Tropical Africa as a cradle for horizontal transfers of transposable elements between species of the genera Drosophila and Zaprionus

Diverse mobilome of Dichotomius (Luederwaldtinia) schiffleri (Coleoptera: Scarabaeidae) reveals long-range horizontal transfer events of DNA transposons

Transposable elements (TEs) are mobile DNA sequences that are able to move from one genomic location to another. These selfish elements are known as genomic parasites, since they hijack the host molecular machinery to generate new copies of themselves. The mobilization of TEs can be seen as a natural mutagen because new TE copies can insert into different loci and impact host genomic structure through different mechanisms. Although our knowledge about TEs is improving with new genomes available, there is still very limited data about the mobilome of species from the Coleoptera order, the most diverse order of insects, including species from the Scarabaeidae family. Therefore, the main goal of this study was to characterize the mobilome of D. (Luederwaldtinia) schiffleri, based on low-coverage genome sequencing, and reconstruct their evolutionary history. We used a combination of four different approaches for TE characterization and maximum likelihood phylogenetic analysis to study their evolution. We found a large and diverse mobilome composed of 38 TE superfamilies, 20 DNA transposon and 18 retrotransposons, accounting for 21% of the genome. Moreover, we found a number of incongruences between the TE and host phylogenetic trees in three DNA transposon TE superfamilies, which represents five TE families, suggesting possible horizontal transfer events between highly divergent taxa. In summary, we found an abundant and diverse mobilome and a number of horizontal transfer events that have shaped the evolutionary history of this species.

High frequency of horizontal transfer in Jockey families (LINE order) of drosophilids

Background

The use of large-scale genomic analyses has resulted in an improvement of transposable element sampling and a significant increase in the number of reported HTT (horizontal transfer of transposable elements) events by expanding the sampling of transposable element sequences in general and of specific families of these elements in particular, which were previously poorly sampled. In this study, we investigated the occurrence of HTT events in a group of elements that, until recently, were uncommon among the HTT records in Drosophila – the Jockey elements, members of the LINE (long interspersed nuclear element) order of non-LTR (long terminal repeat) retrotransposons. The sequences of 111 Jockey families deposited in Repbase that met the criteria of the analysis were used to identify Jockey sequences in 48 genomes of Drosophilidae (genus Drosophila, subgenus Sophophora: melanogaster, obscura and willistoni groups; subgenus Drosophila: immigrans, melanica, repleta, robusta, virilis and grimshawi groups; subgenus Dorsilopha: busckii group; genus/subgenus Zaprionus and genus Scaptodrosophila).

Results

Phylogenetic analyses revealed 72 Jockey families in 41 genomes. Combined analyses revealed 15 potential HTT events between species belonging to different genera and species groups of Drosophilidae, providing evidence for the flow of genetic material favoured by the spatio-temporal sharing of these species present in the Palaeartic or Afrotropical region.

Conclusions

Our results provide phylogenetic, biogeographic and temporal evidence of horizontal transfers of the Jockey elements, increase the number of rare records of HTT in specific families of LINE elements, increase the number of known occurrences of these events, and enable a broad understanding of the evolutionary dynamics of these elements and the host species.

Helena and BS: Two Travellers between the Genera Drosophila and Zaprionus

The frequency of horizontal transfers of transposable elements (HTTs) varies among the types of elements according to the transposition mode and the geographical and temporal overlap of the species involved in the transfer. The drosophilid species of the genus Zaprionus and those of the melanogaster, obscura, repleta, and virilis groups of the genus Drosophila investigated in this study shared space and time at some point in their evolutionary history. This is particularly true of the subgenus Zaprionus and the melanogaster subgroup, which overlapped both geographically and temporally in Tropical Africa during their period of origin and diversification. Here, we tested the hypothesis that this overlap may have facilitated the transfer of retrotransposons without long terminal repeats (non-LTRs) between these species. We estimated the HTT frequency of the non-LTRs BS and Helena at the genome-wide scale by using a phylogenetic framework and a vertical and horizontal inheritance consistence analysis (VHICA). An excessively low synonymous divergence among distantly related species and incongruities between the transposable element and species phylogenies allowed us to propose at least four relatively recent HTT events of Helena and BS involving ancestors of the subgroup melanogaster and ancestors of the subgenus Zaprionus during their concomitant diversification in Tropical Africa, along with older possible events between species of the subgenera Drosophila and Sophophora. This study provides the first evidence for HTT of non-LTRs retrotransposons between Drosophila and Zaprionus, including an in-depth reconstruction of the time frame and geography of these events.

