The origin of a double insertion of homogeneously staining regions in the house mouse (Mus musculus musculus)

Meiotic drive of noncentromeric loci in mammalian meiosis II eggs

The germline produces haploid gametes through a specialized cell division called meiosis. In general, homologous chromosomes from each parent segregate randomly to the daughter cells during meiosis, providing parental alleles with an equal chance of transmission. Meiotic drivers are selfish elements who cheat this process to increase their transmission rate. In female meiosis, selfish centromeres and noncentromeric drivers cheat by preferentially segregating to the egg cell. Selfish centromeres cheat in meiosis I (MI), while noncentromeric drivers can cheat in both meiosis I and meiosis II (MII). Here, we highlight recent advances on our understanding of the molecular mechanisms underlying these genetic cheating strategies, especially focusing on mammalian systems, and discuss new models of how noncentromeric selfish drivers can cheat in MII eggs.

Homogeneously Staining Regions (HSR) in Chromosome 1 of the House Mouse: Synapsis and Recombination at Meiosis

Amplified sequences constitute a large part of mammalian genomes. A chromosome 1 containing 2 large (up to 50 Mb) homogeneously staining regions (HSRs) separated by a small inverted euchromatic region is present in many natural populations of the house mouse (Mus musculus musculus). The HSRs are composed of a long-range repeat cluster, Sp100-rs, with a repeat length of 100 kb. In order to understand the organization and function of HSRs in meiotic chromosomes, we examined synapsis and recombination in male mice hetero- and homozygous for the HSR-carrying chromosome using FISH with an HSR-specific DNA probe and immunolocalization of the key meiotic proteins. In all homozygous and heterozygous pachytene nuclei, we observed fully synapsed linear homomorphic bivalents 1 marked by the HSR FISH probe. The synaptic adjustment in the heterozygotes was bilateral: the HSR-carrying homolog was shortened and the wild-type homolog was elongated. The adjustment was reversible: desynapsis at diplotene was accompanied by elongation of the HSRs. Immunolocalization of H3K9me2/3 indicated that the HSRs in the meiotic chromosome retained the epigenetic modification typical for C-heterochromatin in somatic cells. MLH1 foci, marking mature recombination nodules, were detected in the proximal HSR band in heterozygotes and in both HSR bands of homozygotes. Unequal crossing over within the long-range repeat cluster can cause variation in size of the HSRs, which has been detected in the natural populations of the house mouse.

Zoogeography of the chromosome 1 HSR in natural populations of the house mouse (Mus musculus)

A polymorphism of the central part of chromosome 1 has been described from natural populations of the house mouse (Mus musculus). The region shows up as a C band-positive homogeneously staining region (HSR) under the light microscope. M. m. domesticus mice carry single band HSRs, whereas M. m. musculus animals have double band HSRs. HSR size variations have been described in both subspecies. The frequency of the HSR chromosome 1 in populations varies from 4% to 81%, but none of the large samples examined consisted only of homozygotes. In the subspecies M. m. domesticus, HSRs were found in North Africa and Western Europe, mainly in the hilly regions of Southern Germany and Switzerland. Localities with double HSRs are distributed all over the area of M. m. musculus. Based on the population data presented and DNA similarity of different HSRs, the origin and distribution of HSR chromosomes in the house mouse are discussed.

Chromosome pairing in inter-racial hybrids of the house musk shrew (Suncus murinus, Insectivora, Soricidae)

