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.
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.
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.
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