Abstract
The silkworm Dominant trimolting (Moltinism, M3) mutant undergoes three larval molts and exhibits precocious metamorphosis. In this study, we found that compared with the wild-type (WT) that undergoes four larval molts, both the juvenile hormone (JH) concentration and the expression of the JH-responsive gene Krüppel homolog 1 (Kr-h1) began to be greater in the second instar of the M3 mutant. A positional cloning analysis revealed that only the homeodomain transcription factor gene Sex combs reduced (Scr) is located in the genomic region that is tightly linked to the M3 locus. The expression level of the Scr gene in the brain-corpora cardiaca-corpora allata (Br-CC-CA) complex, which controls the synthesis of JH, was very low in the final larval instar of both the M3 and WT larvae, and exhibited a positive correlation with JH titer changes. Importantly, luciferase reporter analysis and electrophoretic mobility shift assay (EMSA) demonstrated that the Scr protein could promote the transcription of genes involved in JH biosynthesis by directly binding to the cis-regulatory elements (CREs) of homeodomain protein on their promoters. These results conclude that the homeodomain protein Scr is transcriptionally involved in the regulation of JH biosynthesis in the silkworm.
Highlights
Insect larvae generally undergo several molting events before larval-pupal metamorphosis and these molting processes divide the entire larval period into separate stages, which are called instars [1,2,3]
Compared with the WT larvae, which initiated metamorphosis in the fifth instar after four rounds of larval molting, the M3 larvae underwent a precocious metamorphosis in the fourth instar, and the entire larval duration was shortened by approximately six days (Figure 1A)
At the 11th day after hatching, the WT larvae had grown into the third larval molting, whereas the M3 larvae remained at day two of the third larval instar (Figure 1A), but their body size was bigger than that of the WT larvae (Figure 1B)
Summary
Insect larvae generally undergo several molting events before larval-pupal metamorphosis and these molting processes divide the entire larval period into separate stages, which are called instars [1,2,3]. Even within a specific species, such as the silkworm (Bombyx mori), variants of the numbers of larval molts exist [5,6]. This variation in the larval molt number across insects is referred to by the term moltinism, and insect species with different moltinisms can be applied to decipher the genetic basis of larval molting. Insect larval molting and metamorphosis are orchestrated by two types of endocrine hormones, juvenile hormone (JH) and ecdysone [7]. In the final larval instar, the disappearance of JH elicits pupal commitment, a dramatically elevated ecdysone pulse triggers larval-pupal metamorphosis [10]
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