New University of Washington research shows that a regulatory gene named broad, known to be necessary for development of insects that undergo complete metamorphosis, also is key for the maturation of insects that have incomplete metamorphosis. The work appears to present the first molecular evidence that the nymphal stage in lower insects is equivalent to the pupal, or chrysalis, stage of advanced insects such as butterflies.
Metamorphosis evolved in insects about 300 million years ago from ancestors of direct-developing insects such as grasshoppers. Biologists know the broad gene regulates metamorphosis in flies and moths and is found only at the transition between their larval and pupal stages. To understand how metamorphosis evolved in insects, the UW researchers examined how the broad gene functions in direct-developing insects, which don't have a pupal stage.
"We found that it is expressed throughout the nymphal stages, and that it is also required for change," said Deniz Erezyilmaz, a UW biology research associate. "So it looks like metamorphosis evolved in insects by restricting the expression of the broad gene to a short but intense period of change at the transition from larva to pupa."
Normally an insect like the grasshopper that does not undergo complete metamorphosis goes through subtle physical changes during each of its nymphal stages as it progresses to adulthood. If broad is suppressed, the nymph simply repeats the appearance from its previous phase but continues to show normal growth, Erezyilmaz said. The organism eventually becomes an adult, but adult structures such as the wings are severely undersized.