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Biomedical subjects

P Boursot

Publications and source records attributed to P Boursot.

At least 19 recordsLinked to original sources
PubMed2005

Invasion from the cold past: extensive introgression of mountain hare (Lepus timidus) mitochondrial DNA into three other hare species in northern Iberia.

Mitochondrial DNA introgression from Lepus timidus into Lepus granatensis and Lepus europaeus was recently reported in Iberia, although L. timidus presumably retreated from this region at the end of the last ice age. Here we assess the extent of this ancient mtDNA introgression by RFLP analysis of 695 specimens representing the three hare species present in Iberia. The introgressed L. timidus lineage was found in 23 of the 37 populations sampled. It is almost fixed in L. europaeus across its Iberian range in the Pyrenean foothills, and in L. granatensis, which occupies the rest of the peninsula, it is predominant in the north and gradually disappears further south. We also found it in Lepus castroviejoi, a species endemic to Cantabria. Multiple hybridizations and, potentially, a selective advantage for the L. timidus lineage can explain the remarkable taxonomic and geographical range of this mitochondrial introgression.

Animals↗
PubMed2004

Mouse biodiversity in the genomic era.

Comparative genomics has developed by comparison of distantly related genomes, for which the link between the reported evolutionary changes and species development/physiology/ecology is not obvious. It is argued that the mouse (genus Mus) is an optimal model for microevolutionary genomics in vertebrates. This is because the mouse genome sequence, physical and genetic map have been completed, because mouse genetics, morpho-anatomy, pathology, behavior and ecology are well-studied, and because the Mus genus is a diverse, well- documented taxon, allowing comparative studies at the level of individual, population, subspecies, and species. The potential of the interaction between mouse genome and mouse biodiversity is illustrated by recent studies of speciation in the house mouse Mus musculus, and studies about the evolution of isochores, the peculiar pattern of GC-content variation across mammalian genomes.

Animals↗
PubMed2003

Nuclear ribosomal DNA monophyly versus mitochondrial DNA polyphyly in two closely related mite species: the influence of life history and molecular drive.

In two very closely related but reproductively isolated mite species, Tetranychus urticae and T. turkestani, we found nucleotide diversity to be extensive for mitochondrial DNA (mtDNA) cytochrome oxidase 1 (COI) (3-4%) but extremely reduced for nuclear ribosomal DNA (rDNA) internal transcribed spacer (ITS2) (less than 0.5%). By contrast, ITS2 was shown to evolve much faster than COI between species of this genus. Furthermore, we found that these two species are polyphyletic for mtDNA but monophyletic for rDNA. Thus it appears that despite its biparental transmission and multiplicity of copies in the genome, nuclear rDNA has a smaller effective population size than mtDNA in these species. The conjunction of efficient concerted evolution and/or gene conversion in the rDNA cluster, the haplodiploidy of these species and their female-biased sex ratio could account for this apparent contradiction.

Animals↗
PubMed2001

Interpretation of variation across marker loci as evidence of selection.

Population structure and history have similar effects on the genetic diversity at all neutral loci. However, some marker loci may also have been strongly influenced by natural selection. Selection shapes genetic diversity in a locus-specific manner. If we could identify those loci that have responded to selection during the divergence of populations, then we may obtain better estimates of the parameters of population history by excluding these loci. Previous attempts were made to identify outlier loci from the distribution of sample statistics under neutral models of population structure and history. Unfortunately these methods depend on assumptions about population structure and history that usually cannot be verified. In this article, we define new population-specific parameters of population divergence and construct sample statistics that are estimators of these parameters. We then use the joint distribution of these estimators to identify outlier loci that may be subject to selection. We found that outlier loci are easier to recognize when this joint distribution is conditioned on the total number of allelic states represented in the pooled sample at each locus. This is so because the conditional distribution is less sensitive to the values of nuisance parameters.

Animals↗
PubMed2000

A new method for locating changes in a tree reveals distinct nucleotide polymorphism vs. divergence patterns in mouse mitochondrial control region.

A new, model-based method was devised to locate nucleotide changes in a given phylogenetic tree. For each site, the posterior probability of any possible change in each branch of the tree is computed. This probabilistic method is a valuable alternative to the maximum parsimony method when base composition is skewed (i.e., different from 25% A, 25% C, 25% G, 25% T): computer simulations showed that parsimony misses more rare --> common than common --> rare changes, resulting in biased inferred change matrices, whereas the new method appeared unbiased. The probabilistic method was applied to the analysis of the mutation and substitution processes in the mitochondrial control region of mouse. Distinct change patterns were found at the polymorphism (within species) and divergence (between species) levels, rejecting the hypothesis of a neutral evolution of base composition in mitochondrial DNA.

Animals↗
PubMed2000

Comparison of ribosomal ITS regions among Androctonus spp. sorpions (Scorpionida: Buthidae) from Tunisia.

Three species of scorpions have been reported from Tunisia: Androctonus amoreuxi Audoin, A. aeneas Koch, and A. australis L. The latest has been divided in two subspecies: A. australis garzonii Goyffon & Lamy and A. australis hector Koch. Despite the public health importance of these animals, which cause several cases of poisoning each year, nothing is know about the genetic diversity of the group. To gain a better understanding of the group, we studied the variation of rDNA sequences of the ITS1, 5.8S, and ITS2 region of 14 representatives of the four taxa in Tunisia. The main result is the high polymorphism of the ITS regions. In some instances in both intra- and interspecific comparisons, it was difficult to unambiguously align the sequences. However, some representatives of different species were relatively similar, so that it appeared difficult to recognize the species on the basis of these sequences. A phylogenetic analysis was conducted that inquires the validity of the subspecies status of A. australis garzonii and A. australis hector. Moreover, the taxonomic status of A. aeneas is also questioned. Our studies highlighted the need of a revision of the taxonomy of the scorpions in Tunisia; however, the use of other genetic markers will be necessary to solve this question.

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↗
PubMed1998

Species-wide homogeneity of nuclear ribosomal ITS2 sequences in the spider mite Tetranychus urticae contrasts with extensive mitochondrial COI polymorphism.

We compared patterns of intraspecific polymorphism of two markers with contrasted modes of evolution, nuclear ribosomal DNA (rDNA) and mitochondrial DNA (mtDNA), in the phytophagous mite Tetranychus urticae Koch. The second internal transcribed spacer (ITS2) of rDNA and a fragment in the mtDNA gene coding for Cytochrome Oxidase I (COI), were PCR-amplified and sequenced in samples of various geographical origins distributed worldwide. The 15 COI haplotypes found fell into two major phylogenetic lineages differing by an average of 5% nucleotide divergence. Samples from the Mediterranean basin were represented in both lineages, and showed no phylogeographical structure. The other samples, from temperate regions of the northern hemisphere, were clustered in one of the lineages and displayed little variation, indicating a recent colonization of this region. In contrast, no variation at all was found at the ITS2 in this species. We sequenced both COI and ITS2 in four other species of the genus Tetranychus and found that, despite the absence of intraspecific polymorphism, ITS appears to evolve 2.5 times faster than COI. We argue that rDNA homogeneity over the species range of T. urticae results from the high colonization potential of this species, preventing long-term differentiation. Preliminary data on two other mite species (Amphitetranychus viennensis Zacher and Mononychellus progresivus Doreste) with stricter ecological requirements and more restricted colonization potential revealed substantial and concordant geographical differentiation for both ITS2 and COI.

Animals↗
PubMed1997

Social structure of the mound-building mouse Mus spicilegus revealed by genetic analysis with microsatellites.

