Reproductive plasticity and local adaptation in the host–parasite system formed by the toxic Alexandrium minutum and the dinoflagellate parasite Parvilucifera sinerae

Abstract

A parasite threat stimulates adaptive shifts in the life-history strategy (sexual recombination rate) of the toxic bloom-forming dinoflagellate Alexandrium minutum Halim. This microalgae divides asexually when clonal but can also form mobile zygotes (planozygotes) when compatible clones are crossed. Planozygotes usually form resistant dormant stages (resting cysts) although they can also divide. In this study, asexual and sexual cultures were infected with the parasite Parvilucifera sinerae (Perkinsozoa) and the resulting clones classified as susceptible (S), low susceptible (LS), or resistant (R) to the infection. R and LS clones were never of Mediterranean origin, pointing to local adaptation of the parasite. (S × S) crosses were infected faster than either of the parental clones growing asexually. By contrast, (S × R) crosses were resistant to the parasite and produced no resting cysts, even when planozygotes were formed. Therefore, in infected cultures, the planozygotes mainly divided instead of encysting, thus increasing the rate at which recombinant progeny formed. This strategy against infection seems to combine the benefits of quickly producing asexual offspring and increasing recombination. As the susceptibility of the crosses was dependent on parental sexual compatibility, and cultures established by the division of (R × S) planozygotes (F1 offspring) also formed R or LS cultures, resistance may be regulated by several genes or through maternal effects.