Structure and development of the subesophageal zone of the Drosophila brain. II. Sensory compartments

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The subesophageal zone (SEZ) of the brain in flies and other arthropods plays a fundamental role in the control of many different behaviors. The SEZ represents a domain of the brain that results from the fusion of four neuromeres, the three gnathal neuromeres [mandibula, maxilla, and labium; these together forms the subesophageal ganglion (SEG)], and the tritocerebrum. The SEZ houses sensory circuits and central pattern generators for feeding behaviors (Dethier, 1976; Freeman & Dahanukar, 2015; Schachtner & Bräunig, 1993; Rand, Gueijman, Zilberstein, & Ayali, 2008; Rast & Bräunig, 2001; Wright, 2016), in addition to premotor centers that integrate information about wind speed and direction, as well as head and body orientation, to modulate, via descending interneurons, motor circuits for flight and walking/crawling in the thoracic/abdominal ganglia (Ai & Hagio, 2013; Altman & Kien, 1979; Gal & Libersat, 2006; Hustert & Klug, 2009; Kien, 1990; Schröter, Wilson, Srinivasan, & Ibbotson, 2007; Tastekin et al., 2015; Tyrer, Bacon, & Davies, 1979;). In Drosophila, gustatory sensilla, along with a multitude of mechanoreceptors located on the mouthparts and head capsule, project their axons into the neuropil of the SEZ. Gustatory and mechanosensory input is integrated with sensory information from the inner organs (parameters like extension of the gut or nutrient levels in the tissues, which reflect the need for food intake) to generate a motor output controlling feeding behavior (Bader et al., 2007; Colomb, Grillenzoni, Ramaekers, & Stocker, 2007; Gendre et al., 2004; Gordon & Scott, 2009; Harris, Kallman, Mullaney, & Scott, 2015; Hückesfeld, Schoofs, Schlegel, Miroschnikow, & Pankratz, 2015; Stocker, 2008).
In the adult fly, gustatory sensilla are found externally, at the tip of the proboscis (labellum), the anterior wing margin, and the tarsi (feet) of the six legs (Falk, Bleiser‐Avivi, & Atidia, 1976; Nayak & Singh, 1983). Internal gustatory sensilla form three complexes, the labral sensory organ, and the ventral and dorsal cibarial sensory organ (Nayak & Singh, 1983; Singh, 1997; Stocker & Schorderet, 1981; Figure 1a) in the mouth cavity and pharynx. Other sensory organs include olfactory receptors on the antenna and maxillary palp, external mechanosensory bristles on the head capsule, antenna, maxillary palp, and proboscis, and the array of antennal chordotonal organs, known as Johnston's organ, which is receptive to sound and vibration (Kamikouchi, Shimada, & Ito, 2006; Matsuo et al., 2016; Singh & Nayak, 1985; Stocker & Lawrence, 1981). All of these sensilla are formed during metamorphosis; only the internal gustatory receptors are already present in the pharynx of the larva, where they are called dorsal pharyngeal sensory organ (DPS; forerunner of adult labral and ventral cibarial sensory organs) and posterior pharyngeal sensory organ (PPS; forerunner of the adult dorsal cibarial sensory organ; Gendre et al., 2004; Figure 1b). In addition, the larva possesses a ventral pharyngeal sensory organ (VPS), derived from the labial segment, and two external gustatory/olfactory sensory complexes, called the terminal organ (TO) and the dorsal organ (DO), the former derived from the maxillary segment, the latter the antennal segment (Campos‐Ortega & Hartenstein, 1997; Gendre et al., 2004; Singh & Singh, 1984). The VPS, TO, and DO undergo programmed cell death during metamorphosis; neither antennal nor maxillary appendages of the adult regain taste sensilla, but develop olfactory and mechanosensory organs instead. The larval VPS, which is derived from the same segment (labium), and is innervated by the same nerve (labial nerve) as the adult labellum, also undergoes cell death and is replaced by the gustatory sensilla of the labellum.
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