Gastropod Evolutionary Rates and Phylogenetic Relationships assessed using partial 28S rDNA and histone H3 sequences
In this project DNA sequences were used to study the evolutionary relationship between all the major groups of gastropods.
In this project we used DNA sequence data from representatives of all major gastropod clades for testing the major modern hypotheses of gastropod relationships - those of Haszprunar (1988a,b) and Ponder and Lindberg (1997). The work is described in Zoologica Scripta 29: 29-63. The basal dichotomy in the Ponder and Lindberg classification (1997) separates the gastropods into two major groups, the Eogastropoda (comprising the true limpets - Patellogastropoda - and their extinct, probably coiled, ancestors) and the Orthogastropoda -all other gastropods. The major clades within the orthogastropods include the Vetigastropoda (many of the old "Archaeogastropoda"), Neritopsina and Apogastropoda comprising the Caenogastropoda (most "Mesogastropoda"+ "Neogastropoda") and the Heterobranchia (Opisthobranchia + Pulmonata and a few groups previously classified as "mesogastropods").
Sequence data (Download
) for two segments of 28S and Histone H3 from 36 gastropod taxa, a chiton, two bivalves and Nautilus were collected. Statistical results suggest that the accuracy of the available hypotheses could be improved. The data support the monophyly of the Patellogastropoda (true limpets), Euthyneura and the "higher" vetigastropods and the polyphyly of the 'Cocculiniformia'. The division of the gastropods into two major clades (Eogastropoda and Orthogastropoda) is not supported, and neither the Caenogastropoda nor Heterobranchia is well supported. Within the Euthyneura, opisthobranchs are paraphyletic with respect to the pulmonates. The hot vent taxon, Depressigyra, groups with Pleurotomaria in some analyses.
Much of the variability in the 28S rDNA segments lies in discrete areas of the sequence. For one of the segments, these correspond to known expansion regions (D4 and D5). For the other, the variable area, which is found in the patellogastropods, vetigastropods and Nautilus, represents an unreported expansion region.
The data show marked variability in the rate of evolution in both segments of the 28S rDNA, whether or not the expansion regions are included. The variability is largely clade specific. Rates are high in the patellogastropods, vetigastropods, the lower heterobranch "Heterostropha" (Cornirostra and Philippea), Depressigyra and the deep sea cocculinid limpet Coccopigya and substantially lower in other taxa. Rate variation in the histone H3 data is less extreme. The correlation between evolutionary rates in the two 28S rDNA segments is very high, and is also significant for the the pairing of each of the 28S rDNA segments with H3. Rate variability may be due to differential selection but no causative factor has been identified.
The histone H3 data have high codon usage bias. For all amino acids encoded by multiple codons, at least some triplets occur at a frequency of less than a quarter of their expected usage. For all three-, four- and sixfold degenerate amino acids, the most abundant triplet occurs at least twice as frequently as expected. Despite the usage bias, there is a large amount of apparent homoplasy in synonymous alternatives at both the first and third codon positions.
Acknowledgments
Funding for this project has been provided by the Australian Research Council
Participants
Peter Eggler
Don Colgan
Winston Ponder, Centre for Evolutionary Research
Selected references
- Harasewych, M. G., Adamkewicz, S. L., Blake, J. A., Saudek, S., Spriggs, T. & Bult, C. J. (1997a). Phylogeny and relationships of pleurotomariid gastropods (Mollusca: Gastropoda): an assessment based on partial 18S rDNA and cytochrome c oxidase I sequences. Molecular Marine Biology and Biotechnology, 6, 1-20.
- Haszprunar, G. (1985a). The fine morphology of the osphradial sense organs of the Mollusca. Part 1: Gastropoda- Prosobranchia. Philosophical Transactions of the Royal Society of London, series B, 307, 457-496.
- Haszprunar, G. (1985b). The Heterobranchia- a new concept of the phylogeny and evolution of the higher Gastropoda. Zeitschrift für Zoologische Systematik und Evolutionsforsch, 23, 15-37.
- Haszprunar, G. (1988a). On the origin and evolution of major gastropod groups, with special reference to the Streptoneura (Mollusca). Journal of Molluscan Studies, 54, 367-441.
- Haszprunar, G. (1988b). A preliminary phylogenetic analysis of the streptoneurous gastropods. Malacological Review Supplement, 4, 7-16.
- Ponder, W. F. & Lindberg, D. R.. (1997). Towards a phylogeny of gastropod molluscs - a preliminary analysis using morphological characters. Zoological Journal of the Linnean Society, 19, 83-265.
- Rosenberg, G., Tillier, S., Tillier, A., Kuncio, G. S., Hanlon, R. T., Masselot, M. & Williams, C. J. (1997). Ribosomal RNA phylogeny of selected major clades in the Mollusca. Journal of Molluscan Studies, 63, 301-309.
- Thiele J. (1929-31). Handbuch der systematischen Weichtierkunde, Vol. 1. Jena: Gustav Fischer Verlag.
- Tillier, S., Masselot, M., Guerdoux, J. & Tillier, A. (1994). Monophyly of major gastropod taxa tested from partial 28S rRNA sequences, with emphasis on Euthyneura and hot-vent limpets Peltospiroidea. Nautilus 108, Supplement 2, 122-140.
- Winnepenninckx, B., Steiner, G., Backeljau, T. & De Wachter, R. (1998). Details of gastropod phylogeny inferred from 18S rDNA sequences. Molecular Biology and Evolution, 9, 55-63.
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