Coevolution in Mutual Symbionts

Some of the most important evolutionary adaptations to result from coevolution are the mutualistic symbiotic relationships that allow two or more species to thrive through mutually beneficial interactions. An example occurring in one of the most ecologically, economically, and aesthetically important ecosystems on Earth is the coevolution of hermatypic (reef-building) corals and their algal symbionts of the genus Symbiodinium. The algae are incorporated into the coral cells where they produce carbohydrates through photosynthesis, passing this on to the animal tissue and receiving nitrogen and phosphate compounds⁽¹⁾.

For many years it was thought that individual coral species were very specific as to which clade or type of symbiont is hosted*. Recently, with the aid of larger sample sizes more sensitive molecular techniques, it has been shown that corals often host secondary types of symbionts and are able to change which clade is dominant within its cells, especially after periods of stress or coral bleaching⁽²⁾. In "The role of zooxanthellae in the thermal tolerance of corals: a 'nugget of hope' for coral reefs in an era of climate change (2006)," Ray Berkelmans and Madeleine van Oppen tested the hypothesis that this sort of adaptation could help corals survive in a future sure to me marred by global warming.

Acropora millepora(3)

In this study, Berkelmans and van Oppen transplanted colonies of the common, bleaching sensitive coral species Acropora millepora from a cool offshore reef to much warmer inshore sites on the Australian Great Barrier Reef. The result was that many colonies pale, bleached, and subsequently died from the unfamiliar environment. However, a few colonies were able to bounce back from paling and bleaching when the abundance of clade D symbionts, a group much more thermally resistant than the previously dominant clade C, began to increase. Together, A. millepora and clade D Symbiodinium were able to adapt to the changing circumstances and begin to reclaim the dead carbonate skeleton of the bleached part of colonies. This provides some hope that the beautiful coral reefs of the world will be able to survive when our world inevitably continues to increase in temperature.

Results of Berkelmans and van Oppen's 2006 study.

*Type is a transient term, used because some types of Symbiodinium have such genetic similarity they may at some point be proven to be the same species. Clade is an intermediate classification of the Symbiodinium genus into groups A-H. For example one type of algae is B3.

Sources:

1. Pearse, Vicki & Muscatine, Leonard. Role of Symbiotic Algae (Zooxanthellae) in Coral Calcification, 1971. http://www.biolbull.org/content/141/2/350.full.pdf+html

2. Berkelmans, Ray & van Oppen, Madeleine. The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change, (2006). http://rspb.royalsocietypublishing.org/content/273/1599/2305.full

3. http://www.isegretidelmare.it/images/acropora_millepora.jpg

Coevolution may not always explain plant pollination

The mutualistic adaptations that occur in plants and their pollinators has typically been considered to be a coevolutionary development. However, a recent study revealed that in at least one plant and pollinator case there has been adaptation to a niche without a coevolutionary development taking place.

Dr. Florian Schiestl from the University of Zurich found that within the arum family there has been a one way evolutionary mimicking of scents that attract pollinators. Specifically in this case, arum plants secrete scent chemicals that attract scarab beetles to pollinate them. In many such plant-insect interactions the ordinary hypothesis is that these interactions develop through coevolution due to the mutualism involved. The concept is fairly straightforward: the insect derives benefit from the plant via nutrients or other resources, and the plant benefits from the service of protection or spreading the species' seeds or pollen. This interaction benefits both species and they adapt to each other to maximize benefit gained, resulting in coevolutionary development.

Member of the arum family (Araceae). Image Source: Wikipedia

Dr. Schiestl found that within the scarab beetle known to pollinate arum plants there were many of the chemical signals found in the plants. Finding that they were the same scent compounds used to attract the scarabs, he and his colleagues used phylogenetic reconstruction and searched for the evolutionary origin of these compounds. It was discovered that the ancestors to the modern scarab beetles had been using these same scent chemicals since the Jurassic period, and also that these ancestral beetles did not pollinate the arum plants. From this, he concluded that the arum plants themselves simply mimicked the scarab scents and evolved to fit this niche by taking advantage of the scarabs, who have not adapted to the plants at all.

