Molecular mechanisms of silkgland development and compartment specification in the mulberry silkroom, bombyx mori

Abstract

The mulberry silkworm, Bombyx mori, is an emerging model system for studies on genetics of quantitative traits, transcriptional regulation, molecular endocrinology, and development (Willis et al., 1995). It is a holometabolous insect belonging to the order Lepidoptera.

Silk Glands B. mori larvae possess a pair of unique structures called silk glands (SGs), each divided into three distinct regions: anterior (ASG), middle (MSG), and posterior (PSG).

These structures form during embryogenesis, are characteristic of larval stages, and degenerate during pupation.

The compartments are morphologically distinct and functionally specialized:

PSG: synthesizes silk fiber proteins (fibroin heavy and light chains, fibrohexamerin).

MSG: produces glue proteins (sericins).

ASG: serves as a duct to transport silk proteins during secretion.

The MSG is further divided into three sub-compartments: M1, M2, and M3.

M1 synthesizes Sericin-2.

M2 and M3 synthesize Sericin-1 (Couble et al., 1987).

Developmental Biology Silk glands are ectodermal derivatives arising from the labial segment, homologous to Drosophila salivary glands.

Placodes appear during embryonic stage 17-18; growth occurs by cell division at the apex. By stage 26-27, silk glands are fully formed (Matsunami et al., 1999).

Cell division ceases after embryogenesis; growth continues via cell enlargement, leading to polyploidization through endomitosis (Pedrix-Gillot, 1979).

Silk gland nuclei contain up to 2 × 10 11 times more DNA than haploid cells.

Research Focus Silk glands have been widely used to study spatial and temporal regulation of gene expression.

Limited work exists on compartment specification and molecular differences between B. mori silk glands and Drosophila salivary glands.

Endomitosis during late larval development has been studied (Pedrix-Gillot, 1979; Sudhakar & Gopinathan, 2000), but its onset and mechanisms remain unclear.

This study aimed to:

Analyze early events in silk gland morphogenesis and onset of endomitosis.

Investigate compartment specification, focusing on Wnt signaling.

Assess homeobox gene roles and identify novel candidates.

Examine segment polarity genes in embryonic, wing, and gonad development.

Key Findings Cell Cycle Events: Cyclin B and Cyclin E expression revealed mitotic activity at gland apex and restricted regions, followed by switch to endomitosis at stage 26.

Wnt Homologue (Bmwnt-1): Expressed in wing discs, gonads, gut, and MSG sub-compartment M1; implicated in compartment identity.

Cubitus Interruptus (BmCi): Expressed in wings, gonads, and MSG (M1); overlaps with Bmwnt-1 and Sericin-2 domains.

Armadillo/ -catenin: Cloned and characterized; localized to MSG cytosol, confirming canonical Wnt signaling in compartment specification.

Homeobox Genes: Antp expressed in MSG; Ubx and Pax-like homologue in PSG; compartment-specific expression suggests roles in silk gland patterning.

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Willis, J.H. et al. (1995). A brief history of Lepidoptera as model systems. In Molecular Model Systems in the Lepidoptera. Cambridge University Press.