Increasing species sampling in chelicerate genomic-scale datasets provides support for monophyly of Acari and Arachnida

Abstract

Chelicerates are a diverse group of arthropods, represented by such forms as predatory spiders and scorpions, parasitic ticks, humic detritivores, and marine sea spiders (pycnogonids) and horseshoe crabs. Conflicting phylogenetic relationships have been proposed for chelicerates based on both morphological and molecular data, the latter usually not recovering arachnids as a clade and instead finding horseshoe crabs nested inside terrestrial Arachnida. Here, using genomic-scale datasets and analyses optimised for countering systematic error, we find strong support for monophyletic Acari (ticks and mites), which when considered as a single group represent the most biodiverse chelicerate lineage. In addition, our analysis recovers marine forms (sea spiders and horseshoe crabs) as the successive sister groups of a monophyletic lineage of terrestrial arachnids, suggesting a single colonisation of land within Chelicerata and the absence of wholly secondarily marine arachnid orders.

Fig. 1

Saturation plots and Bayesian results. a Saturation plots for Matrices A, B and C, illustrating that Matrix A has the lowest level of saturation. The dots represent the intersection between uncorrected genetic distances and the patristic distances obtained under ML for each pair of the 95 taxa. The lines of best fit show the general trend and direction of the data and above is shown the R² and the slope of the regression line. Matrix A is represented in green, Matrix B in magenta and Matrix C in blue. Source data are provided as a Source Data file. b Schematic representation of the results of the CAT-GTR + G analysis of Matrix A. c Schematic representation of the results of the CAT-GTR + G analysis of Matrix B. d Schematic representation of the results of the CAT-GTR + G analysis of Matrix A, after Dayhoff-6 recoding. b–d Lineages are summarized at the ordinal level. Support values represent posterior probabilities, with lack of a number indicating maximum support. Silhouettes designed by ART

Fig. 2

Phylogenetic tree derived from the CAT-GTR + G analyses of Matrix A after exclusion of six unstable taxa (Calanus, Bothriurus, Liocheles, Pandinus, Superstitionia and Vietbocap). Removal of unstable taxa did not affect topological relationships but increased support levels at key nodes and improved convergence statistics. Support values represent posterior probabilities. Convergence statistics: Burnin = 3000; Total Cycles = 10,000 subsampling frequency = 20. Maxdif = 0.28; Minimal effective size = 63. Silhouettes designed by ART