Robertsonian karyotype variation in wild house mice from Rhaeto-Lombardia

The European longtailed house mouse (M. m. brevirostris and domesticus) in the Rhaetian Alps and Lombardia presents a complex system of Robertsonian (Rb) variation and karyotype diversity, several adjoining populations homozygous for multiple Rb metacentric chromosomes, sites of coexistence of different Rb types, and zones of hybridization with non-Rb populations. The original "tobacco mouse" is just one of many local Rb variants, such as those from other Alpine areas (e.g., Orobian Alps) or from Central Lombardia, where a relatively large region within which the population is homogeneous for multi-Rb metacentrics is found. The present study is based strictly on material in which the chromosome arms were identified by G-banding, so that karyotypes within the areas under investigation could be compared. Altogether 111 mice were studied.     

The phylogeny of Chinese indigenous pig breeds inferred from microsatellite markers

A genetic study of 32 local Chinese, three foreign pig breeds [Duroc (DU), Landrace and Yorkshire], and two types of wild boar (Hainan and Dongbei wild boar) based on 34 microsatellite loci was carried out to clarify the phylogeny of Chinese indigenous pig breeds. The allele frequencies, effective numbers of alleles, and the average heterozygosity within populations were calculated. The results showed that the genetic variability of the Lingao pig was the largest, while the Jiaxing pig was the lowest. The greatest distance between domestic pigs was found between Shanggao and DU pig and the shortest was found between Wuzhishan and Lingao pig, respectively. A neighbour-joining tree constructed from Modified Cavalli-Sforza genetic distances divided Chinese pigs into two clusters; four subclusters were also identified. Our results only partly agree with the traditional types of classification and also provide a new relationship among Chinese local pig breeds. Our data also confirmed that Chinese pig breeds have a different origin from European/American breeds and can be utilized in programmes that aim to maintain Chinese indigenous pig breeds.     

Molecular analysis of nondisjunction in mice heterozygous for a Robertsonian translocation

A Robertsonian translocation results in a metacentric chromosome produced by the fusion of two acrocentric chromosomes. Rb heterozygous mice frequently generate aneuploid gametes and embryos, providing a good model for studying meiotic nondisjunction. We intercrossed mice heterozygous for a (7.18) Robertsonian translocation and performed molecular genotyping of 1812 embryos from 364 litters with known parental origin, strain, and age. Nondisjunction events were scored and factors influencing the frequency of nondisjunction involving chromosomes 7 and 18 were examined. We concluded the following: 1. The frequency of nondisjunction among 1784 embryos (3568 meioses) was 15.9%. 2. Nondisjunction events were distributed nonrandomly among progeny. This was inferred from the distribution of the frequency of trisomics and uniparental disomics (UPDs) among all litters. 3. There was no evidence to show an effect of maternal or paternal age on the frequency of nondisjunction. 4. Strain background did not play an appreciable role in nondisjunction frequency. 5. The frequency of nondisjunction for chromosome 18 was significantly higher than that for chromosome 7 in males. 6. The frequency of nondisjunction for chromosome 7 was significantly higher in females than in males. These results show that molecular genotyping provides a valuable tool for understanding factors influencing meiotic nondisjunction in mammals.     

Robertsonian translocation in a Zebu bull

Karyotypes were prepared for ten Rahaji bulls using cultured peripheral blood lymphocytes. C-banded metaphases were prepared to identify the sex chromosomes and possible translocations. Out of the ten bulls, studied one bull had a 2n number of 59 chromosomes and carried a Robertsonian translocation. The possible importance of the Robertsonian translocation in Zebu cattle in Nigeria is discussed.     

