Table of Contents

Volume 1, Issue 4

  November/December 2011

  Pages 251 - 308

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LETTERS TO THE EDITOR

Open Access Article

Microsporidia: Horizontal gene transfers in vicious parasites

Mohammed Selman and Nicolas Corradi
Pages 251 - 255
http://dx.doi.org/10.4161/mge.18611
Abstract | Full Text | PDF

Microsporidia are obligate intracellular parasites whose genomes have been shaped by an extreme lifestyle. Specifically, their obligate intracellular parasitism has resulted in the loss of many genes and biochemical pathways, but these reductive processes have been often offset by the acquisition of several genes by means of horizontal gene transfer (HGT). Until recently, these HGTs were all found to have derived from prokaryotic donors, but a recent study suggests that some species took advantage of this mechanism to acquire one gene from an animal, which they maintained in their genome for metabolic purposes. The gene encodes for a purine nucleoside phosphorylase, and shows a strong phylogenetic signal of arthropod origin. Here, we briefly review our current knowledge of HGTs discovered across microsporidian genomes and discuss the implications of the most recent findings in this research area for understanding the origin and evolution of this highly adapted group of intracellular parasites. A novel gene potentially transferred by means of HGT to one microsporidia is also reported.

Open Access Article

HGT turbulence: Confounding phylogenetic influence of duplicative horizontal transfer and differential gene conversion

Weilong Hao and Jeffrey Palmer
Pages 256 - 261
http://dx.doi.org/10.4161/mge.19030
Abstract | Full Text | PDF

Horizontal gene transfer (HGT) often leads to phylogenetic incongruence. When “duplicative HGT” introduces a second copy of a pre-existing gene, the two copies may then engage in gene conversion, leading to phylogenetically mosiac genes. When duplicative HGT is followed by differential gene conversion among descendant lineages, as under the DH-DC model, phylogenetic analysis is further complicated. To explore the effects of DH-DC on phylogeny reconstruction, we analyzed two sets of sequences: (1) an augmented set of plant mitochondrial atp1 sequences for which we recently published evidence of DH-DC; and (2) a set of simulated sequences for which we varied the extent of chimerism, the number of chimeric genes and nucleotide substitution rates. We show that the phylogenetic behavior of evolutionarily chimeric genes is highly volatile and depends on both the degree of chimerism and the number of differentially chimeric genes present in the analysis. Furthermore, we show that the presence of chimeric genes in gene trees can spuriously affect the phylogenetic position of purely native sequences, especially by attracting these sequences toward basal positions in trees. We propose the term “HGT turbulence” to describe these complex effects of evolutionarily chimeric genes on phylogenetic results.

Open Access Article

Curiosities of REPINs and RAYTs

Frederic Bertels and Paul B. Rainey
Pages 262 - 268
http://dx.doi.org/10.4161/mge.18610
Abstract | Full Text | PDF

Repetitive extragenic palindromic (REP) sequences are a ubiquitous feature of bacterial genomes. Recent work shows that REPs are remnants of a larger mobile genetic element termed a REPIN. REPINs consists of two REP sequences in inverted orientation separated by a spacer region and are thought to be non-autonomous mobile genetic elements that exploit the transposase encoded by REP-Associated tYrosine Transposases (RAYTs). Complimentarity between the two ends of the REPIN suggests that the element forms hairpin structures in single stranded DNA or RNA. In addition to REPINs, other more complex arrangements of REPs have been identified in bacterial genomes, including the genome of the model organism Pseudomonas fluorescens SBW25. Here, we summarize existing knowledge and present new data concerning REPIN diversity. We also consider factors affecting the evolution of REPIN diversity, the ease with which REPINs might be co-opted by host genomes and the consequences of REPIN activity for the structure of bacterial genomes.




COMMENTARIES

Open Access Article

What Nematode genomes tell us about the importance of horizontal gene transfers in the evolutionary history of animals

Étienne G.J. Danchin
Pages 269 - 273
http://dx.doi.org/10.4161/mge.18776
Abstract | Full Text | PDF

Horizontal gene transfer (HGT), the transmission of a gene from one species to another by means other than direct vertical descent from a common ancestor, has been recognized as an important phenomenon in the evolutionary biology of prokaryotes. In eukaryotes, in contrast, the importance of HGT has long been overlooked and its evolutionary significance has been considered to be mostly negligible. However, a series of genome analyses has now shown that HGT not only do probably occur at a higher frequency than originally thought in eukaryotes but recent examples have also shown that they have been subject to natural selection, thus suggesting a significant role in the evolutionary history of the receiver species. Surprisingly, these examples are not from protists in which integration and fixation of foreign genes intuitively appear relatively straightforward, because there is no clear distinction between the germline and the somatic genome. Instead, these examples are from nematodes, multicellular animals that do have distinct cells and tissues and do possess a separate germline. Hence, the mechanisms of gene transfer appears in this case much more complicated. In this commentary, I will further discuss two recent publications that describe HGT in nematodes, one that highlights the importance of HGT in the emergence of plant parasitism and another one that probably represents the most convincing example of a potential transfer between two different metazoan animals, an insect and a nematode.

