Multiple independent evolutionary losses of XY pairing at meiosis in the grey voles

In many eutherian mammals, X-Y chromosome pairing and recombination is required for meiotic progression and correct sex chromosome disjunction. Arvicoline rodents present a notable exception to this meiotic rule, with multiple species possessing asynaptic sex chromosomes. Most asynaptic vole species belong to the genus Microtus sensu lato. However, many of the species both inside and outside the genus Microtus display normal X-Y synapsis at meiosis. These observations suggest that the synaptic condition was present in the common ancestor of all voles, but gaps in current taxonomic sampling across the arvicoline phylogeny prevent identification of the lineage(s) along which the asynaptic state arose. In this study, we use electron and immunofluorescent microscopy to assess heterogametic sex chromosome pairing in 12 additional arvicoline species. Our sample includes ten species of the tribe Microtini and two species of the tribe Lagurini. This increased breadth of sampling allowed us to identify asynaptic species in each major Microtine lineage. Evidently, the ability of the sex chromosomes to pair and recombine in male meiosis has been independently lost at least three times during the evolution of Microtine rodents. These results suggest a lack of evolutionary constraint on X-Y synapsis in Microtini, hinting at the presence of alternative molecular mechanisms for sex chromosome segregation in this large mammalian tribe.

Chromosomal rearrangements do not seem to affect the gene flow in hybrid zones between karyotypic races of the common shrew (Sorex araneus)

Chromosomal rearrangements are proposed to promote genetic differentiation between chromosomally differentiated taxa and therefore promote speciation. Due to their remarkable karyotypic polymorphism, the shrews of the Sorex araneus group were used to investigate the impact of chromosomal rearrangements on gene flow. Five intraspecific chromosomal hybrid zones characterized by different levels of karyotypic complexity were studied using 16 microsatellites markers. We observed low levels of genetic differentiation even in the hybrid zones with the highest karyotypic complexity. No evidence of restricted gene flow between differently rearranged chromosomes was observed. Contrary to what was observed at the interspecific level, the effect of chromosomal rearrangements on gene flow was undetectable within the S. araneus species.

Natural hybridization between extremely divergent chromosomal races of the common shrew (Sorex araneus, Soricidae, Soricomorpha): hybrid zone in Siberia

Chromosomal races of the common shrew differ in sets of metacentric chromosomes and on contact may produce hybrids with extraordinarily complex configurations at meiosis I that are associated with reduced fertility. There is an expectation that these may be some of the most extreme tension zones available for study and therefore are of interest as potential sites for reproductive isolation. Here, we analyse one of these zones, between the Novosibirsk race (characterized by metacentrics go, hn, ik, jl, mp and qr) and the Tomsk race (metacentrics gk, hi, jl and mn and acrocentrics o, p, q and r), which form hybrids with a chain-of-nine (CIX) and a chain-of-three (CIII) configuration at meiosis I. At the Novosibirsk-Tomsk hybrid zone, the CIX chromosomes form clines of 8.53 km standardized width on average, whereas the cline for the CIII chromosomes was 52.83 km wide. The difference in these cline widths fits with the difference in meiotic errors expected with the CIX and CIII configuration, and we produce estimates of selection against hybrids with these types of configurations, which we relate to dispersal and age of the hybrid zone. The hybrid zone is located at the isocline at 200 m altitude above sea level; this relationship between the races and altitude is suggested at both coarse and fine scales. This indicates adaptive differences between the races that may in turn have been promoted by the chromosome differences. Thus, the extreme chromosomal divergence between the Novosibirsk and Tomsk may be associated with genic differentiation, but it is still striking that, despite the large chromosomal differences, reproductive isolation between the Novosibirsk and Tomsk races has not occurred.

X-Y chromosome synapsis and recombination in 3 vole species of Asian lineage of the genus Microtus (Rodentia: Arvicolinae)

The pattern of X-Y chromosome pairing in male meiosis is an important taxonomic feature of grey voles of the genus Microtus. Asynaptic sex chromosomes have been found in the majority of species of the Palearctic phylogenetic lineage of this genus, while normal X-Y synapsis has been observed in the species of subgenus Pallasiinus belonging to the Asian phylogenetic lineage. We analyzed sex chromosome pairing and recombination in M. maximowiczii, M. mujanensis and M. fortis which also belong to the Asian phylogenetic lineage (subgenus Alexandromys). Using immunostaining for the proteins of the synaptonemal complex (SCP3) and recombination nodules (MLH1) we demonstrated that X and Y chromosomes of these species paired and recombined in a short subtelomeric region. This indicates that the sex chromosomes of these species retain an ancestral fully functional pseudoautosomal region, which has been lost or rearranged in the asynaptic species of the genus Microtus.

