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M Schilthuizen

Publications and source records attributed to M Schilthuizen.

7 recordsLinked to original sources
PubMed2001

The 'rare allele phenomenon' in a ribosomal spacer.

We describe the increased frequency of a particular length variant of the internal transcribed spacer 1 (ITS-1) of the ribosomal DNA in a hybrid zone of the land snail Albinaria hippolyti. The phenomenon that normally rare alleles or other markers can increase in frequency in the centre of hybrid zones is not new. Under the term 'hybrizyme' or 'rare allele' phenomenon it has been recorded in many organisms and different genetic markers. However, this is the first time that it has been found in a multicopy locus. On the one hand, the pattern fits well with the view that purifying selection in hybrid populations works on many loci across the genome and should thus have its effect on many independent molecular markers. On the other hand, the results are puzzling, given that the multiple copies of rDNA are not expected to respond in unison. We suggest two possible explanations for these conflicting observations.

Alleles↗
PubMed2000

Dualism and conflicts in understanding speciation.

Speciation is a central but elusive issue in evolutionary biology. Over the past sixty years, the subject has been studied within a framework conceived by Ernst Mayr and Theodosius Dobzhansky and subsequently developed further by numerous other workers. In this "isolation" theory, the evolution of reproductive isolation is a key element of speciation; natural selection is given only secondary importance while gene flow is considered prohibitive to the process. In this paper, I argue that certain elements in this approach have produced confusion and irreconcilability among students of speciation. The more prominent debates in speciation (i.e., the species definition, sympatry/allopatry, and the role of reinforcement) all derive from an inherent conflict between the "isolation" theory and Darwin's "selection" view on species and speciation (in which disruptive selection is crucial). New data, mainly from field ecology, molecular population genetics, laboratory studies with Drosophila and computer analysis, all suggest that the isolation theory may no longer be the most desirable vantage point from which to explore speciation. Instead, environmental selection in large populations, often unimpeded by ongoing gene flow, appears to be the decisive element. The traditional preoccupation with reproductive isolation has created gaps in our knowledge of several crucial issues, mainly regarding the role of environmental selection and its connection with mate selection.

Animals↗
PubMed1998

Screening mollusks for Wolbachia infection.

We screened 38 species of mollusks for infection by Wolbachia, a bacterium that is a common endosymbiont in arthropods, where it induces alterations in reproduction. Using a PCR assay, we could not detect the symbiont in any of the samples, indicating that, in mollusks, it might be absent.

Animals↗
PubMed1997

Horizontal transmission of parthenogenesis-inducing microbes in Trichogramma wasps.

Complete parthenogenesis (thelytoky) in species of the parasitic wasp Trichogramma is usually caused by the cytoplasmically inherited bacterium Wolbachia. This symbiont induces gamete duplication, which, in these haplodiploid organisms, results in all-female broods. Antibiotic treatment "cures' this condition, restoring normal sexual reproduction. Phylogenetic analysis of Wolbachia has shown that, in contrast with the strains in other host organisms (where the symbiont also induces different reproductive alterations), those in Trichogramma form a monophyletic group. This might be an indication of symbiont-host cocladogenesis. To test this, we performed comparative molecular phylogenetics on 20 parthenogenetic Trichogramma cultures and their Wolbachiae. We conclude that there is, in fact, little evidence for cocladogenesis. Instead, the phylogenetic distribution of the symbionts appears to result from occasional horizontal transmission, which probably takes place inside the hosts of Trichogramma parasitoids (usually lepidopteran eggs). This study therefore suggests that parthenogenesis is not only curable, it can sometimes be contagious also.

Alphaproteobacteria↗
PubMed1995

Phylogenetic relationships inferred from the sequence and secondary structure of ITS1 rRNA in Albinaria and putative Isabellaria species (Gastropoda, Pulmonata, Clausiliidae).

An analysis of the ITS1 sequence variation among five species of terrestrial pulmonate snails was performed to decide between two conflicting hypotheses concerning the phylogeny of these anatomically similar gastropods. It turned out that the so-called genus Isabellaria is a polyphyletic entity; the diagnostic, apomorphic structure of its clausilial apparatus, enabling a nearly complete obstruction of the shell aperture with the animal at rest, apparently evolved more than once from ancestors currently classified with the speciose genus Albinaria. The classification based on general shell shape and sculpture, and distributional patterns, turns out to be the natural one. This study also provides the first data on ITS1 sequences in gastropods. The recently published ITS1 sequence of another molluscan species, a bivalve, is quite different but similar in length to that of the snails.

Animals↗