Where did the pupa come from? The timing of juvenile hormone signalling supports homology between stages of hemimetabolous and holometabolous insects

Abstract

Insect metamorphosis boasts spectacular cases of postembryonic development when juveniles undergo massive morphogenesis before attaining the adult form and function; in moths or flies the larvae do not even remotely resemble their adult parents. A selective advantage of complete metamorphosis (holometaboly) is that within one species the two forms with different lifestyles can exploit diverse habitats. It was the environmental adaptation and specialization of larvae, primarily the delay and internalization of wing development, that eventually required an intermediate stage that we call a pupa. It is a long-held and parsimonious hypothesis that the holometabolous pupa evolved through modification of a final juvenile stage of an ancestor developing through incomplete metamorphosis (hemimetaboly). Alternative hypotheses see the pupa as an equivalent of all hemimetabolous moulting cycles (instars) collapsed into one, and consider any preceding holometabolous larval instars free-living embryos stalled in development. Discoveries on juvenile hormone signalling that controls metamorphosis grant new support to the former hypothesis deriving the pupa from a final pre-adult stage. The timing of expression of genes that repress and promote adult development downstream of hormonal signals supports homology between postembryonic stages of hemimetabolous and holometabolous insects. This article is part of the theme issue 'The evolution of complete metamorphosis'.

Keywords: evolution; hormone receptor; juvenile hormone; metamorphosis; signal transduction; transcription factor.

Figure 1.

Metamorphosis in a snakefly (Raphidioptera), a basal holometabolan. (a) A late (possibly final) larval instar. (b) A mid-stage male pupa from lateral (left) and ventro-lateral (right) views. (c) Adult male. Scale bars, 5 mm.

Figure 2.

Alternative hypotheses for evolutionary relationships between holometabolous stages and those of a hypothetical hemimetabolan ancestor. The green boxes on the right mark the successive embryonic stages (E1, E2) enclosed in the first and second embryonic cuticles, respectively. Left: the ‘de-embryonisation’ concept of Berlese, updated with the pronymph hypothesis [13]. The holometabolous larva hatches at a stage corresponding to E2, which becomes the hemimetabolous pronymph (PN). The pronymph expands into all holometabolous larval instars (yellow) whereas all hemimetabolous juvenile instars are compressed to the pupa (orange). Right: the Hinton concept of homology between the stages. Hemi- and holometabolous larvae hatch at an equal stage (in the third embryonic cuticle). The pupa is a modified late-stage hemimetabolous larva (although not necessarily precisely equal to the single, final instar). Note that the individual instars cannot be homologized.

Figure 3.

Hormones effect juvenile and adult developmental programmes through mutual interactions of transcription factors. Bottom: phenotypes in Pyrrhocoris supporting the pathway scheme. An extranumerary larval instar (L6) can be induced either with the JH mimic methoprene or with E93 RNAi (iE93) early (on the first day) during the final larval instar (L5). E93 knockdown was achieved by injecting 3 µg per animal of double-stranded RNA encompassing the coding region of E93. Note that the wing pads of the aberrant L6 animals are larger relative to control L5 but similarly dark and unarticulated, external genitals undeveloped, and the notum retains the larval pattern. Conversely, precocious development of adult characters at the fifth instar was induced by knocking down Kr-h1 (iKr-h1) early during the L4 instar [21]. For details of RNAi treatments see Smykal et al. [22]. E93 and Kr-h1 are mutual repressors [–25].

Figure 4.

Temporal profiles of the JH titre and expression of the larval determinant Kr-h1 and of the adult determinant E93 support homology of final pre-adult instars, i.e. the pupa (bottom) and the last-instar hemimetabolous larva (top). The recruitment of Br-C to effect the pupal development is a novelty of the Holometabola. Compiled based on published data: representative hormone titres are from [24,48] for hemimetaboly (Blattella germanica) and from [49] for holometaboly (Manduca sexta); approximate mRNA levels are based on [–23,39,50] for hemimetaboly (Blattella germanica, Pyrrhocoris apterus) and from [23,38] for holometaboly (Tribolium castaneum).