16 September 2014

A tangled story: the coevolution of lichen


I'm a sucker for multi-volume books. Whether it is GRR Martin's Game of Thrones, Robert Jordan's Wheel of Time, Tad William's Shadowmarch, Dennis McKiernan's Mithgar or JRR Tolkien's Middle-earth, I like nothing more than getting lost in the depth and sheer volume of story that these authors have created (all that history, setting, scale, and characters, all those interactions). Often these books are in the fantasy line but they can just as easily be science fiction, say David Brin's Uplift, or historical fiction, like Patrick O'Brian's brilliant Master and Commander. Friends often ask me to summarise these series, "What is the story about?". That's a tricky question because their scope defies a meaningful simplification. Most people are familiar with A Game of Thrones now. A simple description of this story (even more complex in the books than the TV series) is either too broad to be helpful (a fantasy version of the War of the Roses) or misses a lot (a bastard, who know nothing, talks to wolves and becomes a leader; a spoiled princess domesticates dragons and sits around in conquered cities; or a dwarf with father issues loses a nose and has a fairly miserable time).  The Wheel of Time series stretches to 13 large books with umpteen viewpoint characters and could be summarised as 'shepherd boy heals the world' but this only takes up a tenth of the story amid all of the braid tugging, gender politics and nonrandomness of events. O'Brian's series about the British navy in the Napoleonic wars stretches to 21 books and follows the career of Aubrey (as captain of larger and larger ships) and Maturin (ship's doctor, naturalist and spy) with little overarching story other than that of following the lives and careers of these fascinating characters. Even The Lord of the Rings, with a reasonably linear story, loses something when condenced to 'short tourist loses heirloom at volcanic destination'. Sometimes, then, stories are complicated and refuse to be simplified. Much the same happens in history and most definitely it happens in natural history, especially when we are dealing with evolution.

The lichen Usnea draps a beech tree
Evolutionary biologists are interested in the history of life, we want to recover the story of how life on Earth ended up as we see it. We use data from fossils and DNA, species distributions and unusual traits to recreate our best guess at summarising the evolutionary story. One area of evolutionary biology that I have been involved with since my PhD is that of coevolution - where we try to recover the story of interacting evolutionary lineages, such as parasites and hosts or pollinators and plants. Sometimes the story is fairly straightforward. The work that I did for my PhD and in various projects since has been with chewing louse species that live on seabird species (like petrels and albatrosses). Lice are insects that live in the feather forests of their hosts and are mostly passed from seabird parents to their offspring. The story of this interaction is reasonably simple, we have two lineages, one insect and the other bird, that codiverge together. When the birds speciate their lice will also become isolated and often cospeciate at the same time. Feather lice have been studied as a nice model of cospeciation for a couple of decades now. We can tell this story well. Sure there are a few quirks here and there but it is now a reasonably predictable story. Louse-host coevolution stories are like nature's little rom-coms. Not all coevolution stories are this simple.
Lichen and mistletoe colonise a beech.

Lichens, commonly seen growing on fences or trees, are composite individuals,  a symbiosis that results when a fungus cohabits with a green algae (or sometimes a cyanobacterium, or, even less commonly, with both an algae and a cyanobacterium). The fungus provides a safe habitat and food for the green algae who, in turn, use their photosynthesis ability to provide products for the fungus. Lichens are the result of this win-win coevolution relationship and would seem a prime candidate for coevolution. Hannah Buckley and her PhD student Arash Rafat joined forces with other Lincoln University ecology staff to recover the coevolution story of lichens which has now been published in the journal PeerJ. Arash collected lichens from around New Zealand. Much of the sampling was from southern beech forests and focused heavily on the lichens Usnea and Ramalina. Genes were analysed from the fungi and algae that made up the lichens. It turns out that when it comes to coevolution in lichens, it's complicated.

Lichen taxonomy is woefully understudied. There were many instances of samples that looked like the same species being genetically very different species, there were many instances of samples that looked like there were from different species being genetically similar. Certain strains of algae were found with lots of different fungal species. Geographical scale, how far sites were from one another, played a role - sites close to one another shared similar diversity. Sometimes. It seems that for lichens to function there only needed to be a fungus and alga paired and it didn't really matter which ones. All of these complications meant that the 'standard' method of analysing coevolution, by comparing phylogenetic trees for fungus and algae, was not going to be very useful, the trees were just too tangled. Hannah and Arash used a distance (principle coordinates) approach to analyse the data rather than a tree-building approach. Surprisingly, give all of the uncertainty of the lichen component species, there was  significant congruence between the genetic variation of the fungal and algal partners. The was particularly true at small spatial scales where fungi and algae species seem to co-disperse and co-adapt with each other. These patterns started to break down at larger scales and were different between lichen genera.
Lichen diversity.

So what is the story with lichens? Well, like a multi-volumed series, it's complicated. Focusing on some of the species provides a story of cospeciation. However, other species have no evidence of codivergence. Sometimes geographical scale is important, sometimes it is not. We can say that there is probably more codivergence occurring than we might have predicted. Overall, though, there is no simple summary, the evolutionary story of lichens are not very predicable. To know more we need to research more. Lichens are not rom-coms, they are art-house. Or a multi-volume series of books.