We compared patterns of intraspecific polymorphism of two markers with contrasted modes of evolution, nuclear ribosomal DNA (rDNA) and mitochondrial DNA (mtDNA), in the phytophagous mite Tetranychus urticae Koch. The second internal transcribed spacer (ITS2) of rDNA and a fragment in the mtDNA gene coding for Cytochrome Oxidase I (COI), were PCR-amplified and sequenced in samples of various geographical origins distributed worldwide. The 15 COI haplotypes found fell into two major phylogenetic lineages differing by an average of 5% nucleotide divergence. Samples from the Mediterranean basin were represented in both lineages, and showed no phylogeographical structure. The other samples, from temperate regions of the northern hemisphere, were clustered in one of the lineages and displayed little variation, indicating a recent colonization of this region. In contrast, no variation at all was found at the ITS2 in this species. We sequenced both COI and ITS2 in four other species of the genus Tetranychus and found that, despite the absence of intraspecific polymorphism, ITS appears to evolve 2.5 times faster than COI. We argue that rDNA homogeneity over the species range of T. urticae results from the high colonization potential of this species, preventing long-term differentiation. Preliminary data on two other mite species (Amphitetranychus viennensis Zacher and Mononychellus progresivus Doreste) with stricter ecological requirements and more restricted colonization potential revealed substantial and concordant geographical differentiation for both ITS2 and COI.

Spider mites, Tetranychidae, represent one of the most cosmopolitan and economically important groups of terrestrial arthropods; however, many aspects of their evolutionary relationships remain uncertain. We sequenced part of the mitochondrial cytochrome oxidase subunit I (COI) gene in 20 species of phytophagous mites belonging to nine genera and two families (Tetranychidae and Tenuipalpidae), including several agricultural pests. As reported in insects, the sequences were extremely rich in A + T (75% on average), especially in the third codon position (95%). However, one of the genera we studied had a significantly lower A+T content (69% on average, 78% in the third codon position), showing that base composition can change substantially over short periods of time. Most interspecific differences were transversions and their number increased steadily with the number of non-synonymous differences, while the number of transitions remained constant. The phylogeny based on COI sequences was inferred using the maximum likelihood method. The results are compatible as a whole with the traditional classification based on morphological characters, but call for some minor taxonomic revisions. Some morphological characters and life history traits (mode of reproduction, adaptation to the host plant) were also analysed within this phylogenetic framework. At the family level, one can see a trend towards thelytoky becoming rarer compared to the general mode of reproduction of the group, arrhenotoky. There is also an evolutionary tendency towards a more complex mode of life, with the production of silk webs and correlated changes of the locomotion apparatus. However, in the Tetranychidae there seems to have been convergent evolution of these morphological characters together with independent development of a common adaptation to this mode of life in different genera.