ASM's "Small Things Considered" Spotlights Our Article, Speciation By Symbiosis

Over at ASM's Small Things Considered Blog, a very popular blog on all things microbes, Elio Schaechter discusses our recent review entitled Speciation by Symbiosis. I'll start this post by saying that I think the review (coauthored by my student Rob Brucker, @liveinsymbiosis) is one of the most important pieces that Ive contributed to. I'll explain more about that opinion below. Elio is favorable to the article. He highlights some of the key aspects of the review, including:

1. Some of the examples they cite are startling. For one, Wolbachia, like some other bacterial symbionts of insects, induces parthenogenesis in the insect host, a form of asexual reproduction that does not involve fertilization and leads to what is called “asexual speciation

2. For another, Drosophila flies reared on different diets house different microbiota, and show strong mating discrimination; ergo, the symbionts dictate who mates with whom. 

3. Bacterial symbionts that we could classify as vertically-transmitted, nutritional mutualists (e.g., insect symbionts in the genus Buchnera) assist in resource exploitation, thereby creating new ecological opportunities for their host

4. Endosymbionts can also induce cytoplasmic incompatibility.  Here the offspring of infected males and uninfected females are sterile, therefore, unproductive. 

5. In other cases, the offspring of hybrid matings become more susceptible to infection than non-hybrids, which may reduce their fertility and viability. We are working on this very issue right now and plan to have a paper ready in the next few weeks.

This review article was actually a decade in the making for me. Speciation by symbiosis was the topic of my Ph.D. thesis at the University of Rochester and along with Jack Werren, we helped put Wolbachia on the map as one of the first cases by which symbionts drive the evolution of reproductive isolation between species. This work was later followed by several other important studies on Wolbachia and speciation and came on the heels of intense evangelizing by Lynn Margulis that symbionts were important to speciation; though I contend that she actually provided little evidence that directly linked symbionts to speciation. In my opinion, Lynn was more interested in showing that symbionts drive adaptations, an undeniable legacy that she has left behind.

After the Wolbachia-associated speciation work came out, there seemed to be a lull in the pace of work on symbionts and speciation. Within the last few years however and the uptick in microbiome studies in all organisms, it has since become clearer that speciation by symbionts in general is a robust field with many new insights to be gleaned in the future. Symbionts drive mate discrimination in Drosophila, rapid evolution of immune genes that in turn cause hybrid maladies in plants, and directly prevent gene flow between closely related species - all discussed in the paper. Our hope is that the review revitalizes the topic for new and old investigators alike, and that as the merger between speciation genetics and symbiosis become seamless, the pioneer of the idea that symbionts drive speciation, Ivan Wallin, be formally recognized. More on Ivan in our article and ASM Small Things Considered Blog.

Finally, we tried to publish this article as open access at various journals, but it ended up in TREE's hands. If anyone needs a full copy of the pdf, do not hesitate to email me. My email is readily available by google search or the lab home page.

Related blog posts:
1. The story behind Speciation by Symbiosis? (April 26, 2012)
2. Is the microbiome part of the organism or the environment?
3. Is the microbiota species specific? (June 23, 2012)

Lab happenings, food, and sites in Krakow, Poland

The International Symbiosis Society 7th Congress is coming to a close. Below are a handful of pictures that Im fond of and will hopefully inspire you to get to Krakow one day. It is a beautiful, lovely, and cozy city. Also very cheap.

The Royal Castle on Wawel Hill in the distance. One of the great symbols of Polish statehood. The former capital and seat of kings as well as the place of their coronation and burial.

Squint and you may see a walking beer holding a stop sign. Inquiring minds want to know more.

The tower is what's left of the old town hall in the Market Square of Krakow.

The biggest square in Europe and Krakow's elegant showpiece.

What the town hall looked like in its completeness. Only the tower in the pictures above remains standing today.

An Espresso Lunch at Hotel Francuski, recommended by a friend of my student. For $6 USD, this was a steal. Freshly squeezed juice, cabbage, ribs, potatoes, and rugula cake.

