Radiations of Pteriomorph Bivalvia (Mollusca)

The study of the evolution of higher taxa of Bivalvia (Mollusca) is notoriously hampered by many cases of morphological convergence and parallel evolution. Molecular characters, said to be less prone to convergency, may therefore contribute greatly to unravel bivalve evolution. The subclass Pteriomorphia is probably the only bivalve taxon with a moderately resolved phylogenetic hypothesis. This group is at least 500 million years old and displays three major, temporally distinct radiations. The existing, morphology-based phylogenetic concept will serve as a landmark for testing the value of molecular data for Pteriomorphia and Bivalvia in general. Two genes will be sequenced, the complete 18S rDNA and a part of the mitochondrial cytochrome-oxidase-I-unit (COI), yielding together about 2500 nucleotides. About 20 pteriomorph species and at least 5 bivalve and other molluscan outgroup organisms will be sequenced. The animals will be collected in Croatia and Costa Rica. Standard methods for DNA extraction, PCR, and cloning will be used. Sequencing will be done on an automatic sequencer. Sequence alignment will be -computer-aided and, for the 18S rDNA, consider secondary structure information. For .the COI,.both. nucleotide and amino acid sequences will be used for phylogenetic inferences. Apart from the orthodox methods of molecular phylogenetic analysis (parsimony, distance and maximum-likelihood), two recently developed techniques for assessing the information contents of sequence data will be applied: spectral analysis and Pattern of Resolved Nodes (PRN). Both methods help quantifying the phylogenetic signals of an alignment and the robustness of nodes in a phylogenetic tree. The PRN software will be modified to use parsimony instead of distance algorithms and, thus, greatly improved. This method also allows calibration of sequence evolution against the fossil record. The sequences will be analyzed both separately and in a total evidence approach. Sequence analysis will shed light on: 1) To what extent do evolutionary rates differ between genes and taxa; 2) What is the optimum time span for each gene to resolve phylogeny; 3) Are the gene trees compatible and, if not, why is this so; 4) Are unresolved areas of the tree(s) due to saturation, too recent divergences or rapid radiations? Available morphological characters will be treated separately and the results compared to those of the molecular data. Stratigraphic information will be used to calibrate speciations. We hope to find answers to the questions: 1) Are morphological and molecular trees congruent; 2) When did the rapid radiations happen; 3) Is there a common pattern in the nucleotide sequences caused by rapid radiations; 4) If so, does the pattern characterize radiations in general, i.e. in other taxa as well? In a sythesis of these results, stratigraphic and paleoecological data, a scenario of pteriomorph evolution will be presented.