Sex allocation is one of the most successful applications of evolutionary game theory. This theory has usually been applied to multicellular organisms; however, conditional sex allocation in unicellular organisms remains an unexplored field of research. Observations at the cellular level are indispensable for an understanding of the phenotypic sex allocation strategy among individuals within clonal unicellular organisms. The diatom Cyclotella meneghiniana, in which the sexes are generated from vegetative cells, is suitable for investigating effects of phenotypic plasticity factors on sex allocation while excluding genetic differences. We designed a microfluidic system that allowed us to trace the fate of individual cells. Sex allocation by individual mother cells was affected by cell lineage, cell size and cell density. Sibling cell pairs tended to differentiate into the same fates (split sex ratio). We found a significant negative correlation between the cell area of the mother cell and sex ratio of the two sibling cells. The male-biased sex ratio declined with higher local cell population density, supporting the fertility insurance hypothesis. Our results characterize multiple non-genetic factors that affect the phenotypic single cell-level sex allocation. Sex allocation in diatoms may provide a model system for testing evolutionary game theory in unicellular organisms.

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We investigated salinity-induced developmental plasticity and difference in the 'reaction norm' of clonal cultures (genotypeenvironment [G Â E] interaction) of Cyclotella meneghiniana, isolated from different locations, and their F 1 offspring. Genetic distances between strains and the phylogenetic position of each strain were investigated by partial 28S rDNA sequencing. In addition, we investigated morphological differences between cells obtained from different sites along a tidal river, from the lower reaches to the river mouth. In laboratory culture, all strains exhibited a greater number of central fultoportulae (CFP ¼ central strutted processes) in saline medium (salinity, 17 psu) than in freshwater medium; the number of striae on the valve and the diameter of the central area increased in all but three strains. A significant G Â E interaction was detected in the number of CFP, the number of striae, the diameter of the central area and the valve diameter. Pair-wise comparison showed significant G Â E interactions for these traits among strain pairs. In the field-collected material, all parameters (CFP number, stria number, central area diameter and valve diameter) were greatest at the estuary mouth.

In cytoplasmic inheritance, structural states of a parent cell could be transmitted to offspring cells via two mechanisms. The first is referred to as the hangover of parent structure, where the structure itself remains and faithfully transmits within offspring cells; the second is structural inheritance, wherein the parent structure functions as a template for development of new offspring structure. We estimated to what extent the parent structure affects the development of offspring structure by structural inheritance, using a clone of the diatom Cyclotella meneghiniana. The cell has two siliceous valves (a cell wall part at both cell poles): one is inherited from the parent and the other is newly formed. We estimated cytoplasmic heritability by comparing valve traits (central fultoportulae (CTFP), striae, central area, and cell diameter) of parent and new offspring valves, using single-cell isolation and valve labelling. Parent-offspring valve trait regressions showed that all traits, except CTFP, were significantly correlated. We formulated a quantitative genetic model considering the diatom inheritance system and revealed short-term rapid evolution compared with other inheritance systems. Diatom structural inheritance will have evolved to enable clonal populations to rapidly acquire and maintain suitable structures for temporal changes in environments and life-cycle stages.

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