Juvenile hormones and the genetic interaction between the transcription factors
<italic>Krüppel homologue 1</italic>
(
<italic>Kr-h1</italic>
) and
<italic>Broad</italic>
(
<italic>Br</italic>
) regulate the transformation of insects from immature to adult forms in both types of metamorphosis (holometaboly with a pupal stage versus hemimetaboly with no pupal stage); however, knowledge about the exact instar in which this occurs is limited. Using the hemimetabolous cricket
<italic>Gryllus bimaculatus</italic>
(
<italic>Gb</italic>
), we demonstrate that a genetic interaction occurs among
<italic>Gb</italic>
′
<italic>Kr-h1</italic>
,
<italic>Gb</italic>
′
<italic>Br</italic>
and the adult-specifier transcription factor
<italic>Gb</italic>
′
<italic>E93</italic>
from the sixth to final (eighth) nymphal instar.
<italic>Gb</italic>
′
<italic>Kr-h1</italic>
and
<italic>Gb</italic>
′
<italic>Br</italic>
mRNAs were strongly expressed in the abdominal tissues of sixth instar nymphs, with precocious adult moults being induced by
<italic>Gb</italic>
′
<italic>Kr-h1</italic>
or
<italic>Gb</italic>
′
<italic>Br</italic>
knockdown in the sixth instar. The depletion of
<italic>Gb</italic>
′
<italic>Kr-h1</italic>
or
<italic>Gb</italic>
′
<italic>Br</italic>
upregulates
<italic>Gb</italic>
′
<italic>E93</italic>
in the sixth instar. By contrast,
<italic>Gb</italic>
′
<italic>E93</italic>
knockdown at the sixth instar prevents nymphs transitioning to adults, instead producing supernumerary nymphs.
<italic>Gb</italic>
′
<italic>E93</italic>
also represses
<italic>Gb</italic>
′
<italic>Kr-h1</italic>
and
<italic>Gb</italic>
′
<italic>Br</italic>
expression in the penultimate nymphal instar, demonstrating its important role in adult differentiation. Our results suggest that the regulatory mechanisms underlying the pupal transition in holometabolous insects are evolutionarily conserved in hemimetabolous
<italic>G</italic>
.
<italic>bimaculatus</italic>
, with the penultimate and final nymphal periods being equivalent to the pupal stage.



This article is part of the theme issue ‘The evolution of complete metamorphosis’.