MULTIPLE TRANS-ARCTIC PASSAGES IN THE RED ALGA PHYCODRYS RUBENS - EVIDENCE FROM NUCLEAR RDNA ITS SEQUENCES

In order to investigate how episodes of geological and climatic change have influenced the distribution and evolutionary diversification of Arctic to cold temperate-North Atlantic seaweed species, intraspecific genetic variation was analyzed among isolates of the sublittoral, ben thic red alga Phycodrys rubens (collected between June 1992 and Januar y 1994). Rooted phylogenetic analyses of nuclear ribosomal DNA interna l transcribed spacer (ITS) sequences and the plastid encoded Rubisco s pacer sequences suggest that P. rubens invaded the North Atlantic from the Pacific shortly after the opening of the Bering Strait (3 to 3.5 million years ago), colonizing both the western and eastern Atlantic c oasts. Based on these data we further hypothesize that P. rubens survi ved along the European coasts during the more recent Pleistocene glaci ations, while becoming locally extinct along the North American Atlant ic coasts. Following retraction of the last ice sheet, the western Atl antic coast was colonized a second time from the Pacific. The presence of two distinct genetic types (based on ITS and Rubisco sequences) al ong the European coasts is postulated to be a result of isolation and subsequent differentiation. This is likely because ice-free areas are known to have existed in northern Scotland and Norway during the last glaciation. The presence of an East Atlantic genetic type along the We st Atlantic coast is believed to be a recent introduction (caused by h uman activity) of P. rubens to Newfoundland.