A recent study led by Dr. Vesna Grujcic has shed light on the intricate relationship between Richelia, a genus of cyanobacteria, and its diatom hosts. This study, just out in the journal Current Biology, looks at how Richelia provides the essential nutrient nitrogen to diatoms. It also sheds new light on one of the evolutionary mechanisms that crafts these symbiotic partnerships.
Under light-field conditions, Richelia forms two types of symbiotic relationships with diatoms, one external and one internal respectively. It can either reside in the periplasmic space between a diatom’s outer cell wall and inner cell membrane, or it can flourish entirely intracellularly within the host. It is precisely these interactions that are critical to revealing how symbiosis evolved, and what genetic adaptations evolve to support this lifestyle.
Dr. Grujcic conducted a comprehensive pangenome analysis of Richelia, identifying its core genome—the set of genes shared by all Richelia species—as well as the accessory genes that differ between them. The combined analysis indicated that Richelia’s genome has undergone extensive genome reduction. It has experienced pseudogenization, the process by which genes accrue mutations and so lose their functional gene-ness.
In addition to exploring Richelia’s unique genome, the team studied the size and distribution of intergenic spacers—non-coding sections of DNA between genes. The results indicated that insertion sequences and transposons have inflated Richelia’s genome size, complicating previous assumptions about its genetic makeup.
Richelia’s deep integration with their hosts have important consequences on the genome size and gene content. It affects the percentage of coding regions—the parts of DNA that have the instructions for making proteins,” noted Mehrshad. Racolta also noted that research into non-coding DNA, like intergenic spacers and pseudogenes, serves as a critical key for understanding Richelia’s evolutionary path.
As explained by uncbiot Professor Rachel Foster, “As symbionts become more dependent on their hosts, they become more and more embedded within the host.” For instance, they can live hiding inside the host cell. In the process, they start shedding redundant genomic material which overlaps with their hosts. This puzzling phenomenon leads to important questions about the role of symbiotic relationships on genetic diversity and evolution.
Daniel Lundin highlighted the significance of this research: “What excites me most with this research is that different steps on the way to a fully integrated symbiont exist at the same time. This allowed us to study the genetics behind how evolution towards a lifestyle characterized by complete dependence of the symbiont on its host happened.”
Richelia euintracellularis, a particularly well-studied species in this genus, beautifully illustrates these dynamics through its symbiosis with the diatom Hemiaulus hauckii. Our research team is studying this multi-sector partnership in detail. They want to learn fundamental things that might improve the use of synthetic biology to create nitrogen-fixing crops.
In explaining these findings, Vesna Grujcic put it quite succinctly, “As Richelia become increasingly reliant on their hosts, the gene repertoire they maintain shifts dramatically, allowing us to track which genes are lost and which persist—a rare opportunity to witness the evolution of such partnerships one step at a time.”