Genetic exchange in eukaryotes through horizontal transfer: connected by the mobilome

Background

All living species contain genetic information that was once shared by their common ancestor. DNA is being inherited through generations by vertical transmission (VT) from parents to offspring and from ancestor to descendant species. This process was considered the sole pathway by which biological entities exchange inheritable information. However, Horizontal Transfer (HT), the exchange of genetic information by other means than parents to offspring, was discovered in prokaryotes along with strong evidence showing that it is a very important process by which prokaryotes acquire new genes.

Main body

For some time now, it has been a scientific consensus that HT events were rare and non-relevant for evolution of eukaryotic species, but there is growing evidence supporting that HT is an important and frequent phenomenon in eukaryotes as well.

Conclusion

Here, we will discuss the latest findings regarding HT among eukaryotes, mainly HT of transposons (HTT), establishing HTT once and for all as an important phenomenon that should be taken into consideration to fully understand eukaryotes genome evolution. In addition, we will discuss the latest development methods to detect such events in a broader scale and highlight the new approaches which should be pursued by researchers to fill the knowledge gaps regarding HTT among eukaryotes.

Living Organisms Author Their Read-Write Genomes in Evolution

Evolutionary variations generating phenotypic adaptations and novel taxa resulted from complex cellular activities altering genome content and expression: (i) Symbiogenetic cell mergers producing the mitochondrion-bearing ancestor of eukaryotes and chloroplast-bearing ancestors of photosynthetic eukaryotes; (ii) interspecific hybridizations and genome doublings generating new species and adaptive radiations of higher plants and animals; and, (iii) interspecific horizontal DNA transfer encoding virtually all of the cellular functions between organisms and their viruses in all domains of life. Consequently, assuming that evolutionary processes occur in isolated genomes of individual species has become an unrealistic abstraction. Adaptive variations also involved natural genetic engineering of mobile DNA elements to rewire regulatory networks. In the most highly evolved organisms, biological complexity scales with "non-coding" DNA content more closely than with protein-coding capacity. Coincidentally, we have learned how so-called "non-coding" RNAs that are rich in repetitive mobile DNA sequences are key regulators of complex phenotypes. Both biotic and abiotic ecological challenges serve as triggers for episodes of elevated genome change. The intersections of cell activities, biosphere interactions, horizontal DNA transfers, and non-random Read-Write genome modifications by natural genetic engineering provide a rich molecular and biological foundation for understanding how ecological disruptions can stimulate productive, often abrupt, evolutionary transformations.

A Horizontally Transferred Autonomous Helitron Became a Full Polydnavirus Segment in Cotesia vestalis

Bracoviruses associate symbiotically with thousands of parasitoid wasp species in the family Braconidae, working as virulence gene vectors, and allowing the development of wasp larvae within hosts. These viruses are composed of multiple DNA circles that are packaged into infective particles, and injected together with wasp's eggs during parasitization. One of the viral segments of Cotesia vestalis bracovirus contains a gene that has been previously described as a helicase of unknown origin. Here, we demonstrate that this gene is a Rep/Helicase from an intact Helitron transposable element that covers the viral segment almost entirely. We also provide evidence that this element underwent at least two horizontal transfers, which appear to have occurred consecutively: first from a Drosophila host ancestor to the genome of the parasitoid wasp C. vestalis and its bracovirus, and then from C. vestalis to a lepidopteran host (Bombyx mori). Our results reinforce the idea of parasitoid wasps as frequent agents of horizontal transfers in eukaryotes. Additionally, this Helitron-bracovirus segment is the first example of a transposable element that effectively became a whole viral circle.

