Endocrine gland growth as the developmental switch for insect diapause

Area of Science:

• Endocrinology
• Insect Physiology
• Molecular Biology

Background:

• Diapause is crucial for animal survival during unfavorable seasons, but the mechanisms linking environmental cues to endocrine changes are unclear.
• In insects, suppressed juvenile hormone (JH) production by the corpora allata (CA) triggers reproductive diapause, yet the regulation of this suppression is poorly understood.

Purpose of the Study:

• To investigate how seasonal photoperiod influences corpora allata (CA) function and juvenile hormone (JH) production in the cabbage beetle (Colaphellus bowringi).
• To identify the molecular mechanisms underlying photoperiod-dependent regulation of CA growth and JH biosynthesis, ultimately controlling seasonal diapause.

Main Methods:

• Generated photoperiod-resolved transcriptomes of the CA and whole bodies to identify photoperiod-responsive genes.
• Utilized RNA interference (RNAi) screening to assess the function of candidate transcriptional regulators in CA growth and JH production.
• Measured CA and nuclear size, hemolymph JH titers, and ovarian status to evaluate the effects of gene silencing and methoprene application.

Main Results:

• Photoperiod directly controls CA growth; long days (LD) induce smaller CA and nuclei, associated with diapause, while short days (SD) promote larger CA and reproductive conditions.
• Identified 'retained' (retn) and 'ventral veins lacking' (vvl) as key factors in the CA that promote DNA replication, gland growth, and JH production.
• Demonstrated that retn acts upstream of vvl, and together they activate 'extra macrochaetae' (emc), a downstream transcription factor that further drives CA growth. Silencing these genes induced diapause-like phenotypes.

Conclusions:

• Endocrine organ growth is a critical switch for seasonal diapause, directly influenced by environmental cues like photoperiod.
• Photoperiod-responsive transcriptional cascades, involving genes like retn, vvl, and emc, regulate endocrine plasticity and mediate seasonal adaptation in insects.
• The findings reveal a novel mechanism where photoperiod controls endocrine gland size to modulate hormone production and reproductive fate, enabling survival through unfavorable seasons.