Alternating nuclear DNA content in chrysophytes provides evidence of their isomorphic haploid-diploid life cycle

Abstract

Across eukaryotic organisms there is a great diversity of life cycles. This particularly applies to unicellular eukaryotes (protists), where the life cycles are still largely unexplored, although this knowledge is key to understanding their biology.To detect the often inconspicuous transitions among life cycle stages, we focused at shifts in ploidy levels within strains of unicellular chrysophyte alga. Representatives of three genera (Chrysosphaerella, Ochromonas, and Synura) were analysed for nuclear DNA contents using a propidium iodide flow cytometry. Selected strains exhibiting ploidy level variation were also surveyed for DNA base composition (GC content) and cell size. Additionally, we tracked ploidy level changes in seven strains under long-term cultivation.An alternation of two ploidy levels was revealed in the life cycle of chrysophytes with both life cycle stages capable of mitotic growth and long-term survival in cultivation. With the exception of a small increase in cell size with higher ploidy, both life cycle stages shared the same phenotype and also had highly similar genomic GC content. Further, we detected three ploidy levels in two Synura species (S. glabra, S. heteropora), where the highest ploidy (putatively 4x) most likely resulted from a polyploidization event.Consequently, chrysophytes have a haploid-diploid life cycle with isomorphic life cycle stages. As far as we know, this is the first report of such life cycle strategy in unicellular algae. Life cycle stages and life stage transitions seem to be synchronized among all cells coexisting within a culture, possibly due to chemical signals. Particular life stages may be more successful under certain environmental conditions, for our studied strains the diploid stage prevailed in cultivation.