Interspecies linkage analysis of mo, a Bombyx mori locus associated with mosaicism and gynandromorphism

Identification of a novel function of the silkworm integument in nitrogen metabolism: Uric acid is synthesized within the epidermal cells in B. mori

During nitrogen metabolism, animals convert toxic ammonia to less toxic forms. Uric acid (UA) is an end product of this process in terrestrial insects. In lepidopteran larvae, a large amount of UA is stored in the integument via a phenomenon known as storage excretion. Physiologically, integumental UA plays crucial roles as a barrier against sunlight and as a white pigment for larval pigmentation patterns. Conventionally, UA is thought to be synthesized in the fat body, the insect equivalent of the liver of vertebrates, and to be transported to the epidermis via the hemolymph. Here, we reconsidered the conventional theory by a mosaic analysis targeting genes governing UA synthesis, using CRISPR/Cas9 mutagenesis and a traditional genetic method in Bombyx mori. Notably, we observed mosaic larvae in which the integument comprised both UA-containing white and UA-lacking translucent areas, indicating that UA synthesis in the epidermis is indispensable to the accumulation of a large amount of highly insoluble UA in the epidermis. Our results thus provide a genetic basis for storage excretion wherein lepidopteran insects use nitrogenous waste to adapt to their environment.

Knockout silkworms reveal a dispensable role for juvenile hormones in holometabolous life cycle

Insect juvenile hormones (JHs) prevent precocious metamorphosis and allow larvae to undergo multiple rounds of status quo molts. However, the roles of JHs during the embryonic and very early larval stages have not been fully understood. We generated and characterized knockout silkworms (Bombyx mori) with null mutations in JH biosynthesis or JH receptor genes using genome-editing tools. We found that embryonic growth and morphogenesis are largely independent of JHs in Bombyx and that, even in the absence of JHs or JH signaling, pupal characters are not formed in first- or second-instar larvae, and precocious metamorphosis is induced after the second instar at the earliest. We also show by mosaic analysis that a pupal specifier gene broad, which is dramatically up-regulated in the late stage of the last larval instar, is essential for pupal commitment in the epidermis. Importantly, the mRNA expression level of broad, which is thought to be repressed by JHs, remained at very low basal levels during the early larval instars of JH-deficient or JH signaling-deficient knockouts. Therefore, our study suggests that the long-accepted paradigm that JHs maintain the juvenile status throughout larval life should be revised because the larval status can be maintained by a JH-independent mechanism in very early larval instars. We propose that the lack of competence for metamorphosis during the early larval stages may result from the absence of an unidentified broad-inducing factor, i.e., a competence factor.