GENETIC-VARIATION WITHIN AND AMONG NORTH-ATLANTIC AND BALTIC POPULATIONS OF THE BENTHIC ALGA PHYCODRYS RUBENS (RHODOPHYTA)

Genetic variation was examined within and among North Atlantic, North Sea and Baltic populations of the benthic red alga Phycodrys rubens us ing allozymes and random amplified polymorphic DNA (RAPD) markers. On western and eastern North Atlantic coasts distinct allozyme types were found, with the exception of western Newfoundland where East and West Atlantic types co-occur. Along the European coasts, two genetic group s were distinguished by fixed allelic differences: an outer oceanic gr oup and a North Sea/Baltic group. The two genetic types co-occur in th e Skagerrak and Kattegat region. Reproductive isolation between the tw o types is suggested by the lack of hybrids in the overlap zones, and they may therefore represent sibling species. Unexpectedly an analysis of RAPD variation was unable to recover the two cryptic species ident ified using allozymes. Within-population RAPD variation was similar to or greater than between-population variation. The lack of structure i n the RAPD data cannot be attributed solely to technical artefacts of the method but appears to reflect real biological variability. Within- population genomic polymorphisms caused by frequent mutational events are discussed, as are high amounts of genetic drift and possible disru ptive selection brought about by stressed habitats. Finally, Baltic an d extra-Baltic salinity ecotypes are known to exist in P, rubens. Howe ver, no correlation between ecotypic variation and allozyme groups was detected.