Rainbow Scarab Beetle. Image Source: http://www.projectnoah.org/spottings/7935873/fullscreen

These findings are remarkable and contradict the current prevailing belief that coevolution is the major factor that induces and shapes mutualistic relationships. Dr. Schiestl noted that coevolution "might well be less common than we thought," and this discovery warrants further research into the evolutionary development of mutualisms.

Primary Literature Source: Florian P. Schiestl, and Stefan Dötterl. The Evolution of Floral Scent and Olfactory Preferences in Pollinators: Coevolution or Pre-Existing Bias? Evolution. International Journal of Organic Evolution. March 12, 2012. doi: 10.1111/j.1558-5646.20

Source link: http://www.mediadesk.uzh.ch/articles/2012/pflanzen-imitierten-duft-von-bestaeubenden-kaefern_en.html
 

Word Count: 330

The Red Queen Hypothesis: It's OK to be a man.

Do we still need men on this planet? A recent article on the New York Times pointed out the limited role of men when it comes to biological reproduction "...since women are both necessary and sufficient for reproduction, and men are neither. From the production of the first cell (egg) to the development of the fetus and the birth and breast-feeding of the child, fathers can be absent." When we consider how safe our world has become since the ages of cavemen, it does make sense to think that the world can live without men as long as there is a sufficient stock of sperm. This wild hypothetical situation leads to one question: why did we evolve to reproduce sexually in the first place?

Sexual reproduction carries some advantages and disadvantages. Dividing a population of organism into males and females creates a fitness disadvantage because males cannot have their own offspring  T. Morran et al. state "Every male takes the place of an offspring-bearing progeny (female or hermaphrodite) that could have been produced. The systematic loss of offspring-bearing progeny can reduce the numerical contribution of a lineage by as much 50%." However, out-crossing (reproduction between different individuals) encouraged by sexual reproduction has an advantage of genetic variability. This genetic exchange between individuals creates a possibility for evolution of new traits, which may be vital for survival and adaptability in a novel environment.

Let's go back to the example of the world with no men. It is not hard to notice how "un-novel" our environment is. Modern medicines and vaccines keep us from getting diseases from most parasites and pathogens. The word "survival" seems very distant from our lives (unless you think making money is directly related to it). Moreover, sexual reproduction in humans seems even deleterious; mistakes in the genetic exchange mechanisms lead to children born with genetic diseases such as Autism and Down's Syndrome. At this point, no man has ever felt this guilty about his existence.

< Above: The 1998 study by Dybdahl and Lively on New Zealand Snails and its parasite trematodes revealed that the parasite will infect the snails with highest genetic frequency in a time-lagged manner.>

The Red Queen Hypothesis may be the answer to our men's despair. It underlines that the "evolutionary arms race" between hosts and their pathogens may be an endless chase. The presence of a pathogen puts a selective pressure on its hosts; the selected hosts that are resistance to the pathogen survive and reproduce. We might then think it is the host that is an eventual winner, but what we don't usually see is the fact that this evolution of hosts also puts the selective pressure on pathogens. As the resistant host increases in population and becomes the most common host, the pathogen counter-evolves for that host, which then has to evolve again to be resistant to the new strain of the pathogen. In the end of the day, the pathogen is endlessly chasing the host. Since there must be a time gap between each selection process, neither one "gets" the other.

How does sexual reproduction link to the Red Queen Hypothesis? 2011 study by T. Morran et al. compared the relationship between the nematode C. elegans and its bacterial parasite S. marcescens in three types of mating conditions: strictly selfing (asexual reproduction), strictly out-crossing (sexual reproduction), and wildtype (control). Each conditional population was exposed to three types of S. marcescens: ancestral strain (no new strains), non-coevolving strain (new strains without selection) and coevolving strain (new strains with selection). Each population was allowed to reproduce up to 30 generations.

