Malpighian tubule function in desert locusts in relation to ecology, disease and plasticity

Malpighian tubules play a fundamental role in insect osmoregulation and the extrusion of potentially noxious substances, which is essential for coping with ingested toxins or metabolic waste. Potentially harmful compounds, such as alkaloids, are actively extruded by ABC transporters, which includes P-glycoprotein transporters. Here, we studied P-glycoprotein transport in the Malpighian tubules of desert locusts, Schistocerca gregaria, which have evolved to feed on a broad variety of plants, including those producing toxic alkaloids. This makes desert locusts an excellent system in which to study P-glycoprotein-mediated extrusion of xenobiotic compounds. We developed a novel method combining a modified Ramsay assay with the application of a specific substrate and inhibitor of P-glycoproteins. We incubated ex-vivo Malpighian tubules in solutions containing the P-glycoprotein substrate, rhodamine B, as a proxy for toxins, in combination with the P-glycoprotein inhibitor, verapamil. By measuring the size and colour of the droplets secreted, we quantified the fluid secretion rate, the rhodamine concentration within the droplet, and the net rhodamine extrusion. We found that locust tubules express P-glycoproteins, the tubules’ surface area positively correlating with their fluid secretion rate, and the fluid secretion rate positively increasing the net rhodamine extrusion. Desert locusts exhibit density-dependent phenotypic plasticity, existing as two extreme phenotypes (with intermediate forms) that differ in their ecology and behaviour depending upon their density. Phenotypic plasticity occurs on multiple time scales; morphology and physiology change within a lifetime or over several generations, while behaviour can change in a few hours. Low densities produce solitarious phase locusts that avoid ingesting potentially harmful compounds, whereas high densities trigger a shift towards a gregarious phase with a broad diet that includes plants producing harmful compounds. During the transition from solitary to gregarious, locusts show active preference for toxic plants. Since P-glycoproteins are inducible transporters and their activity regulated by diet, we compared the net rhodamine extrusion between solitarious, gregarious and transiens phases fed on a diet containing the alkaloid atropine, and on an alkaloid-free diet. We found no difference in the net rhodamine extrusion between gregarious, solitarious and transiens locusts fed on alkaloid-free diet. However, gregarious and transiens locusts upregulated their P-glycoprotein after only three days of exposure to the diet containing the alkaloid, and gregarious locusts reared on the alkaloid diet downregulated their transporters after being switched to an alkaloid-free diet. Parasitic infection can also induce phenotypic plasticity. We studied the performance of Malpighian tubules infected by the protozoan Malpighamoeba locustae, an infection that is known to damage their structure. Infected tubules had greater surface area and higher fluid secretion rate than uninfected ones. Infection also reduced the net rhodamine extrusion per unit of surface area, suggesting that the damage of the brush border likely reduces the P-glycoprotein density. Thus, the work presented in this thesis places the physiology of locust Malpighian tubules in the context of ecology, phenotypic plasticity and disease. We suggest that P-glycoprotein transporters expressed by the Malpighian tubules contribute to xenobiotic extrusion, and that their activity is regulated accordingly to diet. In addition, we showed how a parasitic infection can compromise the Malpighian tubule function, potentially leading to water loss, increased energy consumption, and reduced xenobiotic extrusion.