The Mound-building mouse Mus spicilegus possesses a unique behaviour amongst mice. It constructs large earthen mounds and associated nesting chambers which serve to store food for immature individuals during the winter nesting period. We have used genetic analysis of four autosomal and four X-linked microsatellite loci to determine relationships between individuals inhabiting 40 mounds in Bulgaria. We show that, in almost all cases, individuals in a mound are the product of multiple parentage. We estimate the minimum number of males and female parents contributing offspring to each mound and demonstrate that at least two male and two female parents contribute offspring to a minimum of seven mounds. Analyses of relatedness coefficients and allele sharing values demonstrate that parents of different sibships within mounds are more related than if they had been chosen at random from the population and suggest that it is the female parents that contribute this excess relatedness. These results suggest that the mechanism by which individuals congregate to build mounds is kin-based and that the evolution of mound building and communal nesting in M. spicilegus is due in part to kin selection. This study represents a novel approach to the study of mammalian behavioural ecology. We have used a genetic dataset to construct an outline of social structure in the absence of behavioural data. These inferences can now be used to direct further work on this species.

Alleles↗
PubMed1997

Discordant phylogeographic patterns between the Y chromosome and mitochondrial DNA in the house mouse: selection on the Y chromosome?

We have compared patterns of geographic variation and molecular divergence of mitochondrial DNA (mtDNA) and Y chromosome over the range of the different subspecies of Mus musculus. MtDNA was typed for 305 nucleotides in the control region, the Y chromosome for 834 base pairs (bp) in Zfy introns and 242 bp in Sry, a Zfy2 18-bp deletion, and two microsatellites. Apparent discrepancies exist between the distributions of the lineages of mtDNA and of the two major Y-chromosome lineages thus defined: some subspecies share the same mtDNA lineage but have different Y-chromosome lineages or vice versa. One microsatellite reveals a geographically clustered variation inside the distribution of each Y-chromosome lineage, showing that new Y-chromosome variants can rapidly spread locally. The two major Y-chromosome lineages have a divergence time only about one fourth of that between mtDNA lineages. Although this recent coalescence of the Y chromosomes between subspecies could partly be due to a lower ancestral polymorphism of the Y chromosome, it suggests that secondary introgression after the radiation of the subspecies might have occurred. There is evidence that the differentiation of the Y-chromosome lineages contributes to partial reproductive isolation between subspecies, and patterns of molecular evolution suggest that selection has played a role in the rapid spread across subspecies.

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

Mitochondrial COI sequences in mites: evidence for variations in base composition.

Studies of mitochondrial DNA sequences in a variety of animals have shown important differences between phyla, including differences in the genetic codes used, and varying constraints on base composition. In that respect, little is known of mites, an important and diversified group. We sequenced a portion (340 nt) of the cytochrome oxidase subunit I (COI) encoding gene in twenty species of phytophagous mites belonging to nine genera of the two families Tetranychidae and Tenuipalpidae. The mitochondrial genetic code used in mites appeared to be the same as in insects. As is generally also the case in insects, the mite sequences were very rich in A + T (75% on average), especially at the third codon position (94%). However, important variations of base composition were observed among mite species, one of them showing as little as 69% A + T. Variations of base composition occur mostly through synonymous transitions, and do not have detectable effects on polypeptide evolution in this group.

Animals↗
PubMed1996

[Polytypic species Mus musculus in Transcaucasia].

It has been suggested that the house mouse, Mus musculus, is a polytypic species that originated in the northern part of the Indian sub-continent. Its subspecies have established secondary contact zones in east-centre China and in western Europe. However, the exact colonization routes taken by these subtaxa and their regions of primary differentiation have not yet been identified. We analyzed 89 mice from Transcaucasia at 35 enzyme loci and for polymorphism of the mitochondrial control region and a deletion of the Y chromosome. The various samples analyzed are a mosaic of populations intermediates between M. m. domesticus and M. m. musculus. Trancaucasia appears thus as a broad secondary contact zone, a fact which reinforces the idea that the species has retained large possibilities of remixing.

Animals↗
PubMed1995

Evolutionary correlation between control region sequence and restriction polymorphisms in the mitochondrial genome of a large Senegalese Mandenka sample.

