18S ribosomal RNA genes of insects: Primary structure of the genes and molecular phylogeny of the Holometabola

The 18S ribosomal RNA genes of 19 insect species, including 12 Holometabola, were cloned and sequenced. The genes of the insect species investigated so far vary in length from 1,809 to 3,316 bp. The genes were aligned according to the secondary structure model of the 18S ribosomal RNA. An average of 1,580 aligned nucleotide positions per gene was used for the calculation of phylogenetic trees with sequences of this and previous studies. Neighbor-joining trees were calculated with gamma, substitution rate calibration, and LogDet distances. Informative alignment positions were used for a maximum parsimony analysis. The robustness of the trees was tested by bootstrapping and by branch support calculations. All the major groups of the Holometabola, which are commonly regarded as monophyletic except for the Mecoptera, were supported with the neighbor-joining analysis. Some of these groups were not represented in the most parsimonious trees or had a bootstrap support of <80%. However, a sister group relationship of the Strepsiptera and the Diptera was found with both methods and with corrections for different substitution rates at different sites and for differences in the nucleotide composition.