Somatic activity of the mariner transposable element in natural populations of Drosophila simulans

Pathogen infection alters the gene expression landscape of transposable elements in Drosophila melanogaster

Transposable elements make up substantial proportions of eukaryotic genomes and many are thought to be remnants of ancient viral infections. Current research has begun to highlight the role transposable elements can play in the immune system response to infections. However, most of our knowledge about transposable element expression during infection is limited by the specific host and pathogen factors from each study, making it difficult to compare studies and develop broader patterns regarding the role of transposable elements during infection. Here, we use the tools and resources available in the model, Drosophila melanogaster, to analyze multiple gene expression datasets of flies subject to bacterial, fungal, and viral infections. We analyzed differences in pathogen species, host genotype, host tissue, and sex to understand how these factors impact transposable element expression during infection. Our results highlight both shared and unique transposable element expression patterns between pathogens and suggest a larger effect of pathogen factors over host factors for influencing transposable element expression.

Somatic Mobilization: High Somatic Insertion Rate of mariner Transposable Element in Drosophila simulans

Although transposable elements (TEs) are usually silent in somatic tissues, they are sometimes mobilized in the soma and can potentially have biological consequences. The mariner element is one of the TEs involved in somatic mobilization (SM) in Drosophila and has a high rate of somatic excision. It is also known that temperature is an important factor in the increase of the mariner element SM in the fly. However, it is important to emphasize that excision is only one step of TE transposition, and the final step in this process is insertion. In the present study, we used an assay based on sequencing of the mariner flanking region and developed a pipeline to identify novel mariner insertions in Drosophila simulans at 20 and 28 °C. We found that flies carrying two mariner copies (one autonomous and one non-autonomous) had an average of 236.4 (±99.3) to 279 (±107.7) new somatic insertions at 20 °C and an average of 172.7 (±95.3) to 252.6 (±67.3) at 28 °C. In addition, we detected fragments containing mariner and others without mariner in the same regions with low-coverage long-read sequencing, indicating the process of excision and insertion. In conclusion, a low number of autonomous copies of the mariner transposon can promote a high rate of new somatic insertions during the developmental stages of Drosophila. Additionally, the developed method seems to be sensitive and adequate for the verification and estimation of somatic insertion.

Transposable elements in Drosophila

Drosophila has been studied as a biological model for many years and many discoveries in biology rely on this species. Research on transposable elements (TEs) is not an exception. Drosophila has contributed significantly to our knowledge on the mechanisms of transposition and their regulation, but above all, it was one of the first organisms on which genetic and genomic studies of populations were done. In this review article, in a very broad way, we will approach the TEs of Drosophila with a historical hindsight as well as recent discoveries in the field.

Genomic divergence and adaptive convergence in Drosophila simulans from Evolution Canyon, Israel

Biodiversity refugia formed by unique features of the Mediterranean arid landscape, such as the dramatic ecological contrast of "Evolution Canyon," provide a natural laboratory in which local adaptations to divergent microclimate conditions can be investigated. Significant insights have been provided by studies of Drosophila melanogaster diversifying along the thermal gradient in Evolution Canyon, but a comparative framework to survey adaptive convergence across sister species at the site has been lacking. To fill this void, we present an analysis of genomic polymorphism and evolutionary divergence of Drosophila simulans, a close relative of Drosophila melanogaster with which it co-occurs on both slopes of the canyon. Our results show even deeper interslope divergence in D. simulans than in D. melanogaster, with extensive signatures of selective sweeps present in flies from both slopes but enhanced in the population from the hotter and drier south-facing slope. Interslope divergence was enriched for genes related to electrochemical balance and transmembrane transport, likely in response to increased selection for dehydration resistance on the hotter slope. Both species shared genomic regions that underwent major selective sweeps, but the overall level of adaptive convergence was low, demonstrating no shortage of alternative genomic solutions to cope with the challenges of the microclimate contrast. Mobile elements were a major source of genetic polymorphism and divergence, affecting all parts of the genome, including coding sequences of mating behavior-related genes.

