Multiple surveys employing a new sample‐processing protocol reveal the genetic diversity of placozoans in Japan

Abstract

Placozoans, flat free‐living marine invertebrates, possess an extremely simple bauplan lacking neurons and muscle cells and represent one of the earliest‐branching metazoan phyla. They are widely distributed from temperate to tropical oceans. Based on mitochondrial 16S rRNA sequences, 19 haplotypes forming seven distinct clades have been reported in placozoans to date. In Japan, placozoans have been found at nine locations, but 16S genotyping has been performed at only two of these locations. Here, we propose a new processing protocol, “ethanol‐treated substrate sampling,” for collecting placozoans from natural environments. We also report the collection of placozoans from three new locations, the islands of Shikine‐jima, Chichi‐jima, and Haha‐jima, and we present the distribution of the 16S haplotypes of placozoans in Japan. Multiple surveys conducted at multiple locations yielded five haplotypes that were not reported previously, revealing high genetic diversity in Japan, especially at Shimoda and Shikine‐jima Island. The observed geographic distribution patterns were different among haplotypes; some were widely distributed, while others were sampled only from a single location. However, samplings conducted on different dates at the same sites yielded different haplotypes, suggesting that placozoans of a given haplotype do not inhabit the same site constantly throughout the year. Continued sampling efforts conducted during all seasons at multiple locations worldwide and the development of molecular markers within the haplotypes are needed to reveal the geographic distribution pattern and dispersal history of placozoans in greater detail.

Mitochondrial Genome Evolution of Placozoans: Gene Rearrangements and Repeat Expansions

“…For H2, H9, H11, H17, and H19, animal sampling, DNA extraction, and 16S haplotype determination were performed as previously described ( Miyazawa and Nakano 2018 ). Several short regions (<1 kb) of each mitochondrial genome were first amplified by polymerase chain reaction (PCR) using universal primers ( supplementary table S1 , Supplementary Material online), and the PCR products were Sanger sequenced using FASMAC sequencing service (Atsugi, Japan).…”

“…In total, 20 placozoan 16S haplotypes (H0–H19) have been reported, with 16S-based phylogenetic analyses supporting the subdivision of H1–H19 into seven distinct 16S clades I–VII (see below) ( Eitel et al. 2013 ; Nakano 2014 ; Miyazawa and Nakano 2018 ; Osigus et al. 2019 ).…”

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

“… 15 ), the closely related Trichoplax sp. H2 (16S haplotype H2) 13 is the most abundant and widely distributed placozoan lineage 13 – 15 , 20 . Furthermore, Trichoplax sp.…”

Genome analyses of a placozoan rickettsial endosymbiont show a combination of mutualistic and parasitic traits

“…Despite being almost indistinguishable at the morphological level, the different genetic lineages (“species”) show substantial differences in their geographic distribution and ecological niches. While some lineages seem restricted to coral reef or mangrove habitats, or may even be endemic to certain locations, others show a remarkable broad latitudinal range 2–4,24–28 . When cultured in the lab, different placozoan lineages differ substantially in their demands for diet and water quality (personal observation, Osigus et al .…”

“…We recently sequenced the genome of the placozoan mitochondrial 16S haplotype H2, Trichoplax sp. H2 (strain “Panama”) 5 , which is the most abundant placozoan haplotype around the world and also the most robust lineage for culturing 2–4,28 . The genome sequencing also yielded the novel genome of a single rickettsial endosymbiont which is reported here.…”