Allatostatins (ASTs) are a family of peptides first isolated from the cockroach Diploptera punctata and now known to be widely distributed among insect orders. In cockroaches, ASTs have a number of different functions. Their most widely documented role is the inhibition of juvenile hormone production by endocrine glands called the corpora allata (CA). ASTs also inhibit contractions of visceral muscle. Most recently, they have been implicated as modulators of digestive enzyme production by the midgut. The allatostatins are specific to insects and thus may provide an opportunity to develop novel means of insect pest control.
Left: Allatostatin inhibition of JH biosynthesis by corpora allata of day 2 virgin female Diploptera punctata (Tobe et al. 2000). Centre: Allatostatin inhibition of proctolin-induced contraction of cockroach hindgut (Fuse et al. 1999). Right: Dose-response curve of allatostatin effect on invertase activity in cockroach midgut (Fuse et al. 1999).
In most insect species, juvenile hormone is an important contributor to metamorphosis and reproduction. It is normally required for the maintenance of juvenile characteristics and its absence may be necessary for the final moult from larva to adult. In mature insects, juvenile hormone serves as a regulator of vitellogenesis (egg yolk formation) and thus plays a vital role in reproduction. In cockroaches, oocyte growth can be directly correlated with increasing levels of juvenile hormone during the first part of the gonotrophic cycle. Successful development and reproduction of most insects therefore requires precise control of juvenile hormone. In some other species, like the honey bee, many scientists think JH regulates the behavioral activities of workers through their life. In burying beetles JH has also apparently lost its role in controlling reproduction, instead regulating social behavior.
Left: Effects of corpora allata on larval molts (from here). Right: D. punctata corpora allata.
The D. punctata AST gene has been sequenced, and the gene structure suggests that it codes for thirteen different peptides. Each AST is characterized by a highly conserved C-terminus with the amino acid sequence Tyr/Phe-X-Phe-Gly-Leu/Ile-NH2 (where X=Gly, Ala, Ser, Asp, Asn), which is believed to represent the core "message" sequence of the allatostatins. Each AST has a unique N-terminal sequence of variable length. Modern nomenclature for ASTs is based on the species of origin and the sequence of peptides on the gene. For example, Dippu-AST 7 is the seventh peptide coded for by the Diploptera punctata AST gene.
Top: Diagram of the allatostatin polypeptide precursor from D. punctata. Black rectangles represent individual allatostatins; arrows are sites for endoproteolytic cleavage; white triangles are potential amidation sites. Below: D. punctata allatostatin amino acid sequences deduced from cDNA. Both figures from Donly et al. 1993.