Trichoplax genomes reveal profound admixture and suggest stable wild populations without bisexual reproduction

Abstract

The phylum Placozoa officially consists of only a single described species, Trichoplax adhaerens, although several lineages can be separated by molecular markers, geographical distributions and environmental demands. The placozoan 16S haplotype H2 (Trichoplax sp. H2) is the most robust and cosmopolitan lineage of placozoans found to date. In this study, its genome was found to be distinct but highly related to the Trichoplax adhaerens reference genome, for remarkably unique reasons. The pattern of variation and allele distribution between the two lineages suggests that both originate from a single interbreeding event in the wild, dating back at least several decades ago, and both seem not to have engaged in sexual reproduction since. We conclude that populations of certain placozoan haplotypes remain stable for long periods without bisexual reproduction. Furthermore, allelic variation within and between the two Trichoplax lineages indicates that successful bisexual reproduction between related placozoan lineages might serve to either counter accumulated negative somatic mutations or to cope with changing environmental conditions. On the other hand, enrichment of neutral or beneficial somatic mutations by vegetative reproduction, combined with rare sexual reproduction, could instantaneously boost genetic variation, generating novel ecotypes and eventually species.

Support for a clade of Placozoa and Cnidaria in genes with minimal compositional bias

“…The previously cryptic phylogenetic link between cnidarians and placozoans seen in gene sets less influenced by compositional bias will require further testing with other analyses and data modalities, such as rare genomic changes, which should be ever more visible as highly contiguous assemblies continue to be reported from non-bilaterian animals ( Eitel et al, 2018 ; Kamm et al, 2018 ; Jiang, 2018 ; Leclère, 2018 ). However, if validated, this relationship must continue to raise questions on the homology of certain traits across non-bilaterians.…”

Conserved biophysical features of the CaV2 presynaptic Ca2+ channel homologue from the early-diverging animal Trichoplax adhaerens

“…The same antibodies were then used to detect TCaV2 in protein lysates isolated from Trichoplax whole animals, labeling bands at ~240, 200, 90, 75, 63, 60 and 50 kDa on a Western blot (figure 2G). Although possible that some of the smaller molecular weight bands correspond to offtarget proteins, we note that BLAST searching the epitope protein sequence against Trichoplax transcriptome (50) and genome (14,56) sequences fails to produce significant hits outside of the CaV2 channel. As observed with the HEK-293T lysates, all bands disappeared after peptide pre-incubation, suggesting that although present as a full-length protein, some of the TCaV2 channel protein within the lysates is fragmented.…”

Innate immunity in the simplest animals – placozoans

“…Recently we sequenced and investigated the genome of the placozoan 16S haplotype H2 ( Trichoplax sp. H2; [20]), a close relative of Trichoplax adhaerens , and the genome assembly also yielded the complete genome of a placozoan rickettsial endosymbiont, which is described in Kamm et al, (in prep.). The results of the bacterial genome analyses, in terms of genome size, gene content and composition of metabolic pathways, clearly point to a mutualistic relationship which raises questions about how it has been established and how it is maintained.…”

“…H2 genome for genes involved in innate immune recognition and downstream signaling. The Trichoplax adhaerens reference genome [17] has not been specifically included in our results because both genomes are highly related [20] and the published gene models of the latter were found to be less complete and also inferior with respect to multi-domain genes. Overall, the reference genome did not contribute more information to the presence or absence of immunity related genes.…”