Genetic Basis of Innate Immunity and Host Resistance in Reef Corals
We are investigating the genetics of innate immunity and disease resistance in the staghorn coral Acropora cervicornis using White Band Disease as a disease model. This research follows our finding that up to 6% of staghorn corals are resistant to WBD infection (Vollmer and Kline 2008) while other corals are highly susceptible to WBD. The identification of these two disease phenotypes, i.e. resistant and susceptible corals, allows us to compare how the genetic response these coral phenotypes differ in their genetic response to disease exposure.
We are using next-generation RNA-sequencing to profile the innate immune response of Staghorn corals infected with and exposed to WBD. Our transcriptome profiles indicate that staghorn corals mount a vigorous immune response against WBD involving multiple genetic pathways including some expected immune pathways as well as some unexpected pathways. These results will be out soon.
Microbial communities associated with diseased and healthy corals
Identifying pathogens associated with coral diseases is difficult because of the relatively high microbial diversity on healthy and diseased corals. As a results, Koch’s postulate has only been fulfilled for approximately 6 out of the 20+ described coral diseases. The pathogen causing White Band Disease on the Caribbean Acropora (common names Staghorn and Elkhorn coral) have not been identified, but bacteria in the genus Vibrio and Rickettsia have both been associated with the disease.
Part of our research is trying to identify the WBD pathogen using both culture-dependent and culture-independent techniques. More to come.
Evolution of Pocillopora corals
The Pocillopora corals are a relatively speciose group of coral that spans the Indo-Pacific. They tend to have highly plastic morphologies and can shift their reproductive strategies. For example, throughout the Indo-Pacific they tend to brood their planua larval internal and can produce them both sexually (by outcrossing)and through asexual parthenogenesis. Whereas, in the Eastern Pacific, they are thought to broadcast spawn their gametes.
Littorine snail phylogeography
New species typically arise when two populations become geographically isolated and diverge over time through a process known as allopatric speciation. Another and far less common form of speciation is sympatric speciation, where two species originate within a population because natural selection favored multiple species forms or types. Examples of sympatric speciation are rare, but garner special attention from scientists because of their novelty.
One classic example of sympatric speciation is in the native US fly Ragoletis pomonella, which naturally lived on hawthorn trees, but then split to also inhabit apple trees when they were introduced across the US.
The periwinkle snail Littorina saxatilis, which inhabits the shores of Western and Eastern North Atlantic, is believed to be one of the few marine examples of incipient sympatric speciation. This species split into multiple morphological types (or species) in multiple locations in Europe presumably due to differences in the intensity of crab predation. Essentially, high crab predation has caused the emergence of a thicker shelled form of this species, which is less vulnerable to crab predation, to evolve multiple times across Europe.
Our paper [Doellman, Trussell, Grahame, and Vollmer] published in the Proceedings of the Royal Society of London, Biological Series raises new questions about this classic model of sympatric speciation by showing that L. saxatilis and its periwinkle relatives have a much more complex evolutionary history than previously thought. By analyzing DNA sequences from snails on both sides of the Atlantic, they were able to show that L. saxatilis types, which give live birth to baby snails, apparently originated not once but twice in the past 640,000 years from their egg-laying relatives L. arcana and L. compressa. Both L. saxatilis lineages now co-exist on both sides of the Atlantic in a patchwork influenced by different colonization histories. We even show that live-bearing L. saxatilis may be hybridizing with their egg-laying species.