Pancrustacea

Abstract: The interrelationships of major clades within the Arthropoda remain one of the most contentious issues in systematics, which has traditionally been the domain of morphologists. A growing body of DNA sequences and other types of molecular data has revitalized study of arthropod phylogeny and has inspired new considerations of character evolution. Novel hypotheses such as a crustacean-hexapod affinity were based on analyses of single or few genes and limited taxon sampling, but have received recent support from mitochondrial gene order, and eye and brain ultrastructure and neurogenesis. Here we assess relationships within Arthropoda based on a synthesis of all well sampled molecular loci together with a comprehensive data set of morphological, developmental, ultrastructural and gene-order characters. The molecular data include sequences of three nuclear ribosomal genes, three nuclear protein-coding genes, and two mitochondrial genes (one protein coding, one ribosomal). We devised new optimization procedures and constructed a parallel computer cluster with 256 central processing units to analyse molecular data on a scale not previously possible. The optimal 'total evidence' cladogram supports the crustacean-hexapod clade, recognizes pycnogonids as sister to other euarthropods, and indicates monophyly of Myriapoda and Mandibulata.

Abstract: The complete 18S rDNA gene sequence of Macrobiotus group hufelandi (Tardigrada) was obtained and aligned with 18S rDNA and rRNA gene sequences of 24 metazoans (mainly protostomes) Discrete character (maximum-parsimony) and distance (neighbor-joining) methods were used to infer their phylogeny The evolution of bootstrap proportions with sequence length (pattern of resolved nodes, PRN) was studied to test the resolution of the nodes in neighbor-joining trees The results show that arthropods are monophyletic Tardigrades represent the sister group of arthropods (in parsimony analyses) or they are related with crustaceans (distance analysis and PRN) Arthropoda are divided into two main evolutionary lines, the Hexapoda + Crustacea line (weakly supported), and the Myriapoda + Chelicerata line The Hexapoda + Crustacea line includes Pentastomida, but the internal resolution is far from clear The Insecta (Ectognatha) are monophyletic, but no evidence for the monophyly of Hexapoda is found The Chelicerata are a monophyletic group and the Myriapoda cluster close to Arachnida Overall, the results obtained represent the first molecular evidence for a Tardigrada + Arthropoda clade In addition, the congruence between molecular phylogenies of the Arthropoda from other authors and this obtained here indicates the need to review those obtained solely on morphological characters

Abstract: Mitochondrial DNA (mtDNA) sequences are commonly used for inferring phylogenetic relationships. However, the strand-specific bias in the nucleotide composition of the mtDNA, which is thought to reflect asymmetric mutational constraints, combined with the important compositional heterogeneity among taxa, are known to be highly problematic for phylogenetic analyses. Here, nucleotide composition was compared across 49 species of Metazoa (34 arthropods, 2 annelids, 2 molluscs, and 11 deuterosomes), and analyzed for a mtDNA fragment including six protein-coding genes, i.e., atp6, atp8, cox1, cox2, cox3, and nad2. The analyses show that most metazoan species present a clear strand asymmetry, where one strand is biased in favor of A and C, whereas the other strand has a reverse bias, i.e., in favor of T and G. The origin of this strand bias can be related to asymmetric mutational constraints involving deaminations of A and C nucleotides during the replication and/or transcription processes. The analyses reveal that six unrelated genera are characterized by a reversal of the usual strand bias, i.e., Argiope (Araneae), Euscorpius (Scorpiones), Tigriopus (Maxillopoda), Branchiostoma (Cephalochordata), Florometra (Echinodermata), and Katharina (Mollusca). It is proposed that asymmetric mutational constraints have been independently reversed in these six genera, through an inversion of the control region, i.e., the region that contains most regulatory elements for replication and transcription of the mtDNA. We show that reversals of asymmetric mutational constraints have dramatic consequences on the phylogenetic analyses, as taxa characterized by reverse strand bias tend to group together due to long-branch attraction artifacts. We propose a new method for limiting this specific problem in tree reconstruction under the Bayesian approach. We apply our method to deal with the question of phylogenetic relationships of the major lineages of Arthropoda. This new approach provides a better congruence with nuclear analyses based on 18S rRNA gene sequences. By contrast with some previous studies based on mtDNA sequences, our data suggest that Chelicerata, Crustacea, Myriapoda, Pancrustacea, and Paradoxopoda are monophyletic. (Arthropoda; asymmetry; genome; long-branch attraction artifact; mitochondria; molecular evolution; mutations; phylogeny; strand bias.)

Abstract: Recent morphological and molecular evidence has changed interpretations of arthropod phylogeny and evolution. Here we compare complete mitochondrial genomes to show that Collembola, a wingless group traditionally considered as basal to all insects, appears instead to constitute a separate evolutionary lineage that branched much earlier than the separation of many crustaceans and insects and independently adapted to life on land. Therefore, the taxon Hexapoda, as commonly defined to include all six-legged arthropods, is not monophyletic.