Interspezifische Variabilität bei hypotrichen Ciliaten1

Abstract

Interspecific variability in hypotrichous ciliates The genome organization of hypotrichous ciliates differs fundamentally from those of most other eukaryotic organisms. Every cell has two kinds of nuclei as is characteristic for ciliatese small generative micronuclei (Mi) whose DNA has a high molecular weight and which is organized in chromosomes, and vegetative macronuclei (Ma) which are very rich in DNA. The macronuclear DNA consists of so-called “gene-sized” DNA pieces, an organization which is not found in any other organism. This extraordinary genome organization offers a convenient experimental approach for studying evolutionary divergence at different molecular levels: 1. whole genomes, 2. subfractions of genomes, and 3. enzyme proteins. The comparison of unfractionated genomic DNA of hypotrichous ciliates by Dna-DNA hybridizations has yielded an unsuspected result: species that are closely related according to their morphology show an unusually low amount of sequence homology. The underlying reason might be that hypotrichous species separated early in eukaryotic evolution. Whereas the morphology of “closely related” species has changed only little, molecular evolution has led to major genomic changes that reflect the great evolutionary age of the species. The separation of native macronuclear DNA by gel electrophoresis produces species-specific DNA banding patterns based on different copy numbers of individual “gene-sized” DNA pieces in different species. These banding patterns allow the discrimination of sibling species which are morphologically very similar or even undistinguishable. Higher taxa can also be identified by means of DNA banding patterns. Cloned α- and β-tubulin genes were used in hybridization experiments to study the evolutionary divergence of individual DNA sequences in different hypotrichous species. The unusual Magenome organization makes such an analysis especially convenient. Characteristics of individual genes such as length number of sequence variants, copy number, and pattern of restriction sites can be compared with this method. The digestion of Mi-DNA with restriction endonucleases reveals differences in the repetitive DNA fraction of those genomes. Specific differences can be detected between closely related species and even between different populations of one species. The comparison of evolutionary divergence at the DNA level was supplemented by a comparison at the protein level. Enzyme electrophoresis proved to be a suitable method for the identification of otherwise indistinguishable species. Genetic ivergency (D-values) was estimated on the basis of allozyme data and a dendrogram was constructed reflecting the amount of genetic similarity between the species investigated. The discussion considers advantages and disadvantages of molecular characteristics for attacking taxonomic, phylogenetic, and evolutionary problems.