Mortality without springing a leak: Locust gut epithelia do not become more permeable to fluorescent dextran and bacteria in the cold

Abstract

The insect gut, which plays a role in ion and water balance, has been shown to leak solutes in the cold. Cold stress can also activate insect immune systems, but it is unknown if the leak of the gut microbiome is a possible immune trigger in the cold. We developed a novel feeding protocol to load the gut of locusts (Locusta migratoria) with fluorescent bacteria before exposing them to -2°C for up to 48 h. No bacteria were recovered from the hemolymph of cold-exposed locusts, regardless of exposure duration. To examine this further, we used an ex vivo gut sac preparation to re-test cold-induced fluorescent FITC-dextran leak across the gut and found no increased rate of leak. These results question not only the validity of FITC-dextran as a marker of paracellular barrier permeability in the gut, but also to what extent the insect gut becomes leaky in the cold.

Exploring cross-protective effects between cold and immune stress in Drosophila melanogaster

“…As pertaining to the hypothesized relationship of a actual immune/cold resistant response and a subsequent concurrent upregulation of tot genes, we found a general upregulation of tot genes for all treatments, except the early single cold treatments (i.e., 2 h post cold), which mostly showed non-significant upregulation relative to the control treatment. We therefore have support for the hypothesized notion that tot genes are both cold-and pathogen-responsive genes, as also found in earlier studies [16,18,58,72]. As seen in Figure 7, we found the strongest upregulations for all tot genes in the combined treatment groups at the 8 hour time point (generally 2-to 3-times higher than the singular treatments).…”

“…Consequently, there may have been selection for an anticipatory activation of immunity due to the association between cold and such wounding [44,62]. Recently it was found that leak of fluorescent bacteria did not occur in the gut of cold stressed locusts [18]; however, other leaking immunogenic components such as peptidoglycan and lipopolysaccharide may also trigger antimicrobial response in the host. Another hypothesis revolves around the adaptive nature of pre-emptive immune activation evolving in connection with a mismatch in the thermal performance between pathogen and host organisms, providing protection against cold-active pathogens [20,62].…”