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Adaptation of thermal scavenging ants to severe heat-conditions

par Willot, Quentin
Président du jury André, Bruno
Promoteur Aron, Serge
Co-Promoteur Gueydan, Cyril
Publication Non publié, 2018-12-21

Thèse de doctorat

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Résumé :

Thermal scavenging is a unique behavior restricted to a few desert ant genera. Workers are among the most thermotolerant land animals known to this day, being able to survive body temperatures of sometimes more than 50°C for several minutes. Making use of their remarkable heat-hardiness, they search for food in plain day, a feat that other desert creatures cannot accomplish. They mostly feed on the corpses of heat-stricken, less tolerant arthropods that were unable to survive the blazing sun of the midday desert. Thermal scavenging has evolved independently at least three times in distantly related genera, geographical well segregated inside the different deserts of the world. First, the Cataglyphis genus ranges from the Sahara Desert and extends its distribution to reach minor Asia through the Mediterranean Basin. Second the Ocymyrmex genus can be found in the Namib and Karoo deserts of southern Africa, extending its range to eastern Africa savanna plains. Finally, the Melophorus genus can be found in Australia, with thermal scavenging species distributed in the central desert of the outback region.While this impressive behavior was already well-described by the start of this PhD project, little was known about the mechanisms supporting the remarkable heat-tolerance of workers. Using biophysical and physiological approaches in Cataglyphis and Ocymyrmex, we’ve been able to pinpoint key aspects underlying stress tolerance in those genera. First, from a biophysical standpoint, the Sahara silver ant Cataglyphis bombycina is covered with a unique and dense array of prismatic hairs reflecting visible wavelengths by total internal reflection. This allows reflection of up to 50% of the incident sunlight energy, thus shifting down the ant’s thermal equilibrium and sparing its body a few critical degrees. Second, in a comparative framework, we found numerous genes involved with critical cellular processes to be constitutively expressed or strongly up-regulated to heat in thermal scavenging ants, while their orthologs were not in mesophilic species. Those processes, such as molecular chaperoning, cell-cycle regulation, energy metabolism and muscular functions are keys that allow those ants to meet the higher requirement needed to scavenge for food at both stunning speed and under extreme heat-pressure. Overall, this work investigates the physiological and biophysical basis enabling thermal scavenging ants to survive extreme heat conditions. It provides a deeper understanding of cellular heat-tolerance pathways in non-model animals and contribute to our knowledge of life’s adaptation to extreme conditions.