CHARACTERIZATION OF HERMETIA ILLUCENS (DIPTERA: STRATIOMYIDAE) MIDGUT
Creators: name: BONELLI, MARCO
Publisher: name: Universit� degli Studi di Milano
Year: 2019

description: Introduction The search for new protein sources for animal feed is a mandatory and actual issue, since the current population increase is leading to an increase in global demand for meat with a consequent higher request of protein for feed production. Another serious global problem is food waste, since it is estimated that about one third of the food produced globally is wasted. In this context, in addition to policies to reduce meat consumption and production of food waste, a possible perspective is to consider insects as feed, because they are good agents for organic waste bioconversion into high nutritional value proteins. One of the most promising species for this purpose is Hermetia illucens (Linnaeus, 1758) (Diptera: Stratiomyidae), due to the ability of the larvae to grow on a wide variety of organic substrates, their high efficiency in the bioconversion process and their high content of proteins with high nutritional value, and other macro- and micro-nutrients important for animal feed. Despite the great interest toward this species, the current literature mainly provides information on rearing methods for this insect, the use of larvae for waste treatment, their nutrient composition, and their suitability for feed production, while little is known on the biology of this insect. In particular, a deep understanding of the physiology of the midgut, which is the organ responsible for nutrient digestion and absorption, is essential to better comprehend the extraordinary dietary plasticity of the larva and to better exploit its bioconversion ability. Chapter 1 In the first chapter, an in-depth morphofunctional characterization of the larval midgut of H. illucens is reported. Our results demonstrate that the larval midgut is composed of distinct anatomical regions with different luminal pH and specific morphofunctional features. The midgut epithelium is formed by different cell types that accomplish nutrient digestion and absorption, acidification of the lumen of the middle region, endocrine regulation, and growth of the epithelium. A detailed characterization of the activity of enzymes involved in nutrient digestion and their mRNA expression levels reveals that protein and carbohydrate digestion is associated to specific regions of this organ. Moreover, a significant lysozyme activity in the lumen of the anterior and middle regions of the midgut was detected. This enzyme, together with the strong acidic luminal pH of middle tract, may play an important role in killing pathogenic microorganisms ingested with the feeding substrate. The obtained data allow us to propose a detailed functional model of the larval midgut of H. illucens in which each region is characterized by peculiar features to accomplish specific functions. Chapter 2 The second chapter reports the effects of feeding substrates with different nutritional composition on morphofunctional properties of the larval midgut. Our data show a diet-dependent adaptation process of this organ: differences in cell morphology, activity of digestive enzymes, and accumulation of long-term storage molecules were observed. These results demonstrated the existence of a diet-dependent adaptation process of the midgut. This plasticity may be responsible for the ability of H. illucens larvae to growth and develop on very different organic substrates. Chapter 3 In the third chapters, the effect of diets with different nutrient content on the midgut microbiota of H. illucens larvae, and the impact of the insect feeding activity on the diet microbiota are investigated. In particular, using three feeding substrates with different protein/carbohydrates (P/C) ratio, we established that the diet impacts on the midgut microbiota, in particular a diet with very high P/C ratio do not seem to be optimal for H. illucens rearing, since it induces a dysbiosis. Moreover, the regionalization of this organ influences the microbiota: the bacterial community of the anterior midgut always shows higher diversity than the posterior, whereas bacterial load has an opposite trend, being maximal in the posterior region. We also demonstrated that H. illucens larvae do not affect the microbiota composition of the feeding substrate. The overall data indicate the importance of taking into account the presence of different midgut structural and functional domains, as well as the microbiota of the diet, in any further study that aims at clarifying microbiological aspects concerning H. illucens larval midgut. Chapter 4 In the fourth chapter, the remodeling process of H. illucens larval midgut during metamorphosis, the morphological and functional properties of the adult midgut, and the feeding habits of the fly are investigated. Our results show that the larval midgut is removed during metamorphosis and a new pupal-adult epithelium is formed by proliferation and differentiation of intestinal stem cells. Moreover, at variance with majority of current literature, we demonstrated that the adult insect possesses a functional digestive system: mouthpart allows food ingestion, the bolus can transit along the gut, and the midgut epithelium is endowed with digestive capability. This new scenario, not only opens up the possibility to manipulate the feeding substrate of the adult insect to improve its performances in mass rearing procedures, but could also provide insights into the safety on the use of this insect for feed purposes.