Biomedical subjects

Deborah Charlesworth

Publications and source records attributed to Deborah Charlesworth.

33 records · Page 2Linked to original sources

PubMed2005

Plant self-incompatibility systems: a molecular evolutionary perspective.

Incompatibility recognition systems preventing self-fertilization have evolved several times in independent lineages of Angiosperm plants, and three main model systems are well characterized at the molecular level [the gametophytic self-incompatibility (SI) systems of Solanaceae, Rosaceae and Anthirrhinum, the very different system of poppy, and the system in Brassicaceae with sporophytic control of pollen SI reactions]. In two of these systems, the genes encoding both components of pollen-pistil recognition are now known, showing clearly that these two proteins are distinct, that is, SI is a lock-and-key mechanism. Here, we review recent findings in the three well-studied systems in the light of these results and analyse their implications for understanding polymorphism and coevolution of the two SI genes, in the context of a tightly linked genome region.

Chromosome Mapping↗

PubMed2004

A gradual process of recombination restriction in the evolutionary history of the sex chromosomes in dioecious plants.

To help understand the evolution of suppressed recombination between sex chromosomes, and its consequences for evolution of the sequences of Y-linked genes, we have studied four X-Y gene pairs, including one gene not previously characterized, in plants in a group of closely related dioecious species of Silene which have an X-Y sex-determining system (S. latifolia, S. dioica, and S. diclinis). We used the X-linked copies to build a genetic map of the X chromosomes, with a marker in the pseudoautosomal region (PAR) to orient the map. The map covers a large part of the X chromosomes--at least 50 centimorgans. Except for a recent rearrangement in S. dioica, the gene order is the same in the X chromosomes of all three species. Silent site divergence between the DNA sequences of the X and Y copies of the different genes increases with the genes' distances from the PAR, suggesting progressive restriction of recombination between the X and Y chromosomes. This was confirmed by phylogenetic analyses of the four genes, which also revealed that the least-diverged X-Y pair could have ceased recombining independently in the dioecious species after their split. Analysis of amino acid replacements vs. synonymous changes showed that, with one possible exception, the Y-linked copies appear to be functional in all three species, but there are nevertheless some signs of degenerative processes affecting the genes that have been Y-linked for the longest times. Although the X-Y system evolved quite recently in Silene (less than 10 million years ago) compared to mammals (about 320 million years ago), our results suggest that similar processes have been at work in the evolution of sex chromosomes in plants and mammals, and shed some light on the molecular mechanisms suppressing recombination between X and Y chromosomes.

Animals↗

PubMed2004

Sex determination: balancing selection in the honey bee.

Sequences of alleles of the honey bee's primary sex-determining gene have extremely high diversity, with many amino acid variants, suggesting that different alleles of this gene have been maintained in populations for very long evolutionary times.

Alleles↗

PubMed2004

Plant evolution: modern sex chromosomes.

The first detailed map has been produced of a plant chromosome carrying sex-determining genes. The new data show that, in papaya, these genes lie in a quite extensive non-recombining region. This region is nevertheless a small part of the papaya genome compared with other male-specific genome regions, such as mammalian Y chromosomes.

Carica↗

PubMed2003

Effects of inbreeding on the genetic diversity of populations.

The study of variability within species is important to all biologists who use genetic markers. Since the discovery of molecular variability among normal individuals, data have been collected from a wide range of organisms, and it is important to understand the major factors affecting diversity levels and patterns. Comparisons of inbreeding and outcrossing populations can contribute to this understanding, and therefore studying plant populations is important, because related species often have different breeding systems. DNA sequence data are now starting to become available from suitable plant and animal populations, to measure and compare variability levels and test predictions.

Evolution, Molecular↗

PubMed2003

Haplotype structure of the stigmatic self-incompatibility gene in natural populations of Arabidopsis lyrata.

