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Selective Constraint (selective + constraint)
Selected AbstractsMicroevolutionary support for a developmental hourglass: gene expression patterns shape sequence variation and divergence in DrosophilaEVOLUTION AND DEVELOPMENT, Issue 5 2008Tami Cruickshank SUMMARY A central goal of evolutionary developmental biology (Evo-Devo) is to synthesize comparative molecular developmental genetics and its description of the dynamic relationship between genotype and phenotype with the microevolutionary processes (mutation, random drift, and selection) of population genetics. To this end, we analyzed sequence variation of five gene classes that act sequentially to shape early embryo development in Drosophila: maternal, gap, pair-rule, segment polarity, and segment identity genes. We found two related patterns: (1) a microevolutionary pattern, wherein relative sequence variation within species is 2- to 3-fold higher for maternal-effect genes than for any other gene class; and, (2) a macroevolutionary pattern, wherein the relative sequence divergence among species for maternal-effect genes is 2- to 4-fold greater than for any other gene class. Both patterns are qualitatively and quantitatively consistent with the predictions of microevolutionary theory. Our findings connect within-species genetic variation to between-species divergence and shed light on the controversy over the existence of a "developmental hourglass," where mid-embryonic stages are more evolutionarily constrained than either earlier or later stages. Because maternal-effect genes experience relaxed selective constraint relative to zygotic-effect genes, they explore a wider mutational and phenotypic space. As a result, early acting maternal-effect genes diverge more widely across taxa and thereby broaden the base of the developmental hourglass. In contrast, later acting zygotic genes are relatively more constrained and limited in their diversification across taxa, narrowing the waist of the developmental hourglass. This pattern is obscured by genes with both maternal and zygotic expression, which experience the strongest evolutionary constraint. [source] Variation in synonymous codon use and DNA polymorphism within the Drosophila genomeJOURNAL OF EVOLUTIONARY BIOLOGY, Issue 1 2006N. BIERNE Abstract A strong negative correlation between the rate of amino-acid substitution and codon usage bias in Drosophila has been attributed to interference between positive selection at nonsynonymous sites and weak selection on codon usage. To further explore this possibility we have investigated polymorphism and divergence at three kinds of sites: synonymous, nonsynonymous and intronic in relation to codon bias in D. melanogaster and D. simulans. We confirmed that protein evolution is one of the main explicative parameters for interlocus codon bias variation (r2, 40%). However, intron or synonymous diversities, which could have been expected to be good indicators of local interference [here defined as the additional increase of drift due to selection on tightly linked sites, also called ,genetic draft' by Gillespie (2000)] did not covary significantly with codon bias or with protein evolution. Concurrently, levels of polymorphism were reduced in regions of low recombination rates whereas codon bias was not. Finally, while nonsynonymous diversities were very well correlated between species, neither synonymous nor intron diversities observed in D. melanogaster were correlated with those observed in D. simulans. All together, our results suggest that the selective constraint on the protein is a stable component of gene evolution while local interference is not. The pattern of variation in genetic draft along the genome therefore seems to be instable through evolutionary times and should therefore be considered as a minor determinant of codon bias variance. We argue that selective constraints for optimal codon usage are likely to be correlated with selective constraints on the protein, both between codons within a gene, as previously suggested, and also between genes within a genome. [source] Detecting introgressive hybridization between free-ranging domestic dogs and wild wolves (Canis lupus) by admixture linkage disequilibrium analysisMOLECULAR ECOLOGY, Issue 10 2006A. VERARDI Abstract Occasional crossbreeding between free-ranging domestic dogs and wild wolves (Canis lupus) has been detected in some European countries by mitochondrial DNA sequencing and genotyping unlinked microsatellite loci. Maternal and unlinked genomic markers, however, might underestimate the extent of introgressive hybridization, and their impacts on the preservation of wild wolf gene pools. In this study, we genotyped 220 presumed Italian wolves, 85 dogs and 7 known hybrids at 16 microsatellites belonging to four different linkage groups (plus four unlinked microsatellites). Population clustering and individual assignments were performed using a Bayesian procedure implemented in structure 2.1, which models the gametic disequilibrium arising between linked loci during admixtures, aiming to trace hybridization events further back in time and infer the population of origin of chromosomal blocks. Results indicate that (i) linkage disequilibrium was higher in wolves than in dogs; (ii) 11 out of 220 wolves (5.0%) were likely admixed, a proportion that is significantly higher than one admixed genotype in 107 wolves found previously in a study using unlinked markers; (iii) posterior maximum-likelihood estimates of the recombination parameter r revealed that introgression in Italian wolves is not recent, but could have continued for the last 70 (± 20) generations, corresponding to approximately 140,210 years. Bayesian clustering showed that, despite some admixture, wolf and dog gene pools remain sharply distinct (the average proportions of membership to wolf and dog clusters were Qw = 0.95 and Qd = 0.98, respectively), suggesting that hybridization was not frequent, and that introgression in nature is counteracted by behavioural or selective constraints. [source] | ||||||||||