The Caenorhabditis briggsae genome contains active CbmaT1 and Tcb1 transposons

Divergent evolution profiles of DD37D and DD39D families of Tc1/mariner transposons in eukaryotes

DNA transposons play a significant role in shaping the size and structure of eukaryotic genomes. The Tc1/mariner transposons are the most diverse and widely distributed superfamily of DNA transposons and the structure and distribution of several Tc1/mariner families, such as DD35E/TR, DD36E/IC, DD37E/TRT, and DD41D/VS, have been well studied. Nonetheless, a greater understanding of the structure and diversity of Tc1/mariner transposons will provide insight into the evolutionary history of eukaryotic genomes. Here, we conducted further analysis of DD37D/maT and DD39D (named Guest, GT), which were identified by the specific catalytic domains DD37D and DD39D. Most transposons of the maT family have a total length of approximately 1.3 kb and harbor a single open reading frame encoding a ~ 346 amino acid (range 302-398 aa) transposase protein, flanked by short terminal inverted repeats (TIRs) (13-48 base pairs, bp). In contrast, GTs transposons were longer (2.0-5.8 kb), encoded a transposase protein of ~400 aa (range 140-592 aa), and were flanked by short TIRs (19-41 bp). Several conserved motifs, including two helix-turn-helix (HTH) motifs, a GRPR (GRKR) motif, a nuclear localization sequence, and a DDD domain, were also identified in maT and GT transposases. Phylogenetic analyses of the DDD domain showed that the maT and GT families each belong to a monophyletic clade and appear to be closely related to DD41D/VS and DD34D/mariner. In addition, maTs are mainly distributed in invertebrates (144 species), whereas GTs are mainly distributed in land plants through a small number of GTs are present in Chromista and animals. Sequence identity and phylogenetic analysis revealed that horizontal transfer (HT) events of maT and GT might occur between kingdoms and phyla of eukaryotes; however, pairwise distance comparisons between host genes and transposons indicated that HT events involving maTs might be less frequent between invertebrate species and HT events involving GTs may be less frequent between land plant species. Overall, the DD37D/maT and DD39D/GT families display significantly different distribution and tend to be identified in more ancient evolutionary families. The discovery of intact transposases, perfect TIRs, and target site duplications (TSD) of maTs and GTs illustrates that the DD37D/maT and DD39D/GT families may be active. Together, these findings improve our understanding of the diversity of Tc1/mariner transposons and their impact on eukaryotic genome evolution.

Genotype by sequencing identifies natural selection as a driver of intraspecific divergence in Atlantic populations of the high dispersal marine invertebrate, Macoma petalum

Mitochondrial DNA analyses indicate that the Bay of Fundy population of the intertidal tellinid bivalve Macoma petalum is genetically divergent from coastal populations in the Gulf of Maine and Nova Scotia. To further examine the evolutionary forces driving this genetic break, we performed double digest genotype by sequencing (GBS) to survey the nuclear genome for evidence of both neutral and selective processes shaping this pattern. The resulting reads were mapped to a partial transcriptome of its sister species, M. balthica, to identify single nucleotide polymorphisms (SNPs) in protein-coding genes. Population assignment tests, principle components analyses, analysis of molecular variance, and outlier tests all support differentiation between the Bay of Fundy genotype and the genotypes of the Gulf of Maine, Gulf of St. Lawrence, and Nova Scotia. Although both neutral and non-neutral patterns of genetic subdivision were significant, genetic structure among the regions was nearly 20 times higher for loci putatively under selection, suggesting a strong role for natural selection as a driver of genetic diversity in this species. Genetic differences were the greatest between the Bay of Fundy and all other population samples, and some outlier proteins were involved in immunity-related processes. Our results suggest that in combination with limited gene flow across the mouth of the Bay of Fundy, local adaptation is an important driver of intraspecific genetic variation in this marine species with high dispersal potential.

