Abstract
AbstractAs a model organism, the tobacco hornworm Manduca sexta (Linnaeus 1763) has contributed much to our knowledge of developmental processes in insects, and major developmental changes between different larval instars are generally well understood. Second and later instars of M. sexta do not produce silk, and their spinneret and accessory labial glands (=Lyonet’s glands), structures thought to be key players in silk production in other lepidopterans, are highly reduced. To our knowledge, mouthparts and labial gland morphology of the silk-producing first instar have never been described. In this study, we compared the mouthpart morphology and transcriptome profile of first and later instars of M. sexta to determine whether the loss of silk production correlates with changes in the structure of the spinneret and the labial glands, and with changes in expression of silk-related genes. We found that the first instar, unlike later instars, has a typical, silk-producing spinneret with a tube-like spigot and well developed Lyonet’s glands. Moreover, three known silk protein genes are highly expressed in the first instar but exhibit little to no expression in the embryo or later instars. Thus, the changes in morphology and gene expression presented here, coinciding with changes in larval behavior from silk production to saliva spreading, further our understanding of the developmental processes underlying this transition in this model organism.
Highlights
The self-organization of the silk fiber (Ripley 1924, Wang et al 2016), exhibiting a morphology highly specialized for this function
Morphology we provide a general description of morphological traits of the proximal region of the labial gland and the musculoskeletal system of the labio-hypopharyngeal lobe of M. sexta that are shared with all instars
Lyonet’s Glands and Spigots are Present in M. sexta Larvae Modifications of the Lepidoptera spinneret and Lyonet’s gland between silk-producing and non-silk-producing species and/or instars make these structures feasible models to study how form and function change during the course of both development and evolution (Helm 1876, Ripley 1924, MacKay 1964)
Summary
The self-organization of the silk fiber (Ripley 1924, Wang et al 2016), exhibiting a morphology highly specialized for this function. Previous studies have suggested that both the spigot and the Lyonet’s gland are absent from M. sexta (Leslie and Robertson 1973, Kent et al 1987, Felton et al 2014). This finding is not surprising as it is well known that later instars of M. sexta do not produce silk. The aim of the present study is to explore the anatomy of the spinneret and labial glands in different M. sexta instars and reveal any morphological changes between silk-producing and non-silk-producing instars. We compare available transcriptomes to determine how silk gene expression shifts with changing morphologies in this species
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