Abstract:Following the rapid development of molecular biology techniques, many studies have been carried out of specific gene structures and functions, including those in ecological environments. Juvenile hormone (JH) is an important insect hormone that regulates insect larval development, reproduction, and metamorphosis, although the molecular mechanism behind this remains unclear. The transcription factor Kruppel-homolog 1(Kr-h1) acts downstream of the JH receptor and plays a key role in the JH signaling pathway. It has been identified in over 21 insect species, and sequence analysis has shown it to contain 2-8 C2H2-type zinc-finger motifs, which are highly conserved among Kr-h1 proteins. We previously found that zinc-fingers Zn2-Zn7 are more conserved than others in all insect species we compared. Interestingly, we found that the jewel wasp Nasonia vitripennis only contains two zinc-fingers, Zn4 and Zn7, while the body louse Pediculus humanus corporis contains all zinc-fingers except Zn7. As N. vitripennis and P. humanus were the only two parasitoids among the 21 insect species examined, it is possible that zinc fingers might be lost during evolution. Studies in Drosophila melanogester, Bombyx mori, Tribolium castaneum, and Frankliniella occidentalis revealed that Kr-h1 is required for embryonic development and metamorphosis, and that its down-regulation results in development of the precocious phenotype. Knockdown of Kr-h1 by RNAi in the brown planthopper Nilaparvata lugens showed that this gene is required for normal wing and external genitalia development. Kr-h1 is also involved in neuron development, the regulation of photoreceptor maturation, larval growth, insect development and metamorphosis, and the formation of Drosophila melanogaster neurons. In addition, it controls the foraging behavior of honey bees through affecting the expression of the cGMP-mediated activation of protein kinase G. The expression of Kr-h1 is induced by JH and its mimics. The Bombyx mori juvenile hormone response element (bmJHRE), located 141 bp upstream of Kr-h1, includes an E-box with the sequence CACGTG. bmJHRE was found in the genomic sequence of all seven insect species that we previously analyzed, and the E-box was highly conserved in all species (100%). The E-boxes of Tribolium castaneum Kr-h1(TcKr-h1) and Apis mellifera Kr-h1(AmKr-h1) are close to the Kr-h1 promoter, while those of Bombyx mori Kr-h1(BmKr-h1), Acyrthosiphon pisum Kr-h1(ApKr-h1), Drosophila melanogester Kr-h1(DmKr-h1), Nasonia vitripennis Kr-h1(NvKr-h1), and Nilaparvata lugens Kr-h1(NlKr-h1) are not. JH also interacts with the hormone ecdysone (Ecd), and research from D. melanogaster, B. mori, and T. castaneum suggests that Kr-h1 is a key factor in the JH-Ecd interaction. JH mimics methoprene treatment, which increases the expression of Kr-h1 while reducing that of Broad (B), a transcription factor located downstream of Ecd and its receptor that plays an important role in the ecdysone signaling pathway. This indicates that Br expression might be inhibited by Kr-h1. Although much progress has been made in understanding insect hormone signaling pathways, several key questions still remain. For example, what is the role of Kr-h1 in linking the JH and Ecd signaling pathways? How many other genes form a close connection in the JH signaling cascade? This review highlights the characteristics and functions of Kr-h1 that have been elucidated during the past decade. It also summarizes our understanding of the molecular mechanism of JH, establishes a theoretical basis for the development of biological pesticide, and contributes theoretically to the maintenance of a good ecological environment.