Maintenance of a large pericentric inversion generated by the hobo transposable element in a transgenic line of Drosophila melanogaster

How chromosomal inversions reorient the evolutionary process

Inversions are structural mutations that reverse the sequence of a chromosome segment and reduce the effective rate of recombination in the heterozygous state. They play a major role in adaptation, as well as in other evolutionary processes such as speciation. Although inversions have been studied since the 1920s, they remain difficult to investigate because the reduced recombination conferred by them strengthens the effects of drift and hitchhiking, which in turn can obscure signatures of selection. Nonetheless, numerous inversions have been found to be under selection. Given recent advances in population genetic theory and empirical study, here we review how different mechanisms of selection affect the evolution of inversions. A key difference between inversions and other mutations, such as single nucleotide variants, is that the fitness of an inversion may be affected by a larger number of frequently interacting processes. This considerably complicates the analysis of the causes underlying the evolution of inversions. We discuss the extent to which these mechanisms can be disentangled, and by which approach.

Mobilization of retrotransposons as a cause of chromosomal diversification and rapid speciation: the case for the Antarctic teleost genus Trematomus

Background

The importance of transposable elements (TEs) in the genomic remodeling and chromosomal rearrangements that accompany lineage diversification in vertebrates remains the subject of debate. The major impediment to understanding the roles of TEs in genome evolution is the lack of comparative and integrative analyses on complete taxonomic groups. To help overcome this problem, we have focused on the Antarctic teleost genus Trematomus (Notothenioidei: Nototheniidae), as they experienced rapid speciation accompanied by dramatic chromosomal diversity. Here we apply a multi-strategy approach to determine the role of large-scale TE mobilization in chromosomal diversification within Trematomus species.

Results

Despite the extensive chromosomal rearrangements observed in Trematomus species, our measurements revealed strong interspecific genome size conservation. After identifying the DIRS1, Gypsy and Copia retrotransposon superfamilies in genomes of 13 nototheniid species, we evaluated their diversity, abundance (copy numbers) and chromosomal distribution. Four families of DIRS1, nine of Gypsy, and two of Copia were highly conserved in these genomes; DIRS1 being the most represented within Trematomus genomes. Fluorescence in situ hybridization mapping showed preferential accumulation of DIRS1 in centromeric and pericentromeric regions, both in Trematomus and other nototheniid species, but not in outgroups: species of the Sub-Antarctic notothenioid families Bovichtidae and Eleginopsidae, and the non-notothenioid family Percidae.

Conclusions

In contrast to the outgroups, High-Antarctic notothenioid species, including the genus Trematomus, were subjected to strong environmental stresses involving repeated bouts of warming above the freezing point of seawater and cooling to sub-zero temperatures on the Antarctic continental shelf during the past 40 millions of years (My). As a consequence of these repetitive environmental changes, including thermal shocks; a breakdown of epigenetic regulation that normally represses TE activity may have led to sequential waves of TE activation within their genomes. The predominance of DIRS1 in Trematomus species, their transposition mechanism, and their strategic location in “hot spots” of insertion on chromosomes are likely to have facilitated nonhomologous recombination, thereby increasing genomic rearrangements. The resulting centric and tandem fusions and fissions would favor the rapid lineage diversification, characteristic of the nototheniid adaptive radiation.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4714-x) contains supplementary material, which is available to authorized users.

Mapping Genomic Scaffolds to Chromosomes Using Laser Capture Microdissection in Application to Hawaiian Picture-Winged Drosophila

Next-generation sequencing technologies have led to a decreased cost and an increased throughput in genome sequencing. Yet, many genome assemblies based on short sequencing reads have been assembled only to the scaffold level due to the lack of sufficient chromosome mapping information. Traditional ways of mapping scaffolds to chromosomes require a large amount of laboratory work and time to generate genetic and/or physical maps. To address this problem, we conducted a rapid technique which uses laser capture microdissection and enables mapping scaffolds of de novo genome assemblies directly to chromosomes in Hawaiian picture-winged Drosophila. We isolated and sequenced intact chromosome arms from larvae of D. differens. By mapping the reads of each chromosome to the recently assembled scaffolds from 3 Hawaiian picture-winged Drosophila species, at least 67% of the scaffolds were successfully assigned to chromosome arms. Even though the scaffolds are not ordered within a chromosome, the fast-generated chromosome information allows for chromosome-related analyses after genome assembling. We utilize this new information to test the faster-X evolution effect for the first time in these Hawaiian picture-winged Drosophila species.

Genomic composition and evolution of Aedes aegypti chromosomes revealed by the analysis of physically mapped supercontigs

BACKGROUND

An initial comparative genomic study of the malaria vector Anopheles gambiae and the yellow fever mosquito Aedes aegypti revealed striking differences in the genome assembly size and in the abundance of transposable elements between the two species. However, the chromosome arms homology between An. gambiae and Ae. aegypti, as well as the distribution of genes and repetitive elements in chromosomes of Ae. aegypti, remained largely unexplored because of the lack of a detailed physical genome map for the yellow fever mosquito.

RESULTS

Using a molecular landmark-guided fluorescent in situ hybridization approach, we mapped 624 Mb of the Ae. aegypti genome to mitotic chromosomes. We used this map to analyze the distribution of genes, tandem repeats and transposable elements along the chromosomes and to explore the patterns of chromosome homology and rearrangements between Ae. aegypti and An. gambiae. The study demonstrated that the q arm of the sex-determining chromosome 1 had the lowest gene content and the highest density of minisatellites. A comparative genomic analysis with An. gambiae determined that the previously proposed whole-arm synteny is not fully preserved; a number of pericentric inversions have occurred between the two species. The sex-determining chromosome 1 had a higher rate of genome rearrangements than observed in autosomes 2 and 3 of Ae. aegypti.

