Cells of the rectum of Bombyx mori affected by experimental inoculation with Alphabaculovirus.

Bombyx mori is an insect whose cocoon is used in the sericulture industry, which is an important activity in parts of southern Brazil. When parasitized by Bombyx mori nucleopolyhedrovirus (BmNPV) of the genus Alphabaculovirus (alphaBV), it develops nuclear polyhedrosis disease. In Brazil, an alphaBV was isolated from larvae of B. mori and various target tissues were identified. However, how this geographic viral isolate affects the rectum of silk moths was unknown. The rectum, a component of the cryptonephric system, acts to absorb water and mineral salts, and its importance for the metabolic balance of insects provoked interest in analyzing how it is affected by BmMNPV infection. Fifth instar B. mori larvae were inoculated with a viral suspension, and from the second to the ninth day post-inoculation, segments of the rectum (anterior and anal canal) were examined using light microscopy. The cryptonephric epithelial cells in the anterior region revealed no evidence of infection. However, from the fifth day post-inoculation, cells of the anal canal showed cytopathologies characteristic of alphaBV. Infection of the anal canal and other surrounding tissues led to tissue disorganization, with accumulation of polyhedra in the perinephric space and compartmentalization of the cryptonephric system, promoting changes in the fecal pellets, signalling physiological changes. These observations contribute to our understanding of the infectious cycle of BmMNPV in B. mori.

Bombyx mori pylorus infection by Alphabaculovirus.

Alphabaculovirus is an entomopathogenic virus genus that infects Bombyx mori, which is known as the Bombyx mori multiple nucleopolyedrovirus (BmMNPV). This virus is polyorganotrophic, and a series of tissues are known as targets; however, there is currently no information regarding infection in the pylorus, the segment of the hindgut that is present in the midgut transition and is responsible for food passage control. Thus, in the present study, we aimed to analyze infection of the B. mori pylorus by BmMNPV. To do so, hybrid B. mori larvae were inoculated with a viral suspension of BmMNPV, and segments of the intestine containing the pylorus and its subdivisions, the posterior interstitial ring (PIR), pyloric cone, and pyloric valve, were dissected and processed for light microscopy on different days post inoculation. The results showed that B. mori pylorus subdivisions respond differently to infection, and the anterior area of the PIR is susceptible with these cells being the secondary infection targets. Cytological analysis revealed the presence of viroplasm in the hypertrophic nucleus, followed by the formation and development of viral polyhedra. Cytolysis occurred at the end of the infectious cycle, thereby releasing polyhedra and enabling the spread of the disease. There was no evidence of BmMNPV infection in the posterior area of the PIR, cone, or pyloric valve. These results will contribute to greater understanding of the virus infectious cycle, whose consequent epizootic disease can negatively impact this economically important insect that is used in silk production in Brazil.

Morphology of the Pylorus of Bombyx mori (Linnaeus) (Lepidoptera: Bombycidae).

The pylorus is a segment of the hindgut of great morphological diversity, which is subdivided into the posterior interstitial ring, pyloric cone, and pyloric valve. There is no morphological information on Bombyx mori (L.) hybrids, which is an insect of economic interest to Brazil. Consequently, this study aimed to analyze the morphology of the pylorus of hybrid larvae of fifth instars of B. mori from crossbreeding of Chinese and Japanese races. These were anesthetized, and the pylorus was fixed and processed by light microscopy and scanning electron microscopy. The pylorus presented as a separate compartment, consisting of a simple epithelium, with large variation in cell morphology, covered on the luminal side by an intima with specializations in some areas, the spicules. The musculature was formed by bundles of striated muscle fibers in longitudinal and circular arrangement. These fibers were multinucleated with central and peripheral nuclei, operating in peristalsis. The morphology of the pylorus is a reflection of functional dynamism, and this study provided a better understanding of its physiology, which promotes the regulated movement of the bolus, preventing reflux and directing it towards the ileum in the formation of fecal pellets.