The roles of midgut hormone and allatotropin in the adult black blow fly, Phormia regina Meigen (Diptera: Calliphoridae)
Nutrition greatly affects the reproductive success in many insects. Also, previous studies have shown this nutrition-reproduction connection is mediated via endocrine control. In Phormia regina, a proteinaceous meal is essential for egg maturation. Recent discovery of a midgut hormone, from liver-fed P. regina, established a link between feeding and endocrine function which leads to oogenesis. This midgut hormone is the first such insect gut hormone governing oogenesis. Whether or not this hormone is released into the hemolymph, however, is not determined. To demonstrate the existence of the midgut hormone in the hemolymph, hemolymph was partially purified and its oogenesis-stimulating effect tested. A gonadotropic effect was observed in the hemolymph extracts prepared from 5 and 6h post-liver-fed females. The midgut hormone appears to circulate in the hemolymph to reach its target, presumably the brain neurosecretory cells. Injection of partially purified midgut hormone caused a significant volume increase and changes in the distribution of stainable materials of the type-A brain median neurosecretory cells (MNCs). It is conceivable that these cellular changes reflect the synthesis/release of neurohormones by the type-A MNCs. Thus, results suggested that midgut hormone is hemolymph-borne and has a cerebrotropic function.
The possible role of the brain after being stimulated by midgut hormone was investigated. In vitro studies showed that the brain releases, at 8h after a protein meal, a factor(s) with a strong allatotropic effect (i.e., causing the corpus allatum (CA) to produce 6.9 times more juvenile hormones (JHs) than the control CA). The characteristics of this allatotropic factor(s) were estimated with different experiments. First, Manduca sexta allatotropin (Mas-AT)-immunopositive substances were observed in the lateral neurosecretory cells (LNCs) and the neurosecretory cells located between the pars intercerebralis and oesophagus foramen. Second, synthetic Mas-AT stimulated JH biosynthesis by the fly's CA (2.64-fold activation when stimulated with 20 pmol per test). Third, when brain-released factor(s) was neutralized with anti-Mas-AT antiserum both in vivo and in vitro, the CA still synthesized a considerable quantity of JH. This result suggested that there may be additional non-Mas-AT allatotropic agent in P. regina.
Anatomy & physiology;
0433: Anatomy & physiology