The results on the left highlights that all of the selfing population under coevolving parasite (graph A: c and f) were eliminated within 10 generations (hence the comparison is between ancestral and 10th ). Host mortality for this population also doubled after 10 generations. To the contrast, when the population was allowed to have sexual reproduction (graph C: o and r) the host mortality went down and the population sustained itself after 30 generations. Therefore, we can conclude that sexual reproduction is vital for survival from the continuously evolving pathogens.



As safe as our world today seems like, we are actually still under the threat of pathogen evolution. Development of groundbreaking medicines and antibiotics have also led to the discovery of superbacteria, one that is resistant to the most antibiotics of today. While it may not be possible to fight the bacteria off with reproduction alone, men will still represent the reason why we were able to survive and thrive for the long human history.


762 Words
Bibliography

Dybdahl, Mark F., and Curtis M. Lively. "Host-Parasite Coevolution: Evidence for Rare Advantage and Time-Lagged Selection in a Natural Population." Evolution 52.4 (1998): 1057-066. 

Hampikian, Greg. "Men, Who Needs Them?" The New York Times. The New York Times, 25 Aug. 2012. Web. 20 Feb. 2013.

Morran, L. T., O. G. Schmidt, I. A. Gelarden, R. C. Parrish, and C. M. Lively. "Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex." Science 333.6039 (2011): 216-18.

Image: https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhErfZW45wbMdngzhAmQLUy5J9e8kHiW6ppui2GtAVLXxStw55A4lXIPGQ-mZjRI6wZyCELcPozrHPMeM6HjNQThKqk0zVI2Yi0dfI-eEt7kxnz0ZxWPPjRumPjz8Le0SJ_Ux8NcPmxhkk/s1600/Red-Queen-733517.jpg

Arms Race Chronicles: The Ant Edition

Among the many ant species living in Eurasia are Myrmonexus ravouxi. Scientists call this species dulotic, or “slave-making”, because Myrmonexus kidnap broods of other ant species, bring them back to their own nests and raise the hostages to feed, groom and defend the Myrmonexus brood. Temnothorax longispinosus is the species most often targeted by the slave-making Myrmonexus.

As expected, Temnothorax are under strong selective pressure to evolve parasite-resistant adaptations, just as Myrmonexus are under pressure to overcome such resistance. These mutually competitive host-parasite relationships, which sometimes escalate to so-called “evolutionary arms races”, are an important aspect of coevolution. 

In order to effectively resist parasitic intruders in their colonies, Temnothorax hosts need a way to distinguish between “friends” and “foes”. They do so by analyzing the cuticular hydrocarbon profile, which is the array of chemicals found on the surface of the ant’s body. Intruder Myrmonexus have within their hydrocarbon profile various compounds that are unpleasant to host Temnothorax, who in turn use this chemical signature to identify the parasites. Once they have identified an intruder, Temnothorax aggressively bite and eject it from the colony. In response to this adaptation from the host, parasitic Myrmonexus constantly alter the chemical signature of their bodies by, among other things, adapting the hydrocarbon profile of Temnothorax they had previously captured.

A team of scientists led by Dr Olivier Delattre tried to identify how effectively the Temnothorax are able to guard their nests against parasites. Several dozen ant colonies were observed, including Temnothorax sub-species that did and did not experience frequent parasitic attacks from the slave-making Myrmonexus. The scientists hypothesized that Temnothorax, if they are to keep up with Myrmonexus in the coevolutionary arms race, must act much more aggressively towards parasitic intruders than ants of other Temnothorax subspecies. 

According to the team’s results, Temnothorax do, in fact, react more aggressively towards Myrmonexus in their nests, and they make the distinction based on the cuticular hydrocarbon profiles of the intruders. However, a significant portion of ants that were aggressively bitten and ejected by Temnothorax were actually fellow Temnothorax sub-species from a different colony.