Population-specific dynamics and selection patterns of transposable element insertions in European natural populations

Transposable elements (TEs) are ubiquitous sequences in genomes of virtually all species. While TEs have been investigated for several decades, only recently we have the opportunity to study their genome-wide population dynamics. Most of the studies so far have been restricted either to the analysis of the insertions annotated in the reference genome or to the analysis of a limited number of populations. Taking advantage of the European Drosophila population genomics consortium (DrosEU) sequencing data set, we have identified and measured the dynamics of TEs in a large sample of European Drosophila melanogaster natural populations. We showed that the mobilome landscape is population-specific and highly diverse depending on the TE family. In contrast with previous studies based on SNP variants, no geographical structure was observed for TE abundance or TE divergence in European populations. We further identified de novo individual insertions using two available programs and, as expected, most of the insertions were present at low frequencies. Nevertheless, we identified a subset of TEs present at high frequencies and located in genomic regions with a high recombination rate. These TEs are candidates for being the target of positive selection, although neutral processes should be discarded before reaching any conclusion on the type of selection acting on them. Finally, parallel patterns of association between the frequency of TE insertions and several geographical and temporal variables were found between European and North American populations, suggesting that TEs can be potentially implicated in the adaptation of populations across continents.

The somatic mobilization of transposable element mariner-Mos1 during the Drosophila lifespan and its biological consequences

Transposable elements (TEs) are mobile DNA sequences on genomes. Some elements are able to transpose in somatic cells, a process known as somatic transposition (ST), which has been associated with detrimental biological effects. The mariner-Mos1 element of Drosophila promotes transposition in somatic and germline cells and is an excellent model for studies related to the biological consequence of somatic excision (SE). In this work, we used temperature stress to induce increasing transposition of mariner-Mos1 during different stages of the development of D. simulans, aiming to quantify SE during lifespan. Furthermore, strains of D. melanogaster exhibiting differential expression of mariner-Mos1 were employed for estimating some biological consequences of mariner mobilization. It is shown that SE of mariner-Mos1 was not constant during development; the larval phase had the highest rates while the pupal stage exhibited lower rates, and in the embryonic stage, no difference was detected. SE can be detrimental, as suggested by correlation in SE level and reduction in behavioral activities and embryonic viability. This study showed that mariner-Mos1 SE accumulates during the Drosophila life cycle, and can be involved in detrimental effects.

Effects of heat and UV radiation on the mobilization of transposon mariner-Mos1

There are many complex interactions between transposable elements (TEs) and host genomes. Environmental changes that induce stressful conditions help to contribute for increasing complexity of these interactions. The transposon mariner-Mos1 increases its mobilization under mild heat stress. It has putative heat shock elements (HSEs), which are probably activated by heat shock factors (HSFs). Ultraviolet radiation (UVC) is a stressor that has been suggested as able to activate heat shock protein genes (Hsp). In this study, we test the hypothesis that if UVC induces Hsp expression, as heat does, it could also promote mariner-Mos1 transposition and mobilization. The Drosophila simulans white-peach is a mutant lineage that indicates the mariner-Mos1 transposition phenotypically through the formation of mosaic eyes. This lineage was exposed to UVC or mild heat stress (28 °C) in order to evaluate the induction of mariner-Mos1 expression by RT-qPCR, as well as the mariner-Mos1 mobilization activity based on the count number of red spots in the eyes. The effects of both treatments on the developmental time of flies and cell cycle progression were also investigated. Both the analysis of eyes and mariner-Mos1 gene expression indicate that UVC radiation has no effect in mariner-Mos1 transposition, although heat increases the expression and mobilization of this TE soon after the treatment. However, the expression of Hsp70 gene increased after 24 h of UVC exposure, suggesting different pathway of activation. These results showed that heat promotes mariner-Mos1 mobilization, although UVC does not induce the expression or mobilization of this TE.

