Neurohormones implicated in the control of Malpighian tubule secretion in plant sucking heteropterans: The stink bugs Acrosternum hilare and Nezara viridula
Introduction
Heteropteran insects (the “true” bugs) have sucking mouth parts and are obligate fluid feeders. The majority feed on plant material or other insects, but some, such as the kissing bug Rhodnius prolixus (Heteroptera; Triatomidae), feed on the blood of vertebrates. Plant sucking bugs feed regularly and take relatively small fluid meals, whereas their hematophagous relatives feed irregularly and may imbibe blood meals that are so large the insect is rendered virtually immobile. The different feeding habits of plant sucking and blood sucking bugs impose different requirements on the excretory system for the maintenance of body fluid homeostasis, and this is likely to be reflected in the hormonal control of Malpighian tubule (MT) secretion. This has been intensively studied in R. prolixus, but little is known about the control of MT secretion in plant sucking bugs.
R. prolixus consumes blood meals equivalent in volume to 10–12 times the unfed body weight. To regain maneuverability and to concentrate nutrients (blood cells) in the gut, much of the imbibed plasma (NaCl and water) is rapidly absorbed into the hemolymph from the expanded anterior midgut (a functional crop), transported into the lumen of the upper (secretory) segment of the MT, and voided as NaCl-rich urine from the anus. The rapid diuresis lasts about 3 h, during which time drops of urine are voided from the anus every 2–3 min, and about 50% of the imbibed salt and water are excreted. The volume and composition of the hemolymph change very little after feeding, because rates of absorption of NaCl-rich fluid from the blood meal in the anterior midgut and excretion of NaCl-rich urine are precisely matched.
Pioneering work by Simon Maddrell (reviewed in Ref. [2]) demonstrated that the rapid diuresis is initiated by release of a diuretic hormone (DH) released within seconds of the insect commencing to feed [16] from neurohemal sites on abdominal nerves originating from the fused mesothoracic ganglion mass (MTGM). The DH accelerates MT secretion >1000-fold and also stimulates fluid uptake from the anterior midgut [9], thereby ensuring that the two processes are closely coordinated.
In R. prolixus, the rapid diuresis is initiated by serotonin [13], [18], which is released at neurohemal sites along axons that originate from five dorsal unpaired median (DUM) cells in the MTGM and project into the lateral abdominal nerves. The circulating titer of serotonin increases to >100 nM within 5 min of feeding [13], which is sufficient to maximally stimulate fluid uptake from the anterior midgut and MT secretion [2], [9], [17]. Serotonin levels fall after 5 min, and the rapid diuresis is sustained by release of a corticotropin releasing-factor (CRF)-like DH from the axons of posterior lateral neurosecretory cells in the MTGM that also project into the abdominal nerves [39]. A CRF-like DH of R. prolixus has yet to be identified, but Zoone-DH, a CRF-like peptide from the termite Zootermopsis nevadensis, maximally stimulates fluid transport across both the anterior midgut and the upper MT segment [38], [40]. The rapid diuresis ceases after about 3 h, which was originally thought to be due to the removal of the stimulus for DH release and the degradation/excretion of circulating hormone. More recently, however, evidence has emerged for the release of an antidiuretic peptide toward the end of the rapid diuresis, which decreases stimulated rates of fluid transport across both the upper MT and the anterior midgut [29], [36]. This peptide belongs to the family of cardioacceleratory 2b (CAP2b) peptides, which are designated CAPA peptides, because they are encoded on the capability (capa) gene (capable of encoding CAP2b) of the fruit fly, Drosophila melanogaster. The capa gene of R. prolixus is expressed in three bilaterally paired cells on the ventral surface of the MTGM, and CAPA-like immunoreactive material is released from their axons, which project into the abdominal nerves, toward the end of the rapid diuresis [31], [32].
In contrast to R. prolixus, plant sucking bugs feed on material with low salt content and do not imbibe large volumes of fluid. The rapid diuresis and natriuresis that enables the speedy removal of much of the volume and salt load consumed by R. prolixus is therefore inappropriate for a plant sucking insect. The only study to date of MT function in a plant sucking heteropteran [20] focussed on the handling of the cardiac glycoside ouabain by the large milkweed bug Oncopeltus fasciatus (Heteroptera; Lygaeidae). Little is known of the control of MT secretion in plant sucking bugs, but serotonin appears to be a neurohormone in O. fasciatus, although the serotonergic DUM cells that are found in R. prolixus are absent [21]. Moreover, serotonin does not stimulate cAMP production by O. fasciatus MT, although this is the diuretic second messenger in R. prolixus. The plant sucking bug may, therefore, not use serotonin as a DH, which is consistent with it not requiring the rapid diuresis of its blood feeding relative.