The following explanation could be offered for the large number of non-parasitic ants attacked by Temnothorax: it is possible that at this point in time Myrmonexus are, in fact, ahead in the arms race, having adapted their hosts’ chemical signature to such a degree that even Temnothorax have trouble distinguishing their kin from the slave-makers. The team proposes follow-up studies that would track both the social behavior and hydrocarbon profiles of Temnothorax and Myrmonexus to find out whether the hosts will be able to perfect their ability to discriminate against parasites while keeping their own kind safe.

Word count: 444

Source cited:  Delattre, Olivier, et al. "Do host species evolve a specific response to slave-making ants?" Frontiers in Zoology. 9.38 (2012).

Image Source: National Geographic

Playing nice with invasive species

Alliaria petiolata, more commonly known as garlic mustard, is a highly invasive plant species found throughout much of the of the eastern United States. This European plant was first introduced to America nearly 150 years ago and has been aggressively colonizing large swaths of forest understory (the “ground floor” of the forest) ever since, often running roughshod over the native flora. 

                                                                          Garlic mustard in New York
 
Garlic mustard is characterized primarily by its hardiness-it is capable of reestablishing itself within a year of being physically removed from a location-as well as its toxic attacks on its competitors. The sinigrin compound produced by these plants is used to kill the fungal symbionts that allow native plants to extract and utilize nutrients from the soil, effectively starving them to death and allowing invasive growth to occur relatively uninhibited.

It has long been thought that native plants would never be able to reclaim the competitive advantage in the face of the garlic mustard onslaught, but a recent study published in PNAS (Lankau, 2012) by a team of scientists from the University of Georgia suggests that this might not be true after all.

Instead, they discovered that native clearweed plants (Pilea pumila) located in areas that have been exposed to garlic mustard for extended periods of time have gradually evolved an increased resistance to sinigrin in order to enhance their survival. In response to this challenge of their regional dominance, garlic mustard plants in these areas are attempting to overwhelm clearweed resistance mechanisms by increasing the volume of sinigrin they produce. These findings are significant because they represent the first evidence of coevolution-change in one object triggered by change in another- between native and invasive plant species.

                                                     Clearweed and garlic mustard existing side-by-side                                    

The process of evolution comes with a cost, however. The researchers also determined that sinigrin-resistant clearweed was largely incapable of surviving in areas possessing little to no garlic mustard. In fortifying their defenses, the modified plants appear to have sacrificed some of their fitness relative to their less-resistant brethren.

Despite their narrowed habitat niche, the evolution of sinigrin-resistant clearweed provides hope for the long-term state of ecosystems affected by invasive species. If it is indeed possible for native plants to effectively fight species such as garlic mustard to a relative draw, then it is logical to assume that they will achieve some sort of equilibrium in the future. Whether this means the two species will simply coexist or eventually coevolve new relationships that cause them to integrate remains to be seen, but the long-term outlook for ecosystem health certainly looks much brighter than before. Invasive species will never be completely eradicated from our forests, but thanks to the power of evolution, they can at least be tamed.

Word Count: 445
Source cited: Lankau RA. Coevolution between invasiveand native plants driven by chemical competition and soil biota. PNAS. 2012 Jul10;109(28):11240–5.
Photo credits: Garlic mustard-http://newyorkinvasivespecies.info/plants/GarlicMustard.aspx
Clearweed and garlic mustard-http://laboratoryequipment.tumblr.com/post/26213793859/invasive-species-such-as-kudzu-privet-and-garlic

Man's Best Friend

Ever wondered how the ancestors of your beloved goldendoodle became “man’s best friend?”

-Sena McCrory

Photo by Robert Clark

The domestication of dogs is perhaps one of the greatest examples of co-evolution in human history, and one that has stumped scientists and anthropologist alike for years.

An international team of researchers led by Swedish biochemist Erik Axelsson recently published an article in Nature that describes how genetic changes in wolves could explain the beginning of the relationship between man and canine. The secret lies in a dog’s superior ability to digest starches.

