Inheritance of DNA transferred from American trypanosomes to human hosts

The Trypanosoma cruzi Sylvio X10 strain maxicircle sequence: the third musketeer

Background

Chagas disease has a diverse pathology caused by the parasite Trypanosoma cruzi, and is indigenous to Central and South America. A pronounced feature of the trypanosomes is the kinetoplast, which is comprised of catenated maxicircles and minicircles that provide the transcripts involved in uridine insertion/deletion RNA editing. T. cruzi exchange genetic material through a hybridization event. Extant strains are grouped into six discrete typing units by nuclear markers, and three clades, A, B, and C, based on maxicircle gene analysis. Clades A and B are the more closely related. Representative clade B and C maxicircles are known in their entirety, and portions of A, B, and C clades from multiple strains show intra-strain heterogeneity with the potential for maxicircle taxonomic markers that may correlate with clinical presentation.

Results

To perform a genome-wide analysis of the three maxicircle clades, the coding region of clade A representative strain Sylvio X10 (a.k.a. Silvio X10) was sequenced by PCR amplification of specific fragments followed by assembly and comparison with the known CL Brener and Esmeraldo maxicircle sequences. The clade A rRNA and protein coding region maintained synteny with clades B and C. Amino acid analysis of non-edited and 5'-edited genes for Sylvio X10 showed the anticipated gene sequences, with notable frameshifts in the non-edited regions of Cyb and ND4. Comparisons of genes that undergo extensive uridine insertion and deletion display a high number of insertion/deletion mutations that are likely permissible due to the post-transcriptional activity of RNA editing.

Conclusion

Phylogenetic analysis of the entire maxicircle coding region supports the closer evolutionary relationship of clade B to A, consistent with uniparental mitochondrial inheritance from a discrete typing unit TcI parental strain and studies on smaller fragments of the mitochondrial genome. Gene variance that can be corrected by RNA editing hints at an unusual depth for maxicircle taxonomic markers, which will aid in the ability to distinguish strains, their corresponding symptoms, and further our understanding of the T. cruzi population structure. The prevalence of apparently compromised coding regions outside of normally edited regions hints at undescribed but active mechanisms of genetic exchange.

The limited distribution of Helitrons to vesper bats supports horizontal transfer

Transposable elements (TEs) have the unique ability to move and replicate within the genome and therefore engender dramatic changes to genome architecture. Among different types of TEs, rolling-circle transposons (Helitrons) are well known for their ability to capture and amplify host gene fragments. Bioinformatic analysis revealed that Helitrons constitute ~3% of the Myotis lucifugus, (little brown bat) genome, while no Helitrons were found in any of the other 44+ sequenced mammalian genomes. Recently horizontal transfer has been implicated for some of the M. lucifugus Helitrons, in part explaining this disparate distribution among mammals. The purpose of this work is to determine both the distribution of Helitrons among bats and to estimate the number of independent invasions. We employed a combination of in silico, PCR and hybridization based techniques to identify Helitrons from diverse bat species belonging to ten different families. This work reveals that Helitrons invaded the vesper bat lineage, at least once. Indeed, Helitrons were not identified in the sister taxa 'Miniopterus', which suggests that the amplification of Helibat occurred (30-36 mya) only in the vesper bat lineage. The estimated age of amplification of the Helibats and the rapid radiation of vesper bats are roughly coincidental and suggest that the invasion and amplification of these elements might have influenced their evolutionary trajectory potentially contributing to phenotypic and genotypic diversity.

Promiscuous DNA: horizontal transfer of transposable elements and why it matters for eukaryotic evolution

Horizontal transfer is the passage of genetic material between genomes by means other than parent-to-offspring inheritance. Although the transfer of genes is thought to be crucial in prokaryotic evolution, few instances of horizontal gene transfer have been reported in multicellular eukaryotes; instead, most cases involve transposable elements. With over 200 cases now documented, it is possible to assess the importance of horizontal transfer for the evolution of transposable elements and their host genomes. We review criteria for detecting horizontal transfers and examine recent examples of the phenomenon, shedding light on its mechanistic underpinnings, including the role of host-parasite interactions. We argue that the introduction of transposable elements by horizontal transfer in eukaryotic genomes has been a major force propelling genomic variation and biological innovation.

A role for host-parasite interactions in the horizontal transfer of transposons across phyla

Horizontal transfer (HT), or the passage of genetic material between non-mating species, is increasingly recognized as an important force in the evolution of eukaryotic genomes. Transposons, with their inherent ability to mobilize and amplify within genomes, may be especially prone to HT. However, the means by which transposons can spread across widely diverged species remain elusive. Here we present evidence that host-parasite interactions have promoted the HT of four transposon families between invertebrates and vertebrates. We found that Rhodnius prolixus, a triatomine bug feeding on the blood of various tetrapods and vector of Chagas' disease in humans, carries in its genome four distinct transposon families that also invaded the genomes of a diverse, but overlapping, set of tetrapods. The bug transposons are approximately 98% identical and cluster phylogenetically with those of the opossum and squirrel monkey, two of its preferred mammalian hosts in South America. We also identified one of these transposon families in the pond snail Lymnaea stagnalis, a cosmopolitan vector of trematodes infecting diverse vertebrates, whose ancestral sequence is nearly identical and clusters with those found in Old World mammals. Together these data provide evidence for a previously hypothesized role of host-parasite interactions in facilitating HT among animals. Furthermore, the large amount of DNA generated by the amplification of the horizontally transferred transposons supports the idea that the exchange of genetic material between hosts and parasites influences their genomic evolution.