What is it about?

The transformation of a caterpillar into a pupa and then a butterfly is spectacular and mysterious. This paper shows that the underlying molecular basis of this transformation is due to the workings of a trinity of master genes. Each controls access to the gene set required for its stage and also inhibits the other master genes to insure stage stability. Chinmo, the larval member of the trinity, appears in the embryo and remains through the growth of the larva and suppresses the appearance of the second and third members of the trinity, Broad, the pupal specifier, and E93, the adult specifier. Using the fruit fly Drosophila melanogaster where these factors can be genetically manipulated, we show that in the absence of Chinmo, the metamorphic master genes are prematurely expressed in either the embryo (E93) or the first stage larva (Broad) to direct the animal into metamorphosis rather than larval growth.

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Why is it important?

The insect’s skeleton is its outer covering or cuticle which must be shed (molted) at intervals in order for the larva to grow. Then at metamorphosis, this exoskeleton changes dramatically in shape and form as the larva molts to a pupa that then molts to the adult. Molting is caused by the steroid hormone ecdysone, but the nature of the molt is determined by the presence or absence of a second hormone, juvenile hormone. The elucidation of this three-factor network and its mutually inhibitory interactions that direct metamorphosis provides new insight into how these hormones can ensure an orderly progression through metamorphosis. It also offers new insight into the evolution of metamorphosis. In direct developing insects such as the cockroach Blattella, expression is confined to early embryonic development and gives way to Broad expression as development progresses. The maintenance of Chinmo and the suppression of Broad in larvae of higher insects that have complete metamorphosis provide molecular support for the hypothesis that the larva is based on a persistent embryonic stage.

Perspectives

The “invention” of complete metamorphosis by insects 550 mya is a significant milestone in animal evolution and profoundly shaped the subsequent evolution of plant and animal life on the planet. We hope that our discovery will spur more research into how this fascinating life history strategy evolved.

Professor Lynn Moorhead Riddiford
University of Washington

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This page is a summary of: Chinmo is the larval member of the molecular trinity that directs Drosophila metamorphosis, Proceedings of the National Academy of Sciences, April 2022, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2201071119.
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