Being a lichen is all about give and take
Lichen symbionts sacrifice genes for the greater good. Andrew Masterson reports.
Among human couples it is generally accepted that the main sign that simply sharing a house has progressed into a full-on relationship is when individual collections of kitchenware and books are melded into one, with duplicate items discarded.
A similar process, it turns out, occurs with lichens, although in this case – given that lichens have little use for ginger graters or the novels of Zadie Smith – the duplicate items discarded are genes.
As every high school student knows, lichens are the product of a symbiotic pairing between one species of algae and another of fungi. It is a particularly successful form of mutually dependent relationship. There are so far more than 20,000 recorded types of lichen, collectively covering an estimated 6% of the global land mass.
The evolutionary advantages of the lichen partnership are obvious: in combination, the contributing species are able to colonise environments that neither could if they were operating solo.
The symbiotic bond between the fungus and the algae that constitutes each variety of lichen has long been assumed by biologists to be what is known as an “obligate” partnership – in which at least one of the parties cannot survive without the other. Until now, however, the exact details of the contract entered into (as it were) have remained unknown.
It turns out that in some cases, at least, it is the fungal partner that makes the sacrifice and gets rid of the equivalent of the second rice cooker – a gene from its mitochondrial baggage that is concerned with energy production.
The finding was made by a team led by Cloe Pogoda from Colorado University Boulder, in the US.
Pogoda and colleagues collected 22 different lichen types from the Appalachian Mountains area in the US, and sequenced their mitochondrial DNA. The results were then compared to the mitochondrial genomes of 167 fungus species that do not enter symbiotic partnerships.
The results showed that 10 of the 22 lichenised fungal species lacked a key gene called atp9. Furthermore, the analysis showed that the gene had been lost on more than one occasion.
“The fungus lost a crucial energy-producing gene while the algae retained a full-length copy of this gene," says Pogoda.
“We observed a parallel loss of this gene in three different lichen lineages: the fungus gives up this particular gene while its photosynthetic partner keeps it.”
The loss of apt9 in the fungus partners eliminates the redundancy of having two genes in a lichen doing the same thing. However, it also means that in a very real sense the fungus partner is dependent on the algal partner for its energy supply.
The researchers suggest that this streamlining may give the lichen an ecological advantage by making the genetic division of labour in the partnership more efficient.
“The implications are far-reaching, given how many symbiotic relationships we observe in nature,” says co-author Erin Tripp.
“Now we can expand our scope of study to look for genomic signatures of co-evolution in other organisms.”