Biomedical subjects

A Brehm

Publications and source records attributed to A Brehm.

45 records · Page 3Linked to original sources

PubMed1993

The phylogeny of nine species of the Drosophila obscura group inferred by the banding homologies of the chromosomal regions. IV. Element C.

Homologies of the sections of the polytene chromosomes of element C, among several species of the obscura group of the genus Drosophila, were established according to the similarity of their banding patterns. The information gathered was used to construct an unrooted phylogenetic tree based on qualitative criteria. This tree is compared to three other similar trees derived from independent information provided by the study of chromosomal elements B, D and E. The general congruence of the patterns of these trees proves the well-foundedness of this approach. A single exception to this congruence is discussed. Finally a consensus tree, encapsulating information from all chromosomal elements, is presented and its topology is compared to those derived from electrophoretic data.

Animals↗

PubMed1992

The phylogeny of nine species of the Drosophila obscura group inferred by the banding homologies of chromosomal regions. III. Element D.

The phylogenetic relationships among nine species belonging to the obscura group of the genus Drosophila were deduced, based on similarities of the banding pattern of their polytene chromosomal element D. These similarities were inferred by the comparison of chromosomal photomaps. The phylogenetic reconstruction was the most parsimonious based on seriation by overlapping inversions and on the principle of conservation/disassociation of nearby located segments. The gene sequences of element D for all species studied were relatively easy to recognize in terms of the map of D. obscura, already found to occupy a relative central position in this group. Thus, three clusters of closely related species could be identified: obscura (D. obscura, D. ambigua, and D. tristis), African (D. kitumensis and D. microlabis), and subobscura (D. subobscura, D. madeirensis and D. guanche), with D. subsilvestris standing apart. The results are in agreement with those from the previously studied elements B and E, but element D was found to be much more conclusive concerning the links among the different clusters. Thus, it is inferred that D. guanche occupies an intermediate position between the other two species of its own cluster and all the others. The gene arrangement of D. obscura, directly related to those of the other species, has been identified. In the phylogenetic tree proposed, both the African cluster and D. subsilvestris derive from a hypothetical gene arrangement, intermediate in the pathway between the subobscura and obscura clusters.

Animals↗

PubMed1991

The phylogeny of nine species of the Drosophila obscura group inferred by the banding homologies of chromosomal regions. I. Element B.

The phylogenetic relationships among nine Drosophila species belonging to the obscura group were investigated by establishing the segments displaying banding homologies in their element B (equivalent to the U element of D. subobscura). The phylogenetic ordering of the species was accomplished using overlapping inversions. Two African species, D. kitumensis and D. microlabis, were investigated. These species are homosequential for their element B gene arrangement but differ from that of D. obscura by several rearrangements. Drosophila obscura seems to be most closely related to D. subsilvestris, from which the respective element B gene arrangements differ at least by six inversions. Three species, D. obscura, D. ambigua, and D. tristis, are closely related and form a cluster. Drosophila obscura displays an element B polymorphism for a pericentric inversion for which D. ambigua is fixed for one gene arrangement and D. tristis for the other. Both D. ambigua and D. tristis share a short distal inversion in the small arm of the chromosome, and differ in this respect from D. obscura. Drosophila madeirensis, D. guanche, and D. subobscura all share the same element B gene arrangement, which is acrocentric, but metacentric in all the other species mentioned. It was found that the gene arrangements of the species from the obscura cluster seem to occupy an intermediate position between those of the species of the D. subobscura cluster and those of the African one. The data reported generally are in good agreement with information provided in the literature.

Animals↗

PubMed1990

Evolution of the obscura group Drosophila species. III. Phylogenetic relationships in the subobscura cluster based on homologies of chromosome A.

The Drosophila subobscura cluster comprises D. subobscura, D. madeirensis and D. guanche, species closely related to some interspecific crosses are possible. This paper clarifies definitively the homologies of the segments of the sex chromosome A (= X) among these species and thus permits a phylogenetic seriation of them. This seriation is identical to the one suggested by the study of chromosome O and concordant with similar data for the remaining three rod autosomes (J, U and E), which, however, do not provide qualitative evidence on this subject.

Animals↗

PubMed1990

The phylogeny of nine species of the Drosophila obscura group inferred by the banding homologies of chromosomal regions. II. Element E.

The phylogenetic relationships among nine species of Drosophila belonging to the obscura group were investigated by establishing (according to their banding similarities) the homologous chromosome segments of element E (equivalent to chromosome O of D. subobscura). The phylogenetic relationships were based on the existence of segments in different triads of species, which could only be produced by overlapping inversions. This permitted the ordering of the species belonging to each triad. Drosophila obscura, D. ambigua and D. tristis were found to be very closely related and thus forming a cluster in which D. ambigua occupies an intermediate position between the other two species. Drosophila obscura seems to be the species more directly linked to three other separate lineages, that of D. subsilvestris, the two African species (D. microlabis and D. kitumensis), and the subobscura cluster. The species from this last cluster may be ordered as follows: D. subobscura-D. madeirensis-D. guanche. It is not clear which species of this triad is the direct link to D. obscura. These results completely agree with those produced in an independent study, where element B was considered for the same nine species. Furthermore, the present study clarifies some ambiguities concerning the phylogenetic relationships which remained obscure due to the conservative nature of chromosome B.

