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Biomedical subjects

J Britton-Davidian

Publications and source records attributed to J Britton-Davidian.

At least 19 recordsLinked to original sources
PubMed2006

Complex evolution of X and Y autosomal translocations in the giant mole-rat, Cryptomys mechowi (Bathyergidae).

Cross-species chromosome painting was used to determine homologous chromosomal regions between two species of mole-rat, the naked mole-rat, Heterocephalus glaber (2n = 60), and the giant mole-rat, Cryptomys mechowi (2n = 40), using flow-sorted painting probes representative of all but two of the H. glaber chromosomal complement. In total 43 homologous regions were identified in the C. mechowi genome. Eight H. glaber chromosomes are retained in toto in C. mechowi, and 13 produce two or more signals in this species. The most striking difference in the karyotypes of the two taxa concerns their sex chromosomes. The H. glaber painting probes identified a complex series of translocations that involved the fractionation of four autosomes and the subsequent translocation of segments to the sex chromosomes and to autosomal partners in the C. mechowi genome. An intercalary heterochromatic block (IHB) was detected in sex chromosomes of C. mechowi at the boundary with the translocated autosomal segment. We discuss the likely sequence of evolutionary events that has led to the contemporary composition of the C. mechowi sex chromosomes, and consider these in the light of prevailing views on the genesis of sex chromosomes in mammals.

Animals↗
PubMed2004

Adaptive energetics in house mice, Mus musculus domesticus, from the island of Porto Santo (Madeira archipelago, North Atlantic).

The bioenergetic strategies of house mice (Mus musculus domesticus) from the island of Porto Santo were investigated and compared with those of mice from mainland Portugal. Energy obtained from food ingestion was 18.2% lower in Porto Santo mice than in mainland mice (1.53 vs. 1.87 kJ/g/day). The same pattern was observed for metabolisable energy intake, which was 19.2% lower in island specimens (0.87 vs. 1.08 kJ/g/day for mainland specimens). Apparent digestibility was similar in both groups of mice. Basal metabolic rate (BMR) of Porto Santo individuals was low (1.16 ml O(2)/g/h), representing only 56% of the predicted value, based on body mass, while mainland individuals exhibited a BMR closer to the expected value, corresponding to 87% of the predicted value (1.80 ml O(2)/g/h). Thermoregulatory abilities within the range of 10-28 degrees C ambient temperature did not differ between island and mainland mice. Results suggest an adaptation of Porto Santo mice to the environmental aridity of the island of Porto Santo, leading to a conservative energetic strategy.

Adaptation, Psychological↗
PubMed2002

Chromosomal and allozyme analysis of a hybrid zone between parapatric Robertsonian races of the house mouse: a case of monobrachial homology.

A detailed chromosomal and allozyme analysis of Robertsonian (Rb) populations of the house mouse in Alsace (France) was performed to evaluate the model of speciation by monobrachial centric fusions. The karyological analysis confirmed the existence of a hybrid zone between two Rb races differentiated by monobrachially homologous Rb fusions. The clinical distribution of Rb fusions showed that almost no overlap occurred between the Rb fusions specific to each race, indicating the presence of an acrocentric peak in the center of the hybrid zone. Thus, only chromosomal heterozygotes carrying one to three trivalents were present, instead of the expected highly unfit interracial hybrids, none of which were observed. The effect of karyotypic differentiation on genic divergence was tested across the hybrid zone by comparing levels of genetic structure for chromosomal and allozyme markers according to the isolation by distance model. Results confirmed the highly significant spatial structure for chromosomes, but revealed none for allozymes, indicating that genic diversity was not structured according to karyotype. These data suggest the absence of a strong chromosomal barrier to gene flow, which does not conform with predictions of the model of speciation by monobrachial centric fusions. In addition, the relatedness of these Rb races to neighboring ones, as well as the nonrandom contribution of several chromosomes to the Rb fusions, was analyzed.

Animals↗
PubMed2002

Chromosomal characterization of Arvicanthis species (Rodentia, Murinae) from western and central Africa: implications for taxonomy.

A chromosome study of unstriped grass rats of the genus Arvicanthis (Rodentia, Murinae) in western and central Africa is presented. The observations extend the data available to 242 specimens from 59 localities. All individuals karyotyped belong to four karyotypic forms, or cytotypes, earlier described as ANI-1, ANI-2, ANI-3, and ANI-4 and are presumed to correspond to four distinct species. In order to provide diagnostic characters for these western and one central African Arvicanthis species, we standardized the chromosomal data available and developed a G- and C-banded chromosome nomenclature that allows easy species identification. Each form is characterized by a distinct geographical distribution, roughly following the biogeographical domains of western Africa, although their precise limits remain to be assessed. The sole area of sympatry detected is the region of the inner delta of the Niger River, where both ANI-1 and ANI-3 can be found. It is proposed that the three western African species ANI-1, ANI-3, and ANI-4 be renamed as A. niloticus, A. ansorgei, and A. rufinus, respectively.

