Host-associated transmission favors transition of a commensal toward antagonism.

The impacts of host-associated microbes on their hosts vary along a continuum of antagonistic, neutral, and beneficial interactions. Transmission mode is predicted to contribute to transitions along the continuum by altering opportunities for the alignment of host and microbe fitness interests. Under vertical transmission, microbial evolution is tightly coupled to the host environment, which may facilitate fitness alignment. In contrast, environmentally transmitted microbes spend time in the external environment, outside of hosts, partially decoupling their evolution from the host. This decoupling may misalign host and microbe fitness interests, potentially favoring antagonistic microbial traits. Here, we tested whether transmission environment alters microbial evolution by manipulating the interaction between a commensal Serratia marcescens bacteria and their insect host Anasa tristis, which is the primary vector of these bacteria into plants, where they cause disease. We experimentally evolved S. marcescens through several selection environments. The bacteria were passaged between A. tristis hosts, between A. tristis hosts and soil, through soil, or through standard culture media. We observed rapid evolution of virulence toward hosts across treatments when bacterial evolution occurred within the host environment, indicating that direct host-to-host transmission can increase opportunities for microbes to adapt to hosts and evolve antagonistic traits.

Building consensus around the assessment and interpretation of Symbiodiniaceae diversity.

Within microeukaryotes, genetic variation and functional variation sometimes accumulate more quickly than morphological differences. To understand the evolutionary history and ecology of such lineages, it is key to examine diversity at multiple levels of organization. In the dinoflagellate family Symbiodiniaceae, which can form endosymbioses with cnidarians (e.g., corals, octocorals, sea anemones, jellyfish), other marine invertebrates (e.g., sponges, molluscs, flatworms), and protists (e.g., foraminifera), molecular data have been used extensively over the past three decades to describe phenotypes and to make evolutionary and ecological inferences. Despite advances in Symbiodiniaceae genomics, a lack of consensus among researchers with respect to interpreting genetic data has slowed progress in the field and acted as a barrier to reconciling observations. Here, we identify key challenges regarding the assessment and interpretation of Symbiodiniaceae genetic diversity across three levels: species, populations, and communities. We summarize areas of agreement and highlight techniques and approaches that are broadly accepted. In areas where debate remains, we identify unresolved issues and discuss technologies and approaches that can help to fill knowledge gaps related to genetic and phenotypic diversity. We also discuss ways to stimulate progress, in particular by fostering a more inclusive and collaborative research community. We hope that this perspective will inspire and accelerate coral reef science by serving as a resource to those designing experiments, publishing research, and applying for funding related to Symbiodiniaceae and their symbiotic partnerships.

Intraspecific and interspecific variation in thermotolerance and photoacclimation in Symbiodinium dinoflagellates.

Light and temperature are major drivers in the ecology and biogeography of symbiotic dinoflagellates living in corals and other cnidarians. We examined variations in physiology among 11 strains comprising five species of clade A Symbiodinium We grew cultures at 26°C (control) and 32°C (high temperature) over a duration of 18 days while measuring growth and photochemical efficiency (Fv /Fm ). Responses to thermal stress ranged from susceptible to tolerant across species and strains. Most strains exhibited a decrease in cell densities and Fv /Fm when grown at 32°C. Tolerance to high temperature (T32) was calculated for all strains, ranging from 0 (unable to survive at high temperature) to 1 (able survive at high temperature). There was substantial variation in thermotolerance across species and among strains. One strain had a T32 close to 1, indicating that growth was not reduced at 32°C for only this one strain. To evaluate the combined effect of temperature and light on physiological stress, we selected three strains with different levels of thermotolerance (tolerant, intermediate and susceptible) and grew them under five different light intensities (65, 80, 100, 240 and 443 µmol quanta m-2 s-1) at 26 and 32°C. High irradiance exacerbated the effect of high temperature, particularly in strains from thermally sensitive species. This work further supports the recognition that broad physiological differences exist not only among species within Symbiodinium clades, but also among strains within species demonstrating that thermotolerance varies widely between species and among strains within species.

Circadian clocks in symbiotic corals: the duet between Symbiodinium algae and their coral host.

To date, the association and synchronization between two organismal circadian clocks ticking in parallel as part of a meta-organism (termed a symbiotic association), have rarely been investigated. Reef-building corals exhibit complex rhythmic responses to diurnal, lunar, and annual changes. Understanding circadian, circatidal, and annual regulation in reef-building corals is complicated by the presence of photosynthetic endosymbionts, which have a profound physiochemical influence on the intracellular environment. How corals tune their animal-based clock machinery to respond to external cues while simultaneously responding to internal physiological changes imposed by the symbiont, is not clear. There is insufficient molecular or physiological evidence of the existence of a circadian pacemaker that controls the metabolism, photosynthesis, synchronized mass spawning, and calcification processes in symbiotic corals. In this review, we present current knowledge regarding the animal pacemaker and the symbiotic-algal pacemaker. We examine the evidence from behavioral, physiological, molecular, and evolutionary perspectives. We explain why symbiotic corals are an interesting model with which to study the complexities and evolution of the metazoan circadian clock. We also provide evidence of why the chronobiology of corals is fundamental and extremely important for explaining the biology, physiology, and metabolism of coral reefs. A deeper understanding of these complex issues can help explain coral mass spawning, one of the earth's greatest and most mysterious behavioral phenomena.