Hsp90 and Physiological Stress Are Linked to Autonomous Transposon Mobility and Heritable Genetic Change in Nematodes

Transposable elements (TEs) have been recognized as potentially powerful drivers of genomic evolutionary change, but factors affecting their mobility and regulation remain poorly understood. Chaperones such as Hsp90 buffer environmental perturbations by regulating protein conformation, but are also part of the PIWI-interacting RNA pathway, which regulates genomic instability arising from mobile TEs in the germline. Stress-induced mutagenesis from TE movement could thus arise from functional trade-offs in the dual roles of Hsp90. We examined the functional constraints of Hsp90 and its role as a regulator of TE mobility by exposing nematodes (Caenorhabditis elegans and Caenorhabditis briggsae) to environmental stress, with and without RNAi-induced silencing of Hsp90. TE excision frequency increased with environmental stress intensity at multiple loci in several strains of each species. These effects were compounded by RNAi-induced knockdown of Hsp90. Mutation frequencies at the unc-22 marker gene in the offspring of animals exposed to environmental stress and Hsp90 RNAi mirrored excision frequency in response to these treatments. Our results support a role for Hsp90 in the suppression of TE mobility, and demonstrate that that the Hsp90 regulatory pathway can be overwhelmed with moderate environmental stress. By compromising genomic stability in germline cells, environmentally induced mutations arising from TE mobility and insertion can have permanent and heritable effects on both the phenotype and the genotype of subsequent generations.

Characterization of EamaT1, a member of maT family of transposable elements from the earthworm Eisenia andrei (Annelida, Oligochaeta)

The maT family is a unique clade within the Tc1-mariner superfamily, and their distribution is to date known as being limited to invertebrates. A novel transposon named EamaT1 is described from the genome of the earthworm Eisenia andrei. The full sized EamaT1 was obtained by degenerate and inverse PCR-based amplification. Sequence analysis of multiple copies of the EamaT1, which consisted of 0.9 and 1.4 kb elements, showed that the consensual EamaT1 with inverted terminal repeats (ITRs) of 69 bp was 1,422 bp long and flanked by a duplicated TA dinucleotide. The EamaT1 is present in approximately 120-250 copies per diploid genome but undergoes an inactivation process as a result of accumulating multiple mutations and is nonfunctional. The open reading frame (ORF) of the EamaT1 consensus encoding 356 amino acid sequences of transposase contained a DD37D signature and a conserved paired-like DNA binding motif for the transposition mechanism. The result of ITRs comparison confirmed their consensus terminal sequences (5'-CAGGGTG-3') and AT-rich region on the internal bases for ITRs-transposase interaction.

Identification of Novel Non-autonomous CemaT Transposable Elements and Evidence of their Mobility within the C. elegans Genome

We describe here two new transposable elements, CemaT4 and CemaT5, that were identified within the sequenced genome of Caenorhabditis elegans using homology based searches. Five variants of CemaT4 were found, all non-autonomous and sharing 26 bp inverted terminal repeats (ITRs) and segments (152-367 bp) of sequence with similarity to the CemaT1 transposon of C. elegans. Sixteen copies of a short, 30 bp repetitive sequence, comprised entirely of an inverted repeat of the first 15 bp of CemaT4's ITR, were also found, each flanked by TA dinucleotide duplications, which are hallmarks of target site duplications of mariner-Tc transposon transpositions. The CemaT5 transposable element had no similarity to maT elements, except for sharing identical ITR sequences with CemaT3. We provide evidence that CemaT5 and CemaT3 are capable of excising from the C. elegans genome, despite neither transposon being capable of encoding a functional transposase enzyme. Presumably, these two transposons are cross-mobilised by an autonomous transposon that recognises their shared ITRs. The excisions of these and other non-autonomous elements may provide opportunities for abortive gap repair to create internal deletions and/or insert novel sequence within these transposons. The influence of non-autonomous element mobility and structural diversity on genome variation is discussed.