Linking variation in genome size to cyst size in chrysophytes

Area of Science:

• Eukaryotic genomics
• Algal biology
• Evolutionary biology

Background:

• Eukaryotic genome size varies greatly, often exceeding biological complexity.
• Selection-based hypotheses suggest genome size evolves via life-history traits, independent of gene content.
• Chrysophyte algae (Chrysophyceae) with siliceous stomatocysts offer a model to study genome size-phenotype correlations.

Purpose of the Study:

• To investigate the relationship between genome size and resting cyst size in chrysophyte algae.
• To determine if stomatocyst size can serve as a proxy for genome size in this group.
• To explore the potential of using fossilized stomatocysts to infer ancestral genome sizes.

Main Methods:

• Collected and identified 85 chrysophyte strains (31 Mallomonas species) using nuclear ITS rDNA barcoding.
• Estimated genome sizes via propidium iodide flow cytometry.
• Compiled a dataset of 54 species, combining new measurements with published data on genome size and cyst volume.

Main Results:

• Observed over a 75-fold variation in genome size within Mallomonas (0.15-11.25 pg), with significant intraspecific variation including polyploidization.
• Found a strong positive correlation between genome size and stomatocyst volume in chrysophytes (R^2 adj = 0.626, p < 0.001).
• This genome size-cyst size relationship is stronger than the typical genome size-cell size correlation in eukaryotes.

Conclusions:

• Stomatocyst size is a robust predictor of genome size in chrysophytes.
• This finding supports selection-based hypotheses for genome size evolution.
• Stomatocyst size offers a valuable tool for estimating ancient genome sizes from paleontological records.