The Holometabola (insects with complete metamorphosis: beetles, wasps, flies, fleas, butterflies, lacewings, and others) is a monophyletic group that includes the majority of the world's animal species. Holometabolous orders are well defined by morphological characters, but relationships among orders are unclear. In a search for a region of DNA that will clarify the interordinal relationships we sequenced approximately 1080 nucleotides of the 5' end of the 18S ribosomal RNA gene from representatives of 14 families of insects in the orders Hymenoptera (sawflies and wasps), Neuroptera (lacewing and antlion), Siphonaptera (flea), and Mecoptera (scorpionfly). We aligned the sequences with the published sequences of insects from the orders Coleoptera (beetle) and Diptera (mosquito and Drosophila), and the outgroups aphid, shrimp, and spider. Unlike the other insects examined in this study, the neuropterans have A-T rich insertions or expansion regions: one in the antlion was approximately 260 bp long. The dipteran 18S rDNA evolved rapidly, with over 3 times as many substitutions among the aligned sequences, and 2-3 times more unalignable nucleotides than other Holometabola, in violation of an insect-wide molecular clock. When we excluded the long-branched taxa (Diptera, shrimp, and spider) from the analysis, the most parsimonious (minimum-length) trees placed the beetle basal to other holometabolous orders, and supported a morphologically monophyletic clade including the fleas+scorpionflies (96% bootstrap support). However, most interordinal relationships were not significantly supported when tested by maximum likelihood or bootstrapping and were sensitive to the taxa included in the analysis. The most parsimonious and maximum-likelihood trees both separated the Coleoptera and Neuroptera, but this separation was not statistically significant. The position of the Hymenoptera relative to other orders was not clarified. Including the less derived members in the analysis made the Hymenoptera appear paraphyletic. The two representatives of Neuroptera grouped together as did the two Diptera, both pairs with very significant bootstrap support.
The consequence of this is the flies tend to group with other taxa that also have aberrant substitution levels. The Strepsiptera are a good example, and there has been much controversy about Strepsipteran relationships lately. To oversimplify, the DNA sequence of Strepsiptera and Diptera are so different from other insects that all other insects tend to group together leaving the Strepsiptera and Diptera out on their own (and thus together in phylogenetic analyses).
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See
Carmean, D., and B. Crespi. 1995. Do long branches attract flies? Nature 373: 666. (Scientific Correspondence).
Huelsenbeck, J. P. 1997. Is the Felsenstein Zone a fly trap? Syst. Biol. 46: 69-74. (Data Matrix)
Whiting, M. F. , J. C. Carpenter, Q. D. Wheeler, and W. C. Wheeler. 1997. The Strepsiptera problem: phylogeny of the holometabolous insect orders inferred from 18S and 28S ribosomal DNA sequences and morphology. Syst. Biol. 46: 1-68.
Also see
Friedrich, M. and D. Tautz. 1997. An episodic change of rDNA nucleotide substitution rate has occurred during the emergence of the insect order Diptera.
Mol. Biol. Evol. 14: 644-653
Kristensen, N P. 1995. Forty years' insect phylogenetic systematics... Zoologische Beitraege 36: 83-124.
and papers of Mark Siddall at:
http://limnatis.ummz.lsa.umich.edu/pub/
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http://www.biol.sfu.ca/staff/dc/
http://www.sfu.ca/~carmean/
http://carex.ekbot.umu.se/~dave/