Two chromosome races of the house shrew Suncus murinus that differ from each other for five Robertsonian translocations (8.17, 9.13, 10.12, 11.16, and 14.15), heterochromatic insertions in chromosomes 7 and X, and multiple rearrangements in the Y chromosome were crossed and then intercrossed in captivity to produce a hybrid stock. Electron-microscopic analysis of synaptonemal complexes in fertile and sterile hybrid males was carried out. Meiosis in sterile males did not progress beyond pachytene and was severely disrupted. Meiotic arrest was not determined by structural heterozygosity: heterozygotes for all variant chromosomes distinguishing two parental races were found in both sterile and fertile male hybrids. Fertile hybrids demonstrated an orderly pairing of all chromosomes. In heterozygotes for Robertsonian fusions, completely paired trivalents were formed between the Robertsonian metacentrics and homologous acrocentrics. In heterozygotes for chromosome 7, bivalents with a small buckle were observed in a small fraction of pachytene cells. No differences were found in the morphology and pairing pattern of sex bivalents, composed of the X and Y chromosomes derived from the same or different parental races. Univalents, multivalents, and associations between X and Y chromosomes and autosomal trivalents, as well as associations of autosomal trivalents with each other, were observed in a small fraction of the pachytene cells of fertile males. Our results indicate that the system controlling male sterility in interracial hybrids of S. murinus is of genic rather than of chromosomal type.

The HSR on chromosome 1 of the house mouse, Mus domesticus: distribution and frequency in Switzerland

A total of 357 house mice (Mus domesticus) from 83 localities uniformly distributed throughout Switzerland were screened for the presence of a homogenously staining region (HSR) on chromosome 1. Altogether 47 mice from 11 localities were HSR/+ or HSR/HSR. One sample of 11 individuals all had an HSR/HSR karyotype. Almost all mice with the variant were collected from the Rhone valley (HSR frequency: 61%) and Val Bregaglia (HSR frequency: 81%). For samples from most of the area of Switzerland, the HSR was absent. There was no strong association between the geographic distribution of the HSR and the areas of occurrence of metacentrics. However, at Chiggiogna the HSR was found on Rb (1.3). Possible explanations for the HSR polymorphism are discussed.

Evolution of a long-range repeat family in chromosome 1 of the genus Mus

Copy numbers and variation of a clustered long-range repeat family on chromosome (Chr) 1 have been studied in different species of the genus Mus. The repeat sequence was present in all, as inferred from cross-hybridization with probes derived from the Mus musculus repeat family. Copy numbers determined by dot blot hybridization were very low, from three to six per haploid genome in M. caroli, M. cervicolor, and M. cookii. These species form one branch of the phylogenetic tree in the genus Mus. In the other group of phylogenetically related species--M. spicilegus, M. spretus, M. musculus and M. macedonicus--copy numbers ranged from 6 to 1810 per haploid genome. The repeat cluster is cytogenetically visible as a fine C-band in M. macedonicus and as a C-band positive homogeneously staining region (HSR) in several populations of M. m. domesticus and M. m. musculus. When cytogenetically visible, the clusters contained from 179 to 1810 repeats. Intragenomic restriction fragment length polymorphisms (RFLPs), which reflect sequence variation among different copies of the long-range repeat family, increased with higher copy numbers. The high similarity of the RFLP pattern among genomes with C-band positive regions in Chr 1 of M. m. musculus, M. m. domesticus, and M. macedonicus points to a close evolutionary relationship of their Chr 1 repeat families.

Chiasma distribution in the first bivalent of mice carrying a double insertion of homogeneously-staining regions in homo- and heterozygous states

An examination of the meiotic pattern of chromosome 1 isolated from a feral mouse population and containing a double insertion (Is) of homogeneously-staining regions (HSRs) was carried out. In a previous study is was shown that the region delineated by the proximal breakpoint of Is(HSR;1C5) 1Icg and the distal one of Is(HSR;1D)2Icg is unpaired during early pachytene and heterosynapsed at midpachytene. No synaptic disturbances were revealed in homozygotes in this study. Chiasmata number per first bivalent in heterozygous (1.87) and homozygous (1.88) males was shown to be higher than in normal ones (1.61). In normal males a single chiasma is located in the medial part of chromosome 1. In heterozygotes this segment is heterosynapsed and unavailable for recombination. This leads to a significant decrease in the frequency of bivalents bearing a single chiasma and an increase in the frequency of bivalents bearing double chiasmata located mostly at subcentromeric and subtelomeric regions of the chromosome. In homozygous males the frequency of double chiasmata is also increased, and even triple chiasmata become possible because of the increase in the physical length of the bivalents. Thus insertion of heterochromatic regions, which are inert with respect to recombination, leads to an increase in the length of the genetic map of the chromosome because of relaxation of interference restrictions.