Parental origin and timing of de novo Robertsonian translocation formation

Robertsonian translocations (ROBs) are the most common chromosomal rearrangements in humans. ROBs are whole-arm rearrangements between the acrocentric chromosomes 13-15, 21, and 22. ROBs can be classified into two groups depending on their frequency of occurrence, common (rob(13q14q) and rob(14q21q)), and rare (all remaining possible nonhomologous combinations). Herein, we have studied 29 case subjects of common and rare de novo ROBs to determine their parental origins and timing of formation. We compared these case subjects to 35 published case subjects of common ROBs and found that most common ROBs apparently have the same breakpoints and arise mainly during oogenesis (50/54). These probably form through a common mechanism and have been termed "class 1." Collectively, rare ROBs also occur mostly during oogenesis (7/10) but probably arise through a more "random" mechanism or a variety of mechanisms and have been termed "class 2." Thus, we demonstrate that although both classes of ROBs occur predominantly during meiosis, the common, class 1 ROBs occur primarily during oogenesis and likely form through a mechanism distinct from that forming class 2 ROBs.     

Genetic variation and identification of promising sour cherries inferred from microsatellite markers

The aim of this study was to identify the group of highly polymorphic microsatellite markers for identification of promising sour cherries. From among 30 tested microsatellite (SSR) markers, 19 were selected to profile genetic variation in sour cherries due to high polymorphisms. Results indicated a high level of polymorphism of the accessions based on these markers. Totally 148 alleles were generated at 19 SSR loci which 122 alleles were polymorphic. The number of total alleles per locus ranged from 2 to 15 with an average of 7.78 and polymorphism percentage varied from 50 to 100% with an average of 78.76%. Also, PIC varied from 0.47 to 0.89 with an average of 0.79 and heterozygosity ranged from 0.35 to 0.55 with a mean of 0.45. According to these results, these markers specially PMS3, PS12A02, PceGA34, BPPCT021, EMPA004, EMPA018, and Pchgms3 produced good and various levels of amplifications and showed high heterozygosity levels. By the way, the genetic similarity showed a high diversity among the sour cherries. Cluster analysis separated improved cultivars from promising sour cherries, and the PCoA supported the cluster analysis results. Since the studied sour cherries were superior to the improved cultivars and were separated from them in most groups, these sour cherries can be considered as distinct genotypes for further evaluations in the framework of breeding programs and new cultivar identification in cherries. Results also confirmed that the set of microsatellite markers employed in this study demonstrated usefulness of microsatellite markers for the identification of sour cherry genotypes.     

Developmental stability in house mice heterozygous for single Robertsonian fusions

Fluctuating asymmetry (FA) of tooth traits has been reported to be increased in Down syndrome patients as well as hybrids between chromosomal races of the house mouse differing in several Robertsonian (Rb) fusions. Developmental stability, assessed by FA, is thus thought to be impaired by spontaneous chromosomal abnormality or by chromosomal heterozygosity. Although the effect of a single fusion on developmental stability could theoretically be expected, it has never been documented. Crosses involving two chromosomal races of the house mouse diverging for one Rb fusion were performed to assess developmental stability in parental homozygous races as well as in their hybrids. Moreover, the occurrence of a spontaneous chromosomal mutation (WART type-b) allowed us to study the instantaneous effect of such a translocation on developmental stability. No difference in fluctuating asymmetry levels was detected among the groups considered in this study. This result suggested that a single stable or spontaneous balanced structural rearrangement did not inherently disturb developmental stability. In addition, the differential effect on developmental stability of one versus many heterozygous Rb fusions highlights the role of their quantitative accumulation in the disruption of coadaptation in chromosomal hybrids.     

Interregional differentiation of chum salmon from Sakhalin and South Kurils inferred from microsatellite markers

Variability at ten microsatellite loci was examined in wild and hatchery populations of chum salmon (Oncorhynchus keta Walbaum) from the Sakhalin Island and Southern Kuril Islands, Iturup and Kunashir. Substantial genetic differences between Sakhalin and South Kurils chum salmon (the differentiation reached 6.0%) were revealed. Statistically significant differences between chum salmon from Iturup and those from Kunashir were demonstrated, as well as between the chum salmon populations from different rivers within the islands. It was shown that in different types of population comparisons, different marker sets were most informative.     

