Inheritance of organelles in artificial hybrids of the isogamous multicellular chromist alga Ectocarpus siliculosus (Phaeophyceae)

Our knowledge about the sexual transmission of mitochondria and plastids (hereafter organelles) in isogamous eukaryotes comes mostly from studies of the yeast Saccharomyces cerepisiae and the green alga Chlamydomonas reinhardtii which are both unicellular species. To investigate organelle inheritance in a multicellular organism with morphological isogamy, we studied the filamentous brown alga Ectocarpus siliculosus, in which each gamete contributes one plastid and at least one mitochondrion to the zygote. We crossed strains whose organelle genotypes were distinguishable by PCR. Hybrids contained only maternal mitochondrial genotypes, indicating uniparental inheritance of this organelle. In contrast, hybrids were chimerical for the plastid genome. In 64 to 75% of cases examined, the two zygotic plastids were partitioned into the two halves of the sporophyte, which developed from the zygote. A smaller number of hybrids deviated from this pattern, suggesting more complex mechanisms such as irregular division, segregation, or exchange of genetic material between plastids. E. siliculosus is the first isogamous heterokont eukaryote in which inheritance of organelles has been determined with molecular markers. The deterministic pseudo-Mendelian mechanism of plastid inheritance in E. siliculosus is unusual among eukaryotes.