Phenotypic Analysis and Molecular Characterization of Enlarged Cell Size Mutant in Nannochloropsis oceanica .

The cell cycle is the fundamental cellular process of eukaryotes. Although cell-cycle-related genes have been identified in microalgae, their cell cycle progression differs from species to species. Cell enlargement in microalgae is an essential biological trait. At the same time, there are various causes of cell enlargement, such as environmental factors, especially gene mutations. In this study, we first determined the phenotypic and biochemical characteristics of a previously obtained enlarged-cell-size mutant of , which was designated . Whole-genome sequencing analysis of the insertion sites of indicated that the insertion fragment is integrated inside the 5'-UTR of U/P-type cyclin and significantly decreases the gene expression of this cyclin. In addition, the transcriptome showed that is a highly expressed cyclin. Furthermore, cell cycle analysis and RT-qPCR of cell-cycle-related genes showed that maintains a high proportion of 4C cells and a low proportion of 1C cells, and the expression level of in wild-type (WT) cells is significantly increased at the end of the light phase and the beginning of the dark phase. This means that is involved in cell division in the dark phase. Our results explain the reason for the larger size. Mutation of leads to the failure of to fully complete cell division in the dark phase, resulting in an enlargement of the cell size and a decrease in cell density, which is helpful to understand the function of in the cell cycle.