UV2 and LW opsin genes mediate phototactic responses in the Asian lady beetle, Harmonia axyridis.

Abstract

Phototaxis is a critical behavior in insects and is closely linked to vision and environmental adaptation. Understanding how insects perceive light and exhibit phototactic responses is crucial for assessing the ecological impact of artificial light at night. However, the molecular and neural mechanisms that regulate phototactic responses in insects remain largely unknown. Here, we investigated the phototactic response of the Asian lady beetle Harmonia axyridis using wavelength-specific behavioral assays, combined with gene RNA interference (RNAi) and pharmacological rescue experiments. Our results indicated that adult beetles exhibited phototactic response peaks at 365, 420, and 520 nm. Knockdown of opsin genes UV1, UV2, and LW demonstrated that UV2 and LW play essential roles in wavelength-specific phototactic responses, while no specific role for UV1 has been identified in the adult phototactic response. Knockdown of one opsin gene affected the expression of the other opsin genes, especially highlighting a regulatory interaction between UV2 and LW. Histidine decarboxylase (HDC) is responsible for the synthesis of histamine. HDC knockdown impaired phototactic responses without affecting locomotor activity. Exogenous histamine successfully rescued the phototactic response in HDC-RNAi adults, confirming that histamine was required for visual signal transduction. Furthermore, UV2 or LW knockdown significantly reduced HDC expression, suggesting an interaction between photoreception and histaminergic signaling. These findings suggest that UV2 and LW play key roles in the phototactic response and that histamine is required for visual signal transduction in H. axyridis. This study provides insights into the visual and neuromodulatory pathways that drive adult phototactic behavior in insects.