Michael Jonathan Carlson and Dr. Keith A. Crandall, Zoology
Freshwater crayfish of the family Parastacidae consist of 14 genera distributed in the Southern Hemisphere: nine are endemic to Australia, one to Madagascar, one to New Zealand, and three to South America (southern Chile and Brazil). Although this family has been studied for several years (e.g. Huxley, 1880), the evolutionary relationships among the taxa are still unclear (Crandall et al., 1999). Recently, Crandall et al. (2000) demonstrated the utility of combining mitochondrial (16S) and nuclear (18S and 28S) ribosomal genes to resolve deep and shallow phylogenetic relationships among crayfish. Thus, a more extensive molecular study, using these regions and additional genes could allow us to resolve the phylogenetic relationships among the 14 Parastacidae genera. Moreover, a robust phylogeny will also be useful for addressing another important question concerning to the center of origin of the family.
Therefore the aims of this project are to:
1) Collect new data from mitochondrial (12S and 16S), and nuclear genes (18S and 28S) to study the phylogenetic relationships among all of the Parastacidae genera.
2) Use the obtained phylogeny to test the center of origin of the Parastacidae family as being Australia
Nucleotide sequences were aligned using Clustal X and then adjusted by eye. Data sets from different gene regions were combined. Phylogeny relationships were estimated using neighbor-joining and maximum parsimony. We used the approach outlined by Huelsenbeck and Crandall (1997) to test hypotheses relating to the molecular evolution of the nucleotide sequences examined in this study implemented in PAUP* (Swofford, 2000) and Modeltest 3.0 (Posada and Crandall, 1998). Maximum parsimony searches were heuristic with 100 random taxon additions. Confidence in the resulting relationships was assessed using the bootstrap procedure (Felsenstein, 1985).
Our sequencing efforts resulted in 56 new DNA sequences from 28 crayfish species representing all the Parastacidae genera and four Astacidae and Cambaridae species.
The maximum likelihood hypothesis testing procedure resulted in the rejection of all seven null hypotheses tested except transition rates are equal, thus our justified model of evolution was the TVM model plus gamma distributed rate heterogeneity plus a significant proportion of invariable sites (TVM+Γ+I).
Phylogenetic relationships among Parastacidae taxa were estimated using neighbor-joining and incorporating this model of molecular evolution, and using maximum parsimony. The latter procedure resulted in one most parsimonious tree with a tree length of 2835 steps. Phylogenies based on both phylogenetic methods place Astacoides species from Madagascar in the most basal position of the tree. South American and New Zealand crayfish genera form sister groups, and Australian crayfish are not considered to be a monophyletic group.
Parastacidae genera from Australia do not form a monophyletic group as could be expected under a vicariance scenario of phylogenetic separations subsequent to the fragmentation of the Gondwana.
Crayfish from Madagascar (i.e., Astacoides) are the most basal group. This agrees with Tectonic Theory about the earliest separation of this continent from Gondwana. However, this conflicts with the proposal of Australia as being the center of origin of the Southern Hemisphere crayfish.
The sister relationships of South American and New Zealand crayfish conflicts with the accepted pattern of continental fragmentation.
New genes must be sequenced to strongly establish deep relationships among these 14 Parastacidae genera. A robust phylogeny and the use of molecular clocks will allow us to test vicariant and dispersal hypotheses about the biogeography of the Parastacidae crayfish.
- Crandall, K. A., Fetzner, J. W. J., Lawler, S. H., Kinnersley, M. and Austin, C. M. (1999). Phylogenetic relationships among the Australian and New Zealand genera of freshwater crayfishes (Decapoda: Parastacidae). Australian Journal of Zoology 47, 199-214.
- Crandall, K. A., Harris D. J. and Fetzner, J. W. (2000). The monophyletic origin of freshwater crayfish estimated from nuclear and mitochondrial DNA sequences. The Royal Society 267, 1679-1686.
- Felsenstein, J. (1985). Confidence limits on phylogenies: an approach using the bootstrap. Evolution 39, 783-791.
- Huelsenbeck, J.P. and Crandall, K. A. (1997). Phylogeny estimation and hypothesis testing using maximum likelihood. Annual Review of Ecology and Systematics 28, 437-466.
- Huxley, T. H. 1880 The Crayfish : An Introduction to the study of Zoology. New York:
- Posada, D. and Crandall, K. A. (1998). Modeltest: Testing the model of DNA substitution. Bioinformatics 14, 817- 818.
- Swofford, D. L. (1999). PAUP* Phylogenetic analysis using parsimony and other methods. Sinauer Associates, Sunderland, MA.