Midgut morphological changes and autophagy during metamorphosis in sand flies

Abstract

During metamorphosis, holometabolous insects undergo significant remodeling of their midgut and become able to cope with changes in dietary requirements between larval and adult stages. At this stage, insects must be able to manage and recycle available food resources in order to develop fully into adults, especially when no nutrients are acquired from the environment. Autophagy has been previously suggested to play a crucial role during metamorphosis of the mosquito. Here, we investigate the overall morphological changes of the midgut of the sand fly during metamorphosis and assess the expression profiles of the autophagy-related genes ATG1, ATG6, and ATG8, which are associated with various steps of the autophagic process. Morphological changes in the midgut start during the fourth larval instar, with epithelial degeneration followed by remodeling via the differentiation of regenerative cells in pre-pupal and pupal stages. The changes in the midgut epithelium are paired with the up-regulation of ATG1, ATG6 and ATG8 during the larva-adult transition. Vein, a putative epidermal growth factor involved in regulating epithelial midgut regeneration, is also up-regulated. Autophagy has further been confirmed in sand flies via the presence of autophagosomes residing within the cytoplasmic compartment of the pupal stages. An understanding of the underlying mechanisms of this process should aid the future management of this neglected tropical vector.

Autophagy-dependent cell death

“…In Lepidoptera (silk worm), the induction of autophagy and apoptosis gene expression during metamorphosis occurs in response to ecdysone 20E [93]. In other Diptera (sand flies), the upregulation of Atg gene expression coincides with increased ecdysone during midgut removal; however, it is yet to be established if they are regulated by ecdysone [94]. There may be additional roles for ecdysone in the regulation of autophagy pathway activity as in H. armigera conjugation of ATG12-ATG5 occurs in response to ecdysone in a concentration-and time-dependent manner [57].…”

Cell death during complete metamorphosis

“…In haematophagous Diptera, the remodelling of the midgut during metamorphosis is fundamental to produce an organ that is able to cope with the change in dietary requirements between the larval and the adult stage. Malta et al [89] demonstrated that metamorphic changes in the larval midgut epithelium of sand flies are associated with the upregulation of several ATG genes owing to the presence of ecdysone-responsive elements in their promoter, as occurs in the silkworm. This evidence confirms that the mechanisms controlling the onset of metamorphosis are conserved across multiple orders of holometabolous insects.…”

De novogenome assembly of the Tobacco Hornworm moth (Manduca sexta)

“…However, we did identify an increase in expression of genes within the autophagic pathway beginning at the L5 pre-wandering stage (G-L5-preW-S), which coincides with the decline in digestive protease expression at the L5 wandering stage (G-L5-W-S/G-L5-W) ( Figure 4B). These results indicate the potential involvement of the autophagic pathway in M. sexta midgut during metamorphosis, at the same time in development that autophagy has been shown to remodel the midgut of other insects such fruit flies [61] , silkmoths [56] , and sand flies [62] . [25] .…”