Novel repeated DNAs in the antarctic polyplacophoran Nuttallochiton mirandus (Thiele, 1906)

Within the scope of a project on the characterization of satellite DNAs in polar mollusks, the Antarctic chiton Nuttallochitonmirandus (Thiele, 1906) was analyzed. Two novel families of tandemly repeated DNAs, namely NmH and NmP, are described in their structure and chromosomal localization, and, furthermore, their presence was analyzed in related species. Data reported here display a particular variability in the structural organization of DNA satellites within this species. Processes driving satellite evolution, which are likely responsible for the intriguing variability of the identified satellite DNAs, are discussed.

Transposable element small RNAs as regulators of gene expression

Transposable elements (TEs) are a source of endogenous small RNAs in animals and plants. These TE-derived small RNAs have been traditionally treated as functionally distinct from gene-regulating small RNAs, such as miRNAs. Two recent reports in Drosophila and Arabidopsis have blurred the lines of this distinction. In both examples, epigenetically and developmentally regulated bursts in TE expression produce gene-regulating small RNAs. In the Drosophila early embryo, maternally deposited TE-derived PIWI-interacting small RNAs (piRNAs) play a role in regulating the nanos mRNA through small RNA binding sites in the nanos 3' untranslated region (UTR). In Arabidopsis, when Athila retrotransposons are epigenetically activated, their transcripts are processed into small RNAs, which directly target the 3'UTR of the genic oligouridylate binding protein 1B (UBP1b) mRNA. Based on these two examples, we suggest that other TE-derived small RNAs regulate additional genes and propose that, through small RNAs, the epigenetic status of TEs could widely influence the genic transcriptome.

The impact of transposable elements on eukaryotic genomes: from genome size increase to genetic adaptation to stressful environments

Transposable elements (TEs) are present in roughly all genomes. These mobile DNA sequences are able to invade genomes and their impact on genome evolution is substantial. The mobility of TEs can induce the appearance of deleterious mutations, gene disruption and chromosome rearrangements, but transposition activity also has positive aspects and the mutational activities of TEs contribute to the genetic diversity of organisms. This short review aims to give a brief overview of the impact TEs may have on animal and plant genome structure and expression, and the relationship between TEs and the stress response of organisms, including insecticide resistance.

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.

The mariner transposable element in natural populations of Drosophila simulans

In cosmopolitan species, geographical variations in copy number and/or level of transposition activity have been observed for several transposable elements (TEs). Environment, history and population structure can contribute to such variation in ways that are difficult to tease apart. For the mariner element, previous studies of the geographic variation of its somatic activity in natural populations of Drosophila simulans have shown contradictory results (latitudinal clines of divergent orientations or no apparent structure). To try and resolve these inconsistencies, we gathered all available data on the mariner somatic activity of worldwide natural populations. This includes previously published results by different groups and also new data. The correlations between the level of activity and several geoclimatic factors were tested. Although no general effect of temperature was found, a relationship with the invasion history was detected. It was also shown that recent invasive populations have a higher level of activity than the putative ancestral ones. Our results strongly suggest that variability of the mariner somatic activity among natural populations of D. simulans is mainly due to populational and historical factors probably related to the recent world colonization of this species. Indeed, this activity is correlated to the main route out of Africa (the Nile route) and the recent colonization of continents such as Australia and South America.

Deleterious transposable elements and the extinction of asexuals

The genomes of virtually all sexually reproducing species contain transposable elements. Although active elements generally transpose more rapidly than they are inactivated by mutation or excision, their number can be kept in check by purifying selection if its effectiveness becomes disproportionately greater as their copy number increases. In sexually reproducing species, such synergistic selection can result from ectopic crossing-over or from homologous recombination under negative epistasis. In addition, there may be controls on transposon activity that are associated with meiosis. Because a sexual lineage that abandons sex must lack such mechanisms, it may be driven to extinction by the unchecked proliferation of deleterious transposons inherited from its sexual progenitor. An important component of the evolutionary advantage of sex over asex may therefore lie in the ability of sex, despite facilitating the spread of deleterious elements within interbreeding populations, also to restrain their intragenomic proliferation.

