Testing Hypotheses of Early Arthropod Evolution

University Of Maryland Biotechnology Institute, Baltimore MD

Investigators

Abstract

9981970 Regier & Shultz Arthropods have played a central role in recent studies of development and evolution, particularly, for identifying mechanisms conserved across animals. Arthropods should be equally valuable for studies of diversification, given their wide range of morphologies and physiologies, their species richness, and a fossil record that extends to the Cambrian. However, studies of diversity (from gene to morphology) across Arthropoda are hindered by the absence of clearly identifiable, monophyletic groups. As a result, many fundamental aspects of arthropod evolutionary history remain uncertain. For example, do suites of morphological characters found across subphyla and which are associated with terrestrialization, such as features of respiration and locomotion, represent convergences or shared, derived features? A robust resolution of arthropod relationships would represent a major advance for historical biology and would provide a means of testing these and many other evolutionary hypotheses. Recent molecular studies, including our own NSF-supported study, provide compelling evidence that Crustacea and Hexapoda (called Pancrustacea) are closely related, contra the traditional hypothesis that the terrestrial groups Hexapoda and Myriapoda are more closely related. Together with our strong support for the monophyly of Chelicerata (including Pycnogonida) and Myriapoda, we now propose to resolve the basal arthropod trichotomy among Pancrustacea, Chelicerata, and Myriapoda. Additionally, we propose to resolve basal relationships within Pancrustacea, particularly the placement of Hexapoda with respect to the crustacean classes; current molecular (and morphological) evidence is not compelling for a monophyletic Crustacea. Our current and proposed phylogenetic studies utilize nucleotides and inferred amino acids from multiple protein-encoding, nuclear gene sequences, including elongation factor-1a (1092 nt), RNA polymerase II (largest subunit, 1038 nt to ~1600 nt), and elongation factor-2 (1899 nt). Fifty-four species of arthropods and their near-outgroups (i.e., Onychophora and Tardigrada) will be sampled, including 8 hexapods, 23 crustaceans, 6 chelicerates, 5 myriapods, 9 tardigrades, and 3 onychophorans. Published morphological character matrices will be reanalyzed in light of our strongly supported molecular results.