Robertsonian translocations introduced into an island population of house mice

In April 1982, 77 house mice from the Orkney Island of Eday were released by R. J. Berry and his associates on the Isle of May, Firth of Forth. The Isle of May had a standard house mouse karyotype (2n = 40), while those from Eday are homozygous for three centric fusions (2n = 34). Within 18 months of introduction (September 1983), each centric fusion had increased in frequency from an estimated starting value of 8% to a value close to 50%, and they were segregating in accordance with Hardy–Weinberg expectations. In essence, the transformed population was behaving as a panmictic unit. The frequencies of introduced chromosomes had apparently stabilized by September 1986 with values around 65% for all three fusions. The cytogenetic data obtained in the Isle of May introduction experiment accord well with data for single gene loci (represented by allozyme data) and morphometric data. Male Eday–May F1 hybrids were found to have a low frequency of non-disjunction (13%). This study is unusual because the successful introduction of mice into an established population, and the introgression and stabilization of three centric fusions, could not have been predicted from previous studies on the mouse.     

Robertsonian chromosomal variation and identification of metacentric chromosomes in feral mice

Cytogenetic studies of feral mice (M. musculus) from various but predominantly Alpine areas of Switzerland, carried out on random samples collected by spot-checks, established the widespread existence of metacentric chromosomes in the somatic karyotype. Despite the finding of the common occurrence of some of the metacentrics in different places, the examination of the possible homology or heterology by breeding procedures revealed the surprising fact that independence, partial or heterobrachial homology of the metacentric chromosomes prevail among mice from different geographical areas. Thus, the general picture is that of an array of different metacentric chromosomes derived from independent events of Robertsonian variation in the process of evolution. — While heterozygosity with independent metacentrics within a Robertsonian system may have a bearing on the fertility rate of a given mouse population, a more severe impairment of the reproductive capacity must be taken into account in mouse populations which possess different metacentrics with mono- or heterobrachial homologies. These conditions favour the assumption of the existence of a selective system of reproductive barriers further subdividing the species in many, more or less stable, micro-populations. — The chromosomal arms (telocentrics) involved in the formation of the metacentric chromosomes could be identified by Q- and G-banding techniques in combination with the results of crossbreeding, and were assigned to the corresponding telocentric autosomes of the mouse (Comm. Standard. Genet. Nomenclat. for Mice, 1972). Most of the telocentric autosomes of the mouse are included in one or more of the metacentrics found in the feral populations. By means of their isolation in separate lines, these metacentrics may be useful in experimental biology as marker chromosomes of defined identity carrying known linkage groups.     

Genetically diversified populations in the loach Misgurnus anguillicaudatus inferred from newly developed microsatellite markers

The loach Misgurnus anguillicaudatus is a freshwater fish which include natural polyploids and clones besides most bisexually reproducing diploids. Recent genetic studies using allozymes differentiated the populations in northern region of Hokkaido from other populations in Japan. Here, we developed 23 new sets of polymerase chain reaction primers to detect polymorphic microsatellites loci and tested them in 20 individuals from Memanbetsu town (northern Hokkaido) and Kita village (southern Hokkaido). These markers can be potentially diagnostic for identification of two populations.     

Features of evolution and expansion of modern humans,inferred from genomewide microsatellite markers

We study data on variation in 52 worldwide populations at 377 autosomal short tandem repeat loci, to infer a demographic history of human populations. Variation at di-, tri-, and tetranucleotide repeat loci is distributed differently, although each class of markers exhibits a decrease of within-population genetic variation in the following order: sub-Saharan Africa, Eurasia, East Asia, Oceania, and America. There is a similar decrease in the frequency of private alleles. With multidimensional scaling, populations belonging to the same major geographic region cluster together, and some regions permit a finer resolution of populations. When a stepwise mutation model is used, a population tree based on TD estimates of divergence time suggests that the branches leading to the present sub-Saharan African populations of hunter-gatherers were the first to diverge from a common ancestral population (approximately 71-142 thousand years ago). The branches corresponding to sub-Saharan African farming populations and those that left Africa diverge next, with subsequent splits of branches for Eurasia, Oceania, East Asia, and America. African hunter-gatherer populations and populations of Oceania and America exhibit no statistically significant signature of growth. The features of population subdivision and growth are discussed in the context of the ancient expansion of modern humans.     

Male meiosis and gametogenesis in wild house mice (Mus musculus domesticus) from a chromosomal hybrid zone; a comparison between simple Robertsonian heterozygotes and homozygotes.

