A New Partner Found in Lichens

Lichens look like plants, usually plastered flat against substrates or stringing off into the air as spanish moss does. Lichens are something more than a plant, they are a superorganism, sensu latu. A superorganism is a unique system where individuals’ behaviors and processes are so coordinated that the system takes on the qualities of an individual itself. The classic example is a eusocial ant colony, where individuals perform distinct functions, insufficient to support a single ant, but in concert perform all functions necessary to sustain life. Such is the case with lichens.

Biologists heretofore saw lichens as two species symbiotic systems. Symbiotic species interactions occur when two or more species have complementary needs and processes, that, when combined, help both to live and reproduce. In lichens, those species were thought to be a fungus and a photosynthesizing algae and/or bacteria. The fungus, the structure visible to the naked eye provides a shelter to the photosynthesizer while the photosynthesizing organism provides food in the form of sugar to the fungus–something to eat and somewhere to live.

Photo of Bryoria lichen.
B. tortousa lichen. The yellow coloration indicates high concentration vulpinic acid, a toxin. Research on Bryoria lichens by Spribille, et al. revealed a yeast partner in the lichen system. (Photo credit: Millifolium, CC-BY-SA 3.0)

Work by Toby Spribille, et al, published in Science last July, identified a third component to the lichen system: yeast (yeasts are also of the fungi kingdom). The investigators selected the lichens B. fremontii and B. tortuosa for analysis, noting that the latter produces vulpinic acid and appears yellow, rather than brown in the case of the former. Examining messenger RNA from the two species did not reveal differences to explain the distinct phenotypes. Further, the lack of difference in level of gene expression reinforced earlier suspicions that the two species are indeed one and the same, however when examining relative gene expression across all fungi, certain basidiomycota gene transcripts were found to be present in both lichen phenotypes, with higher expression in the vulpinic acid phenotype (B. tortuosa). The implication of this finding being that the the Bryoria lichens have another endosymbiont, and that it might be driving the phenotypic difference.

Spribille and colleagues then looked for and observed the presence of ribosomal RNA markers in other species of lichens collected from Montana, adjacent to where the Bryoria had been collected. Moreover, they found unique sequences of basidiomycetes associated each species of lichen, raising the possibility of intimate coevolution between the classical fungus-algae/bacteria and the yeast partners. Adding yet another layer of interest to the research, the identified basidiomycetes sequence, Cyphobasidium, was a sister to Cystobasidium minutum, the only known member of Basidiomycetes (its region of the fungi kingdom) associating with lichens, and pathologically at that. The fossil record and molecular clock put the split in lineages at 200 million years ago (for comparisons, the human and chimpanzee lineages share a most recent common ancestor from about 5 million years ago). How the most recent common ancestor to Cyphobasidium (identified within lichens) and Cystobasidium (identified living in growths on lichens) made its living, and how the diverging lineages got to make their livelihoods so differently, is a mystery.

Neither able to see microscopically nor culture the basidiomycetes, the researchers used FISH (fluorescent in situ hybridization) to attach markers to the basidiomycetes which could be visualized, thereby confirming its presence in both B. fremontii and B. tortuosa, enhanced presence in B. tortuosa (consistent with gene expression results), and allowing researchers to see localization to the lichen cortex. That these yeasts localize to the lichen cortex and that in vitro lichen models have used only classically identified fungi and algal/bacterial constituents without these yeasts, may be the reason that such cultured lichens have underdeveloped cortexes.

The virtually universal presence of the yeast Cyphobasidiales in Bryoria and that unique species of yeasts are found in different species of lichens radically changes our understanding of a long known symbiotic system. It reminds us that ecosystems are complex, and the genes necessary to support life are varied. In the symbiotic partnership within lichens, we see the presence of vulpinic acid associated with Cyphopasidiales, rendering B. tortuosa toxic. Whether it is produced by the yeast, its production controlled by the yeast, or related in some other way, the third partner to the lichen seems to exchange defense for shelter. Whereas lichens have a morphology distinct from that of their symbiotic partners, they are able to support the partners in environments where they could not survive individually. The discoveries by Spribille, et al, will certainly reshape our thinking about the simplicity, or better put complexity, of symbiotic relationships.

