Internal ploidy and heritable chromosome fragmentation in Parachlorella kessleri

Parachlorella kessleri is a unicellular green alga. Its ability to produce starch and neutral lipids makes it a valuable bioresource for biomass production. In green algae, the cell cycle with multiple fission shows different patterns in different species. Nevertheless, the basic understanding of the cell cycle and division in P. kessleri is still incomplete. Seven A chromosomes and three B chromosomes ( n = 7 + 3B) were identified by microscopic observation and pulsed-field gel electrophoresis. The nuclear DNA content was then calculated. When the mean nuclear DNA content in the stationary phase was designated as 1C, the nuclear DNA content in the logarithmic growth phase ranged from 1.1C to 9.7C. In contrast, the chromosome number remained constant during the logarithmic growth phase, indicating that P. kessleri chromosomes underwent endoreduplication during multiple fission. P. kessleri cells were irradiated with heavy-ion beams at 640, 290, and 30 keV/μm linear energy transfer (LET). Chromosome fragmentation was observed for each irradiation group. The number of chromosomes in the cells tended to increase with increasing LET and with increasing dose. Some fragmented chromosomes were observed to be stably inherited after repeated subculture. To facilitate mutation analysis, we refined the reference genome sequence of P. kessleri , resulting in 52 scaffolds with 2.5 Mbp of N50 and a 98.3 % BUSCO value. Genome-wide mutation analysis of the mutants with stable fragmented chromosomes revealed translocations. In the two mutants irradiated with 290 keV/μm LET, translocations affecting telomeric repeat sequences were detected, which may have contributed to the stability of the fragmented chromosomes. In addition, large-scale duplication and deletion were observed in the two mutants irradiated with 290 keV/μm LET. These findings indicated the potential utility of a heavy-ion beam as a mutagen for microalgae, inducing large-scale mutations in numerous genes leading to novel phenotypic mutants. • Microscopic observation revealed chromosome number of Parachlorella kessleri . • Endoreduplication was induced during multiple fission. • Inheritable chromosome fragmentation is induced by heavy-ion beam. • Translocations involving telomeric repeats may contribute to the inheritance.