We describe analyses of almost full-length sequences (including both the kinase domain and the S-domain) of the putative SRK incompatibility gene of the self-incompatible plant Arabidopsis lyrata. In A. lyrata, the SRK S-domain controls the pistil recognition specificity, as in self-incompatible Brassica species. In alleles from plants derived from natural A. lyrata populations, nonsynonymous and synonymous site diversity values are very high in both domains; even in exons 3 to 7 of the kinase domain, which probably have no recognition functions, 39% of the amino acids are polymorphic. Within populations, diversity between alleles is high, as expected for an incompatibility locus, which should be under frequency-dependent selection within populations, whereas within the different putative allelic classes polymorphism is very low, as predicted from theoretical models when recombination is rare. Nonsynonymous site variability declines in the kinase domain with increasing distance from the S-domain border, although synonymous diversity remains high, and the introns are unalignable. A decline in nonsynonymous diversity is expected due to selective constraints in the kinase domain, in combination with recombination (allowing diversity to decrease at sites distant from those under balancing selection). However, it is unclear whether recombination occurs in the SRK locus, and interpretation of the observed diversity pattern is complicated by apparent gene conversion with a paralogous gene (or genes). Patterns of linkage disequilibrium in our SRK sequences do not support the conclusion that recombination occurs, which was suggested from previous analyses based on Brassica SLG sequences.

Alleles↗

PubMed2003

Duplicative transfer of a MADS box gene to a plant Y chromosome.

Y chromosomes carry genes with functions in male reproduction and often have few other loci. Their evolution and the causes of genetic degeneration are of great interest. In addition to genetic degeneration, the acquisition of autosomal genes may be important in Y chromosome evolution. We here report that the dioecious plant Silene latifolia harbors a complete MADS box gene, SlAP3Y, duplicated onto the Y chromosome. This gene has no X-linked homologs but only an autosomal paralog, SlAP3A, and sequence divergence suggests that the duplication is a quite old event that occurred soon after the evolution of the sex chromosomes. Evolutionary sequence analyses using homologs of closely related species, including hermaphroditic Silene conica and dioecious Silene dioica and Silene diclinis, suggest that both SlAP3A and SlAP3Y genes encode functional proteins. Indeed, quantitative RT-PCR and in situ hybridization analyses showed that SlAP3A is expressed specifically in developing petals, but SlAP3Y is much more strongly expressed in developing stamens. The S. conica homolog, ScAP3A, is expressed in developing petals, suggesting subfunctionalization with evolution of male-specific functions, possibly due to evolutionary change in regulatory elements. Our results suggest that the acquisition of autosomal genes is an important event in the evolution of plant Y chromosomes.

Chromosomes, Plant↗

PubMed2003

Subdivision and haplotype structure in natural populations of Arabidopsis lyrata.

High levels of inbreeding are expected to cause a strong reduction in levels of genetic variability, effective recombination rates and in adaptation compared with related outcrossing populations. We examined patterns of DNA polymorphism at five nuclear loci and one chloroplast locus within and between four populations of the outcrossing plant Arabidopsis lyrata, a close relative of the highly self-fertilizing model species A. thaliana. The observed patterns are compared with species-wide polymorphism at orthologous loci, as well as within- and between-population patterns at other studied loci in A. thaliana. In addition to evidence for much higher average within-population diversity, species-wide levels of silent polymorphism are generally higher in A. lyrata than in A. thaliana, unlike the results from a previous study of the ADH locus. However, polymorphism is also low in the North American A. lyrata subspecies lyrata compared with the European subspecies petraea, suggesting either a population bottleneck in North American populations or recent admixture involving diverged European populations. Differentiation between the two subspecies is strong, although there are few fixed differences, suggesting that their isolation is recent. Estimates of intralocus recombination rates and analysis of haplotype structure in European A. lyrata populations indicate lower recombination than predicted based on the variability together with physical recombination rates estimated from A. thaliana. This may be due to strong population subdivision, or to recent departures from demographic equilibrium such as a bottleneck or population admixture. Alternatively, there may be consistently lower recombination rates in the outcrossing species. In contrast, estimates of recombination rates from species-wide samples of A. thaliana are close to the values expected assuming a high rate of self-fertilization. Complex population histories in both A. thaliana and A. lyrata complicate theoretical predictions and empirical tests of the effects of inbreeding on polymorphism and molecular evolution.

