The pupal specifier broad directs progressive morphogenesis in a direct-developing insect

Abstract

A key regulatory gene in metamorphosing (holometabolous) insect life histories is the transcription factor broad (br), which specifies pupal development. To determine the role of br in a directdeveloping (hemimetabolous) insect that lacks a pupal stage, we cloned br from the milkweed bug, Oncopeltus fasciatus (Of'br). We find that, unlike metamorphosing insects, in which br expression is restricted to the larval-pupal transition, Of'br mRNA is expressed during embryonic development and is maintained at each nymphal molt but then disappears at the molt to the adult. Induction of a supernumerary nymphal stage with a juvenile hormone (JH) mimic prevented the disappearance of br mRNA. In contrast, induction of a precocious adult molt by application of precocene II to third-stage nymphs caused a loss of br mRNA at the precocious adult molt. Thus, JH is necessary to maintain br expression during the nymphal stages. Injection of Of'br dsRNA into either early third-or fourthstage nymphs caused a repetition of stage-specific pigmentation patterns and prevented the normal anisometric growth of the wing pads without affecting isometric growth or molting. Therefore, br is necessary for the mutable (heteromorphic) changes that occur during hemimetabolous development. Our results suggest that metamorphosis in insects arose as expression of br, which conveys competence for change, became restricted to one postembryonic instar. After this shift in br expression, the progressive changes that occur within the nymphal series in basal insects became compressed to the one short period of morphogenesis seen in the larva-to-pupa transition of holometabolous insects.evolution of metamorphosis ͉ heteromorphosis ͉ Oncopeltus ͉ juvenile hormone ͉ allometry L ife history strategies are highly plastic within animal phyla; some groups develop directly, whereas related taxa pass through a metamorphosis. Regulation of stage-specific differences may be under either environmental or hormonal control, but relatively little is known of the molecular switches involved or how changes in the timing of these switches can lead to evolutionary change (1). In insects, metamorphosis arose once from a direct-developing ancestor Ϸ300 million years ago (2). A key regulatory gene in metamorphosing (holometabolous) insect life histories is the transcription factor broad (br) (3-7). In both moths and flies, epidermal expression of br is restricted to the larval-pupal transition (3, 5-7), and its expression at this time is required for activation of pupal-specific gene expression, as well as suppression of larval-and adult-specific gene expression (3, 7). Accordingly, Drosophila null mutants never enter the pupal stage; instead, they remain in a prolonged larval state (8). In addition, gynander larvae mosaic for br null and br ϩ tissue produced mosaic larval and pupal tissue, respectively, at the larval-pupal transition (9). Loss of br also prevents the larvalpupal transition in the silkmoth; tissues that were transformed with a vector driving br RNA interferenc...

Distinct roles of Broad isoforms in regulation of the 20-hydroxyecdysone effector gene, Vitellogenin, in the mosquito Aedes aegypti

“…The br mutants exhibit abnormal regulation of many intermolt, early and late gene activities (Karim et al, 1993;Zhou and Riddiford, 2002). More recently, it has been demonstrated that br is expressed during each nymphal molt of the direct-developing insect Oncopeltus fasciatus (Erezyilmaz et al, 2006). Much less is known about the role of br in adult insects.…”

Nuclear receptor BgFTZ‐F1 regulates molting and the timing of ecdysteroid production during nymphal development in the hemimetabolous insect Blattella germanica

“…In more basal hemimetabolous, direct-developing insects data on the mechanisms of 20E action are practically nonexistent, with few exceptions (e.g., Erezyilmaz et al, 2006). The postembryonic development of hemimetabolous species shows significant differences with respect to holometabolous, especially because growth and maturation occur simultaneously throughout successively nymphal stages until the imaginal molt, without an intermediate pupal stage.…”

Isoform-Specific Regulation of a Steroid Hormone Nuclear Receptor by an E3 Ubiquitin Ligase inDrosophila melanogaster

“…The A and B1 isoforms are expressed in complementary patterns, with one notable exception: the prothoracic gland, which exhibits expression of both isoforms (Talbot et al 1993). The other nuclear receptor, USP, is expressed ubiquitously and exhibits a DNA-binding motif that is required for repression, but dispensable for activation, of metamorphosis (Schubiger and Truman 2000;Ghbeish et al 2001;Erezyilmaz et al 2006). The mechanisms that lead to the activation of a specific receptor or, more often, a receptor isoform are still poorly known for ecdysone and most other steroid hormones across species.…”