Relative ploidy levels in Prymnesium parvum and P. patelliferum (Haptophyta) analyzed by flow cytometry

The haptophytes Prymnesium parvum Carter and Prymnesium patelliferum Green, Hibberd et Pienaar, which can be distinguished only by minor details in scale morphology as seen in transmission electron microscopy (TEM), recurrently form mixed blooms in the Sandsfjord system, southwestern Norway. DNA sequences reported so far indicate that the two forms are genetically identical in both coding and noncoding regions. We therefore hypothesize that P. parvum and P. patelliferum could be stages in the life cycle of a single species. The relative DNA content per cell in three strains of P. parvum and three strains of P. patelliferum was examined by flow cytometry to investigate whether the two ‘species’ could be joined in a sexual life cycle. Two basic ploidy levels were found that differed by a factor of c. two and that were operationally defined as haploid and diploid. All three P. patelliferum strains were haploid by this definition, whereas one P. parvum strain proved to be haploid and the remaining two diploid. We did not observe a transition from one species to the other in the six cultures over time. In a mating experiment, the six strains were mixed pairwise in all possible combinations, and the mixed cultures were examined by TEM and by flow cytometry over a period of 18 d. Mixtures of haploid P. patelliferum strains did not produce P. parvum cells, nor did they result in diploid cell populations. Unexpectedly, a mixture of the two diploid P. parvum strains yielded haploid cells, and the same mixture of diploid P. parvum strains, as well as a mixture of one diploid and one haploid P. parvum strain, produced P. patelliferum cells. A haplo–diploid life cycle embracing the two forms is proposed, based mainly on the presence of the basic ploidy levels. This hypothesis is supported by the apparent genetic identity of P. parvum and P. patelliferum, and by their co-occurrence in nature.