Abstract
High population density alters insect prophylactic immunity, with density-dependent prophylaxis (DDP) being reported in many polyphonic insects. However, the molecular mechanism for DDP remains unclear. In current study, the role of tyramine β-hydroxylase (Tβh) in the immune response of M. separata larvae that were subject to different rearing densities conditions was investigated. The tyramine β-hydroxylase activity of larvae from high density treatments (10 and 30 larvae per jar) was significantly higher than that of the larvae from low density treatments (one, two, and five larvae/jar). A tyramine β-hydroxylase (designated MsTβh) containing a 1779 bp open reading frame was identified. Multiple sequence alignment and phylogenetic analysis indicated that MsTβh was orthologous to the Tβh that was found in other lepidopterans. Elevated MsTβh expression was observed in larvae under high density (10 larvae per jar). Silencing MsTβh expression by the injection of dsRNA in larvae from the high density treatment produced a 25.1% reduction in octopamine levels, while at the same time, there was a significant decrease in phenoloxidase (PO) and lysozyme activity, total haemocyte counts, and survival against Beauveria infection 56.6%, 88.5%, 82.0%, and 55.8%, respectively, when compared to control larvae. Our findings provide the first insights into how MsTβh mediates the octopamine level, which in turn modulates the immune response of larvae under different population densities.
Highlights
Octopamine (OCT) is among a group of compounds that is known as biogenic amines
OCT is biosynthesized from tyrosine by tyrosine decarboxylase and tyramineβ-hydroxylase enzymes
Mechanical stimulation stress led to an increase in tyramine β-hydroxylase (Tβh) activity and OCT level in Periplaneta americana [14]
Summary
Octopamine (OCT) is among a group of compounds that is known as biogenic amines. OCT is an invertebrate structural analog of vertebrate norepinephrine, and it is present in both vertebrate and invertebrate nervous systems [1]. Our previous results showed that OCT plays an important role in the larvae of Mythimna sepatrata, from high density conditions to be resistant to pathogens [9]. Larvae that experience crowded conditions are more resistant to pathogens when compared to counterparts at low-densities, and this phenomenon is known as “density-dependent prophylaxis (DDP)” [22]. Our previous research suggested that immune indicators, such as phenoloxidase (PO) activity and total haemocyte counts and the level of OCT, increased in the larvae of M. separata from high-density conditions. We investigated the Tβh enzyme activity of larvae from different densities in rearing chambers. 2019, 20, 1553 gene expression in the larval density dependent prophylaxis, and it provides information needed to explore the mechanism of octopamine on the larval immune function in M. separata Sci. 2019, 20, 1553 gene expression in the larval density dependent prophylaxis, and it provides information needed to explore the mechanism of octopamine on the larval immune function in M. separata
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