DNA ANALYSIS OF EUKARYOTIC ALGAL SPECIES1

Abstract

ABSTRACTPlastid genomes of algae resemble those of terrestrial plants in form, size, and rate of nucleotide sequence change. They are circular and range in size from 73 kilobases (kb) to over 400 kb. Their many copies per cell can compose >15% of total cell DNA. Mitochondrial genomes, like plastid genomes, are present in high copy number in preparations of total algal cell DNA. Almost all known algal mitochondrial DNA genomes are relatively small, <50 kb; in some species they are linear, whereas in others they are circular.One of the persistent perplexities for phycologists is the question of what relationship two clones or two groups of organisms bear to each other. Several relatively simple techniques can reveal whether or not two organisms belong to the same clone. Total mitochondrial genome size can be compared directly between isolates, although identity in size does not necessarily mean identity in sequence. Restriction endonuclease digestion combined with probing permits comparison of DNA fragment patterns to see if there is identity or near identity between two samples. This methodology can be applied both to organelle genomes and to nuclear genomes.So far, restriction endonucleases cleave plastid and mitochondrial DNA of organisms belonging to the same gene pool into nearly identical fragment patterns, whereas organisms nearly or totally incapable of interbreeding display patterns wherein ≫ 50% of restriction fragments differ in position on an agarose gel after electrophoresis. Thus, organelle genomes may be the first choice for comparing both total size and restriction endonuclease fragment patterns to obtain an indication of whether two organisms are closely related. This methodology can be applied both to organisms in which interbreeding is easy to test and to the many algae in which homothallism or lack of sexual clones has precluded standard breeding analyses. With further data on variability levels within and between fertile populations, it may be possible to state with confidence whether a sample of morphologically similar organisms shares a common gene pool, even if their breeding cannot be manipulated experimentally.