Dating the arthropod tree based on large-scale transcriptome data

Abstract

Molecular sequences do not only allow the reconstruction of phylogenetic relationships among species, but also provide information on the approximate divergence times. Whereas the fossil record dates the origin of most multicellular animal phyla during the Cambrian explosion less than 540millionyearsago(mya), molecular clock calculations usually suggest much older dates. Here we used a large multiple sequence alignment derived from Expressed Sequence Tags and genomes comprising 129genes (37,476 amino acid positions) and 117taxa, including 101arthropods. We obtained consistent divergence time estimates applying relaxed Bayesian clock models with different priors and multiple calibration points. While the influence of substitution rates, missing data, and model priors were negligible, the clock model had significant effect. A log–normal autocorrelated model was selected on basis of cross-validation. We calculated that arthropods emerged ∼600mya. Onychophorans (velvet worms) and euarthropods split ∼590mya, Pancrustacea and Myriochelata ∼560mya, Myriapoda and Chelicerata ∼555mya, and ‘Crustacea’ and Hexapoda ∼510mya. Endopterygote insects appeared ∼390mya. These dates are considerably younger than most previous molecular clock estimates and in better agreement with the fossil record. Nevertheless, a Precambrian origin of arthropods and other metazoan phyla is still supported. Our results also demonstrate the applicability of large datasets of random nuclear sequences for approximating the timing of multicellular animal evolution.