Thursday, 24 July 2014

Evolution Arms Race - Moose Spit Detox

Ungulate saliva inhibits a grass–endophyte mutualism

A fascinating example of both evolutionary cooperation and an evolutionary arms race was published in Royal Society Biology Letters yesterday.

Abstract*

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"

Gary Felton, Penn State University, Pennsylvania, USA
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.

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.[1] 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.

Red fescue
The other aspect is the arms race between this mutually evolved defence mechanism which clearly benefits the grass by deterring herbivores from grazing it and both the moose and the reindeer which needs to eat a large volume of vegetation, especially the males which depend on a large size and large antlers to defend their females against other males and which feature in female sex selection, so needing a good supply of nutrients including calcium.

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

'via Blog this'

submit to reddit

1 comment :

  1. Yes, creationist have a lot to explain. And they try hard by proposing intricate and complicated explanations. Here is an example: http://creation.com/what-about-parasites .

    Some quotes:

    QUOTE: The first question, therefore, is “Why did God, if He is good and loving, create disease-causing parasites—not to mention all the other disease, violence, suffering and death which we see around us in nature?”

    The straightforward, biblical answer is that these evils did not exist in the original creation. They came into the world only after the Fall, when Adam and Eve disobeyed God (Genesis 3:14–24; Romans 8:18–25). They were part of the “Curse” (Revelation 22:3).

    MY COMMENT: That is a very far-fetched explanation. Probably you must be a true God believer to "buy"/accept that kind of reasoning.

    (...)

    QUOTE: The second question is this: If disease-causing parasites did not exist in the original creation, how did they come to be here now? We do not know the answer for certain, but we can reasonably speculate. I think it is unlikely that they were specially created after the Fall, as Genesis 2:1–3 says that God finished his work of creation “by the seventh day” (cf. Exodus 20:11).

    If that is correct, these parasites must have been benign and beneficial in their original form. Perhaps some were independent and free-living, and others had beneficial symbiotic relationships with animals or humans. But when Adam and Eve sinned, things began to go wrong. These once-harmless creatures degenerated, and became parasitic and harmful. The Fall and subsequent Curse were unique events which took place soon after the beginning of human history. So there is no compelling reason why God should not have caused these organisms to become pathogenic (disease-causing) miraculously.

    MY COMMENT: So parasitism was caused by the sinful behavior of Adam and Eve?

    QUOTE: Perhaps some became parasitic as a result of mutations. They degenerated, lost their ability to live independently or symbiotically, and became harmfully dependent on their hosts. Note that mutations overwhelmingly cause loss of genetic information.

    MY COMMENT: This is a misleading and deceptive way of reasoning. See for example http://www.newscientist.com/article/dn13673-evolution-myths-mutations-can-only-destroy-information.html#.U9H0pfmSxmM

    A quote from that article: Most biologists would see this as a gain in information: a change in environment (the availability of cow's milk as food) is reflected by a genetic mutation that lets people exploit that change (gaining the ability to digest milk as an adult). Creationists, however, dismiss this as a malfunction, as the loss of the ability to switch off the production of the milk-digesting enzyme after childhood.

    And check out this link: http://science.howstuffworks.com/life/evolution/evolution9.htm .

    A quote from that article: One line of research in this area focuses on transposons, or transposable elements, also referred to as "jumping genes." A transposon is a gene that is able to move or copy itself from one chromosome to another. The book "Molecular Biology of the Cell" puts it this way:

    Here's another quote from that HSW article: Another area of research involves polyploidy. Through the process of polyploidy, the total number of chromosomes can double, or a single chromosome can duplicate itself. This process is fairly common in plants, and explains why some plants can have as many as 100 chromosomes.

    See also Rosa Rubicondior's blog article http://rosarubicondior.blogspot.se/2013/09/how-early-cells-may-have-got-complex-dna.html . Use this blog's search function. For example search for "christmas tree", for "mutations" etc - and you'll find many more of Rosa's excellent articles on this intriguing topic.

    ReplyDelete

Obscene, threatening or obnoxious messages, preaching, abuse and spam will be removed, as will anything by known Internet trolls and stalkers or by known sock-puppet accounts.

A claim made without evidence can be dismissed without evidence. Remember: your opinion is not an established fact unless corroborated.

Related Posts Plugin for WordPress, Blogger...
Web Analytics