ИСТИНА

It is hypothesized that nervous system was preceded in evolution by “volume transmission system” - array of scattered glandular cells secreting various signal molecules, including present-day neurotransmitters and hormones. Studies of peptide signalling in Trichoplax (phylum Placozoa) have shown that these nerveless animals are clear extant example of volume transmission system, coordinating behaviour in absence of neurons and synapses. We have sequenced, assembled and annotated complete genome of another placozoan isolate, Hoilungia sp. H4, and compared it with published genomes of Trichoplax adhaerens and Hoilungia hongkongensis. We have found very high rate of gene duplications in Placozoa, up to 1805 in Hoilungia. Functional spectra of duplicated genes is surprisingly dominated by neuronal components (receptors and vesicular transporters), which is unknown for any other animals. These extensive duplications underscore adaptive value of neuronal genes in nerveless animals. Fast evolution of receptors in volume transmission system may underlie evolution of species-specific behaviour, which we indeed found in Placozoa.