Showing papers by "Alex Widmer published in 2003"

The genetic architecture necessary for transgressive segregation is common in both natural and domesticated populations

Abstract: Segregating hybrids often exhibit phenotypes that are extreme or novel relative to the parental lines. This phenomenon is referred to as transgressive segregation, and it provides a mechanism by which hybridization might contribute to adaptive evolution. Genetic studies indicate that transgressive segregation typically results from recombination between parental taxa that possess quantitative trait loci (QTLs) with antagonistic effects (i.e. QTLs with effects that are in the opposite direction to parental differences for those traits). To assess whether this genetic architecture is common, we tabulated the direction of allelic effects for 3252 QTLs from 749 traits and 96 studies. Most traits (63.6%) had at least one antagonistic QTL, indicating that the genetic substrate for transgressive segregation is common. Plants had significantly more antagonistic QTLs than animals, which agrees with previous reports that transgressive segregation is more common in plants than in animals. Likewise, antagonistic QTLs were more frequent in intra- than in interspecific crosses and in morphological than in physiological traits. These results indicate that transgressive segregation provides a general mechanism for the production of extreme phenotypes at both above and below the species level and testify to the possible creative part of hybridization in adaptive evolution and speciation.

Gene flow across species boundaries in sympatric, sexually deceptive Ophrys (Orchidaceae) species

Abstract: Orchids of the genus Ophrys (Orchidaceae) are pollinated by male bees and wasps through sexual deception. The Ophrys sphegodes group encompasses several closely related species that differ slightly in floral morphology and are pollinated by different solitary bee species. Populations representing different species of the O. sphegodes group often flower simultaneously in sympatry. To test whether gene flow across the species boundaries occurs in these sympatric populations, or whether they are reproductively isolated, we examined the distribution of genetic variation within and among populations and species of this group. We collected at each of five different localities in southern France and Italy two sympatric, co-flowering Ophrys populations, representing six Ophrys species in total. The six microsatellite loci surveyed were highly variable. Genetic differentiation among geographically distant populations of the same species was lower than differentiation among sympatric populations of different species. However, the strength of genetic differentiation among species was among the lowest reported for orchids. Genotype assignment tests and marker-based estimates of gene flow revealed that gene flow across species boundaries occurred and may account for the low observed differentiation among species. These results suggest that sexual deceit pollination in Ophrys may be less specific than thought, or that rare mistakes occur.

Fine‐scale phylogeographical analysis of Mediterranean Anacamptis palustris (Orchidaceae) populations based on chloroplast minisatellite and microsatellite variation

Abstract: The phylogeographical history of the rare marsh orchid Anacamptis palustris (Orchidaceae) was reconstructed using highly polymorphic chloroplast minisatellite and microsatellite loci. Allelic variation at chloroplast microsatellite loci was due to length variation in poly(A/T) repeats and was informative on a regional scale, but was not sufficient to unravel relationships among populations on a local geographical scale. The minisatellite locus, however, was found to be highly variable. Nine distinct repeat types were found and variation in repeat number occurred in five repeat types. The distribution of chloroplast haplotypes, combining microsatellite and minisatellite repeat type variation, provided a clear phylogeographical picture on a large geographical scale, whereas length variation in one highly polymorphic minisatellite repeat type provided fine-scale phylogeographical information. Mediterranean populations could be divided into four main lineages, a western European lineage, a northern and central Italian lineage, a well-isolated southern Italian (Apulian) lineage, and an eastern European lineage. Variation at the most variable minisatellite repeat type N revealed 19 alleles and allowed the study of seed-mediated gene flow and an estimation of the ratio of pollen to seed flow among neighbouring populations.

Variation at a chloroplast minisatellite locus reveals the signature of habitat fragmentation and genetic bottlenecks in the rare orchid Anacamptis palustris (Orchidaceae)

Abstract: Geoclimatic changes during the Oligocene and more recent anthropogenic influences have shaped the current distribution and population structure of Mediterranean plant species. Anacamptis palustris (Orchidaceae) is a typical member of coastal wetlands, which have become increasingly fragmented and isolated. As a consequence, this orchid has become rare in the recent past. Length variation at a chloroplast minisatellite locus was used to estimate genetic variation within and between the largest extant populations of A. palustris. Genetic diversity was positively correlated with population size. Estimation of observed and expected gene diversity and analyses of haplotype number and haplotype frequency distributions provided evidence for population bottlenecks in the history of small populations. Comparison with an earlier study suggests that nuclear allozyme diversity was most likely lost during the Oligocene and could not recover subsequently due to low mutation rates, whereas genetic variation was restored at the highly variable chloroplast minisatellite locus. Population bottlenecks indicated by cpDNA variation occurred most likely as a consequence of more recent anthropogenic changes. The comparison of molecular markers with different levels of polymorphism provided valuable insights into the processes shaping genetic diversity and population structure in this rare orchid.

Molecular Evolution of a Plastid Tandem Repeat Locus in an Orchid Lineage

Abstract: The molecular evolution of a chloroplast minisatellite locus in the Anacamptis palustris (Orchidaceae) lineage and haplotype variation in two Italian A. palustris populations were investigated. A phylogenetic analyses of the chloroplast tRNALEU intron, where the minisatellite locus is located, revealed that a deletion in the ancestor of the A. palustris lineage led to the formation of two noncontiguous, complementary sequence motifs. We propose a model to explain the initial formation of the minisatellite repeat motif, starting with the two noncontiguous, complementary sequence motifs. A survey of minisatellite variation in four species of the A. palustris lineage revealed several haplotypes that differed not only in repeat number, but also in repeat organization. A haplotype network suggests that three different minisatellite loci evolved independently at the same position in the tRNALEU intron. A secondary structure model revealed that the A. palustris minisatellite repeat forms a stem region of the tRNALEU intron, which allows its notable expansion without negatively affecting splicing. Minisatellite variation was high in the two examined A. palustris populations where 20 haplotypes were detected, whereas no length variation was detected in a neighboring poly (A) microsatellite locus. We estimated a chloroplast minisatellite mutation rate of 3.2 × 10−3 mutations per generation. Southern blot analyses did not find evidence for chloroplast heteroplasmy. Based on the analysis of the largest known, extant A. palustris population, a stepwise mutation model (SMM) was inferred.

Rare orchid anacamptis palustris (orchidaceae)1

Abstract: Geoclimatic changes during the Oligocene and more recent anthropogenic influences have shaped the current distribution and population structure of Mediterranean plant species. Anacamptis palustris (Orchidaceae) is a typical member of coastal wetlands, which have become increasingly fragmented and isolated. As a consequence, this orchid has become rare in the recent past. Length variation at a chloroplast minisatellite locus was used to estimate genetic variation within and between the largest extant populations of A. palustris. Genetic diversity was positively correlated with population size. Estimation of observed and expected gene diversity and analyses of haplotype number and haplotype frequency distributions provided evidence for population bottlenecks in the history of small populations. Comparison with an earlier study suggests that nuclear allozyme diversity was most likely lost during the Oligocene and could not recover subsequently due to low mutation rates, whereas genetic variation was restored at the highly variable chloroplast minisatellite locus. Population bottlenecks indicated by cpDNA variation occurred most likely as a consequence of more recent anthropogenic changes. The comparison of molecular markers with different levels of polymorphism provided valuable insights into the processes shaping genetic diversity and population structure in this rare orchid.