© Peter Roehrich, 2016

Crows Use Tools

Crows are often vilified, seen in popular as bad omens, sometimes of death. (To be fair to the crow, this is not universal nor has it always been so.) They do not get the credit they deserve for their intelligence, especially tool use. I’m astounded by crows ingenuity when it comes to fashioning and using tools.

Photo of crow on branch
Crow perched on branch. (Photo credit: werner22brigitte)

Until recently we believed tool use to be uniquely human; flattery, really. While we thought it differentiated us from other animals, recent discoveries tell us we are not unique. 

A growing body of literature documents New Caledonian crows’ tool use. Examples include fashioning a hook from a piece of wire to retrieve a container of food from a confined space (this was in an experimental context). Nature published an account of tool use by the Hawaiian Alala crow this week.
Interestingly, this study found that the Alala crows made tools as juveniles without modelling from adults. This raises the question: are the crows instinctually ‘programmed’ to use tools, or are they capable of complex problem solving? This remains to be uncovered. The end result, however, remains the same in that crows enhance their reproductive fitness (more about reproductive fitness later, for now know that it means the ability to live long enough to successfully raise offspring) by utilizing objects found in their environment.

This study also notes that the New Caledonian and Alala crows live in similar island habitats but are distant relatives. If the intermediate relatives don’t use tools; if it is only found on these tips of this particular branch on the shrub of life, it suggests that the trait emerged twice to reach a similar outcome. The authors pose the explanation that the similarities in habitat allowed the tool use trait to develop in both species.

© Peter Roehrich, 2016

Evolution Over the Next 2-3 Billion Years

Evolution has been at work since life first arose in small, warm puddles some 4 billion years ago. Evolution is the change in allele (variation of a gene) frequency in a population over time. It is the outcome of four forces (selection, genetic drift, gene flow, and mutation), the definitions of which aren’t the topic of this post, although I suspect many of you are familiar with selection. An interesting article by Jodi Brewster, Thomas Finn, Miguel Ramirez, and Wayne Patrick (I’ll refer to the authors collectively as Brewster et al going forward) raises possibilities for evolution in the future. Where we often think of evolution as bringing us to our current state, Brewster et al notes that our planet has as much as 3+ billion years before the sun expands to eradicate all life; evolution still has time to work.

It’s easy to think that evolution renders an organism to its most fit of all possible states. After all, we say colloquially that a species has evolved to its environment as a result of selection. It is true that selection will favor more fit phenotypes, often meaning more fit genotypes will become more common, but this should not be taken to mean that selection will ‘optimize’ a population. Selection, indeed all evolutionary mechanisms, only work on the genes available. Sometimes genes that would be very beneficial have never been ‘proposed’ by mutation, or getting to the ‘optional’ allele would force the population through an impassible fitness valley (more on these phenomena in future posts). 

Photo of plant cells with chloroplasts visible.
Micrograph of plant cells. Chloroplasts, where plant cells carryout photosynthesis, are visible as small green orbs. Note the cells’ rectangular shape. (Photo credit: Vierschilling)

As Brewster et al points out, many biochemical processes, those processes within a cell, have room for improvement, and synthetic biology demonstrated potential evolutionary innovations. Among those they report on, a particularly interesting case is that of carbon dioxide fixation, a fascinating process that all life depends on, directly or indirectly, but is notoriously inefficient. Biological engineering has demonstrated that carbon fixing structures within some bacteria could be transferred into hardy plants. The result: an organism that has now resistance to environmental factors but the carbon fixing efficiency of the bacteria. Could nature bring this efficiency to green plants in the future billions of years? Time will tell.

As mentioned above, stay tuned for more discussions of evolutionary mechanisms and trends.

© Peter Roehrich, 2016