Whole genome duplication is now accepted as an important evolutionary force, but the genetic factors and the life history implications affecting the existence and abundance of polyploid lineages within species are still poorly known. Polyploidy has been mainly studied in plant model species in which the sporophyte is the dominant phase in their life history. In this study, we address such questions in a novel system (Porphyra, red algae) where the gametophyte is the dominant phase in the life history. ThreePorphyraspecies (P.dioica,P.umbilicalis, andP.linearis) were used in comparisons of ploidy levels, genome sizes and genetic differentiation using flow cytometry and 11 microsatellite markers among putative polyploid lineages. Multiple ploidy levels and genome sizes were found inPorphyraspecies, representing different cell lines and comprising several cytotype combinations among the same and different individuals. InP.linearis, genetic differentiation was found among three polyploid lineages: triploid, tetraploid and mixoploids, representing different evolutionary units. We conclude that the gametophytic phase (n) inPorphyraspecies is not haploid, contradicting previous theories. New hypotheses for the life histories ofPorphyraspecies are discussed.

Caulerpa species are marine green algae, which often act as invasive species with rapid clonal proliferation when growing outside their native biogeographical borders. Despite many publications on the genetics and ecology of Caulerpa species, their life history and ploidy levels are still to be resolved and are the subject of large controversy. While some authors claimed that the thallus found in nature has a haplodiplobiontic life cycle with heteromorphic alternation of generations, other authors claimed a diploid or haploid life cycle with only one generation involved. DAPI-staining with image analysis and microspectrophotometry were used to estimate relative nuclear DNA contents in three species of Caulerpa from the Mediterranean, at individual, population and species levels. Results show that ploidy levels and genome size vary in these three Caulerpa species, with a reduction in genome size for the invasive ones. Caulerpa species in the Mediterranean are polyploids in different life history phases; all sampled C. taxifolia and C. racemosa var. cylindracea were in haplophasic phase, but in C. prolifera, the native species, individuals were found in both diplophasic and haplophasic phases. Different levels of endopolyploidy were found in both C. prolifera and C. racemosa var. cylindracea. Life history is elucidated for the Mediterranean C. prolifera and it is hypothesized that haplophasic dominance in C. racemosa var. cylindracea and C. taxifolia is a beneficial trait for their invasive strategies.

Optimum culture conditions for conchocelis growth and conchospore production were determined for the winter annual Porphyra linearis (Rhodophyta, Bangiaceae) from western Ireland. Conchocelis cultures were established on scallop shells under a range of temperatures (10°C, 13°C, 15°C and 20°C), photon irradiances (5, 10, 20 and 40 μmol photons m⁻² s⁻¹) and daylengths (8:16, 11:13, 12:12, 13:11 and 16:8 h light-dark). Both conchocelis area (mm²) and conchosporangial density (number of conchosporangial colonies per mm² of conchocelis area) were assessed using an interfaced digital image processor. Under all conditions, conchocelis area increased up to 40 d, but thereafter no further increase was observed. The number of conchosporangial colonies peaked after 40 d for all treatments and then decreased. This reduction in density coincided with conchospore release. Conchosporangial density was highest at 20°C, intermediate at 15°C and 13°C and lowest at 10°C. At all temperatures, conchosporangial density was greatest in treatments established under long-day conditions. There was no difference in conchosporangial growth between treatments. The transfer of cultures grown at different daylengths for 30 d to daylengths of 16:8 h light-dark did not stimulate any increase in conchosporangial density or conchospore release at any temperature. The results indicated clearly that the optimal conditions for conchocelis growth and conchospore production in P. linearis from Ireland are 20°C and long days (l6:8 h), and suggest that intertidal conditions in summer are ideal for growth of this phase in the wild.