 Senior graduate student, Rob Brucker, giving his talk on "Gut Bacteria Enhance Postmating Reproductive Isolation"

Perhaps the best chicken cesar salad Ive ever had.

Group picture of Lisa, me, Rob, and Jason in front of Lisa's poster "Mom knows best: Maternal regulation of Wolbachia titers"

I'll add just one scientific thought to this post. This meeting has convinced me more than ever that we live in an age where symbiosis is a centerpiece to Biology in the 21st century. A profound, modern synthesis is underway in which the century old merger of evolution and genetics is being updated with symbiosis as the third pillar.

Science of a Superorganism - my article in Bare Essentials

Bare Essentials - A Free Online Magazine Promoting Life Science and Conservation:  Bare Essentials is an open access online journal that has a commitment to spreading scientific literacy along with some snazzy designs/layouts in their articles. The editor of Bare Essentials asked for a contribution on the relationships between the microbiota and evolution. The article, entitled Science of a Superorganism, is downloadable here.

BE has progressive principles that deserve giving a shot out to, including an affiliation with the Australian Zoo Wildlife Warriors, a conservationist organization that was established in 2002 by the late Steve Irwin and his wife Terri Irwin to involve and educate others in the protection of injured, threatened or endangered wildlife. From the Bare Essentials website:

"Beyond our online resources and publication, we help co-ordinate fundraising opportunities for our conservation partners inviting individuals, groups and sponsors to affiliate with and help raise awareness for preservation efforts through our Wildlife Warriors Initiative."

Spotlighting the Stories of the Microbiota: There's so much to say about the microbiota that it was impossible to cover all the work that should be covered; and there are many good articles already out there. My approach was to boil down the basics and shine the lights on the importance of the microbiota for the general audience. I included topics such as how microbes weigh over 5000x more than humans do on the planet, how microbes make you attractive to mosquitoes, how women's hands are microbial dirtier than men's hands, among others.

I also tried to pick up some of the things that my student is studying on gut microbiota and evolution. Here are two more scientific points that are touched upon towards the end of the Bare Essentials piece but dont get much real estate in the article:

1. First, the influence of gut bacteria on animal speciation is one of the major foci of our lab's research. We are studying how changes in the number and types of gut bacteria change during the process of animal speciation, and how the gut bacterial community may in fact cause speciation events by reducing interbreeding between two animal species. Recently we published a paper in the journal Evolution in which we showed that gut bacteria in closely related species of insects, Nasonia parasitoid wasps, increases in diversity over development from larvae to pupae and adults. The gut bacteria essentially becomes more diverse by colonization of new bacterial types as the insects develop, similar to what happens within humans in which an infant rapidly accumulates different species of bacteria over the course of their first year. 
2. We also test whether diet or animal genes have a more important impact on the composition of the types and abundance of bacterial species in their guts. If diet affects the gut microbiota, then by rearing closely related species on the same diet, the null hypothesis would be that all three species harbor the same types of bacteria. We did not observe this. Instead, what we observed is that when the Nasonia wasps were reared on the same diet (fleshfly hosts), the bacterial communities became slightly different in each of the wasp species; and the relationships of these slightly different gut communities between the Nasonia species parallels the relationships of the Nasonia chromosomal genes (see conceptual figure below based on findings in the Evolution paper). Therefore, we conclude that bacterial communities diverge in parallel with the wasp's genes over evolutionary time-frames that span the formation of new species. The implications are significant. In particular, a host's bacterial population is not transient or unstable. Instead, it is species-specific at some levels and likely selected for by the immune system to perform functions within the host that may be slightly different from the functions in closely-related host species. What happens to these bacterial communities in hybrids and how the bacteria affect hybrid problems such as mortality between Nasonia species is what we currently have our heads buried in.

The figure above by Robert Brucker and I shows the codivergence between the genes of the wasp on the left and their gut microbial communities on the right. The three Nasonia species differ in their wing sizes and the fleshfly host on the bottom is Sarcophaga bullata. The bolded colors on the circular trees indicate the different types of bacterial species present in the insects.