Arabidopsis↗

PubMed2003

Diversity and linkage of genes in the self-incompatibility gene family in Arabidopsis lyrata.

We report studies of seven members of the S-domain gene family in Arabidopsis lyrata, a member of the Brassicaceae that has a sporophytic self-incompatibility (SI) system. Orthologs for five loci are identifiable in the self-compatible relative A. thaliana. Like the Brassica stigmatic incompatibility protein locus (SRK), some of these genes have kinase domains. We show that several of these genes are unlinked to the putative A. lyrata SRK, Aly13. These genes have much lower nonsynonymous and synonymous polymorphism than Aly13 in the S-domains within natural populations, and differentiation between populations is higher, consistent with balancing selection at the Aly13 locus. One gene (Aly8) is linked to Aly13 and has high diversity. No departures from neutrality were detected for any of the loci. Comparing different loci within A. lyrata, sites corresponding to hypervariable regions in the Brassica S-loci (SLG and SRK) and in comparable regions of Aly13 have greater replacement site divergence than the rest of the S-domain. This suggests that the high polymorphism in these regions of incompatibility loci is due to balancing selection acting on sites within or near these regions, combined with low selective constraints.

Alleles↗

PubMed2002

Self-incompatibility: how to stay incompatible.

Plant self-incompatibility is controlled by different genes for the recognition reactions of pollen and stigmas, yet correct association of the two genes have been maintained in two Brassica species.

Brassica↗

PubMed2002

Plant sex determination and sex chromosomes.

Sex determination systems in plants have evolved many times from hermaphroditic ancestors (including monoecious plants with separate male and female flowers on the same individual), and sex chromosome systems have arisen several times in flowering plant evolution. Consistent with theoretical models for the evolutionary transition from hermaphroditism to monoecy, multiple sex determining genes are involved, including male-sterility and female-sterility factors. The requirement that recombination should be rare between these different loci is probably the chief reason for the genetic degeneration of Y chromosomes. Theories for Y chromosome degeneration are reviewed in the light of recent results from genes on plant sex chromosomes.

Evolution, Molecular↗

PubMed2002

Molecular variation at the self-incompatibility locus in natural populations of the genera Antirrhinum and Misopates.

The self-incompatibility system of flowering plants is a classic example of extreme allelic polymorphism maintained by frequency-dependent selection. We used primers designed from three published Antirrhinum hispanicum S-allele sequences in PCR reactions with genomic DNA of plants sampled from natural populations of Antirrhinum and Misopates species. Not surprisingly, given the polymorphism of S-alleles, only a minority of individuals yielded PCR products of the expected size. These yielded 35 genomic sequences, of nine different sequence types of which eight are highly similar to the A. hispanicum S-allele sequences, and one to a very similar unpublished Antirrhinum S-like RNase sequence. The sequence types are well separated from the S-RNase sequences from Solanaceae and Rosaceae, and also from most known "S-like" RNase sequences (which encode proteins not involved in self-incompatibility). An association with incompatibility types has so far been established for only one of the putative S-alleles, but we describe evidence that the other sequences are also S-alleles. Variability in these sequences follows the pattern of conserved and hypervariable regions seen in other S-RNases, but no regions have higher replacement than silent diversity, unlike the results in some other species.

Alleles↗

PubMed2002

Substitution rates in the X- and Y-linked genes of the plants, Silene latifolia and S. dioica.

