Evolution of a transposon in Daphnia hybrid genomes

Becoming a Selfish Clan: Recombination Associated to Reverse-Transcription in LTR Retrotransposons

Transposable elements (TEs) are parasitic DNA bits capable of mobilization and mutagenesis, typically suppressed by host’s epigenetic silencing. Since the selfish DNA concept, it is appreciated that genomes are also molded by arms-races against natural TE inhabitants. However, our understanding of evolutionary processes shaping TEs adaptive populations is scarce. Here, we review the events of recombination associated to reverse-transcription in LTR retrotransposons, a process shuffling their genetic variants during replicative mobilization. Current evidence may suggest that recombinogenic retrotransposons could beneficially exploit host suppression, where clan behavior facilitates their speciation and diversification. Novel refinements to retrotransposons life-cycle and evolution models thus emerge.

Standing chromosomal variation in Lake Whitefish species pairs: the role of historical contingency and relevance for speciation

The role of chromosome changes in speciation remains a debated topic, although demographic conditions associated with divergence should promote their appearance. We tested a potential relationship between chromosome changes and speciation by studying two Lake Whitefish (Coregonus clupeaformis) lineages that recently colonized postglacial lakes following allopatry. A dwarf limnetic species evolved repeatedly from the normal benthic species, becoming reproductively isolated. Lake Whitefish hybrids experience mitotic and meiotic instability, which may result from structurally divergent chromosomes. Motivated by this observation, we test the hypothesis that chromosome organization differs between Lake Whitefish species pairs using cytogenetics. While chromosome and fundamental numbers are conserved between the species (2n = 80, NF = 98), we observe extensive polymorphism of subtle karyotype traits. We describe intrachromosomal differences associated with heterochromatin and repetitive DNA, and test for parallelism among three sympatric species pairs. Multivariate analyses support the hypothesis that differentiation at the level of subchromosomal markers mostly appeared during allopatry. Yet we find no evidence for parallelism between species pairs among lakes, consistent with colonization effect or postcolonization differentiation. The reported intrachromosomal polymorphisms do not appear to play a central role in driving adaptive divergence between normal and dwarf Lake Whitefish. We discuss how chromosomal differentiation in the Lake Whitefish system may contribute to the destabilization of mitotic and meiotic chromosome segregation in hybrids, as documented previously. The chromosome structures detected here are still difficult to sequence and assemble, demonstrating the value of cytogenetics as a complementary approach to understand the genomic bases of speciation.

Patterns of genome size variation in snapping shrimp

Although crustaceans vary extensively in genome size, little is known about how genome size may affect the ecology and evolution of species in this diverse group, in part due to the lack of large genome size datasets. Here we investigate interspecific, intraspecific, and intracolony variation in genome size in 39 species of Synalpheus shrimps, representing one of the largest genome size datasets for a single genus within crustaceans. We find that genome size ranges approximately 4-fold across Synalpheus with little phylogenetic signal, and is not related to body size. In a subset of these species, genome size is related to chromosome size, but not to chromosome number, suggesting that despite large genomes, these species are not polyploid. Interestingly, there appears to be 35% intraspecific genome size variation in Synalpheus idios among geographic regions, and up to 30% variation in Synalpheus duffyi genome size within the same colony.

Impact of ploidy level on the distribution of Pokey element insertions in the Daphnia pulex complex

Background

Transposable elements (TEs) play a major role in genome evolution. Their capacity to move and/or multiply in the genome of their host may have profound impacts on phenotypes and dramatic consequences on genome structure. The population dynamics and distribution of TEs are influenced by their mode of transposition, the availability of niches in host genomes, and host population dynamics. Theories predict an increase in the number of TE insertions following hybridization or polyploidization. Evolution of TEs in hybrids and polyploids has mostly been studied in plants; few studies have examined the impacts of hybridization and/or polyploidization on TEs in animals. Hybrids and polyploids have arisen multiple times in the Daphnia pulex complex and are thought to reproduce by obligate parthenogenesis. Our study examines the effects of ploidy level on polymorphism and number of Pokey element insertions in diploid and polyploid hybrid isolates from the Daphnia pulex complex.

Results

The polymorphism of Pokey insertion sites did not depend solely on either the ploidy level or the genetic background of their host; therefore, it may be the result of interactions between these parameters and other parameters such as Pokey activity, selection and/or drift. No significant effect of ploidy level was found on the number of Pokey insertions using TE display and qPCR. However, the load of Pokey insertion sites and the number of unique insertion sites were slightly (but not significantly) higher in polyploids than in diploids.

Conclusions

These results suggest a lack of increase in the number of Pokey insertions following polyploidization but higher availability of Pokey insertion sites in polyploids than in diploids. Compared to previous TE display and qPCR results, the load of Pokey insertions in hybrid diploids was higher than in non-hybrid sexual and asexual diploids, which suggests an increase in the density of Pokey insertions following hybridization.

In and out of the rRNA genes: characterization of Pokey elements in the sequenced Daphnia genome

Background

Only a few transposable elements are known to exhibit site-specific insertion patterns, including the well-studied R-element retrotransposons that insert into specific sites within the multigene rDNA. The only known rDNA-specific DNA transposon, Pokey (superfamily: piggyBac) is found in the freshwater microcrustacean, Daphnia pulex. Here, we present a genome-wide analysis of Pokey based on the recently completed whole genome sequencing project for D. pulex.

Results

Phylogenetic analysis of Pokey elements recovered from the genome sequence revealed the presence of four lineages corresponding to two divergent autonomous families and two related lineages of non-autonomous miniature inverted repeat transposable elements (MITEs). The MITEs are also found at the same 28S rRNA gene insertion site as the Pokey elements, and appear to have arisen as deletion derivatives of autonomous elements. Several copies of the full-length Pokey elements may be capable of producing an active transposase. Surprisingly, both families of Pokey possess a series of 200 bp repeats upstream of the transposase that is derived from the rDNA intergenic spacer (IGS). The IGS sequences within the Pokey elements appear to be evolving in concert with the rDNA units. Finally, analysis of the insertion sites of Pokey elements outside of rDNA showed a target preference for sites similar to the specific sequence that is targeted within rDNA.

Conclusions

Based on the target site preference of Pokey elements and the concerted evolution of a segment of the element with the rDNA unit, we propose an evolutionary path by which the ancestors of Pokey elements have invaded the rDNA niche. We discuss how specificity for the rDNA unit may have evolved and how this specificity has played a role in the long-term survival of these elements in the subgenus Daphnia.

Genetic and epigenetic changes involving (retro)transposons in animal hybrids and polyploids

Transposable elements (TEs) are discrete genetic units that have the ability to change their location within chromosomal DNA, and constitute a major and rapidly evolving component of eukaryotic genomes. They can be subdivided into 2 distinct types: retrotransposons, which use an RNA intermediate for transposition, and DNA transposons, which move only as DNA. Rapid advances in genome sequencing significantly improved our understanding of TE roles in genome shaping and restructuring, and studies of transcriptomes and epigenomes shed light on the previously unknown molecular mechanisms underlying genetic and epigenetic TE controls. Knowledge of these control systems may be important for better understanding of reticulate evolution and speciation in the context of bringing different genomes together by hybridization and perturbing the established regulatory balance by ploidy changes. See also sister article focusing on plants by Bento et al. in this themed issue.