A fascinating example of both evolutionary cooperation and an evolutionary arms race was published in Royal Society Biology Letters yesterday.
Fungal endophytes modify plant–herbivore interactions by producing toxic alkaloids that deter herbivory. However, studies have neglected the direct effects herbivores may have on endophytes. Antifungal properties and signalling effectors in herbivore saliva suggest that evolutionary pressures may select for animals that mitigate the effects of endophyte-produced alkaloids. Here, we tested whether saliva of moose (Alces alces) and European reindeer (Rangifer tarandus) reduced hyphal elongation and production of ergot alkaloids by the foliar endophyte Epichloë festucae associated with the globally distributed red fescue Festuca rubra. Both moose and reindeer saliva reduced the growth of isolated endophyte hyphae when compared with a treatment of distilled water. Induction of the highly toxic alkaloid ergovaline was also inhibited in plants from the core of F. rubra's distribution when treated with moose saliva following simulated grazing. In genotypes from the southern limit of the species' distribution, ergovaline was constitutively expressed, as predicted where growth is environmentally limited. Our results now present the first evidence, to our knowledge, that ungulate saliva can combat plant defences produced by a grass–endophyte mutualism.
Andrew J. Tanentzap, Mark Vicari, and Dawn R. Bazely; Ungulate saliva inhibits a grass–endophyte mutualism
Biol. Lett. July, 2014 10 7 20140460; doi:10.1098/rsbl.2014.0460 1744-957X
The team from the Department of Plant Sciences, University of Cambridge, UK and the Department of Biology, York University, Toronto, Canada were specifically looking for evidence that Moose and Reindeer could counteract the defence mechanism which some grasses have evolved in cooperation with a fungus. The fungus is known to produce the highly toxic alkaloid ergovaline which can cause, amongst other things, hooved animals to lose their hooves.
If these animals continually graze the same plants, then it would be quite beneficial. We are still uncovering novel roles for saliva"They found evidence that moose saliva not only significantly reduced the concentration of ergovaline by between 41% and 70% but it also inhibited a signalling system whereby grazing the grass stimulates growth of the fungus and so the production of the toxin. This is one of the few example discovered so far of a vertebrate being able to do what some insects are known to do - chemically influence the metabolism of a plant. It has also been shown that sheep saliva can stimulate growth in at least one grass, Leymus chinensis.
Gary Felton, Penn State University, Pennsylvania, USA
The fascinating things here are the example of mutualism, whereby a fungus has formed a mutually beneficial association with a grass to defend the grass against being eaten, and the example of an arms race where the species this defence evolved against has evolved a counter-measure.
First the cooperative, or symbiotic alliance:
Epichloë species and their close relatives, the Neotyphodium species, are systemic and constitutive symbionts of cool-season grasses (Poaceae subfamily Pooideae), and belong to the fungal family Clavicipitaceae. Among the Clavicipitaceae, many species are specialized to form and maintain systemic, constitutive (long-term) symbioses with plants, often with limited or no disease incurred on the host. The best-studied of these symbionts are associated with the grasses and sedges, in which they infect the leaves and other aerial tissues by growing between the plant cells (endophytic growth) or on the surface above or beneath the cuticle (epiphytic growth). An individual infected plant will generally bear only a single genetic individual clavicipitaceous symbiont, so the plant-fungus system constitutes a genetic unit called a symbiotum (pl. symbiota).
The evolution of symbiosis probably goes through a stage of parasitism. In this case a fungal infection of grass. However, if something produced by the parasite benefits the host more than the parasite harms it, the host genes which help ensure the parasite is not attacked will do better than those which attack it, so the host will evolve tolerance rather than resistance. Meanwhile, the parasite genes which increase the benefit and/or reduce the harm will also prosper at the expense of those which do the opposite. Hence, the parasite will also tend to evolve towards cooperation with the host and parasitism will move towards symbiosis.
This is all readily understandable in terms of what benefits the genes of both parties. Cooperation serves the 'selfish' needs of both sets of genes.
As the authors of this paper point out, the endophytic fungus probably originated some 30-40 million years ago and accompanied the grass as they became the dominant plant type about 10 million years later. This was followed by the evolution of large herbivores which diversified into the new niches provided by grasslands. This in turn would have provided a selective pressure on the fungus to maximise the persistence of their hosts.
Meanwhile, some larger herbivores would have been diversifying yet again to occupy niches found in forests, so leaving the grass-eating herbivores with no alternatives and high selection pressure to evolve counter-measures. The classic conditions for an evolutionary arms race and again fully understandable in terms of 'selfish' genes and a natural selection process which ensures those best able to survive and reproduce are the ones who leave the most descendants.
What is not at all understandable is how either this evolution of mutualism or the evolutionary arms race can be explained in terms of the work of an intelligent designer. Why would an intelligent designer design parasites in the first place, so creating the initial conditions for mutualism to evolve in which the parasite needs to defend its host because if its host get eaten, it gets eaten too? If an intelligent designer wanted to protect grass from being eaten by herbivores, why did it also create herbivores, and it it didn't want the herbivores to eat grass, why did it give them the mechanism for doing so?
Similar arguments apply to the notion of intelligent design and arms races. Ultimately, arms races are about two mutually hostile systems coexisting and needing to run to stand still because each is creating a hostile environment for the other and so selection pressure to overcome it. Arms races are ultimately to neither side's benefit and yet are entirely predictable. As a piece of design, arms races such as that between the grass-fungus complex and the moose, are manifestly stupid, lacking both foresight and ultimate purpose.
Creationist proponents of the notion of intelligent design need to explain parasitism, mutualism and arms races. Evolutionary biologists, on the other hand, have a perfectly rational, easily understood and well-evidenced explanation for them. They are all entirely predicted from the basic principles of evolution by natural selection.
*Copyright © 2014, The Royal Society
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