Differentiation of oocyte- and somatic-type 5S rRNAs in animals

In some amphibians and bony fishes, oocyte- and somatic-type 5S rRNA genes are expressed differently in oocytes and somatic cells. In order to determine at what stage of animal evolution this differential expression system appeared and how it is regulated, the sequences of oocyte and somatic 5S rRNAs from three invertebrates (sea urchin, sea hare, and silkworm) and two vertebrates (lamprey and chick) were analyzed. It was found that the oocyte 5S rRNA from lamprey consists of two components, while its somatic 5S rRNA consists of only one. In other animals, such differential expression of 5S rRNA in oocytes and somatic cells was not seen. A phylogenetic tree of 63 animal 5S rRNAs was constructed by means of the parsimony method, and the evolution of oocyte and somatic-type 5S rRNAs was discussed.