The present study examines the control of MT secretion in two related plant sucking bugs (Heteroptera: Pentatomidae), the green stink bug Acrosternum hilare, and the Southern green stink bug Nezara viridula, polyphagous pentatomids that have a detrimental effect on product quality and yield in cotton and other row, fruit and nut crops [33]. More recently, adult southern green stink bugs have been shown to vector plant pathogens in cotton [19]. These bugs feed by inserting their proboscis into the host plant and sucking up nutrients. They employ a macerate-and-flush strategy [10]; the mandibular and maxillary stylets contained within the proboscis sheath cut into the plant and saliva, which contains digestive enzymes, is injected into the wound to liquefy the tissues [19]. The macerated and partially digested material is “flushed out” by the saliva and ingested by sucking. Relatively small volumes of liquefied material, much of which is the injected saliva, are imbibed and there is little obvious abdominal distension during feeding. In this paper, attention has focussed on the effects of serotonin and of neuropeptides implicated in the control of MT secretion, namely kinin, CRF-related peptides and CAPA peptides. Kinin and CRF-related peptides have yet to be identified from stink bugs, but two CAPA peptides (Nezvi-CAPA-1 and -2) are known from N. viridula [34] and identical peptides are present in A. hilare [35]. The effect of CAPA peptides on stink bug MT was of particular interest since these peptides have diuretic activity in dipteran insects [6], [22], [27], [26], but antidiuretic activity in R. prolixus. The immunocytochemical localization of CAPA cells in N. viridula is identical to that described in R. prolixus and the presence of neurohemal release sites on the abdominal nerves suggests CAPA peptides function as circulating hormones.
Section snippets
Insects
Adult green stink bugs (A. hilare) were captured in 40 W light traps (with live insect canisters) located adjacent to fields cultivated with corn, cotton, sorghum and soybeans in Burleson County, Texas. Adult Southern green stink bugs (N. viridula) were obtained from a colony maintained at the USDA Invasive Insects Biocontrol & Behavior Laboratory, Beltsville Agricultural Research Center in Beltsville, MD.
Fluid secretion assay
Stink bugs have four MTs that are situated in the dorsal abdomen with their closed distal
Performance of A. hilare MT in O. fasciatus saline
The MT of A. hilare functioned well in O. fasciatus saline as illustrated in Fig. 1, which shows the mean rate of secretion by four tubules of similar length removed from one insect. The rate of secretion measured over the first 30 min was 1.83 ± 0.52 nL min−1, which does not differ significantly from that measured after 3 h (1.31 ± 0.31 nL min−1; P = 0.418, unpaired t-test).
Effect of serotonin
Serotonin acts via cAMP to stimulate maximal secretion by MT of the blood sucking bug R. prolixus, but does not increase cAMP
Discussion
Although the stink bugs, A. hilare and N. viridula, and the triatomid bug, R. prolixus, are all heteropteran insects, their feeding habits are very different as are the requirements placed on their excretory systems for hemolymph homeostasis. R. prolixus imbibes a large volume and salt (NaCl) load, which must be voided for the insect to regain its maneuverability and this is achieved during the rapid postprandial diuresis and accompanying natriuresis. Stink bugs, on the other hand, feed
Acknowledgements
We wish to acknowledge the able technical assistance of Allison Strey and Chris Parker, and the financial support of a Collaborative Research Grant (#LST.CLG.979226) from the North Atlantic Treaty Organization (NATO) (GMC, RJN) a Binational Agricultural Research and Development Grant (BARD # IS-4205-09C) (RJN), and a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (IO).
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2013, Advances in Insect PhysiologyCitation Excerpt :In contrast to the dipteran species, Manduca CAP2b at low doses yielded antidiuretic responses in MTs of R. prolixus (Quinlan et al., 1997) and T. molitor (Wiehart et al., 2002). Also CAPA peptides, which were isolated and identified from the Southern green stink bug Nezara viridula (Predel et al., 2006, 2008), demonstrated antidiuretic activity (Coast et al., 2010) and most likely act in vivo to terminate cAMP-stimulated diuresis. It has been established that unlike CRF-like DHs, CAP2b neuropeptides act via cGMP as a second messenger (Broderick et al., 2003; Davies et al., 1995, 1997; Dow et al., 1994; Pollock et al., 2004; Quinlan et al., 1997).
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2012, General and Comparative EndocrinologyNatriuresis and diuretic hormone synergism in R. prolixus upper Malpighian tubules is inhibited by the anti-diuretic hormone, RhoprCAPA-α2
2012, Journal of Insect PhysiologyCitation Excerpt :Although less studied, anti-diuretic factors which act upon the hindgut to increase reabsorption of selected ions and water have also been identified (Audsley et al., 1992; Fournier and Girardie, 1988; Meredith et al., 1996; Phillips et al., 1980; Spring and Phillips, 1980). In addition, peptidergic anti-diuretic factors which inhibit fluid secretion by MTs (Coast et al., 2010, 2011; Eigenheer et al., 2003; Paluzzi and Orchard, 2006; Paluzzi et al., 2008; Quinlan et al., 1997) and absorption by the anterior midgut (Ianowski et al., 2010; Orchard and Paluzzi, 2009) have been described. In Rhodnius prolixus, two diuretic hormones have been identified, including the biogenic amine serotonin (5-hydroxytryptamine, 5HT) and a vertebrate corticotropin releasing factor-related peptide, RhoprDH (Lange et al., 1989; Maddrell et al., 1991; Te Brugge et al., 2002, 2011b).