Open Access Article

Role of piRNAs in the Drosophila telomere homeostasis

Sergey Shpiz and Alla Kalmykova
Pages 274 - 278
http://dx.doi.org/10.4161/mge.18301
Abstract | Full Text | PDF

Drosophila telomeres are maintained as a result of transpositions of specialized telomeric retrotransposons. The abundance of telomeric retroelement transcripts, as well as the frequency of their transpositions onto the chromosome ends, is controlled by a PIWI-interacting RNA (piRNA) pathway. In our recent report, we demonstrate strong evidence of piRNA-mediated transcriptional silencing of telomeric repeats in the Drosophila germline. Telomerase-generated repeats serve as a platform for recruiting specialized DNA-binding proteins which are involved in chromosome end protection and in the telomere length control. No specific proteins are known to bind to heterogeneous long sequences of the Drosophila telomeric retrotransposons. The importance of the piRNA silencing mechanism in the formation of telomeric chromatin along the region of the retrotransposon array will be discussed. We propose that Drosophila telomeric retrotransposon HeT-A serves as a template for the piRNA-mediated assembly of the specific protein complex, which is functionally similar to the recruiting of the DNA-binding telomeric proteins by the telomerase-generated repeats. The role of the piRNA pathway components in the assembly of the telomere capping complex was recently unveiled. Taken together, these data elucidate the importance of the piRNA pathway in the Drosophila telomere homeostasis.

Open Access Article

Is somatic retrotransposition a parasitic or symbiotic phenomenon?

Kyle R. Upton, J. Kenneth Baillie and Geoffrey J. Faulkner
Pages 279 - 282
http://dx.doi.org/10.4161/mge.18422
Abstract | Full Text | PDF

The extraordinary evolutionary success of transposable elements (TEs) invites us to question the nature of the co-evolutionary dynamics between TE and host. Although sometimes assumed to be wholly parasitic, TEs have penetrated and spread throughout eukaryotic genomes at a rate unparalleled by other parasites. This near-ubiquity, occurring despite the potentially deleterious effects of insertional mutagenesis, raises the possibility that a counterbalancing benefit exists for the host. Such a benefit may act at the population level to generate genomic diversity within a species and hence greater adaptability under new selective pressures, or at the level of primary gain for the individual. Recent studies have highlighted the occurrence of retrotransposition events in the germline and discovered a surprisingly high rate of mobilization in somatic cells. Here we examine the available evidence for somatic retrotransposition and discuss how this phenomenon may confer a selective advantage upon an individual or species.

Open Access Article

Bacterial toxin-antitoxin systems: Translation inhibitors everywhere

Julien Guglielmini and Laurence Van Melderen
Pages 283 - 290
http://dx.doi.org/10.4161/mge.18477
Abstract | Full Text | PDF

Toxin-antitoxin (TA) systems are composed of two elements: a toxic protein and an antitoxin which is either an RNA (type I and III) or a protein (type II). Type II systems are abundant in bacterial genomes in which they move via horizontal gene transfer. They are generally composed of two genes organized in an operon, encoding a toxin and a labile antitoxin. When carried by mobile genetic elements, these small modules contribute to their stability by a phenomenon denoted as addiction. Recently, we developed a bioinformatics procedure that, along with experimental validation, allowed the identification of nine novel toxin super-families. Here, considering that some toxin super-families exhibit dramatic sequence diversity but similar structure, bioinformatics tools were used to predict tertiary structures of novel toxins. Seven of the nine novel super-families did not show any structural homology with known toxins, indicating that combination of sequence similarity and three-dimensional structure prediction allows a consistent classification. Interestingly, the novel super-families are translation inhibitors similar to the majority of known toxins indicating that this activity might have been selected rather than more detrimental traits such as DNA-gyrase inhibitors, which are very toxic for cells.

