Annelids win again: the first evidence of Hox antisense transcription in Spiralia.

To date it is becoming more and more obvious that multiple non-coding RNAs, once considered to be transcriptional noise, play a huge role in gene regulation during animal ontogenesis. Hox genes are key regulators of embryonic development, growth and regeneration of all bilaterian animals. It was shown that mammalian Hox loci are transcribed in both directions and noncoding RNAs maintain and control the normal functioning of Hox clusters. We revealed antisense transcripts of most of Hox genes in two lophotrochozoans, errant annelids Alitta virens and Platynereis dumerilii. It is for the first time when non-coding RNAs associated with Hox genes are found in spiralian animals. All these asRNAs can be referred to as natural antisense transcripts (NATs). We analyzed the expression of all detected NATs using sense probes to their Hox mRNAs during larval and postlarval development and regeneration by whole mount in situ hybridization (WMISH). We managed to clone several asRNAs (Avi-antiHox4-1, Avi-antiHox4-2 and Avi-antiHox5) of these annelids and analyzed their expression patterns as well. Our data indicate variable and complicated interplay between sense and antisense Hox transcripts during development and growth of two annelids. The presence of Hox antisense transcription in the representatives of different bilaterian clades (mammals, myriapods and annelids) and similar expression relationships in sense-antisense pairs suggest that this can be the ancestral feature of Hox cluster regulation.