A universal insect clock: under our noses all the time!

Abstract

Evolutionary studies of many major parasites are hampered by the lack of a fossil record. For example, theories relating to the evolution of protozoa have, for most of the 20th century, been based on morphological and life cycle data, despite their known limitations. It is only recently that advances in molecular methodology, notably the wide availability of accurate, automated DNA sequencing, have made it possible to deduce the evolutionary relationships of extant species from their genes. Of course, the use of molecular data to determine the timing of evolutionary events comes with its own set of problems, not least being the reliance of such studies on molecular clocks, calibrated by a variety of methods, and the inherently variable rates at which different genetic markers evolve. Independent methods of calibrating molecular clocks, by reference to fossil records (but see above) and biogeographical data, can help overcome many of the problems of gene- and/or lineage-dependent clock speeds. In the case of parasites, reference to host and/or vector evolution, which in many cases is better understood, can also be invaluable for understanding patterns of parasite evolution. It is in this context then that the article by Gaunt and Miles [1xAn insect molecular clock dates the origin of the insects and accords with palaeontological and biogeographic landmarks. Gaunt, M.W. and Miles, M.A. Mol. Bio. Evol. 2002; 19: 748–761Crossref | PubMedSee all References[1] is so interesting, because they present, for the first time, a robust nucleotide and amino acid mitochondrial molecular clock, encompassing five insect orders including the Diptera, the vectors of many important parasites.The study assesses the clock-like characteristics of a range of commonly used genetic markers (i.e. 16S, 18S, cytochrome b, cytochrome oxidase I and elongation factor 1α), with an eye to the availability of sequence data for insect orders Blatteria and/or Odonata, both of which offer early, extensive fossil records, crucial for the calibration of the molecular trees being assessed. Having compared constrained molecular clock phylogenies, derived from both nucleotide and amino acid data sets, against a range of unconstrained models for the various genetic markers, the authors found that only cytochrome oxidase I recovered global molecular clocks for at least some of the taxa examined. Moreover, in phylogenies constructed from cytochrome oxidase I data, regardless of the method (maximum-likelihood or parsimony) or sequences (nucleotide or amino acid) used, very similar topologies were recovered within each insect order. The key divergence dates are shown to be robust against both the fossil record and proposed biogeographic events, including the break-up of Gondwanaland and the Cambrian explosion.Having established the suitability of cytochrome oxidase I as a global molecular clock for insects, the authors use this information to revisit key questions relating to the evolution of triatomine bugs, the vectors of Chagas disease and, in particular, the antiquity of the two main tribes of triatomines, the Rhodniini and Triatomini. The authors conclude that the date of divergence (99.7 million to 93.6 million years ago) between the tribes coincides with the break-up of Gondwanaland, a result somewhat at odds with previous estimates by Schofield [2xTrypanosoma cruzi – the vector–parasite paradox. Schofield, C.J. Mem. Inst. Oswaldo Cruz. 2000; 95: 535–544Crossref | PubMedSee all References[2], based primarily on ecological associations. Whatever, the findings of Gaunt and Miles provide a solid foundation on which to evaluate (and, undoubtedly, re-evaluate) the timing of key events in insect vector phylogenies and evolution, which in turn, without question, will affect our understanding of their associated parasites.