A new genome-wide method to track horizontally transferred sequences: application to Drosophila

Because of methodological breakthroughs and the availability of an increasing amount of whole-genome sequence data, horizontal transfers (HTs) in eukaryotes have received much attention recently. Contrary to similar analyses in prokaryotes, most studies in eukaryotes usually investigate particular sequences corresponding to transposable elements (TEs), neglecting the other components of the genome. We present a new methodological framework for the genome-wide detection of all putative horizontally transferred sequences between two species that requires no prior knowledge of the transferred sequences. This method provides a broader picture of HTs in eukaryotes by fully exploiting complete-genome sequence data. In contrast to previous genome-wide approaches, we used a well-defined statistical framework to control for the number of false positives in the results, and we propose two new validation procedures to control for confounding factors. The first validation procedure relies on a comparative analysis with other species of the phylogeny to validate HTs for the nonrepeated sequences detected, whereas the second one built upon the study of the dynamics of the detected TEs. We applied our method to two closely related Drosophila species, Drosophila melanogaster and D. simulans, in which we discovered 10 new HTs in addition to all the HTs previously detected in different studies, which underscores our method’s high sensitivity and specificity. Our results favor the hypothesis of multiple independent HTs of TEs while unraveling a small portion of the network of HTs in the Drosophila phylogeny.

An evaluation of the ecological relationship between Drosophila species and their parasitoid wasps as an opportunity for horizontal transposon transfer

Evidences of horizontal transfer, the exchange of genetic material between reproductively isolated species, have accumulated over the last decades, including for multicellular eukaryotic organisms. However, the mechanisms and ecological relationships that promote such phenomenon is still poorly known. Host-parasite interaction is one type of relationship usually pointed in the literature that could potentially increase the probability of the horizontal transfer between species, because the species involved in such relationships are generally in close contact. Transposable elements, which are well-known genomic parasites, are DNA entities that tend to be involved in horizontal transfer due to their ability to mobilize between different genomic locations. Using Drosophila species and their parasitoid wasps as a host-parasite model, we evaluated the hypothesis that horizontal transposon transfers (HTTs) are more frequent in this set of species than in species that do not exhibit a close ecological and phylogenetic relationship. For this purpose, we sequenced two sets of species using a metagenomic and single-species genomic sampling approach through next-generation DNA sequencing. The first set was composed of five generalist Drosophila (D. maculifrons, D. bandeirantorum, D. polymorpha, D. mercatorum and D. willistoni) species and their associated parasitoid wasps, whereas the second set was composed of D. incompta, which is a flower specialist species, and its parasitoid wasp. We did not find strong evidence of HTT in the two sets of Drosophila and wasp parasites. However, at least five cases of HTT were observed between the generalist and specialist Drosophila species. Moreover, we detected an HT event involving a Wolbachia lineage between generalist and specialist species, indicating that these endosymbiotic bacteria could play a role as HTT vectors. In summary, our results do not support the hypothesis of prevalent HTT between species with a host-parasite relationship, at least for the studied wasp-Drosophila pairs. Moreover, it suggests that other mechanisms or parasites are involved in promoting HTT between Drosophila species as the Wolbachia endosymbiotic bacteria.

hobo-brothers elements and their time and place for horizontal transfer

Transposable elements (TEs) are ubiquitous components of nearly all genomes studied. These elements are highly variable in copy number, molecular structure and transposition strategies. They can move within and between genomes, thus increasing their copy numbers and avoiding being eliminated by stochastic and deterministic processes. hobo is a class II element isolated from Drosophila melanogaster. Previous phylogenetic analyses have shown that the canonical hobo element from D. melanogaster has a sister group formed by sequences found in D. willistoni (called howilli2) and D. mojavensis (called homo1). In the present study, we investigated 36 Drosophilidae species for sequences similar to howilli2 and homo1 using degenerate primers. Additionally, in silico searches were performed in 21 available Drosophila genomes. The obtained sequences formed a monophyletic sister group with the canonical hobo element; we termed these sequences 'hobo-brothers' elements. These elements showed a patch distribution and incongruities with the TE and host species phylogenies, suggesting possible cases of horizontal transfer (HT). Species that possess hobo-brothers sequences are from the New World, mainly Neotropical areas. In addition, the estimated divergence of the sequences found showed that these elements are or were recently active; the large number of HT events observed suggests that these elements could be experiencing an expansion process in Neotropical genomes. A comparison of these results with the literature is discussed with regard to the importance of the time and location of horizontal transposon transfer events.

