DNA ANALYSIS-METHODS FOR RECOGNIZING SPECIES INVASION - THE EXAMPLE OF CODIUM, AND GENERALLY APPLICABLE METHODS FOR ALGAE

Analysis of DNA can help to distinguish those morphological characters indicative of species difference from those representing retained tra its or parallel evolution. This can be of great value in detecting rec ent invaders. The choice of which DNA characters to examine not only d ictates the methodology to be used but must also be appropriate for th e detection level sought. Restriction endonuclease fragment comparison s of plastid DNA have been used to assess Codium species; the results show C. fragile subsp. tomentosoides from east and west coast North Am erica to be identical while sympatric endemic Codium species each disp lay their own unique set of fragments. For species of other algae, pla stid DNA fragment patterns are not necessarily identical across a morp hological species, e.g. Pandorina morum. Such repetitive element probe s as M13 and the use of RAPDs are more appropriate for analysis of pop ulations within species. DNA base sequence comparisons of nuclear rDNA genes often yield too few variant bases between closely related speci es for reliable identifications. Analysis of the more variable Interna l Transcribed Spacer (ITS) region, lying between the small and large r ibosomal subunit genes in nuclear DNA, yields more extensive base pair variation between species and relatively little within species; it ma y be an alternative choice for endonuclease restriction fragment analy sis or for sequencing.