We present here the first comparative analysis at the population level between Restriction Fragment Length Polymorphism (RFLP) and control region sequence polymorphism in a large and homogeneous Senegalese Mandenka sample. Eleven RFLP haplotypes and 60 different sequences are found in 119 individuals, revealing that a very high level of mtDNA diversity can be maintained in a small population. A sequence neighbor-joining tree and an analysis of molecular variance show that sequences associated with a given restriction haplotype are evolutionarily highly correlated: sequencing generally leads to the subtyping of RFLP haplotypes. Evolutionary relationships among RFLP haplotypes inferred from restriction site differences are in good agreement with those inferred from sequence data. A single difference is observed and is likely due to a single restriction homoplasy having occurred in the control region. Selective neutrality tests on both RFLP and sequence data accept the hypotheses of mtDNA neutrality and population equilibrium. The deep coalescence times (exceeding 50,000 yr) of sequences associated with the two most frequent restriction haplotypes confirm that the Niokolo Mandenka population has not passed through a recent bottleneck and that gene flow is maintained among West African populations despite ethnic differences.

Base Sequence↗
PubMed1995

Population subdivision and gene flow in Danish house mice.

Genetic subdivision in local populations of the European house mice, Mus musculus domesticus and M. m. musculus, was analysed to study patterns of gene flow. The data consisted of frequencies of microsatellite alleles in 16 samples (250 individuals) from a total of 11 sites in Jutland, which included successive samples from three sites. Sequences of the control region of mitochondrial DNA in three successive samples from one site were also analysed. Microsatellite genotype frequencies within samples were close to Hardy-Weinberg expectations. Levels of microsatellite differentiation among samples (theta = 0.05-0.21) corresponded to limited gene flow at migration-drift equilibrium (Nm = 1-5). Weak isolation by distance for microsatellites in M.m. musculus suggested that gene flow tends to occur among neighbouring sites. Estimates of effective population size over a few generations were much lower than those corresponding to the long periods needed for arrival at mutation-drift equilibrium. This suggested that subpopulations had been influenced by gene flow since formation, or had originated recently from genetically diverse founders.

Analysis of Variance↗
PubMed1994

Speciation and paraphyly in western Mediterranean hares (Lepus castroviejoi, L. europaeus, L. granatensis, and L. capensis) revealed by mitochondrial DNA phylogeny.

Mitochondrial DNA (mtDNA) variation among specimens of the northwestern African hare (Lepus capensis schlumbergeri) and three European hares sampled in Spain (L. castroviejoi and L. granatensis, which are endemic to the Iberian Peninsula, and L. europaeus) was analyzed using seven restriction endonucleases. Fourteen haplotypes were found among the 34 animals examined. Restriction site maps were constructed and the phylogeny of the haplotypes was inferred. mtDNA of L. capensis was the most divergent, which is consistent with its allopatric African distribution and with an African origin of European hares. We estimated that mtDNA in hares diverges at a rate of 1.5-1.8% per MY assuming that the European and African populations separated 5-6 MYBP. Maximum intraspecies nucleotides divergences were 1.3% in L. capensis, 2.7% in L. castroviejoi, and 2.3% in L. granatensis but 13.0% in L. europaeus. The latter species contained two main mtDNA lineages, one on the branch leading to L. castroviejoi and the other on that leading to L. granatensis. The separation of these two lineages from the L. castroviejoi or L. granatensis lineages appears to be much older than the first paleontological record of L. europaeus in the Iberian peninsula. This suggests that the apparent polyphyly of L. europaeus is due not to secondary introgression, but to the retention of ancestral polymorphism in L. europaeus. The results suggest that L. europaeus either has evolved as a very large population for a long time or has been fractionated. Such a pattern of persistence of very divergent lineages has also been reported in other species of highly mobile terrestrial mammals. As far as mtDNA is concerned. L. europaeus appears to be the common phylogenetic trunk which has diversified during dispersion over the European continent and from which L. castroviejoi and L. granatensis speciated separately in southwest Europe.

Animals↗