Physiological resistance to salinity in the invasive ground beetle Merizodus soledadinus at the Kerguelen Islands; or how the physiology helps insect species to invade new habitats?
Salinity is one of the main abiotic factors that impact survival and fitness of terrestrial insects in coastal environments. Indeed, hypersaline environments induce osmotic stress disturbing the cellular homeostasis, the structure and fluidity of cell membranes or again denaturating proteins.
Meanwhile, some terrestrial insect species have developed the ability to survive such hypersaline environments, and counterbalance osmotic stress by intra- and extracellular buildups of organic osmolytes. A good example is the ground beetle Merizodus soledadinus which is originate from South America and distribute in forests and riparian zones, where salinity levels are considerably low. However, this species has been introduced at the Kerguelen Islands a century ago, where it colonized coastal areas (tide drift lines), and must thus withstand salinity variations due to tide, spray, and organic matter deposited therein.
Accordingly, the research study I addressed in 2013 during my MSc aimed to answer the question of physiological resistance to salinity in this invasive ground beetle. Indeed, we supposed an high physiological plasticity of this species explain in part the quick distribution of Merizodus soledadinus in the coastal areas of the Kerguelen Islands, and its high invasive abilities in the Islands.
In this work, I monitored both body water content and survival in adult M. soledadinus experimentally subjected to different salinity amounts (0, 35, 70‰). The study revealed a progressive drop in body water content in individuals exposed to 35 and 70‰, as opposed to the controls exposed to 0‰. However, this drop appeared few weeks after we exposed the insects to high salinity levels suggesting ground beetle are able to maintain water homeostasis and survival under 35 and 70‰ using physiological resistance strategies.
We hypothesized that this invasive ground beetle can withstand a broad range of salinity conditions thanks to the plastic accumulation of compatible solutes. Therefore, I then investigated possible metabolic adjustments involved in the three contrasted salinity levels (0, 35, 70‰). Indeed, some terrestrial insects can endure and survive saline levels up to 70-80‰ by accumulating several osmolyte classes named compatible solutes, including monosaccharides, disaccharides, sugar alcohols, quaternary amino acids. Altogether, these compatible solutes help to maintain the osmotic homeostasis among body compartments without altering intracellular biochemistry and physiology.
Metabolic fingerprints of Merizodus soledadinus thus showed compatible solute (e.g.erythritol, alanine, glycine and proline) accumulation at medium (35‰) and high (70‰) salinity conditions.
The conclusion of this work is therefore that the osmo-induced accumulation of amino acids and polyols was likely to modulate the ground beetles’ body water balance on medium (35‰) saline substrates, thus enhancing their survival ability and explaining their successful colonization of the tide drift line at the Kerguelen Islands.
More details of this work is available at:
