Posts Tagged ‘Symbiosis’

A recent review on this topic by Robert Bucker and Seth Bordenstein, directed my attention to this little, taught and debated, fact. We consider that the process of speciation to be, one which divides an existing single species into two, or more clearly, the emergence of a new species. Biological species concept is one of the most favoured ones, but others are not uncommon and each has its own arguments making you wonder why these many species concepts. However, all the species concepts agree at one point that a new “species” is formed, through reproductive isolation (biological species concept), or a lineage evolving separately (as in the evolutionary species concept and which is true even for asexual beings), even the phylogenetic species concept advocates monophyly of a group to consider it a species where again a species is formed. Our peek today is not into the species concepts but one of the least appreciated and more important causes of speciation, an organism’s associated microbial community or symbionts.

Clownfish-Sea Anemone mutualism: if the host and symbiont has a specific preference could it lead to ecological divergence and thereby speciation?

The idea of symbiosis as an integral part of speciation, be it in reproductive isolation (sensu Biological Species concept and many others ), or in niche divergence (sensu Ecological species concept), can be easily comprehended. This review paper addresses the importance of symbionts in the whole process of speciation. We all know that changes in the genes (mutations) are fixed in the genome if advantageous (substitutions), which leads to adaptive divergence of the population and this slow process (millions of years) could lead to speciation. However, the authors argue here for another “genetic” component other than the nuclear genes the “symbionts”.

It is known that symbionts/microbes are omnipresent among the eukaryotes, which we recently come to call as microbiome, and is often clubbed together with the genome as called the hologenome of the organism. Here we need to recognize that microbial community of the organism can be decisive in determining the reproductive isolation between its sister (isolated) population harboring a different microbiota. While reading the paper we are convinced that the immune genes that are constantly facing adaptive evolution do so due to also the influence of the pathogen/microbial community of the organism as one of the factors. Thus, considering the immune genes as reproductive isolation locii could lead us to appreciate the importance of symbionts.

This paper cites different examples of Wolbachia symbionts (and many others) in the arthropods and the adaptive divergence and reproductive isolation between populations, and even ecological and behavioral isolation. The authors also point out that the hybrid incompatibilities caused due to symbionts are a “third” genetic factor. Cytoplasmic incompatibility between hybrids is a reality when we look at vertical transfer of symbionts or pathgens. In short, symbiotic association can be akin to allopatry in one sense, and aid speciation. The figure in this post (nemo) could be misleading and is just an example of mutualism, and is different from microbial association and speciation, the readers are directed to read the “trends review for better comprehension of the problem.

Wolbachia, a major symbiont of arthropods and touted to be involved in speciation by symbiosis one among a long list of microbial symbionts/microbiome.

The review synthesizes the symbiont reproductive incompatibility issue its extent its frequency and the hybrid incompatibility angle, OR pre and post reproductive isolation by symbionts. It is a good read, and an educating review of literature and introduction to concepts for students of evolutionary biology.

Reference:

Robert M. B., & S. R. Bordenstein, 2012. Speciation by symbiosis. Trends in ecology and evolution, dx.doi.org/10.1016/j.tree.2012.03.011

 


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A group of predominantly Dutch scientists have revealed the reasons for the ecological success of seagrasses and in turn its associated organism the Lucinid molluscs. Seagrass meadows, as we know is an important kind of habitat for various organisms like coral reef fishes, reptiles (like turtles), waterbirds and mammals (dugongs, manatees), and is a basic environment for these organisms’ survivor-ship.

Sea-grass meadow and an associated puffer fish.

They survive till now, but as the paper points out it is a mystery how they do it. The sediments trap high organic matter content which in turn is fodder for some bacteria that revel in oxygen lacking environment and take up the sulfite present and produce sulfides as an end product of their metabolism. This sulfide is toxic to seagrass, so how do seagrass survive? This is the question asked by the researchers, they analysed data from world-wide, and formulated hypothesis and did experiments to prove their guess.

In their meta-analysis they found that a specific type of bivalve (Lucinidae) is associated with seagrass in more than 90% of the tropical and subtropical seagrass beds and in more than 50% of temperate seagrass meadows. This points that temperature-dependent sulfide deposition in tropics favours the association of the bivalve with the seagrass.

The bivalves harbours a symbiotic bacteria that metabolises sulfide and in turn benefits the mollusc, which sequesters sulfide and oxygen for the symbiont, by providing sugars. This association has been dated back to the Silurian (416 million years ago [Ma]; see the paper for detailed references), however the diversification of the mollusc and its associated symbiont is dated back only to the Cretaceous (145-65 Ma) when the seagrass emerged, the diversification of seagrass was in the Eocene but the symbiosis between them and the mollusc still continues, the probable diversification was aided by the help rendered by their symbiont mollusc to stabilize at first hand some 50 Ma .

The authors hypothesised that the association between the mollusc (with the endosymbiont which metabolises sulfides), could have helped in the survival of the seagrass which would otherwise have perished due to the high sulfide content in the sediments. The do experiments and prove that is the case.

We read yet another paper which really observes, hypothesizes and proves. It provides evidence of diversification of the mollusc and seagrass were interdependent, while 11 out of 12 seagrass genera harboured associated molluscs 18 genera (~50% of Lucinidae genera) of the molluscs are associated with seagrass. Basic research like this would help the restoration programs for sea-grasses which is not yet a big success.  Such basic research into the function of the ecosystem, its components, interactions etc. are the need of the hour.

Reference:

Tjisse van der Heide, Laura L. Govers, Jimmy de Fouw, Han Olff, Matthijs van der Geest, Marieke M. van Katwijk, Theunis Piersma, Johan van de Koppel, Brian R. Silliman, Alfons J. P. Smolders, Jan A. van Gils, 2012. A Three-Stage Symbiosis Forms the Foundation of Seagrass Ecosystems. Science, 336:6087, 1432-1434.