Animals↗
PubMed2001

Molecular studies on the colonization of the Madeiran archipelago by house mice.

To study the colonization history of the house mouse (Mus musculus domesticus) on the Madeiran archipelago, complete mitochondrial D-loop sequences were obtained for 44 individuals from Madeira, Porto Santo and Ilhas Desertas. Altogether, 19 D-loop haplotypes were identified which formed part of a single clade in a phylogeny incorporating haplotypes from elsewhere in the range of M. m. domesticus, indicating that the Madeiras were colonized from a single source. Similarities between the sequences found in the Madeiras and those in Scandinavia and northern Germany suggest that northern Europe was the source area, and there is the intriguing possibility that the Vikings may have accidentally brought house mice to the archipelago. However, there is no record of Vikings visiting the Madeiras; on historical grounds, Portugal is the most likely source area for Madeiran mice and further molecular data from Portugal are needed to rule out that possibility.

Animals↗
PubMed2001

[Probable origin of the Robertsonian phenomena in domestic mice in Tunisia].

The Robertsonian phenomenon in house mice (Mus musculus domesticus) from Tunisia consists in the presence of only one 22-chromosome Robertsonian race (22Rb) carrying the maximum number of fusions observed until now. The 22Rb populations exclusively occupy urban centers in the Eastern-Central region of Tunisia where standard population with 40-all acrocentric chromosomes (40Std) occur in surrounding neighborhoods and rural environments. In addition to the habitat partition, allozyme and mitochondrial DNA analyses showed that the 22Rb populations were genetically differentiated from the 40Std ones. This differentiation mostly stemmed from an important decrease in genetic variability in the 22Rb populations from the Sahel towns. The extent of morphological, ecological and genetical divergence observed between these chromosomal races in Tunisia is in agreement with the predictions of the chromosomal speciation model of White which advocates that karyotypic differentiation between taxa can lead to their reproductive isolation and independent evolution. Such a process is verified if the Rb process in Tunisia results from local differentiation which is supported by both the genetic and morphological data. However, the hypothesis of an origin by introduction of these mice from another region of Tunisia or from another country cannot be totally dismissed. In this study, an allozymic analysis of mice (22Rb and 40Std) from the geographically distant city of Kairouan was performed. Results showed that 22Rb and 40Std mice from Kairouan shared the same high degree of variability, and were not genetically differentiated. This contrasts with the results registered in the two chromosomal races in the Sahel towns. Such data argue in favor of a local differentiation of the Robertsonian process in Tunisia and suggest that the decrease in variability of the structural nuclear genes in the Sahel 22Rb populations can be related to an introduction from Kairouan into a Sahel locality resulting in a founder effect or followed by a severe bottleneck prior to its dispersion throughout the Sahel region.

Animals↗
PubMed2000

Spontaneous occurrence of a Robertsonian fusion involving chromosome 19 by single whole-arm reciprocal translocation (WART) in wild-derived house mice.

Chromosomal races of the house mouse (Mus musculus domesticus) bear Robertsonian (Rb) fusions, which consist of centric translocations between two non-homologous acrocentric chromosomes. The high level of diversity of these fusions in house mice is generated by de-novo formation of Rb fusions and subsequent whole-arm reciprocal exchanges (WARTs). This paper describes the spontaneous occurrence of a new Rb fusion, Rb(4.19), in progeny of wild-derived house mice segregating for Rb(4.12). The chromosomal mutation was traced to a female which exhibited germline and somatic mosaicism indicating an early embryonic origin of the mutation. FISH analysis of centromerically-located ribosomal genes suggested that no modification was observed on chromosomes 12 and 19 prior to or following the occurrence of Rb(4.19). Distribution of telomeric sequences showed that both Rb fusions lacked telomeres in their centromeric regions. It is argued that this spontaneous mutation most likely originated by single whole-arm reciprocal translocation (WART) between Rb(4.12) and an acrocentric chromosome 19, resulting in Rb(4.19) and a neo-acrocentric chromosome 12. Sequences required for centromeric function and proximal telomeres would have been transferred to the neo-chromosome 12 from chromosome 19 during the translocation. The existence of such WARTs which generate derived acrocentric chromosomes has several implications for chromosomal evolution in house mice.

