EVOLUTIONARY AND ECOLOGICAL DIFFERENTIATION IN THE PANTROPICAL TO WARM-TEMPERATE SEAWEED DIGENEA SIMPLEX (RHODOPHYTA)

Genetic differentiation among geographic isolates of the pantropical t o warm-temperate red alga Digenea simplex (Wulfen) C. Agardh was inves tigated using random amplified polymorphic DNA (RAPD) markers, crossin g studies, and temperature tolerances experiments. Eleven isolates rep resenting populations from the Caribbean, eastern Atlantic, and Indo-W est Pacific were compared. RAPD analysis clearly revealed an Indo-West Pacific group, a Caribbean/Cape Verde Islands group, and a Canary Isl ands group. Crossing studies showed different levels of interfertility . In most crosses between Western Australian and Atlantic isolates, no hybrid tetrasporophytes were formed. In crosses between Caribbean and Cape Verde Islands isolates, tetrasporophytes developed, but the viab ility of tetraspores was reduced. Full sexual compatibility was observ ed among Cape Verde Islands isolates and among isolates from Bonaire. Temperature tolerance studies indicate that Pacific isolates have a br oader temperature survival range than Atlantic isolates, which may be correlated to local temperature extremes. Despite the reduced level of sexual compatibility between Caribbean and Cape Verde Islands isolate s, their shared position in the RAPD analysis and similar temperature responses suggest trans-Atlantic dispersal in the near geological past . In addition to their discrete position in the RAPD distance analysis , the Canary Islands isolates were significantly more cold-tolerant th an the other Atlantic isolates. This finding is consistent with the hy pothesis that the Canary Islands were recolonized from cold-adapted ea stern Mediterranean populations after the last Pleistocene glaciation.