STRONGLY EXPANDED 18S RIBOSOMAL-RNA GENES CORRELATED WITH A PECULIAR MORPHOLOGY IN THE INSECT ORDER OF STREPSIPTERA

The 18S rRNA genes of the insects Xenos vesparum (Strepsiptera), its h ost Polistes dominulus (Insecta, Hymenoptera) and the beetle Melee pro scarabaeus (Coleoptera) were cloned and sequenced. The Xenos gene was 3 316 bp long with a G+C content of 28.1%. The low G+C content is caus ed by eight extremely A+T rich elements all intercalated into sites kn own to be variable in eukaryotic 18S rRNAs. Southern analysis showed t hat all 18S rRNA genes of Xenos contain A+T rich elements. 18S rRNA wa s reverse transcribed, the product amplified by the polymerase chain r eaction and sequenced. This showed that the A+T rich elements were als o presented in the mature rRNA. The 18S rRNA gene of another strepsipt eran, Stylops melittae, was also shown to be longer than 3 kb and to c ontain A+T rich elements. The 18S rRNA genes of the host Polistes domi nulus were 1 919 bp and those of Melee proscarabaeus 1 934 bp long wit h G+C contents of 49.3% and 48.2% respectively. Phylogenetic trees wer e constructed with the sequences of Xenos vesparum and six other insec t species using maximum parsimony and the Fitch-Margoliash method. Bot h methods showed that Xenos groups together with Diptera and not with Coleoptera as suggested by a few morphological traits.