M.H.M. (Martijn) Holterman: "Phylogenetic relationships within the phylum Nematoda as revealed by ribosomal DNA, and their biological implications"

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7 Mar 2008 11:00
Unit: Wageningen University
Location: Aula, building 362, Gen. Foulkesweg 1, Wageningen
Organisation: Wageningen University
Promotor: prof.dr.ir. J. Bakker (Nematology)
Co Promotor: Dr.ir. J. Helder

 In this thesis we have endeavoured to make a significant contribution to the field of nematode taxonomy and evolution. A phylogenetic tree was constructed of 349 taxa spanning the entire phylum Nematoda using the small subunit ribosomal DNA. Twelve major clades could be distinguished with the Enoplida and Triplonchida forming the most basal clade. Further trees were constructed for the Dorylaimia, Chromadoria and Tylenchomorpha. For the Dorylaimida a tree using the more variable large subunit ribosomal DNA was constructed. In most cases nematode relationships could be elucidated with good support, although some areas in the trees remained unresolved. Furthermore, the suitability of ribosomal DNA for a (semi-) quantative molecular identification method was demonstrated. Plant parasitism has arisen several times within the Nematoda (once in the Triplonchida, at least twice in the Dorylaimida and at least twice in the Tylenchomorpha) and in most cases plant parasites were associated with fungal feeding nematodes. The generally accepted hypothesis that plant parasites evolved from fungal feeding ancestors could not be corroborated however. Analysis revealed that transitions from a thalassic to a limnoterrestrial habitat (and vice versa) have taken place at least 11 times in the Chromadoria and these transitions are apparently fairly easy to achieve for nematodes. Finally a framework was laid out to study correlations between nematode traits and stress tolerance and the importance of accounting for the effects of phylogeny was demonstrated. This is a first step towards a transparent, ecological grouping of free-living nematodes.
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