Wild male house mice Mus musculus domesticus were collected from the hybrid zone between the John o'Groats race (2n = 32) and the standard race (2n = 40) in northern Scotland. Meiosis in both homozygotes (2n = 32, 36, and 40) and single Robertsonian heterozygotes (2n = 33, 35, and 37) was found to be orderly. At prophase/metaphase I in heterozygotes, a trivalent was formed from the metacentric and two homologous acrocentrics. At pachytene, this trivalent usually had a single side arm at the position of the centromeres, as a result of nonhomologous pairing of the acrocentrics. This side arm persisted into diplotene. Generally only a single chiasma was formed between each acrocentric and the metacentric. Anaphase I nondisjunction frequencies were estimated as 1.5% for the homozygotes and 2.7% for the heterozygotes. The extent of germ cell death between the pachytene and round spermatid stages was 18% greater in heterozygotes than in homozygotes. Our results concur with previous studies which indicate that single Robertsonian heterozygotes in wild house mice have near-normal fertility.     

The evolution and phylogeography of the African elephant inferred from mitochondrial DNA sequence and nuclear microsatellite markers

Recent genetic results support the recognition of two African elephant species: Loxodonta africana, the savannah elephant, and Loxodonta cyclotis, the forest elephant. The study, however, did not include the populations of West Africa, where the taxonomic affinities of elephants have been much debated. We examined mitochondrial cytochrome b control region sequences and four microsatellite loci to investigate the genetic differences between the forest and savannah elephants of West and Central Africa. We then combined our data with published control region sequences from across Africa to examine patterns at the continental level. Our analysis reveals several deeply divergent lineages that do not correspond with the currently recognized taxonomy: (i) the forest elephants of Central Africa; the forest and savannah elephants of West Africa; and (iii) the savannah elephants of eastern, southern and Central Africa. We propose that the complex phylogeographic patterns we detect in African elephants result from repeated continental-scale climatic changes over their five-to-six million year evolutionary history. Until there is consensus on the taxonomy, we suggest that the genetic and ecological distinctness of these lineages should be an important factor in conservation management planning.     

The FMR1 CGG repeat and linked microsatellite markers in two Basque valleys

Fragile X syndrome is associated with an unstable CGG repeat sequence in the 5' untranslated region of the first exon of the FMR1 gene. The present study involved the evaluation of factors implicated in CGG repeat stability in a normal sample from two Basque valleys (Markina and Arratia), to discover whether the Basque population shows allelic diversity and to identify factors involved, by using the data in conjunction with previous findings. The study was based on a sample of 204 and 58 X chromosomes from the Markina and Arratia valleys, respectively. The CGG repeat, the AGG interspersion and two flanking microsatellite markers, FRAXAC1 and DXS548, were examined. In the Markina valley, gray zone alleles (> or =35 CGG repeats) were associated with anchoring AGGs, with the longest 3' pure CGG repeats of the valley (=15), with the 5' instability structure 9+n and with one principal fragile X FRAXAC1-DXS548 haplotype 42-50. In the Arratia valley, gray zone alleles (> or =35 CGG repeats) showed the highest frequency among the Basque samples analyzed, and were associated with anchoring AGGs, with the longest 3' pure repeats (> or =20), with the 5' instability structure 9+n and with one "normal" FRAXAC1-DXS548 haplotype 38-40 (these data from Arratia suggest the existence of a "protective" haplotype). The results showed, on the one hand, differences between Markina and Arratia in factors implicated in CGG repeat instability and, on the other hand, a great similarity between the general Basque sample from Biscay and the Markina valley.     

Reciprocal translocation at the origin of a Robertsonian translocation 15;22 by the loss of a metacentric chromosome. Genetic counseling

One of the children of a t(15;22) (q111;p11) woman has lost the minute metacentric der(15) without any clinical consequence, indicating the inocuity of the 15pter----q111 and 22pter----p11 monosomies. The segregation mechanism of this monosomy and, from this family, the relation between reciprocal translocations and Robertsonian translocations are discussed. Another subject with r(22) in the same family questions on an hypothetic common origin.