In brief (August 2013, part II) – microbiome and speciation, and more viruses

Picks for this month, second part:
gut bacteria keeping species apart, or how the microbiota may promote speciation by contributing to offspring lethality when two species are interbred,
new genera of ocean-dwelling phages, or how analysis of an aquatic bacterium led to the discovery of twelve new genera of bacteria-infecting viruses.

  • Gut bacteria keeping species apart

The microbiome (all of the bacteria living e.g. in your gut or on your skin, and their genes) is currently the subject of much attention. While it is mainly studied from a health perspective, e.g. how it interacts with its host’s immune system, little is known on how it evolves along with the host it lives in/on. Two American researchers, interested in the possibility of the microbiota contributing to animal evolution and the origin of new species, set out to investigate the role of the microbiome in the hybrid lethality that is observed when two particular species of wasps are interbred (closely-related organisms are considered different species if they cannot produce viable and fertile offspring when interbred). Their results appeared in the journal Science in August.

Brucker and Bordenstein used two parasitic wasp species of the genus Nasonia that diverged about a million years ago, called N. vitripennis and N. giraulti. When individuals of these two species are interbred, about 90% of the F2 hybrid males die during larval development (see cartoon for a visual representation of what F1 and F2 hybrids are). By contrast, crossing of the two Nasonia species N. giraulti and N. longicornis that diverged more recently, about 400,000 years ago, results in only about 8% of F2 hybrid lethality.

F1 and F2When the researchers looked at the bacterial species making up the microbiota of the N. vitripennis and N. giraulti hybrids, they observed that they differed both in diversity and in abundance from the bacteria found in the microbiota of either parental species. To test whether the microbiome was involved in hybrid death, the researchers devised a way to rear the wasps in germ-free conditions. In absence of microbiota, 50-60% of hybrids survived, but when bacteria naturally present in the parental species were inoculated back in larvae that had been first reared under germ-free conditions, the survival rate decreased again, indicating that the microbiome does indeed contribute to hybrid lethality. The researchers found that innate immune genes were on average less expressed in germ-free hybrids compared to the hybrids raised conventionally, suggesting that interactions between the immune system and the microbiome may play a role in hybrid death. However, they note that there are very possibly other functional categories of genes that they have not looked at in their study that could contribute to microbiome-gene interactions detrimental to hybrid survival.

Hybrid lethality when crossing two species is generally thought to stem from genetic incompatiblities between the parental species, resulting in negative gene-gene interactions and the demise of the offspring. Brucker and Bordenstein speculate that, in addition to this phenomenon, the interactions between host genes and microbiome may amplify the potential hybrid genetic incompatibilities and thus contribute to speciation. An interesting point in this idea is that microbes can evolve very rapidly, so they could quickly enforce the split between two species progressively emerging from a common ancestor.

(Brucker & Bordenstein, Science 9 August 2013, doi: 10.1126/science.1240659)

  • New genera of ocean-dwelling phages

In summertime, when you blissfully indulge in some sea-bathing, little do you think of the crowd of algae and bacteria floating around you, and you most likely think even less of the millions of bacteria-infecting viruses, known as phages, that are part of the oceans’ ecosystems. In fact, not much is known about ocean-dwelling viruses, despite the important role they play in the marine nutrient cycles by controlling the way the bacteria they infect grow and interact with the environment.

In a study published online in July in PNAS, Holmfeldt et al. describe the discovery of twelve new genera of ocean phages. The researchers isolated one particular strain of aquatic bacteria, Cellulophaga baltica, from a sample collected from the strait of water between Sweden and Denmark and analyzed the genetic material it contained. They identified 31 phages, which fell into 12 distinct groups that each represented a new genus.

Though such a diversity is beyond what is known about the few marine phage systems studied so far, the researchers note that it is similar to the diversity of phages infecting human-associated bacteria. They further suggest that it represents but a small window on the marine viral diversity that is yet to be discovered.

(Holmfeldt et al., PNAS 15 July 2013, doi: 10.1073/pnas.1305956110)


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