Words: Véronique Paris
Almost every child is familiar with the famous story ‘The Very Hungry Caterpillar’, illustrating the transformation of an unremarkable hungry caterpillar into a beautiful butterfly. What exactly takes place within the pupal cocoon during the transformation remains a mystery. The scientific definition of this dramatic change is known as ‘complete metamorphosis’, where most of the caterpillars tissues are completely reorganized during pupation. The journal Philosophical Transactions of the Royal Society of Science B recently published a themed issue discussing the current state of our scientific understanding around the evolution of this extraordinary phenomenon.
The two major forms of insect metamorphosis are hemimetaboly (incomplete metamorphosis) and holometaboly (complete metamorphosis). Hemimetabolous insects, such as grasshoppers, transform into adults by passing through several nymphal stages, increasingly resembling the final adult. More than 80% of all described insect species are holometabolous, undergoing complete metamorphosis. In the latter case, the change from the larval stage to adult are more drastic: rather than several incremental stages, they pupate in a cocoon, during which the insect goes through a radical reorganization and remodelling of their external and internal anatomy.
The adaptive value of complete metamorphosis is to decouple the larval and the adult stages, allowing independent adaptation to distinct environments or niches within an environment. While occupying more than one niche as the same individual is clearly beneficial, a shift in niches is often accompanied by a shift in diet – as we learned from The Very Hungry Caterpillar. What we probably didn’t consider as kids, and where it gets a bit more complicated, is that this process requires significant anatomical changes of the digestive tract to make such a diet change possible. Remodelling the larval gut to its adult form creates a problem for a holometabolous insect: the gut harbours beneficial microbes which are important for digestion of food, protection from parasites, pathogens and much more. The insect host must control its gut microbiota to avoid an infection by its own gut bacteria, but a complete eradication of the gut microbiota risks the loss of important beneficial bacteria. Previous studies have shown there is rapid induction of bactericidal activity in the gut at the onset of pupation. Mainly mediated by antimicrobial peptides (AMPs) and lysozymes, suggesting the host immune system and its microbiota interact during gut metamorphosis.
The incomplete metamorphosis in hemimetabolous insects contrasts with holometaboly in that it entails much less dramatic physiological and morphological reorganisation. While both types of metamorphosis are mediated by the same major pathways, little is known about the regulation of immune effectors during moults and especially during the final moult into an adult. Indirect evidence of differences in the regulation of immunity in the gut is provided by studies which quantified the changes in gut microbiota during development. While a constant increase in microbial density and diversity during development is reported for hemimetabolous insects, complete metamorphosis drives strong reductions in bacterial density and diversity as well as changes the composition of the microbiota.
I studied metamorphosis during my masters research at the Freie Universität Berlin (my PhD research at PEARG now is rather different). In the study resulting from my masters work, we compared the temporal dynamics of immune effector gene expression in the midgut throughout the final larval moult in the holometabolous Galleria moth (Galleria mellonella) with a hemimetabolous field cricket (Gryllus bimaculatus). Using RNA sequencing we identified a number of immune effectors expressed in the gut of both species. During complete metamorphosis we found stark changes in gene expression: The upregulation of three antimicrobial peptides and a lysozyme coincide with the delamination of the larval gut in the Galleria moth.
Immune effector gene expression in the midgut during the larval– pupal moult of Galleria mellonella. Arrows denote the onset of the larval– pupal moult.
By contrast, no such upregulation was detectable in the hemimetabolous cricket we studied.
(a, b) Immune effector gene expression in the midgut during the nymphal– adult moult of the cricket Gryllus bimaculatus. Plotted values represent the coefficients and 95% confidence intervals from negative binomial generalized linear models. Arrows denote the onset of the nymph– adult moult.
These findings support the hypothesis that during the metamorphosis of holometabolous insects the remodeling of the larval gut is associated with a strong immune response in the gut. This has helped to clarify how insects gut immunity is adapted to the different types of metamorphosis. It would be very interesting to look at whether the increase in activity of the gut’s immunity functions as a protection from infection or actively regulates diet shifts (or both).
With these discoveries, we continue to move forward on our journey to unravel the mystery of what is happening during The Very Hungry Caterpillar’s remarkable transformation.