Shining a Light on Exploitative Host Control in a Photosynthetic Endosymbiosis

Endosymbiosis allows hosts to acquire new functional traits such that the combined host and endosymbiont can exploit vacant ecological niches and occupy novel environments [1, 2]; consequently, endosymbiosis affects the structure and function of ecosystems [3, 4]. However, for many endosymbioses, it is unknown whether their evolutionary basis is mutualism or exploitation [5-9]. We estimated the fitness consequences of symbiosis using the interaction between the protist host Paramecium bursaria and the algal symbiont Chlorella sp. [10]. Host fitness was strongly context dependent: whereas hosts benefited from symbiosis at high light intensity, carrying endosymbionts was costly to hosts in the dark and conferred no benefit over growing autonomously at intermediate light levels. Autonomous Chlorella densities increased monotonically with light intensity, whereas per-host symbiont load and symbiont abundance peaked at intermediate light levels and were lowest at high light intensity. This suggests that hosts controlled the costs of symbiosis by manipulating symbiont load according to light intensity. Photosynthetic efficiency was consistently lower for symbiotic compared to autonomous algae, suggesting nutritional constraints upon algae in symbiosis. At intermediate light levels, we observed the establishment of small populations of free-living algae alongside the hosts with endosymbionts, suggesting that symbionts could escape symbiosis, but only under conditions where hosts didn't benefit from symbiosis. Together, these data suggest that hosts exerted strong control over endosymbionts and that there were no conditions where this nutritional symbiosis was mutually beneficial. Our findings support theoretical predictions (e.g., [5, 9]) that controlled exploitation is an important evolutionary pathway toward stable endosymbiosis

Elongation, Alignment, and Guided Electrophoretic Migration of ds-DNA in Flow-Aligned Hexagonal F127 Gels

Elongation, alignment, and electrophoretic migration of double stranded DNA (ds-DNA) are investigated within flow aligned hexagonal Pluronic F127 mesophases contained in microfluidic channels. The DNA molecules are stained with YOYO-1 for visualization of their positions, conformations, and motions, which are recorded by wide-field fluorescence video microscopy. The videos show that the ds-DNA molecules are elongated in flow aligned hexagonal F127 mesophases, with the long axis of the DNA molecules aligned parallel to the flow direction. Elongation and alignment are most prevalent near the channel surface in the hexagonal mesophase. In contrast, little or no alignment is observed for the cubic mesophase. DNA elongation and alignment may involve adsorption of one strand end to the glass surface, or its capture by an adsorbed, structured surface layer of F127. Subsequent stretching of the DNA would then occur within the steep flow profile that exists near the glass surface during filling of the microfluidic channels. Videos recorded under the influence of applied electric fields demonstrate that the electrophoretic motions of the elongated, aligned DNA are strongly guided by the hexagonal mesophase structure. Electrophoretic migration is observed to occur exclusively along the local flow alignment direction within hexagonal mesophases for fields applied at 0, 45 and 90° to the flow alignment direction. These results show that ds-DNA interacts strongly with the micelles comprising the gel. These observations will lead to a better understanding of macromolecular interactions with nanostructured gels like those now being investigated for use in drug delivery and chemical separations

Odorant binding proteins promote flight activity in the migratory insect, Helicoverpa armigera

Migratory insects are capable of actively sustaining powered flight for several hours. This extraordinary phenomenon requires a highly efficient transport system to cope with the energetic demands placed on the flight muscles. Here, we provide evidence that the role of the hydrophobic ligand binding of odorant binding proteins (OBPs) extends beyond their typical function in the olfactory system to support insect flight activity via lipid interactions. Transcriptomic and candidate gene analyses show that two phylogenetically clustered OBPs (OBP3/OBP6) are consistently over-expressed in adult moths of the migrant Old-World bollworm, Helicoverpa armigera, displaying sustained flight performance in flight activity bioassays. Tissue-specific over-expression of OBP6 was observed in the antennae, wings and thorax in long-fliers of H. armigera. Transgenic Drosophila flies over-expressing an H. armigera transcript of OBP6 (HarmOBP6) in the flight muscle attained higher flight speeds on a modified tethered flight system. Quantification of lipid molecules using mass spectrometry showed a depletion of triacylglyerol and phospholipids in flown moths. Protein homology models built from the crystal structure of a fatty acid carrier protein identified the binding site of OBP3 and OBP6 for hydrophobic ligand binding with both proteins exhibiting a stronger average binding affinity with triacylglycerols and phospholipids compared with other groups of ligands. We propose that HarmOBP3 and HarmOBP6 contribute to the flight capacity of a globally invasive and highly migratory noctuid moth, and in doing so, extend the function of this group of proteins beyond their typical role as chemosensory proteins in insects