DNA from 12 wolves and 60 dogs (from 14 different breeds) were sequenced in this study. The team detected over 3.7 million single nucleotide polymorphisms, over 1.7 million of which were unique to domesticated dogs. They searched for reduced heterozygostity—a measure of how recently the polymorphism developed—however, the researchers did note that it is difficult to distinguish allele fixations caused by genetic drift from those alleles chosen by natural or artificial selection.

Thirty-six “domestication regions,” which included 122 separate genes, were identified by the researchers. Several of these genes explain differences in brain development and thus behavioral changes.

However, three genes involved in the breakdown and uptake of starches were found to differ significantly between the wolf and dog DNA. Dogs carry many more copies than wolves of the alpha-2B-amylase encoding gene which in involved in the breakdown of starches. In addition, an extension of the MGAM gene is likely another adaptive mutation that allowed dogs to adopt a more omnivorous diet (this extended gene is also found in herbivorous and omnivorous animals like rabbits and rats). And finally, changes in the SGLT1 gene are responsible for higher rates of glucose uptake in dogs than wolves.

The mutations in these genes allow dogs to leave behind some of their carnivorous tendencies (e.g. your faithful pooch views the biscuit and not your hand as food) and also help their digestive systems cope with “human food” like roots and grains.

There are still questions about whether these genetic adaptations came about as a result of a scavenging lifestyle or selective breeding of wolves, and it is likely that further studies will focus on pinning down the exact timing of these genetic changes that led to the co-evolution of man and dog.

Photo by Robert Clark, National Geographic article "Wolf to Woof"  

Link to original article in Nature

Word count: 379

Attine Ants and Their Fungi

The mutually beneficial interaction between fungus-growing ants (Hymenoptera: Formicidae:  Attini) and their fungi (Agaricales: mostly Lepiotaceae: Luecocoprineae) is said to have started 45-60 million years ago. In this symbiotic relationship, the fungus-growing, or attine ants use leaves, flowers, and detritus as well as dead insects and their feces to mature their fungal gardens. In addition, they disinfect these added organic materials to protect the fungi from bacterial infection by licking and chewing them. This also begins the decomposition process.  The disinfection is attributed to the secretion of antibiotic substance such as phenylacetic acid, 3-hydroxydecanoic and indoleacetic acid from the ant’s mandibular glands and metapleaural glands. The thick whitesh granular deposits on the surface of some of the attine ants are actually actinomycetes-filamentous bacteria, which also secrete anti-biotic substances. Collectively, these anti-biotic substances play a big role in suppressing anti-fungal pathogens such as Escovopsis.  In return for the protection and gardening, the fungi produce specialized structures called gongylidia -rich in lipids and carbohydrates- for the attine ants to consume.  

 

The mutualism between the attine ants and their cultivars has been conserved for over 45 million years mainly due to ‘vertical transmission’ of the cultivars. It is referred to as vertical transmission because attine queen ants carry fungal inocula from their parents to their new colonies. Lateral transmission of cultivars between colonies has also been reported and is used to explain incidences when distantly related ant species cultivate the same fungi or incidences when a single ant species cultivates distantly related cultivars.

Comparison of free living fungi and ant-associated fungi has shown similarities between the two. This suggests that any observed differences between them arose due to domestication of formerly free living fungi by ants.  In relative terms, recently domesticated fungi show closer links to currently free living fungi, while fungi domesticated earlier on show closer links to ancient free living fungi. This data can be explained by co-evolution: fungi cultivated by attine ants evolved differently than free living fungi. For example, over time, the cultivated fungi have lost their independent resistance to infection due to the anti-biotic substances and constant grooming provided by the attine ants. On the other hand, free living fungi have retained the independent resistance, likely passed down from ancient species.

 Word count: 373
Reference 

Currie, C R. “A Community of Ants, Fungi, and Bacteria: a Multilateral Approach to Studying Symbiosis.” Annual Review of Microbiology 55 (2001): 357–380