spore killers). Among 99 isolates from nature, six of these meiotic drive elements occurred in a local population. Spore killers comprise 23% of the natural population of P. anserina in Wageningen, The Netherlands, sampled from 1991 to 1997. One Spore-killer type was also found in a French strain dating from 1937. All other isolates found so far are sensitive to spore killing. All seven Spore killer types differ in the percentage of asci that show killing and in their mutual interactions. Interactions among Spore killer types showed either mutual resistance or dominant epistasis. Most killer elements could be assigned to linkage group III but are not tightly linked to the centromere.

Centromere↗

The meiotic drive system on maize abnormal chromosome 10 contains few essential genes.

In maize, a distal portion of abnormal chromosome 10 (Ab10) causes the meiotic drive of itself as well as many unlinked heterochromatic regions known as knobs. The Ab10 drive system, which encodes trans- as well as cis-acting components, occupies a large region of chromosome 10L equivalent to approximately 3% of the genome. Here we describe five new structural mutations of Ab10 (five deletions and a duplication) that arose from a screen for meiotic drive mutants. The high frequency of breakage events, detected both genetically and cytologically, suggest that the chromosome may be especially unstable. Very large deletions within the drive system are female-transmissible and plants homozygous for deficiencies lacking much of this interval can be grown to maturity. The data suggest that few genes required for normal growth and development lie within the portion of Ab10 responsible for meiotic drive. These and other published data suggest that meiotic drive systems tend to evolve in gene-sparse or otherwise information-poor regions of the genome where they are less likely to negatively affect individual fitness.

Chromosome Aberrations↗

Spore killing in the fungus Podospora anserina: a connection between meiotic drive and vegetative incompatibility?

Fungi in which the haploid nuclei resulting from meiosis are linearly arranged in asci provide unique opportunities to analyse abnormal segregation. Any meiotic drive system in such fungi will be observed in a cross between a driving and a sensitive strain as spore killing: the degeneration of half the ascospores in a certain proportion of the asci. In a sample of some 100 strains isolated from a single natural population we have discovered at least six different meiotic drive elements (van der Gaag et al., 2000). Here we report results of research that was aimed at elucidating a possible correlation between meiotic drive and vegetative incompatibility in eight different Spore killer strains from this population. We show that there is a strong correlation between these two phenotypes, although the precise genetic nature of the correlation is not yet clear. We discuss the implications of our results for the understanding of the population genetics of meiotic drive in Podospora.

Chromosome Segregation↗

[Meiotic drive of t-haplotypes: segregation of chromosomes in mice with partial trisomy].

The properties of the t haplotypes, specific mutant states of the proximal region of chromosome 17 in the house mouse keep renewing interest. One such property is increased transmission of the t haplotype from heterozygous t/+ males to their offspring. By means of reciprocal translocation T (16; 17)43H, we have constructed males with tertiary trisomy 17 (+T43/++/RB7+) carrying Robertsonian translocation Rb(16.17)7Bnr. The offspring of these males was viable when sperm of +T43/++ and Rb7+ was used. The segregation patterns in the offspring of t-bearing trisomics were analysed on days 16-18 of embryonic development. It was found that in the case when the t haplotype is on the normal acrocentric (male male ++T43/+t12+/Rb7++), its presence in the gamete +t12+/++T43 does not produce meiotic drive. However, when t6 is on Rb7, meiotic drive was equal to 80%. It is concluded that the presence of a normal homolog and a t-bearing chromosome in sperm does not result in meiotic drive. Possible mechanisms of meiotic drive of the t haplotypes are discussed.

Animals↗

Heritability of the maternal meiotic drive system linked to Om and high-resolution mapping of the Responder locus in mouse.

Matings between (C57BL/6 x DDK)F(1) females and C57BL/6 males result in a significant excess of offspring inheriting maternal DDK alleles in the central region of mouse chromosome 11 due to meiotic drive at the second meiotic division. We have shown previously that the locus subject to selection is in the vicinity of D11Mit66, a marker closely linked to the Om locus that controls the preimplantation embryo-lethal phenotype known as the "DDK syndrome." We have also shown that observation of meiotic drive in this system depends upon the genotype of the sire. Here we show that females that are heterozygous at Om retain the meiotic drive phenotype and define a 0.32-cM candidate interval for the Responder locus in this drive system. In addition, analysis of the inheritance of alleles at Om among the offspring of F(1) intercrosses indicates that the effect of the sire is determined by the sperm genotype at Om or a locus linked to Om.

Animals↗