Rhodnius, Golden Oil, and Met: A History of Juvenile Hormone Research

Abstract

Juvenile hormone (JH) is a unique sesquiterpenoid hormone which regulates both insect metamorphosis and insect reproduction. It also may be utilized by some insects to mediate polyphenisms and other life history events that are environmentally regulated. This article details the history of the research on this versatile hormone that began with studies by V. B. Wigglesworth on the "kissing bug" Rhodnius prolixus in 1934, through the discovery of a natural source of JH in the abdomen of male Hyalophora cecropia moths by C. M. Williams that allowed its isolation ("golden oil") and identification, to the recent research on its receptor, termed Methoprene-tolerant (Met). Our present knowledge of cellular actions of JH in metamorphosis springs primarily from studies on Rhodnius and the tobacco hornworm Manduca sexta, with recent studies on the flour beetle Tribolium castaneum, the silkworm Bombyx mori, and the fruit fly Drosophila melanogaster contributing to the molecular understanding of these actions. Many questions still need to be resolved including the molecular basis of competence to metamorphose, differential tissue responses to JH, and the interaction of nutrition and other environmental signals regulating JH synthesis and degradation.

Keywords: Hyalophora cecropia; Manduca sexta; Methoprene-tolerant; Rhodnius prolixus; juvenile hormone.

FIGURE 1

Top: Vincent B. Wigglesworth (left); non-fed (left) and blood-fed (right) nymphs of Rhodnius prolixus (center); cartoon of the normal development of a nymph which after feeding on a blood meal (orange) molts to the next nymphal instar 6 days later. Bottom: Cartoon of decapitation experiments shows that decapitation immediately after feeding prevents the molting but decapitation 3 days after feeding does not. Photos of Wigglesworth from Phillips (2004) and of Rhodnius taken by Timothy Bradley and Catherine Loudon from Knight (2014) (reproduced with permission).

FIGURE 2

Top: Diagram of brain and corpus allatum (CA) of Rhodnius (modified from Figure 2 in Wigglesworth, 1934); Rhodnius 5th instar (N5), supernumerary 6th instar (N6), and parabiosed, fed and decapitated (decap) 4th instar (N4) (top one decapitated one week after feeding, the bottom 1 day after feeding) nymphs [photos from Plates I and II in Wigglesworth (1954), reproduced with permission]. Bottom: Cartoon of two parabiosis experiments: (1) (top) a 4th instar nymph decapitated 1 day after feeding and parabiosed to a 5th instar nymph decapitated 8 days after feeding (thus ready to molt) molted to a precocious adult; (2) a parabiosed 4th instar nymph with only the tip of the head removed (leaving the CA intact) 6 days after feeding parabiosed to a 5th instar nymph decapitated only 1 day after feeding caused the latter to molt to a supernumerary 6th instar nymph (Wigglesworth, 1934). See text for details. ao, aorta; eso, esophagus; sg, sympathetic ganglion; SOG, subesophageal ganglion.

FIGURE 3

Top: (Left) Diagram of effects of removal of the corpora allata (CA) (allatectomy) from the silkworm Bombyx mori. (A) Normal 5th instar larva and pupa; (B) allatectomized 4th instar larva formed a precocious pupa; (C) allatectomized 3rd instar larva formed a precocious pupa (figure from Bounhiol, 1938). (Right) Jean-Jacques Bounhiol (above); photo of pupae formed from allatectomized 4th (left) and 3rd (right) instar larvae (from Bounhiol, 1938) (bottom). Bottom: (Left) Hans Piepho. (Center) Result of an implant of 5th instar Galleria integument (epidermis plus cuticle) into another 5th instar Galleria larva that subsequently metamorphosed to a pupa and an adult. The implanted epidermis formed a pupal cuticle, then an adult cuticle with scales (from Piepho, 1938a). (Right) Result of an implant of 1^st instar integument into a 5th instar larva after metamorphosis of the host. The implant formed a pupal, then an adult cuticle (from Piepho, 1938b). Photo of Bounhiol from Lamy and Delsol (1980) (reproduced with permission from Elsevier Masson) and that of Piepho from Hintze-Podufal (1996) (reproduced with permission from www.schweizerbart.de/journals/entomologia).

FIGURE 4

Left: (Top) Carroll M. Williams. Photo from his memoir in the National Academy of Sciences Memoir collection. (Bottom) Cecropia male abdomen parabiosed to a chilled diapausing Cecropia pupa (left); after 21 days, the “second pupa” had formed and the abdomen had molted to a scaleless adult abdomen (right, arrow) (from the original slides taken by Muriel V. Williams that were used for the black-and-white Figures 3, 4 in Williams, 1963). Center: Diagram of the Cecropia male moth whose corpora allata (CA) synthesize and secrete juvenile hormone (JH) acid and from whose abdomen the “golden oil” that contained JH was extracted (drawing of the moth modified from a figure in Williams, 1958). Right: Reproductive system of a male saturniid moth (modified Figure 1 in Shepherd, 1974). The accessory glands of Cecropia males synthesize JH from the JH acid that is secreted from the CA, then store the hormone (Shirk et al., 1976; Peter et al., 1981).

FIGURE 5

Structures of the natural juvenile hormones (JH) (left) and some JH analogs known as insect growth regulators (IGRs) (right). JHB₃, JH bisepoxide; JHSB₃, JH skipped bisepoxide; MF, methyl farnesoate. Prepared by Xavier Belles.

FIGURE 6

(A) Top: (Left) Tom Wilson (photo was taken by Emily Wilson and used with her permission). The adult abdominal sternites of the Methoprene-tolerant (Met⁺) mutant of Drosophila melanogaster after the larvae fed on diet containing methoprene (modified from Figure 2 of Wilson and Fabian, 1986). +/+, homozygous wild type; +/Met, heterozygous Met mutant; Met/Met, homozygous Met mutant. Bottom: Barbora Konopova and Marek Jindra and the precocious pupa they obtained after giving Met RNAi to a 4th instar larva of Tribolium castaneum (right) compared with a normal pupa that formed after egfp RNAi had been given to a 4th instar larva (left) (modified from Figure 3 in Konopova and Jindra, 2007). Photos of Barbora Konopova from her Research Gate file (used with her permission) and of Marek Jindra taken by Masako Asahina (used with Dr. Jindra’s permission). (B) Top: Cartoon of the Met protein showing its various domains: bHLH, basic helix-loop-helix domain; the Period-Arylhydrocarbon receptor nucleotide translocator-Single-minded (PAS) A and B domains; the C terminal domain. The bHLH domain binds to the juvenile hormone (JH) response element (JHRE) on the target gene promoter; JH binds to the Pas B domains of Met (Charles et al., 2011) and Germ cell-expressed (Gce) [the paralog of Met (Baumann et al., 2010)] (Bittova et al., 2019). Bottom: Cartoon of events occurring when JH enters a target cell. The unliganded Met is thought to exist as a homodimer (Godlewski et al., 2006). When JH is present, it binds to Met causing the dissociation of the dimer and the binding of Taiman (Tai) to Met, then both Tai and Met bind to the JHRE on the promoters of Krüppel homolog 1 (Kr-h1) and other JH target genes (Jindra et al., 2015a).

FIGURE 7

Summary diagram of milestones in juvenile hormone research. See text for details. CA, corpus allatum; IGR, insect growth regulator; JH, juvenile hormone; Kr-h1, Krüppel homolog 1; Met, Methoprene-tolerant; Tai, Taiman.