Animals↗
PubMed1999

Microgeographical distribution of two chromosomal races of house mice in Tunisia: pattern and origin of habitat partitioning.

Two chromosomal races of the house mouse occur in Tunisia, a standard morph (40St) found all over the country, and a derived morph (22Rb) occurring only in central Tunisia. In this region, habitat partitioning between the two morphs was investigated by a microgeographical analysis of their distribution, assessing habitat characteristics and demographic parameters. Results showed that the 22Rb mice always occurred in the oldest sections of towns (medinas), often extending to more recent surrounding neighbourhoods where the 40St morph was most abundant. The latter was never trapped within the medinas. The transition between the two morphs was located within cities in the more recent areas, the hybrid zone being estimated at less than 0.5 km in width by a clinal analysis of chromosomal data. Although differences between habitats exist, almost no demographic differences were found between populations of the two morphs when they occurred in the same or in different habitats. Two hypotheses are discussed to account for the origin of habitat partitioning. The first relies on competitive exclusion of the 40St mice from the medinas by the derived 22Rb mice; the second is based on stochastic processes related to historical evolution of Tunisian urban communities.

Animals↗
PubMed1998

Population genetic structure in a Robertsonian race of house mice: evidence from microsatellite polymorphism.

Genetic evidence was assessed for inbreeding and population subdivision in a Robertsonian fusion (Rb) race of the western European form of house mouse, Mus musculus domesticus, in central Belgium. Inbreeding, and the factors responsible for subdivision (genetic drift and extinction-recolonization) can theoretically influence the fixation of underdominant Rb variants. The data consisted of allele frequencies of eight microsatellite loci and of the Rb(4.12) and Rb(5.10) chromosomes. Six populations were sampled once, and a seventh was sampled successively over 3 years. No evidence for inbreeding within populations was found. Levels of between-population subdivision were high (theta = 0.15-0.39), and showed no association with either karyotype or geographical distance over 8-60 km. In addition, low values of effective size were found in the successively sampled population (Ne = 5-20). Cases of significant two-locus disequilibria were associated with the most closely linked pair of microsatellite loci (r = 0.15): also consistent with small effective sizes. These results suggest that both the lack of inbreeding, and the combined effects of genetic drift and extinction-recolonization, may promote Rb polymorphism in M. m. domesticus.

Animals↗
PubMed1997

Chromosomal transmission bias in laboratory hybrids between wild strains of the two European subspecies of house mice.

Laboratory crosses between wild strains of the two European house mouse subspecies Mus musculus domesticus (2n = 34) and M. m. musculus (2n = 40) were performed to analyze the selective processes involved in the non-introgression of centromeric regions of Robertsonian (Rb) fusions in the Danish hybrid zone. The chromosomal analysis of 226 backcross progeny from 22 reciprocal crosses showed that the segregation of the three Rb fusions present did not significantly differ from Mendelian expectations. However, a significant negative correlation was found between Rb transmission rates and the average litter sizes of the F1 pairs. Among the different models of selection discussed, the most likely one supported the existence of two opposing selective factors resulting in an overall compensation of chromosomal types in the backcross progeny. A two-phase selective process involving embryo competition was postulated with non-Rb carriers being favored during pre-implantation but disadvantaged after implantation. Such balanced selective pressures acting on musculus non-Rb centromeres are compatible with the steep slope and off-centered position of the chromosomal cline observed in the Danish hybrid zone. These results suggested that these selective factors may be more related to centromere origin (musculus or domesticus) than to centromere structure (Rb or non-Rb).

Animals↗
PubMed1996

Genomic incompatibilities in the hybrid zone between house mice in Denmark: evidence from steep and non-coincident chromosomal clines for Robertsonian fusions.

The pattern of chromosomal variation is investigated in house mice from the Danish hybrid zone between the translocation-prone Mus musculus domesticus and the chromosomally conservative M. m. musculus. The cytogenetic analysis confirmed the non-introgression of three pairs of Robertsonian (Rb) fusions from M. m. domesticus into the M. m. musculus genome. The geographic distribution of two of these Rb fusions was shown to follow staggered chromosomal clines which increased in steepness the closer they were to the centre of the hybrid zone as defined by allozymes. Analysis of alternate hypotheses suggests that chromosomal differentiation of the Danish domesticus occurred after contact was established with musculus. The staggering of the clines would reflect the order of arrival of the Rb fusions into the hybrid zone. Several models with different processes of underdominance of the chromosomal heterozygotes are discussed to account for the difference in width between clines. A selective model with increasing levels of genomic underdominance due to interaction with a progressively enriched musculus genome provides the best fit for the observed pattern. Selection against Rb fusions with little effect on the recombination of linked allozyme markers supports the view that no reduction in gene flow due to chromosomal heterozygosity is yet apparent through the hybrid zone and that only the centromeric segments of the Rb fusions are incompatible with the musculus genome.