Mobilization of the active MITE transposons mPing and Pong in rice by introgression from wild rice (Zizania latifolia Griseb.)

Hybridization between different species plays an important role in plant genome evolution, as well as is a widely used approach for crop improvement. McClintock has predicted that plant wide hybridization constitutes a "genomic shock" whereby cryptic transposable elements may be activated. However, direct experimental evidence showing a causal relationship between plant wide hybridization and transposon mobilization has not yet been reported. The miniature-Ping (mPing) is a recently isolated active miniature inverted-repeat transposable element transposon from rice, which is mobilized by tissue culture and gamma-ray irradiation. We show herein that mPing, together with its putative transposase-encoding partner, Pong, is mobilized in three homologous recombinant inbred lines (RILs), derived from hybridization between rice (cultivar Matsumae) and wild rice (Zizania latifolia Griseb.), harboring introgressed genomic DNA from wild rice. In contrast, both elements remain immobile in two lines sharing the same parentage to the RILs but possessing no introgressed DNA. Thus, we have presented direct evidence that is consistent with McClintock's insight by demonstrating a causal link between wide hybridization and transposon mobilization in rice. In addition, we report an atypical behavior of mPing/Pong mobilization in these lines, i.e., the exclusive absence of footprints after excision.

Stress and transposable elements: co-evolution or useful parasites?

The activity of transposable elements can be induced by environmental and population factors and in particular by stresses in various organisms. A consequence of the increase in transposable element mobility is the creation of new genetic variability that can be useful in the face of stressful conditions. In this review, results supporting this hypothesis are presented and discussed. The main question is how stress induces the activity of transposable elements. We discuss hypotheses based upon the existence of promoters or fixation sites of transcription activators in the untranslated regions of transposable elements, similar to those found in regulatory regions of host defence genes.

Genetic variation and differentiation at microsatellite loci in Drosophila simulans. Evidence for founder effects in new world populations

Drosophila simulans isofemale lines from Africa, South America, and two locations in North America were surveyed for variation at 16 microsatellite loci on the X, second, and third chromosomes, and 18 microsatellites, which are unmapped. D. simulans is thought to have colonized New World habitats only relatively recently (within the last few hundred years). Consistent with a founder effect occurring as colonizers moved into these New World habitats, we find less microsatellite variability in North and South American D. simulans populations than for an African population. Population subdivision as measured at microsatellites is moderate when averaged across all loci (FST = 0.136), but contrasts sharply with previous studies of allozyme variation, which have showed significantly less differentiation in D. simulans than in D. melanogaster. There are substantially fewer private alleles observed in New World populations of D. simulans than seen in a similar survey of D. melanogaster. In addition to possible differences in population size during their evolutionary histories, varying colonization histories or other demographic events may be necessary to explain discrepancies in the patterns of variation observed at various genetic markers between these closely related species.

A temperature cline in copy number for 412 but not roo/B104 retrotransposons in populations of Drosophila simulans

The copy number of the retrotransposable element 412 of Drosophila simulans from populations collected worldwide shows a negative correlation with minimum temperature. No association was detected for the roo/B104 element. The possibility that selective pressures might regulate the 412 copy number in these natural populations is supported by detection of selection against the detrimental effects of 412 insertions (estimated by the proportion of insertions on the X chromosome in comparison with the autosomes) but not roo/B104. These data reveal different spatial patterns for two element families, and strongly suggest that some factors in the environment, such as temperature, may interfere with the control of retrotransposition, thus affecting important aspects of genomic evolution.