Population dynamics of aberrant chromosome 1 in mice

Natural populations of two semispecies of house mouse, Mus musculus domesticus and M.m. musculus, were found to be polymorphic for an aberrant chromosome 1 bearing a large inserted block of homogeneously staining heterochromatin. Strong meiotic drive for the aberrant chromosome from M.m. musculus was previously observed in heterozygous female mice. There are at least three meiotic drive levels determined by different allelic variants of distorter. Homozygotes had low viability and females showed low fertility. Both homo- and heterozygous males had normal fertility and their segregation patterns did not deviate from normal. Computer simulations were performed of the dynamics of aberrant chromosome 1 in demes and populations. The data demonstrate that a spontaneous mutation (inversion) of an aberrant chromosome 1, once arisen, has a high probability of spreading in a population at high coefficients of meiotic drive and migration. In the long-term, the population attains a stationary state which is determined by the drive level and migration intensity. The state of stable genotypic equilibrium is independent of deme and population size, as well as of the initial concentration of the aberrant chromosome. As populations initially polymorphic for the distorters approach the stationary state, the stronger distorter is eliminated. The frequencies of the aberrant chromosome determined by computer analysis agree well with those obtained for the studied Asian M.m. musculus populations. The evolutionary pathways for the origin and fixation of the aberrant chromosome in natural populations are considered.

Effect of sperm genotype on chromatid segregation in female mice heterozygous for aberrant chromosome 1

An aberrant chromosome 1 with two large homogeneously staining insertions was isolated from wild populations of Mus musculus musculus. The specific features of the aberrant chromosome have been described elsewhere (Agulnik et al. 1990). These include its preferential entry into the oocyte of heterozygous females, increased mortality of homozygotes and decreased fertility of homozygous females. Here we present data indicating that chromatid segregation in heterozygous females depends upon which sperm enters the oocyte before the second meiotic division: meiotic drive is powerful when it is sperm bearing normal chromosome 1, and segregation normalizes during MII when it is sperm bearing chromosome 1 with the extra segment. Experimental data are adduced and explanations offered for the observed phenomenon.

Meiotic drive on aberrant chromosome 1 in the mouse is determined by a linked distorter

An aberrant chromosome 1 carrying an inverted fragment with two amplified DNA regions was isolated from wild populations of Mus musculus. Meiotic drive favouring the aberrant chromosome was demonstrated for heterozygous females. Its cause was preferential passage of aberrant chromosome 1 to the oocyte. Genetic analysis allowed us to identify a two-component system conditioning deviation from equal segregation of the homologues. The system consists of a postulated distorter and responder. The distorter is located on chromosome 1 distally to the responder, between the ln and Pep-3 genes, and it acts on the responder when in trans position. Polymorphism of the distorters was manifested as variation in their effect on meiotic drive level in the laboratory strain and mice from wild populations.

Synapsis in single and double heterozygotes for partially overlapping inversions in chromosome 1 of the house mouse

Electron microscopic (EM) analysis of synaptonemal complexes (SC) in single and double heterozygotes for the partially overlapping inversions In(1)1Icg, In(1)1Rk and In(1)12Rk in chromosome 1 of the house mouse reveals that synapsis and synaptic adjustment are dependent on the size and location of the inversions and interaction between the latter. Is(1)1Icg contains insertions of the inverted repeats Is(HSR;1C5)1Icg and Is(HSR;1D)2Icg and an inverted euchromatic region. Synaptic adjustment of the D-loops by shortening of the asynapsed segments of the lateral elements belonging to the insertions occurs at the late zytogene to early pachytene stage. Synaptic adjustment of the inversion loops takes place at early to late pachytene. A delay in adjustment was found in the double heterozygotes In(1)1Icg/In(1)1Rk and In(1)1Icg/In(1)12Rk. A correspondence between the lifespan of asynapsis in inverted regions and the probability of association of XY and heteromorphic bivalents was revealed.