Processing of soft pupae and uneclosed pharate adults of Drosophila for scanning electron microscopy

An apocrine mechanism delivers a fully immunocompetent exocrine secretion

Apocrine secretion is a recently discovered widespread non-canonical and non-vesicular secretory mechanism whose regulation and purpose is only partly defined. Here, we demonstrate that apocrine secretion in the prepupal salivary glands (SGs) of Drosophila provides the sole source of immune-competent and defense-response proteins to the exuvial fluid that lies between the metamorphosing pupae and its pupal case. Genetic ablation of its delivery from the prepupal SGs to the exuvial fluid decreases the survival of pupae to microbial challenges, and the isolated apocrine secretion has strong antimicrobial effects in "agar-plate" tests. Thus, apocrine secretion provides an essential first line of defense against exogenously born infection and represents a highly specialized cellular mechanism for delivering components of innate immunity at the interface between an organism and its external environment.

Fine infrastructure of released and solidified Drosophila larval salivary secretory glue using SEM

The Golgi-derived large secretory granules of Drosophila salivary glands (SGs) constitute the components of the salivary glue secretion (Sgs). The Sgs represents a highly special and unique extracellular composite glue matrix that has not yet been identified outside of Cyclorrhaphous Dipterans. For over half a century, the only major and unambiguously documented function of the larval salivary glands was to produce a large amount of mucinous glue-containing secretory granules that, when released during pupariation, serves to affix the freshly formed puparia to a substrate. Besides initial biochemical characterization of the Sgs proteins and cloning of their corresponding Sgs genes, very little is known about other properties and functions of the Sgs glue. We report here observations on the fine SEM-ultrastructure of the Sgs glue released into to the lumen of SGs, and after it has been expectorated and solidified into the external environment. Surprisingly, in contrast to long held expectations, it appears to be a highly structured bioadhesive mass with an internal spongious to trabecular infrastructure, reflecting the state of its hydratation. We also found that in addition to its cementing properties, it is highly efficient at glueing and trapping microorganisms, and thus may serve a potentially very important immune and defense role. High hydration capacity, the speed by which this glue can dry, uniqueness of its protein composition and spongious infrastructure can provide inspiration for development of potential biomimetics that can attach completely different or incompatible surfaces with high efficiency and strength.

A protocol for processing the delicate larval and prepupal salivary glands of Drosophila for scanning electron microscopy

Although scanning electron microscopy (SEM) has been broadly used for the examination of fixed whole insects or their hard exoskeleton-derived structures, including model organisms such as Drosophila, the routine use of SEM to evaluate vulnerable soft internal organs and tissues was often hampered by their fragile nature and frequent surface contamination. Here, we describe a simple four-step protocol that allows for the reliable and reproducible preparation of the larval and prepupal salivary glands (SGs) of Drosophila for SEM devoid of any surface contamination. The steps are to: first, proteolytically digest the adhering fat body; second, use detergent washes to remove contaminating coarse tissue fragments, including sticky remnants of the fat body; third, use nonionic emulsifying polysorbate emulsifiers to remove fine contaminants from the SGs surface; and fourth, use aminopolycarboxylate-based chelating agents to detach sessile hemocytes. Short but repeated rinses in 100 μL of a saline-based buffer between steps ensure efficient removal of remnants removed by each treatment. After these steps, the SGs are fixed in glutaraldehyde, postfixed in osmium tetroxide, dehydrated, critically point-dried, mounted on aluminum stubs, sputter coated with gold-palladium alloy and examined in the SEM.

Calliphora vicina (Diptera: Calliphoridae) pupae: a timeline of external morphological development and a new age and PMI estimation tool

The minimum postmortem interval (PMI(min)) is commonly estimated using calliphorid larvae, for which there are established age estimation methods based on morphological and development data. Despite the increased duration and sedentary nature of the pupal stage of the blowfly, morphological age estimation methods are poorly documented and infrequently used for PMI determination. The aim of this study was to develop a timeline of metamorphosis, focusing on the development of external morphology (within the puparium), to provide a means of age and PMI estimation for Calliphora vicina (Rob-Desvoidy) pupae. Under controlled conditions, 1,494 pupae were reared and sampled at regular time intervals. After puparium removal, observations of 23 external metamorphic developments were correlated to age in accumulated degree hours (ADH). Two age estimation methods were developed based on (1) the combination of possible age ranges observed for each characteristic and (2) regression analyses to generate age estimation equations employing all 23 characteristics observed and a subset of ten characteristics most significantly correlated with age. Blind sample analysis indicated that, using the combination of both methods, pupal age could be estimated to within ±500 ADH with 95% reliability.

Apocrine secretion in Drosophila salivary glands: subcellular origin, dynamics, and identification of secretory proteins

In contrast to the well defined mechanism of merocrine exocytosis, the mechanism of apocrine secretion, which was first described over 180 years ago, remains relatively uncharacterized. We identified apocrine secretory activity in the late prepupal salivary glands of Drosophila melanogaster just prior to the execution of programmed cell death (PCD). The excellent genetic tools available in Drosophila provide an opportunity to dissect for the first time the molecular and mechanistic aspects of this process. A prerequisite for such an analysis is to have pivotal immunohistochemical, ultrastructural, biochemical and proteomic data that fully characterize the process. Here we present data showing that the Drosophila salivary glands release all kinds of cellular proteins by an apocrine mechanism including cytoskeletal, cytosolic, mitochondrial, nuclear and nucleolar components. Surprisingly, the apocrine release of these proteins displays a temporal pattern with the sequential release of some proteins (e.g. transcription factor BR-C, tumor suppressor p127, cytoskeletal β-tubulin, non-muscle myosin) earlier than others (e.g. filamentous actin, nuclear lamin, mitochondrial pyruvate dehydrogenase). Although the apocrine release of proteins takes place just prior to the execution of an apoptotic program, the nuclear DNA is never released. Western blotting indicates that the secreted proteins remain undegraded in the lumen. Following apocrine secretion, the salivary gland cells remain quite vital, as they retain highly active transcriptional and protein synthetic activity.