LIFE HISTORY AND HYBRIDIZATION STUDIES ONGIGARTINA STELLATAANDPETROCELIS CRUENTA(RHODOPHYTA) IN THE NORTH ATLANTIC1

Abstract

ABSTRACTFourteen isolates of the crustose marine red algaPetrocelis cruentaJ. Agardh from various localities in the British Isles, France (including the type locality), Spain and Portugal gave rise in culture to dioecious foliose plants identifiable asGigartina stellata(Stackhouse) Batters although two isolates formed only sterile foliose blades. A total of 145 isolates ofGigartina stellatawere also grown in culture from various localities in the U.S.A. (Maine), the British Isles, Iceland, Denmark, France, Spain and Portugal using both carpospores and vegetative blade apices. Two basic types of life history were found among these isolates: a direct‐type life history involving the formation of further foliose plants from carpospores, some isolates of which also form spermatangia on the same papillae as the cystocarps; and a heteromorphic‐type in which only crustose plants resemblingPetrocelis cruentaare formed from carpospores. Only heteromorphic‐type life histories were found from Spain and Portugal. Both life history types were found in plants from the U.S.A., the British Isles and northern France. Only direct‐type life histories were found in plants from Iceland and Denmark. SomePetrocelis‐like crusts derived from field collectedG. stellatacarpospores andPetroceliscrusts of hybrid progeny formed tetrasporangia in 8:16 h LD, 10° C but not in 8:16 h LD, 15° C; 16:8 h LD 10° C or 15° C; and 10:6.5:1: 6.5 h LDLD, 10° C. The spores thus formed were viable and produced normal dioecious male and female gametophytes. Short day and low temperature conditions appear necessary for tetrasporogenesis. The results from crossing experiments with 32 male and 27 female isolates of the heteromorphic‐type derived from bothG. stellataandP. cruentashowed that two virtually non‐interbreeding populations with a high degree of geographical separation exist in the north‐eastern Atlantic. Morphological differences between plants from each population are described. On the basis of culture and crossing results,Petrocelis cruentaJ. Agardh is placed in synonymy withGigartina stellata(Stackhouse in Withering) Batters.