Animals↗

PubMedDate not supplied

Genetic diversity of the Macaronesian leafy liverwort Porella canariensis inferred from RAPD markers.

Plant colonization of the North Atlantic raises the intriguing question of the relationships between extant island species with their continental counterparts (European, African, and American), which may provide clues to past geographic distribution and colonization history. It has been suggested that during past glaciations, many plant species with typical Mediterranean distributions survived in the Atlantic islands that belong to what is today known as Macronesia. We used random amplified polymorphic DNA (RAPD) markers to study 12 populations of the liverwort Porella canariensis partly covering its present-day distribution (Azores, Madeira, Canary and Cape Verde Islands, and Iberian Peninsula). Unweighted pair-group (UPGMA) and principal component (PCO) analyses showed a similar geographical pattern that suggested a close relationship between Iberian populations and those from the Canaries and Cape Verde Islands. Populations from Madeira had more genetic variation than those from the Azores, a result from either a richer diversity of habitats in Madeira, which prompted more population diversification, successive colonization waves from different origins, or an older colonization of Madeira. The data show that continuous patches of liverworts are often comprised of more than one individual. Finally, RAPDs can be used to investigate intraspecific diversity within a comparatively large geographic area and, with utmost care, can be used to infer a historic context to explain the patterns observed.

Atlantic Islands↗

PubMedDate not supplied

Population genetic structure and colonization sequence of Drosophila subobscura in the Canaries and Madeira Atlantic Islands as inferred by autosomal, sex-linked and mtDNA traits.

The genetic structure in Atlantic Islands and continental populations of Drosophila subobscura has been studied using autosomal and sex-linked allozymes and mitochondrial DNA (mtDNA) haplotypes. From the data it is deduced that whereas the Canary islands have long been isolated, the neighboring island of Madeira has been subjected to continuous migration from the mainland. In addition, sex-linked allozymes and mtDNA data show a large divergence between the geologically younger western islands of the Canarian Archipelago and the older central ones, finding strong founder effects in the former. Divergence rates of sex-linked and mitochondrial genes relative to autosomic loci several times higher than expected under neutrality have been explained by differential migration between sexes. The Canarian Archipelago colonization fits in well with a stepping-stone model of a directional east-west migration that parallels the geological origin of these Islands.

Animals↗

PubMedDate not supplied

Inversion polymorphism in Drosophila obscura.

The inversion polymorphism of Drosophila obscura Fallen, a European species of the obscura group of the subgenus Sophophora, is described. A total of 21 inversions have been recorded; they are located in the five large chromosomes of the species (a dot chromosome is also present) and form 25 gene arrangements present in the species' natural populations. Strains from five different countries were studied. Two of these inversions were found to be pericentric, and the remaining were paracentric. The presence of "hot" points (multibreaks) was noticed. The distribution of the relative lengths of inversions conformed to the Van Valen-Levins distribution, contrary to what happens in D. subobscura. Observations also showed that there is no crossover inhibition between nonoverlapping inversions. The phylogeny of chromosome C was reconstructed; the existence of several arrangements not found was postulated; and the primitive gene arrangement, linking D. obscura to its closely related species was identified, as well as the primitive gene arrangements of the other chromosomes. Photographic maps of the chromosomes of D. obscura are provided here.

Animals↗

PubMedDate not supplied

Lack of congruence between morphometric evolution and genetic differentiation suggests a recent dispersal and local habitat adaptation of the Madeiran lizard Lacerta dugesii.

Genetic differentiation among nine populations of the endemic lizard Lacerta dugesii Milne-Edwards 1829 (Lacertidae) from four groups of islands constituting the Archipelago of Madeira, was investigated by protein electrophoresis at 23 enzyme loci. Among twenty polymorphic loci, the total genetic diversity was due primarily to intra-population variation. The allele and genotypic frequencies among populations showed some heterogeneity, allowing the species to present a structuring pattern compatible with their geographical clustering. Some evidence suggests that selection acting on some loci in different ecological conditions may be responsible for the clustering of the populations studied. There was no apparent isolation effect expected under an "island" model of population divergence, and no correlation was found between genetic and geographic distances among populations. Morphological variation of the proposed three L. dugesii subspecies is not congruent with the allozyme analysis. This most probably suggests a rapid colonization of the islands followed by a strong effect of selection operating over the morphological characters used to define the subspecies.

Adaptation, Physiological↗