A- and B-chromosome pairing and recombination in male meiosis of the silver fox (Vulpes vulpes L., 1758, Carnivora, Canidae)

We examined A- and B-chromosome pairing and recombination in 12 males from the farm-bred population of the silver fox (2n = 34 + 0-10 Bs) by means of electron and immunofluorescent microscopy. To detect recombination at A and B chromosomes, we used immunolocalisation of MLH1, a mismatch repair protein of mature recombination nodules, at synaptonemal complexes. The mean total number of MLH1 foci at A-autosomes was 29.6 foci per cell. The XY bivalent had one MLH1 focus at the pairing region. Total recombination length of the male fox genome map was estimated as 1,530 centimorgans. We detected single MLH1 foci at 61% of linear synaptic configurations involving B chromosomes. The distribution of the foci along B- and A-bivalents was the same. This may be considered as a first molecular evidence that meiotic recombination does occur in mammalian B chromosomes. There was no correlation between the number of synaptic configurations involving B chromosomes per cell and the recombination rate of the A-genome.

Predicting human height by Victorian and genomic methods

In the Victorian era, Sir Francis Galton showed that 'when dealing with the transmission of stature from parents to children, the average height of the two parents, ... is all we need care to know about them' (1886). One hundred and twenty-two years after Galton's work was published, 54 loci showing strong statistical evidence for association to human height were described, providing us with potential genomic means of human height prediction. In a population-based study of 5748 people, we find that a 54-loci genomic profile explained 4-6% of the sex- and age-adjusted height variance, and had limited ability to discriminate tall/short people, as characterized by the area under the receiver-operating characteristic curve (AUC). In a family-based study of 550 people, with both parents having height measurements, we find that the Galtonian mid-parental prediction method explained 40% of the sex- and age-adjusted height variance, and showed high discriminative accuracy. We have also explored how much variance a genomic profile should explain to reach certain AUC values. For highly heritable traits such as height, we conclude that in applications in which parental phenotypic information is available (eg, medicine), the Victorian Galton's method will long stay unsurpassed, in terms of both discriminative accuracy and costs. For less heritable traits, and in situations in which parental information is not available (eg, forensics), genomic methods may provide an alternative, given that the variants determining an essential proportion of the trait's variation can be identified.

[Telomeric DNA allocation in chromosomes of common shrew Sorex araneus, Eulipotyphla]

Recently we have displayed shrew species, Iberian shrew S. granarius, with telomeres of unusual for mammals structure, including long telomeres on the short acrocentrics arms containing 213 kb on average and short telomeres (3.8 kb) on the other chromosomal ends (Zhdanova et al., 2005, 2007). However, it is not clear if such telomeres are characteristic of all shrew species or only of S. granarius. S. granarius and common shrew Sorex araneus are the sibling species. In this investigation by using modified Q-FISH, we demonstrated that telomeres in S. araneus from different chromosomal races differing in the numbers of metacentrics contain 6.8-15.2 kb of telomeric tracts. Thus, the S. araneus telomere lengths appeared to correspond with telomere lengths both in shrews and majority wild mammalian species, and S. granarius has telomeres with unique or scarce structure. Furthermore, using DNA and RNA modified with probe high specificity to telomeric repeats (PNA and LNA) we showed that interstitial telomeric sites in S. araneus chromosomes contained mainly telomeric DNA and their localization coincided with some evolutionary breakpoints. Interstitial telomeric DNA in S. granarius chromosomes was not revealed. Thus, distribution of telomeric DNA can greatly differ even in closely related species whose chromosomes are composed from almost identical chromosomal arms.

[Immunofluorescent analysis of meiotic recombination and interference in the domestic cat]

The aim of this work was an analysis of frequency, density and distribution of recombination sites in male meiosis of the domestic cat. The study was carried out using immunofluorescent staining of synaptonemal complex (SC) proteins, centromeric proteins and mismatch repair protein MLH1, a reliable marker of the sites of crossing over. We mapped 2633 sites of crossing over at 1098 individual autosomes. On the basis of these data the total length of the domestic cat genetic map was estimated as 2176 centimorgans. We found a typical for all mammals studied positive correlation between the length of SC and the number of recombination sites. The domestic cat demonstrated the highest among mammals density of recombination and the lowest interference.