CONCLUSIONS

The study developed a physical map of 45% of the Ae. aegypti genome and provided new insights into genomic composition and evolution of Ae. aegypti chromosomes. Our data suggest that minisatellites rather than transposable elements played a major role in rapid evolution of chromosome 1 in the Aedes lineage. The research tools and information generated by this study contribute to a more complete understanding of the genome organization and evolution in mosquitoes.

Genome landscape and evolutionary plasticity of chromosomes in malaria mosquitoes

BACKGROUND

Nonrandom distribution of rearrangements is a common feature of eukaryotic chromosomes that is not well understood in terms of genome organization and evolution. In the major African malaria vector Anopheles gambiae, polymorphic inversions are highly nonuniformly distributed among five chromosomal arms and are associated with epidemiologically important adaptations. However, it is not clear whether the genomic content of the chromosomal arms is associated with inversion polymorphism and fixation rates.

METHODOLOGY/PRINCIPAL FINDINGS

To better understand the evolutionary dynamics of chromosomal inversions, we created a physical map for an Asian malaria mosquito, Anopheles stephensi, and compared it with the genome of An. gambiae. We also developed and deployed novel Bayesian statistical models to analyze genome landscapes in individual chromosomal arms An. gambiae. Here, we demonstrate that, despite the paucity of inversion polymorphisms on the X chromosome, this chromosome has the fastest rate of inversion fixation and the highest density of transposable elements, simple DNA repeats, and GC content. The highly polymorphic and rapidly evolving autosomal 2R arm had overrepresentation of genes involved in cellular response to stress supporting the role of natural selection in maintaining adaptive polymorphic inversions. In addition, the 2R arm had the highest density of regions involved in segmental duplications that clustered in the breakpoint-rich zone of the arm. In contrast, the slower evolving 2L, 3R, and 3L, arms were enriched with matrix-attachment regions that potentially contribute to chromosome stability in the cell nucleus.

CONCLUSIONS/SIGNIFICANCE

These results highlight fundamental differences in evolutionary dynamics of the sex chromosome and autosomes and revealed the strong association between characteristics of the genome landscape and rates of chromosomal evolution. We conclude that a unique combination of various classes of genes and repetitive DNA in each arm, rather than a single type of repetitive element, is likely responsible for arm-specific rates of rearrangements.

Applying mobile genetic elements for genome analysis and evolution

Transposable elements (TEs) are ubiquitous components of all living organisms, and in the course of their coexistence with their respective host genomes, these parasitc DNAs have played important roles in the evolution of complex genetic networks. The interaction between mobile DNAs and their host genomes are quite diverse, ranging from modifications of gene structure and regulation to alterations in general genome architecture. Thus during evolutionary time these elements can be regarded as natural molecular tools in shaping the organization, structure, and function of eukaryotic genes and genomes. Based on their intrinsic properties and features, mobile DNAs are widely applied at present as a technical "toolbox," essential for studying a diverse spectrum of biological questions. In this review, we aim to summarize both the evolutionary impact of TEs on genome evolution and their valuable and diverse methodological applications as molecular tools.

Fluorescence in situ hybridization analysis of hobo, mdg1 and Dm412 transposable elements reveals genomic instability following the Drosophila melanogaster genome sequencing

The genome of Drosophila melanogaster strain y cn bw sp has been sequenced and the transposable elements insertion sites have been determined. We hybridized fluorescence-labeled probes directed to the hobo transposon, Dm412 and mdg1 retrotransposons to polytene chromosomes and compared the observed sites to those published in the annotated genome sequence. We observed an almost twofold increase in the number of hobo hybridization sites (46 found as compared to 24 annotated sites). There was no evidence that the hobo transposition rate is slowing over the 10-year period. The patterns of Dm412 and mdg1 sites have changed less dramatically since the time of genome sequencing. Three novel Dm412 hybridization sites were detected while 4 out of 30 annotated sites were missing. Only one additional mdg1 site was found, while 1 out of 29 annotated sites has been lost.

Rates and patterns of chromosomal evolution in Drosophila pseudoobscura and D. miranda

Comparisons of gene orders between species permit estimation of the rate of chromosomal evolution since their divergence from a common ancestor. We have compared gene orders on three chromosomes of Drosophila pseudoobscura with its close relative, D. miranda, and the distant outgroup species, D. melanogaster, by using the public genome sequences of D. pseudoobscura and D. melanogaster and approximately 50 in situ hybridizations of gene probes in D. miranda. We find no evidence for extensive transfer of genes among chromosomes in D. miranda. The rates of chromosomal rearrangements between D. miranda and D. pseudoobscura are far higher than those found before in Drosophila and approach those for nematodes, the fastest rates among higher eukaryotes. In addition, we find that the D. pseudoobscura chromosome with the highest level of inversion polymorphism (Muller's element C) does not show an unusually fast rate of evolution with respect to chromosome structure, suggesting that this classic case of inversion polymorphism reflects selection rather than mutational processes. On the basis of our results, we propose possible ancestral arrangements for the D. pseudoobscura C chromosome, which are different from those in the current literature. We also describe a new method for correcting for rearrangements that are not detected with a limited set of markers.