Ancestral polymorphism and recent invasion of transposable elements in Drosophila species

BACKGROUND

During the evolution of transposable elements, some processes, such as ancestral polymorphisms and horizontal transfer of sequences between species, can produce incongruences in phylogenies. We investigated the evolutionary history of the transposable elements Bari and 412 in the sequenced genomes of the Drosophila melanogaster group and in the sibling species D. melanogaster and D. simulans using traditional phylogenetic and network approaches.

RESULTS

Maximum likelihood (ML) phylogenetic analyses revealed incongruences and unresolved relationships for both the Bari and 412 elements. The DNA transposon Bari within the D. ananassae genome is more closely related to the element of the melanogaster complex than to the sequence in D. erecta, which is inconsistent with the species phylogeny. Divergence analysis and the comparison of the rate of synonymous substitutions per synonymous site of the Bari and host gene sequences explain the incongruence as an ancestral polymorphism that was inherited stochastically by the derived species. Unresolved relationships were observed in the ML phylogeny of both elements involving D. melanogaster, D. simulans and D. sechellia. A network approach was used to attempt to resolve these relationships. The resulting tree suggests recent transfers of both elements between D. melanogaster and D. simulans. The divergence values of the elements between these species support this conclusion.

CONCLUSIONS

We showed that ancestral polymorphism and recent invasion of genomes due to introgression or horizontal transfer between species occurred during the evolutionary history of the Bari and 412 elements in the melanogaster group. These invasions likely occurred in Africa during the Pleistocene, before the worldwide expansion of D. melanogaster and D. simulans.

Horizontal transposon transfer in eukarya: detection, bias, and perspectives

The genetic similarity observed among species is normally attributed to the existence of a common ancestor. However, a growing body of evidence suggests that the exchange of genetic material is not limited to the transfer from parent to offspring but can also occur through horizontal transfer (HT). Transposable elements (TEs) are DNA fragments with an innate propensity for HT; they are mobile and possess parasitic characteristics that allow them to exist and proliferate within host genomes. However, horizontal transposon transfer (HTT) is not easily detected, primarily because the complex TE life cycle can generate phylogenetic patterns similar to those expected for HTT events. The increasingly large number of new genome projects, in all branches of life, has provided an unprecedented opportunity to evaluate the TE content and HTT events in these species, although a standardized method of HTT detection is required before trends in the HTT rates can be evaluated in a wide range of eukaryotic taxa and predictions about these events can be made. Thus, we propose a straightforward hypothesis test that can be used by TE specialists and nonspecialists alike to discriminate between HTT events and natural TE life cycle patterns. We also discuss several plausible explanations and predictions for the distribution and frequency of HTT and for the inherent biases of HTT detection. Finally, we discuss some of the methodological concerns for HTT detection that may result in the underestimation and overestimation of HTT rates during eukaryotic genome evolution.

Taxonomic and evolutionary analysis of Zaprionus indianus and its colonization of Palearctic and Neotropical regions

Zaprionus indianus is a dipteran (Drosophilidae) with a wide distribution throughout the tropics and temperate Palearctic and Nearctic regions. There have been proposals to reclassify the genus Zaprionus as a subgenus or group of the genus Drosophila because various molecular markers have indicated a close relationship between Zaprionus species and the immigrans-Hirtodrosophila radiation within Drosophila. These markers, together with alloenzymes and quantitative traits, have been used to describe the probable scenario for the expansion of Zaprionus indianus from its center of dispersal (Africa) to regions of Asia (ancient dispersal) and the Americas (recent dispersal). The introduction of Z. indianus into Brazil was first reported in 1999 and the current consensus is that the introduced flies came from high-latitude African populations through the importation of fruit. Once in Brazil, Z. indianus spread rapidly throughout the Southeast and then to the rest of the country, in association with highway-based fruit commerce. These and other aspects of the evolutionary biology of Z. indianus are addressed in this review, including a description of a probable route for this species’ dispersal during its recent expansion.