Mitochondrial Genome of a Japanese Placozoan

Abstract

Placozoans are marine invertebrates found in tropical and subtropical waters. Their body plan is among the simplest of free-living animals. The present study determined the mitochondrial genome sequence of a placozoan collected on the coast of Shirahama, Wakayama, Honshu, Japan, and compared it with those of Trichoplax adhaerens from the Red Sea and of three strains from the Caribbean Sea. The sequences of mitochondrial respiratory chain of the Japanese placozoan genes are very similar to those of the BZ49 strain from the Caribbean Sea. However, there are distinct differences in gene arrangement, such as the location of two open reading frames. This Japanese placozoan is therefore distinguishable from the other strains. Based on current knowledge of placozoan 16S diversity our 'Shirahama' strain most likely represents the H15 lineage, known from the Philippines. In the mitochondrial genome of placozoans, substitution rates are slower than in bilaterians, whereas the rate of rearrangements is faster.

Survey of the Japanese Coast Reveals Abundant Placozoan Populations in the Northern Pacific Ocean

“…Substrate at Sesoko was unique in that coral skeletons consisted a large portion, whereas they were absent in the four other sites, as well as Shimoda. Of the five sites, placozoans have been previously reported from Shirahama212223 and Sesoko24252627.…”

“…There have been sporadic reports of placozoans from Japan2021222324252627, with many of them just mentioned briefly in conference proceedings or as personal communications in scientific papers. They have also been reported from the Russian coast of the Sea of Japan33, but they were found in aquaria in Moscow containing animals from the Sea of Japan and not from the Sea of Japan per se.…”

“…However, little research has been performed on Japanese placozoans since. Over the years, some research has been performed on the Shirahama population, with behavior21, seasonal variations in population size22, and mitochondrial genome23 being reported. There have also been a number of personal communications of placozoans being discovered in aquaria and tanks around the country.…”

Deep RNA sequencing reveals the smallest known mitochondrial micro exon in animals: The placozoan cox1 single base pair exon

“…[7]), for instance, possess highly reduced circular mitochondrial genomes [8], while different Porifera and Cnidaria may have linear and sometimes even fragmented mitochondrial genomes [9, 10]. Some of the largest animal mitochondrial genomes are found in the phylum Placozoa [11–13]. As large circular mitochondrial genomes are also found in unicellular organisms (including the choanoflagellate Monosiga brevicollis [14]) it has been postulated that placozoan mitochondrial genomes have retained several ancestral characteristics of metazoan mitochondrial genomes [11].…”

“…Another remarkable feature is the variable number of introns in some mitochondrial genes ( cox1 , nad5 and 16S rDNA) in different placozoans, recommending placozoans as an ideal model system to study the evolution of metazoan mitochondrial introns and mRNA processing or splicing mechanisms. Former analyses of the placozoan cox1 gene already revealed the presence of several cis-splicing group I and group II introns [11–13]. Cox1 introns are also found in some Porifera and Cnidaria, but the cox1 fragmentation in Placozoa is unique among metazoans.…”