Open Access Article

The evolution and consequences of snaR family transposition in primates

Andrew M. Parrott and Michael B. Mathews
Pages 291 - 295
http://dx.doi.org/10.4161/mge.18478
Abstract | Full Text | PDF

The small NF90 associated RNA (snaR) family of small noncoding RNAs (ncRNA) appears to have evolved from retrotransposon ancestors at or soon after pivotal stages in primate evolution. snaRs are thought to be derived from a FLAM C-like (free left Alu monomer) element through multiple short insertion/deletion (indel) and nucleotide (nt) substitution events. Tracing snaR’s complex evolutionary history through primate genomes led to the recent discovery of two novel retrotransposons: the Alu/snaR related (ASR) and catarrhine ancestor of snaR (CAS) elements. ASR elements are present in the genomes of Simiiformes, CAS elements are present in Old World Monkeys and apes, and snaRs are restricted to the African Great Apes (Homininae, including human, gorilla, chimpanzee and bonobo). Unlike their ancestors, snaRs have disseminated by multiple rounds of segmental duplication of a larger encompassing element. This process has produced large tandem gene arrays in humans and possibly precipitated the accelerated evolution of snaR. Furthermore, snaR segmental duplication created a new form of chorionic gonadotropin β subunit (CGβ) gene, recently classified as Type II CGβ, which has altered mRNA tissue expression and can generate a novel short peptide.

Open Access Article

Expression of DNA transposable elements during nervous system development: A discussion about its possible functions

Fernando Faunes, Dasfne Lee-Liu and Juan Larrain
Pages 296 - 300
http://dx.doi.org/10.4161/mge.18550
Abstract | Full Text | PDF

Transposable elements (retrotransposons and DNA transposons) comprise a large proportion of animal genomes, for example 20% in D. melanogaster, 36% in X. tropicalis and 45% in humans. After invading a new genome, the transposable element increases its copy number and subsequently accumulates mutations. These may eventually result in inactive copies. Until recent days transposons have been considered “junk” DNA and no clear function have been assigned for this important amount of information on genomes.


Open Access Article

Tnt1 retrotransposon tagging of STF in Medicago truncatula reveals tight coordination of metabolic, hormonal and developmental signals during leaf morphogenesis

Million Tadege and Kirankumar S. Mysore
Pages 301 - 303
http://dx.doi.org/10.4161/mge.18686
Abstract | Full Text | PDF

Tnt1 (transposable element if Nicotiana tabaccum cell type 1) is one of the very few active LTR retrotransposons used for gene tagging in plants. In the model legume Medicago truncatula, Tnt1 has been effectively used as a gene knock-out tool to generate several very useful mutants. stenofolia (stf) is such a mutant identified by Tnt1 insertion in a WUSCHEL-like homeobox transcription factor. STF is required for blade outgrowth, leaf vascular patterning and female reproductive organ development in barrel medic and woodland tobacco. Using transcript profiling and metabolite analysis, we uncovered that mutant leaves are compromised in steady-state levels of multiple phytohormones, sugar metabolites and derivatives including flavonoids and polyamines. In the lam1 mutant (caused by deletion of the STF ortholog in Nicotiana sylvestris), while glucose, fructose, mannose, galactose, myo-inositol and aromatic aminoacids are dramatically reduced, sucrose is comparable to wild-type levels, and glutamine, proline, putrescine, nicotine and sorbitol are highly increased. We demonstrated that both stf and lam1 mutants accumulate reduced levels of free auxin and ABA in their leaves, and ectopic expression of STF in tobacco leads to auxin and cytokinin overproduction phenotypes including formation of tumors on the roots and crown. These data suggest that STF mediated integration of metabolic and hormonal signals are required for lateral organ morphogenesis and elaboration.

Open Access Article

Goods-thinking vs. tree-thinking: Finding a place for mobile genetic elements

James McInerney, Carla Cummins and Leanne Haggerty
Pages 304 - 308
http://dx.doi.org/10.4161/mge.19153
Abstract | Full Text | PDF

While it has become increasingly clear that the Tree of Life hypothesis has limitations in its ability to describe the evolution of all evolving entities on the planet, there has been a marked reluctance to move away from the tree-based language. Ironically, while modifying the idea of the Tree of Life to the extent that it is only very distantly related to its original descriptions, there has been a very careful attempt to retain the language of tree-thinking. The recent movement away from a tree-thinking language toward a goods-thinking language and perspective is a significant improvement. In this commentary, we describe how goods-thinking can provide better descriptions of evolution, can integrate evolution with environment more closely and can offer an equal place for Mobile Genetic Elements and chromosomal elements in discussions of evolutionary history.