On Rice, Mosquito Ferns, and Cyanobacteria...

This blog has been veering towards the theme of food in recent posts. Perhaps the most important staple food in Asia is rice, traditionally grown in irrigated paddy fields. A piece of natural history trivia that has stuck with me from my childhood fascination with ferns is how paddy farmers encourage the growth of the mosquito fern Azolla, because it has nitrogen-fixing cyanobacteria that live in symbiosis with it, thereby contributing to the nitrogen content of the soil.



Trivia by definition is shallow knowledge. The genome of the cyanobacterium symbiont of Azolla has recently been sequenced, and the paper describing it has taught me more fascinating facts about these unassuming organisms (Azolla is one of the smallest ferns around).

First off: the cyanobacterium is an extracellular symbiont but is vertically transmitted. That means it's passed down from one host generation directly to its offspring, and is not acquired from the environment. Vertical transmission is usually an indication of a close history of association; it's not just any cyanobacterium that sets up shop with Azolla. Cyanobacteria in general often form thick-walled spores, called akinetes, as a mechanism for weathering stress or promoting dispersal. In this species, motile cells of the cyanobacterium migrate through channels in the megasporangium of Azolla and differentiate into akinetes on top of the megaspores, and accompany them to their eventual germination.

Next: unlike most extracellular symbionts, but like many vertically transmitted symbionts, the genome of the cyano is undergoing a process of 'erosion'. Pseudogenes, which are genes which are no longer functional but still recognizably related to known genes, are present in a proportion higher than free-living species. Transposable elements are also present in higher number. In obligate symbionts that have close physiological relationships with their hosts, loss of genes associated with free-living lifestyle is expected.

On the basis of these findings, the authors conclude that the cyanobacterial symbiont of Azolla is undergoing a coevolutionary specialization with its host. And, because of its use in paddy fields, we are very much beneficiaries of that symbiosis too, as much as Azolla itself is. Now that's food for thought.