Linking variation in genome size to cyst size in chrysophytes

Genome size varies tremendously across eukaryotes, which often contain far more DNA than expected from their biological complexity. To explain this paradox, selection-based hypotheses propose that genome size evolves through selection acting on life-history traits correlated with the phenotypic effects, independent of its genic content. To test the association between genome size and resting cyst size-a structure with morphology that may reflect overall body size-we selected chrysophyte algae (Chrysophyceae) producing siliceous stomatocysts as a model. In this study, we obtained and identified 85 chrysophyte strains representing 31 Mallomonas species using nuclear ITS rDNA region barcoding and estimated their genome sizes using propidium iodide flow cytometry. Within this genus, we observed more than a 75-fold variation in genome size (0.15-11.25 pg) and frequent substantial intraspecific variation, in some cases consistent with whole-genome doubling (polyploidization). By summarizing the published genome size records, adding our new measurements and combining them with published cyst sizes, we assembled a data set for 54 chrysophyte species to examine the relationship between genome size and cyst volume. Linear regression on log-transformed data revealed a strong positive correlation ( $R_{\mathrm{adj}}^2$  = 0.626, p < 0.001), showing that species with larger genomes tend to produce larger stomatocysts. In chrysophytes, this relationship is considerably stronger than the widely documented genome size-cell size correlation observed across eukaryotes. Our findings suggest that stomatocyst size is a more informative predictor of genome size in this group and highlight the potential for using stomatocyst sizes to infer ancestral genome sizes from the fossil record.