RESUMO
A selective sweep is the result of strong positive selection driving newly occurring or standing genetic variants to fixation, and can dramatically alter the pattern and distribution of allelic diversity in a population. Population-level sequencing data have enabled discoveries of selective sweeps associated with genes involved in recent adaptations in many species. In contrast, much debate but little evidence addresses whether "selfish" genes are capable of fixation-thereby leaving signatures identical to classical selective sweeps-despite being neutral or deleterious to organismal fitness. We previously described R2d2, a large copy-number variant that causes nonrandom segregation of mouse Chromosome 2 in females due to meiotic drive. Here we show population-genetic data consistent with a selfish sweep driven by alleles of R2d2 with high copy number (R2d2(HC)) in natural populations. We replicate this finding in multiple closed breeding populations from six outbred backgrounds segregating for R2d2 alleles. We find that R2d2(HC) rapidly increases in frequency, and in most cases becomes fixed in significantly fewer generations than can be explained by genetic drift. R2d2(HC) is also associated with significantly reduced litter sizes in heterozygous mothers, making it a true selfish allele. Our data provide direct evidence of populations actively undergoing selfish sweeps, and demonstrate that meiotic drive can rapidly alter the genomic landscape in favor of mutations with neutral or even negative effects on overall Darwinian fitness. Further study will reveal the incidence of selfish sweeps, and will elucidate the relative contributions of selfish genes, adaptation and genetic drift to evolution.
Assuntos
Proteínas Nucleares/genética , Proteínas de Ligação a RNA/genética , Sequências Repetitivas de Ácido Nucleico , Adaptação Fisiológica/genética , Alelos , Animais , Evolução Biológica , Variações do Número de Cópias de DNA/genética , Evolução Molecular , Feminino , Variação Genética , Genética Populacional , Masculino , Camundongos , Modelos Genéticos , Mutação , Seleção GenéticaRESUMO
A new repeated DNA from Microtus thomasi, Mth-Alu2.2, was cloned and characterized and is presented here for the first time. Digestion of genomic DNA from M. thomasi with AluI restriction enzyme revealed a 2.2-kb repetitive DNA sequence with a high AT content (69%). This sequence consists of a tandemly repeated nonanucleotide of the consensus sequence CACAATGTA, which constitutes approximately 93-95% of the total unit length. The location of the Mth-Alu2.2 sequence in the karyotype was determined by FISH, demonstrating strong hybridization signals in the pericentromeric regions of all chromosomes and in the heterochromatin blocks of several X chromosome variants. In addition, the distribution of the 4 pericentromeric repeat sequences Msat-160, Mth-Alu900, Mth-Alu2.2, and interstitial telomeric repeats was analyzed by in situ hybridization in M. thomasi, in order to shed light on the complex composition of the chromosomal pericentromeric regions in this species. The order and organization of these sequences in the pericentromeric regions are conserved, with slight variations in both the degree of overlapping and the amount of each repeated DNA in the chromosomes. Specifically, Mth-Alu2.2 is localized in the terminal regions of the chromosomes, with Msat-160 occupying the immediately inner region, partially intermixed with Mth-Alu2.2. The sequence Mth-Alu900 is found in internal positions below Msat-160, and the interstitial telomeric repeats are located close to the long-arm euchromatin of the chromosomes.
Assuntos
Arvicolinae/genética , Centrossomo/ultraestrutura , Heterocromatina/química , Animais , Arvicolinae/metabolismo , Linhagem da Célula , Centrômero/ultraestrutura , Cruzamentos Genéticos , DNA/química , Feminino , Hibridização in Situ Fluorescente , Cariotipagem , Masculino , Fenótipo , Sequências Repetitivas de Ácido Nucleico , Telômero/ultraestruturaRESUMO
In the subfamily Arvicolinae (Cricetidae, Rodentia) the satellite DNA Msat-160 has been so far described in only some species from the genus Microtus and in one species from another genus, Chionomys nivalis. Here we cloned and characterized this satellite in two new arvicoline species, Microtus (Terricola) savii and Arvicola amphibius (terrestris). We have also demonstrated, by PCR and FISH, its existence in the genomes of several other species from both genera. These results suggest that Msat-160 already occurred in the common ancestor of the four genera/subgenera of Arvicolinae (Microtus, Chionomys, Arvicola, and Terricola). In Arvicola and Terricola, Msat-160 showed the basic monomer length of 160 bp, although a higher-order repeat (HORs) of 640 bp could have been probably replacing the original monomeric unit in A. a. terrestris. Msat-160 was localized by FISH mostly on the pericentromeric regions of the chromosomes, but the signal intensity and the number of carrier chromosomes varied extremely even between closely related species, resulting in a species-specific pattern of chromosomal distribution of this satellite. Such a variable pattern most likely is a consequence of a rapid amplification and contraction of particular repeats in the pericentromeric regions of chromosomes. In addition, we proposed that the rapid variation of pericentromeric repeats is strictly related to the prolific species radiation and diversification of karyotypes that characterize Arvicolinae lineage. Finally, we performed phylogenetic analysis in this group of related species based on Msat-160 that results to be in agreement with previously reported phylogenies, derived from other molecular markers.