A possible relationship between the phylogenetic branch lengths and the chaetognath rRNA paralog gene functionalities: Ubiquitous, tissue-specific or pseudogenes

Abstract

Chaetognaths constitute a small marine phylum exhibiting two classes of 18S and 28S ribosomal RNA genes which is highly unusual in animal genomes. In situ hybridizations of the chaetognath Spadella cephaloptera have shown that 18S class I genes are expressed in the whole body, corresponding to housekeeping genes, whereas 18S and 28S class II genes are expressed in oocytes specifically. Moreover, a heterologous probe against 28S genes revealed only a single signal in a distinct area of intestinal cells. In chaetognaths, the cell gut and oocytes are the cell types which require the greatest translational activities. These results strongly suggest that each type of 18S and 28S paralog is important for specific cellular functions. In addition, phylogenetical analyses have shown a coupling between sequence evolution rate and expression pattern; 28S class II paralogs clearly have the longest branches, indicating a more rapid evolution than class I paralogs. © 2008 Springer Berlin Heidelberg.