Inheritance of litter size at birth in farmed arctic foxes (Alopex lagopus, Canidae, Carnivora)

Natural populations of the arctic fox (Alopex lagopus, Canidae, Carnivora) differ drastically in their reproductive strategy. Coastal foxes, which depend on stable food resources, produce litters of moderate size. Inland foxes feed on small rodents, whose populations are characterized by cycling fluctuation. In the years with low food supply, inland fox populations have a very low rate of reproduction. In the years with high food supply, they undergo a population explosion. To gain insight into the genetic basis of the reproductive strategy of this species, we performed complex segregation analysis of the litter size in the extended pedigree of the farmed arctic foxes involving 20,665 interrelated animals. Complex segregation analysis was performed using a mixed model assuming that the trait was under control of a major gene and a large number of additive genetic and random factors. To check the significance of any major gene effect, we used Elston-Stewart transmission probability test. Our analysis demonstrated that the inheritance of this trait can be described within the frameworks of a major gene model with recessive control of low litter size. This model was also supported by the pattern of its familial segregation and by comparison of the distributions observed in the population and that expected under our model. We suggest that a system of balanced polymorphism for litter size in the farmed population might have been established in natural populations of arctic foxes as a result of adaptation to the drastic fluctuations in prey availability.

Identification of all pachytene bivalents in the common shrew using DAPI-staining of synaptonemal complex spreads

A major problem in studies of synaptonemal complexes (SC) is the difficulty in distinguishing individual chromosomes. This problem can be solved combining SC immunostaining with FISH of chromosome-specific sequences. However, this procedure is expensive, time-consuming and applicable only to a very limited number of species. In this paper we show how a combination of SC immunostaining and DAPI staining can allow identification of all chromosome arms in surface-spreads of the SC of the common shrew (Sorex araneus L.). Enhancement of brightness and contrast of the images with photo editing software allowed us to reveal clear DAPI-positive and negative bands with relative sizes and positions similar to DAPI landmarks on mitotic metaphase chromosomes. Using FISH with DNA probes prepared from chromosome arms m and n we demonstrated correct recognition of the chromosomes mp and hn on the basis of their DAPI pattern. We show that the approach we describe here may be applied to other species and can provide an important tool for identification of individual bivalents in pachytene surface-spreads.

[Changes in litter size in Kerry blue terrier dogs with abnormal dentition]

The pleiotropic effects of mutations resulting in abnormal dentition were analyzed in Kerry Blue Terrier. A decrease in litter size was demonstrated for dogs with dentition anomalies. The mean litter size was 5.72 puppies when both parents had normal dentition and 3.64 puppies when the parents had hypodontia. Analysis showed that the decrease in litter size cannot be fully explained by the effect of inbreeding and is most probably associated with the pleiotropic effect of the genes controlling teeth development on the embryonic viability.

[Polymorphism of dental formula and segregation of its variants in a pedigree of kerry blue terrier dogs]

Polymorphism of the dental formula was analyzed in a sophisticated pedigree of Kerry Blue Terrier. A lack of one or more lower premolars was observed in some dogs. Two different patterns of missing teeth were identified. One pattern consisted in agenesis of a second premolar, often in combination with agenesis of neighbor teeth, including the fourth premolar. In the second pattern, agenesis of a fourth premolar was expressed as an isolated abnormality. It was shown previously that the first pattern is inherited as a recessive trait with near complete penetrance. In this work, the control of a major-gene was demonstrated for the second pattern. This abnormality develops in 70-80% of mutant homozygotes and in no more than 20% of heterozygotes and wild-type homozygotes. It was shown that the two dentition abnormalities are controlled by different genes, which were designated LPA2 and LPA4 (Lower Premolar Agenesis).

Reproductive isolation due to the genetic incompatibilities between Thrichomys pachyurus and two subspecies of Thrichomys apereoides (Rodentia, Echimyidae)

We tested intrinsic reproductive isolation between 3 taxa of the South American caviomorph rodent Thrichomys (Rodentia, Echimyidae): T. pachyurus, T. apereoides subsp. apereoides and T. apereoides subsp. laurentius. They were mated in captivity and produced viable progeny. Some F1 hybrid females were fertile, whereas all F1 males were sterile. Histological examination revealed meiotic arrest at the primary spermatocyte stage. No sperm was detected in testes or epididymes. Electron microscopic analysis of surface spread synaptonemal complexes revealed a complete failure of chromosome pairing in F1 hybrids of T. pachyurus with T. apereoides subsp. laurentius and T. apereoides subsp. apereoides. In the male hybrids between T. apereoides subsp. apereoides and T. apereoides subsp. laurentius, meiosis did not proceed beyond diplotene, although all of the chromosomes, including heteromorphic ones, paired in an orderly fashion. Backcross males with homomorphic karyotypes showed segregation in meiosis progression. This indicates that male hybrid sterility is due to genetic, but not chromosomal, incompatibility of the parental taxa.Key words: hybrid sterility, speciation, chromosome rearrangements, meiosis, spermatogenesis, synaptonemal complex, Thrichomys.

