Arthropod phylogeny based on eight molecular loci and morphology

The interrelationships of major clades within the Arthropoda remain one of the most contentious issues in systematics, which has traditionally been th e domain of morphologists(1,2). A growing body of DNA sequences and other t ypes of molecular data has revitalized study of arthropod phylogeny(3-7) an d has inspired new considerations of character evolution(8,9). Novel hypoth eses such as a crustacean-hexapod affinity(4,10-12) were based on analyses of single or few genes and limited taxon sampling, but have received recent support from mitochondrial gene order(13), and eye and brain ultrastructur e and neurogenesis(14,15). Here we assess relationships within Arthropoda b ased on a synthesis of all well sampled molecular loci together with a comp rehensive data set of morphological, developmental, ultrastructural and gen e-order characters. The molecular data include sequences of three nuclear r ibosomal genes, three nuclear protein-coding genes, and two mitochondrial g enes (one protein coding, one ribosomal). We devised new optimization proce dures(16,17) and constructed a parallel computer cluster with 256 central p rocessing units(18) to analyse molecular data on a scale not previously pos sible. The optimal 'total evidence' cladogram supports the crustacean-hexap od clade, recognizes pycnogonids as sister to other euarthropods, and indic ates monophyly of Myriapoda and Mandibulata.