Polyploid polynuclear consecutive cell-cycle enables large genome-size in Haematococcus pluvialis

Abstract

Studies revealed and keep revealing a staggering diversity of life-cycle patterns and genome-sizes in microalgae. Even within a single order like the Volvocales with Volvox carteri, Chlamydomonas reinhardtii, and Dunaliella salina, differences are significant. Additional knowledge gaps in microalgal physiology and genetics hinder our understanding of their evolution and valorization. Herein, we present for the first time, the multiple fission cell-cycle with corresponding DNA quantities of the economically important Volvocales Haematococcus pluvialis. Further, we provide two straightforward protocols for efficient culture synchronization, based on filtration or skimming of zoospores. The cell-cycle and nuclear DNA content where monitored by flow cytometry and fluorescence microscopy after staining with dsDNA dye PICO- or SYBR-green®. The average nuclear DNA content was 3.6–4.1 and 22.6–25.2 pg in zoospores/coenobial daughter-cells and aplanospores, respectively. We found a single DNA replication step for all coenobial daughter cells prior mitosis, followed by one consecutive cytokinesis. Thus, our results suggest a new Haematococcus-type Cn cell-cycle with unusual polyploid and polynuclear phases. This cell-cycle regime might allow for the large genome-size of Haematococcus and is consistent with the mutational-hazard hypothesis for Volvocales microalgae.