[Genetic control of chromosome synapsis in mice heterozygous for a paracentric inversion]

Frequencies of formation of inversion loops and their relative sizes were studied in laboratory mice heterozygous at paracentric inversion In1(1)Rk in chromosome 1, depending on the genetic background. Homozygotes In1/In1 were crossed with mice from five inbred strains (A/HeJ, BALB/cJ, C3H/HeJ, C57BL/6J, DBA2/J). The frequency of formation of inversion loops, their relative sizes, and the dependence of these parameters on the stage of pachitene were analyzed on electron-microscopic slides of spread spermatocytes in first-generation hybrids. It was shown that the genetic background and cross direction statistically significantly influenced the duration of individual pachitene stages and the frequency of inversion loops, but not relative loop size. Using a database on SNP distribution in the inbred strains examined, we carried out in silico mapping of genes affecting the genotype-dependent characters. We have found that the efficiency of synapsis in the inversion does not depend on interstrain differences in homology of the chromosome 1 region involved in the inversion. Genes controlling the inversion loop frequency in the inversion heterozygotes were mapped to chromosome 7, and genes controlling the duration of individual pachitene stages, to chromosomes 2 and 5.

Pattern of X-Y chromosome pairing in microtine rodents

Pairing of X and Y chromosomes at meiotic prophase in 14 species of the subfamily Microtinae (Clethrionomys rufocanus, C. rutilus, C. glareolus, Arvicola terrestris, Microtus guentheri, M. socialis, M. afghanus, M. bucharicus, M. oeconomus, M. arvalis, M. rossiaemeridionalis, M. kirgisorum, M. transcaspicus, M. (Pitymys) majori) was analysed in relation to their taxonomic position and variation in the morphology of their sex chromosomes. The sex chromosomes formed a synaptonemal complex (SC) at pachytene in all Clethrionomys species, Arvicola terrestris, and M. oeconomus, while they did not pair at all in M. (Pitymys) majori, Microtus socialis, M. guentheri, M. afghanus, M. bucharicus, M. arvalis, M. rossiaemeridionalis, M. kirgisorum, and M. transcaspicus. The X chromosome of these species varied in centromere position independently of pairing pattern. Insertion of heterochromatin of different size and location was found in some, but not in all species with asynaptic sex chromosomes. It is suggested that the sex chromosomes lost their ability to pair at male meiosis in the common ancestor of palearctic species of the genus Microtus. This event was not caused by a gross chromosomal rearrangement.

Inheritance of White Head Spotting in Natural Populations of South American Water Rat (Nectomys squamipes Rodentia: Sigmodontinae)

Specimens with white head spots are present at low frequency in the natural populations of South American water rat (Nectomys squamipes) and absent in the sibling species Nectomys rattus. We analyzed the pattern of inheritance of the phenotype using complex segregation analysis of pedigrees of a captive-bred population of N. squamipes. We found that the inheritance of the white head spot in this species can be described within the framework of the major gene recessive model with incomplete penetrance of genotypes.

B chromosomes of Korean field mouse Apodemus peninsulae (Rodentia, Murinae) analysed by microdissection and FISH

Organization of B chromosomes in the Korean field mouse Apodemus peninsulae was analyzed. We painted its metaphase chromosomes with whole and partial chromosome paints generated by microdissection and DOP-PCR. The results of the painting indicated that all B chromosomes contained a large amount of repeated DNA sequences. The repeats could be classified in terms of their homology and predominant location. Pericentromeric repeats of B chromosomes were present in many copies in pericentromeric C-blocks of all autosomes and in non-centromeric C-blocks of the sex chromosomes. B arm specific type 1 repeats comprised the main body of the arms of almost all B chromosomes and were present in the arms of A chromosomes as interspersed sequences. B arm-specific type 2 repeats were found at the ends of some B chromosomes that did not undergo compaction at the interphase- metaphase transition and remained uncondensed. On the basis of comparative analysis of localization of B chromosome repeats in the chromosomes of two related species, A. peninsulae and A. agrarius, we suggest a hypothesis of B chromosome origin and evolution in the genus Apodemus.

Inheritance of litter size at birth in the Brazilian grass mouse (Akodon cursor, Sigmodontinae, Rodentia)

By means of complex segregation analysis we studied the inheritance of litter size in two large pedigrees of captive-bred colonies of the Brazilian grass mouse Akodon cursor. Genetic analysis has revealed a highly significant influence of genetic factors on the variation of litter size (heritability, h2, was estimated as 0.44). The inheritance followed the classical polygene model: neither the major-gene model nor the polygene with unequal contribution model described the data significantly better.