Abstract
Developmental, genetic and endocrine data from diverse taxa provide insight into the evolution of insect metamorphosis. We equate the larva–pupa–adult of the Holometabola to the pronymph–nymph–adult of hemimetabolous insects. The hemimetabolous pronymph is a cryptic embryonic stage with unique endocrinology and behavioural modifications that probably served as preadaptations for the larva. It develops in the absence of juvenile hormone (JH) as embryonic primordia undergo patterning and morphogenesis, the processes that were arrested for the evolution of the larva. Embryonic JH then drives tissue differentiation and nymph formation. Experimental treatment of pronymphs with JH terminates patterning and induces differentiation, mimicking the processes that occurred during the evolution of the larva. Unpatterned portions of primordia persist in the larva, becoming imaginal discs that form pupal and adult structures. Key transcription factors are associated with the holometabolous life stages: Krüppel-homolog 1 (Kr-h1) in the larva, broad in the pupa and E93 in the adult. Kr-h1 mediates JH action and is found whenever JH acts, while the other two genes direct the formation of their corresponding stages. In hemimetabolous forms, the pronymph has low Broad expression, followed by Broad expression through the nymphal moults, then a switch to E93 to form the adult.This article is part of the theme issue ‘The evolution of complete metamorphosis’.
Highlights
During their evolution, insects have progressed through a number of life-history strategies, some of which persist in present day orders [1]
A reason for such differences is suggested by recent experiments in the silkworm, Bombyx mori, using genetic means to prevent juvenile hormone (JH) production or JH action by removing the JH receptor Methoprene-tolerant (Met) or the Krüppel-homolog 1 (Kr-h1) transcription factor that mediates JH action [39,40]
An attempt to remove JH from the cockroach, Blattella germanica, embryos using maternal RNA interference (RNAi) treatment to suppress the production of juvenile hormone acid methyl transferase (JHAMT) produced variable results but 23% of the treated embryos could not progress beyond the pronymph stage [55]
Summary
Insects have progressed through a number of life-history strategies, some of which persist in present day orders [1] (figure 1). As described above for the eye and the leg, these arise from a unitary embryonic primordium, part of which is used to make a larval structure and part of which is carried forward in the larva as an imaginal primordium (figure 2b) Such cells are best studied in the epidermis of Drosophila in which they comprise small clusters of 20–30 cells termed ‘polyclones’ [27]. The ancestral condition for the timing of disc formation is probably similar to that seen in the beetles, T. molitor [35] and Tribolium castaneum [36], in which the wing primordia arise as late-forming discs in the final larval instar Flies, such as Drosophila, are at the other end of the spectrum in that all the adult structures, except the abdomen, come from early forming discs [27].
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