Theory predicts that selection should be less effective in the nonrecombining genes of Y-chromosomes, relative to the situation for genes on the other chromosomes, and this should lead to the accumulation of deleterious nonsynonymous substitutions. In addition, synonymous substitution rates may differ between X- and Y-linked genes because of the male-driven evolution effect and also because of actual differences in per-replication mutation rates between the sex chromosomes. Here, we report the first study of synonymous and nonsynonymous substitution rates on plant sex chromosomes. We sequenced two pairs of sex-linked genes, SlX1-SlY1 and SlX4-SlY4, from dioecious Silene latifolia and S. dioica, and their non-sex-linked homologues from nondioecious S. vulgaris and Lychnis flos-jovis, respectively. The rate of nonsynonymous substitutions in the SlY4 gene is significantly higher than that in the SlX4 gene. Silent substitution rates are also significantly higher in both Y-linked genes, compared with their X-linked homologues. The higher nonsynonymous substitution rate in the SlY4 gene is therefore likely to be caused by a mutation rate difference between the sex chromosomes. The difference in silent substitution rates between the SlX4 and SlY4 genes is too great to be explained solely by a higher per-generation mutation rate in males than females. It is thus probably caused by a difference in per-replication mutation rates between the sex chromosomes. This suggests that the local mutation rate can change in a relatively short evolutionary time.

Amino Acid Substitution↗

PubMed2002

Rates and patterns of molecular evolution in inbred and outbred Arabidopsis.

The evolution of self-fertilization is associated with a large reduction in the effective rate of recombination and a corresponding decline in effective population size. If many spontaneous mutations are slightly deleterious, this shift in the breeding system is expected to lead to a reduced efficacy of natural selection and genome-wide changes in the rates of molecular evolution. Here, we investigate the effects of the breeding system on molecular evolution in the highly self-fertilizing plant Arabidopsis thaliana by comparing its coding and noncoding genomic regions with those of its close outcrossing relative, the self-incompatible A. lyrata. More distantly related species in the Brassicaceae are used as outgroups to polarize the substitutions along each lineage. In contrast to expectations, no significant difference in the rates of protein evolution is observed between selfing and outcrossing Arabidopsis species. Similarly, no consistent overall difference in codon bias is observed between the species, although for low-biased genes A. lyrata shows significantly higher major codon usage. There is also evidence of intron size evolution in A. thaliana, which has consistently smaller introns than its outcrossing congener, potentially reflecting directional selection on intron size. The results are discussed in the context of heterogeneity in selection coefficients across loci and the effects of life history and population structure on rates of molecular evolution. Using estimates of substitution rates in coding regions and approximate estimates of divergence and generation times, the genomic deleterious mutation rate (U) for amino acid substitutions in Arabidopsis is estimated to be approximately 0.2-0.6 per generation.

Arabidopsis↗

PubMedDate not supplied

The inter-specific hybrid Silene latifolia x S. viscosa reveals early events of sex chromosome evolution.

The dioecious plant species Silene latifolia has a sex determination mechanism based on an active Y chromosome. Here, we used inter-specific hybrids in the genus Silene to study the effects of gene complexes on the Y chromosome. If the function of Y-linked genes has been maintained in the same state as in the hermaphrodite progenitor species, it should be possible to substitute such genes by genes coming from a related hermaphrodite species. In the inter-specific hybrid, S. latifolia x S. viscosa, anthers indeed develop far beyond the early bilobal stage characteristic of XX S. latifolia female plants. The S. viscosa genome can thus replace the key sex determination gene whose absence abolishes early stamen development in females (loss of the stamen-promoting function, SPF), so that hybrid plants are morphologically hermaphrodite. However, the hybrids have two anther development defects, loss of adhesion of the tapetum to the endothecium, and precocious endothecium maturation. Both these defects were also found in independent Y-chromosome deletion mutants of S. latifolia. The data support the hypothesis that the evolution of complete gender dimorphism from hermaphroditism involved a major largely recessive male-sterility factor that created females, and the appearance of new, dominant genes on the Y chromosome, including both the well-documented gynoecium-suppressing factor, and two other Y specific genes promoting anther development.

Biological Evolution↗