Animals↗
PubMed1996

Unconventional organization of amplified esterase B gene in insecticide-resistant mosquitoes of the Culex pipiens complex.

The amplified esterase B gene responsible for resistance to organophosphorus insecticides in Culex mosquitoes is shown to form a DNA "puff" in salivary gland polytene chromosomes. This "puff" is absent in susceptible mosquitoes lacking the amplified gene, and unlike all puffs known today, it is neither related to development nor involved in protein synthesis. The "puff" therefore corresponds to a special arrangement of the very large number of copies (possibly up to 250,000) of the 25-30-kb DNA fragment encompassing the esterase B structural gene in polytene chromosomes. In addition, the amplified and nonamplified esterase B gene(s) were found to be localized on chromosome 2L, and flanked by identical banding patterns, indicating the absence of an important chromosome rearrangement before, during, or after gene amplification.

Animals↗
PubMed1993

Fertility estimates in the Tunisian all-acrocentric and Robertsonian populations of the house mouse and their chromosomal hybrids.

The reproductive features of wild all-acrocentric and 2n = 22 Robertsonian (Rb) house mice (M. m. domesticus) from Tunisia were studied. The aim was to examine the possibility of a reproductive selective advantage associated with chromosomal change as well as to measure the effect of heterozygosity for a large number of Rb fusions on the fertility of hybrids. Results showed that litter sizes were significantly smaller in Rb than in all-acrocentric mice. This difference, which may represent a favourable demographic strategy related to the habitat segregation observed in the Tunisian mice, needs to be studied further. The F1 hybrids between the two chromosomal races showed a significantly reduced reproductive success and litter size (respectively, 53 per cent and 60 per cent less than either parental race). Analysis of the testicular histology of F1 and backcross males showed in some cases a breakdown of spermatogenesis. The degree of this disturbance was not related to the level of chromosomal heterozygosity suggesting that genetic incompatibilities between the two genomes might be involved. The strong reduction in fertility measured in these hybrids represents a reproductive isolating mechanism effectively reducing gene flow between the all-acrocentric and 22Rb mice populations of Tunisia.

Analysis of Variance↗
PubMed1991

[New Robertsonian populations of the house mouse (Mus musculus domesticus) from north of the Alps].

New Robertsonian (Rb) populations of the house mouse (Mus musculus domesticus) carrying different combinations of centric fusions are reported in France, Switzerland, and Germany. In Alsace (France), the diploid number varied from 2n = 38 to 34; four fusions were present, with Rb(4.12) homozygous in all populations whereas Rb(5.10), Rb(5.7), or Rb(10.14) were found to be segregating. In Switzerland, only all-acrocentric mice (2n = 40) were present in Bern while Rb(5.7) and Rb(9.16) occurred in Basel. In the Konstanz locality from southern Germany, all the mice were homozygous for nine Rb fusions: Rb(1.18), Rb(2.5), Rb(3.6), Rb(4.12), Rb(7.15), Rb(8.17), Rb(9.14), Rb(10.11), and Rb(13.16). The phylogenetic relationship of these new Rb populations with those already known is discussed.

Animals↗
PubMed1990

Kinetics of oogenesis in mice heterozygous for Robertsonian translocation.

The total number of oocytes at different postmating time intervals (18-40 days) was determined in mice homozygous and heterozygous for different Robertsonian (Rb) translocations, of both laboratory and feral origin. The number of oocytes was lower in heterozygous than in homozygous mice throughout the period studied. Independently of the genetic background (i.e. laboratory or feral), structural heterozygosity had a progressive detrimental effect on oocyte numbers: open, or chain diakinetic configurations had a greater detrimental effect than close, or ring, configurations. The genetic background, however, affected the ovarian constitution in terms of the total number of germ cells, which are more numerous in laboratory than in feral mice. The kinetics of oogenesis seems to be faster in feral than in laboratory mice. At the light of the data here presented, and of those already available from the literature on male and female gametogenesis in conditions of structural heterozygosity, it appears that factors other than unsaturation of pairing sites or interference with pachytene X-chromosome inactivation have to be considered. In the wild, the reduced oocyte numbers in Rb heterozygous female can contribute to the retention of isolated populations in contact zones.

Animals↗