Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic

Abstract

Recent molecular analyses indicate that crustaceans and hexapods form a clade (Pancrustacea or Tetraconata), but relationships among its constituent lineages, including monophyly of crustaceans, are controversial. Our phylogenetic analysis of three protein-coding nuclear genes from 62 arthropods and lobopods (Onychophora and Tardigrada) demonstrates that Hexapoda is most closely related to the crustaceans Branchiopoda (fairy shrimp, water fleas, etc.) and Cephalocarida + Remipedia, thereby making hexapods terrestrial crustaceans and the traditionally defined Crustacea paraphyletic. Additional findings are that Malacostraca (crabs, isopods, etc.) unites with Cirripedia (barnacles, etc.) and they, in turn, with Copepoda, making the traditional crustacean class Maxillopoda paraphyletic. Ostracoda (seed shrimp)--either all or a subgroup--is associated with Branchiura (fish lice) and likely to be basal to all other pancrustaceans. A Bayesian statistical (non-clock) estimate of divergence times suggests a Precambrian origin for Pancrustacea (600 Myr ago or more), which precedes the first unambiguous arthropod fossils by over 60 Myr.

Figure 1

Maximum-likelihood, 3-gene, amino acid (‘protml’) tree selected from 91673 most-favoured maximum-parsimony trees. Nodes are arbitrarily numbered on the cladogram (left) for reference to table 1. Branch lengths from the same analysis are shown in phylogram format (right). Averaged absolute divergence time estimates are for nodes identified by half-filled circles only. Note that variance for time estimates is substantial (see figure S1 for full results). Nodes that are either ‘supported’ or ‘well supported’ (defined in text) are basally connected to solid branch lines. Less well-supported nodes are connected to dashed lines. Recovery of nodes 21, 24 and 33 is of particular interest and their numbers are in bold type.