A Bug's Life History

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Science  12 May 2006:
Vol. 312, Issue 5775, pp. 817
DOI: 10.1126/science.312.5775.817a

Direct-developing insects progress through nymphal and adult stages, where nymphs are similar to but smaller than adults, whereas other insects experience a dramatic transition—metamorphosis—with distinct larval and pupal stages giving rise to the adult form. The transcription factor broad is known to play a critical role in metamorphosis: Its expression is limited to the larval-pupal transition, where it activates pupal-specific genes and specifies pupal development. But what does broad do in direct-developing insects?

Erezyilmaz et al. have cloned the broad gene from the direct-developing milkweed bug Oncopeltus fasciatus, which passes through five nymphal instars before molting into the adult. The broad gene is expressed during embryogenesis and the nymphal stages; expression peaks during the nymphal molts, but broad RNA is not present in the latter part of the fifth and final nymphal instar or in the subsequently formed adult.

RNAi knockdown of broad blocks the morphological transition from one nymphal instar to the next, although it does not alter the number of nymphal instars or the transition to the adult. Metamorphosis in insects is thought to have arisen in a direct-developing ancestor some 300 million years ago and may have been caused in part by changes in the expression of broad, from its temporally complex pattern in the milkweed bug, which directs differential growth between nymphal instars, to the highly restricted pattern during the last larval instar of insects that undergo metamorphosis. — GR

Proc. Natl. Acad. Sci. U.S.A. 103, 6925 (2006).

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