Molecular and Potential Regulatory Mechanisms of Melanin Synthesis in Harmonia axyridis

Abstract

Melanization is a common phenomenon in insects, and melanin synthesis is a conserved physiological process that occurs in epidermal cells. Moreover, a comprehensive understanding of the mechanisms of melanin synthesis influencing insect pigmentation are well-suited for investigating phenotype variation. The Asian multi-colored (Harlequin) ladybird beetle, Harmonia axyridis, exhibits intraspecific polymorphism based on relative levels of melanization. However, the specific characteristics of melanin synthesis in H. axyridis remains elusive. In this study, we performed gene-silencing analysis of the pivotal inverting enzyme, tyrosine hydroxylase (TH), and DOPA decarboxylase (DDC) in the tyrosine metabolism pathway to investigate the molecular and regulatory mechanism of melanin synthesis in H. axyridis. Using RNAi of TH and DDC genes in fourth instar larvae, we demonstrated that dopamine melanin was the primary contributor to the overall body melanization of H. axyridis. Furthermore, our study provides the first conclusive evidence that dopamine serves as a melanin precursor for synthesis in the early pupal stage. According to transcription factor Pannier, which is essential for the formation of melanic color on the elytra in H. axyridis, we further demonstrated that suppression of HaPnr can significantly decrease expression levels of HaTH and HaDDC. These results in their entirety lead to the conclusion that transcription factor Pannier can regulate dopamine melanin synthesis in the dorsal elytral epidermis of H. axyridis.that different phenotypic individuals exhibited varying behavioral traits. Thus, elucidating the molecular mechanism of melanin synthesis in H. axyridis would provide crucial information on the sole ontogenies and evolutionary histories of the attendant melanization.Melanin is the final product in the melanization process and has a prominent role in wound healing, cuticle sclerotization, innate immunity, defensive reactions, cuticular coloration, and camouflage in insects [16][17][18]. Melanin synthesis is a conserved physiological process in insects that occurs in epidermal cells [19,20], and in the melanin synthesis pathway, the precursor DOPA (3, 4-dihydroxyphenylalanine) and dopamine are synthesized from tyrosine hydroxylase (TH) and DOPA decarboxylase (DDC) enzymes, respectively [21]. Tyrosine hydroxylase and DOPA decarboxylase enzymes are abound in central nervous system and epidermal cells of insects. Dopamine is an important catecholamine neurotransmitter in invertebrates and vertebrates [18]. In insects, dopamine is the major contributor for melanin biosynthesis, while in mammals DOPA is dominated [17,18]. The biochemistry of tyrosine metabolism is abounding in insects because tyrosine is the initiatory material of melanin formation. Tyrosine hydroxylase is the first key enzyme in this pathway that can convert tyrosine to DOPA. Then, DOPA decarboxylase catalyzes DOPA to dopamine. DOPA and dopamine are crucial precursors of sclerotization and melanization in insects. There was research re...

Silencing tyrosine hydroxylase or dopa decarboxylase gene disrupts cuticle tanning during larva‐pupa‐adult transformation in Henosepilachna vigintioctopunctata

“…6). Consistent with this result, dysfunction of TH also causes deformed and separated elytra 3, 7, 13 …”

“…Biochemically, the cuticle tanning in most insects results from a combination of melanins and sclerotins 3,5–12 . These compounds are produced from tyrosine precursor by a branched biochemical pathway 13 .…”

The Molecular Properties and Roles of Pannier in Harmonia axyridis’s Metamorphosis and Melanin Synthesis

“…A previous study demonstrated that the elytra color pattern variation in H. axyridis is a melanization phenomenon in the cuticle ( True, 2003 ). In our previous studies, we have confirmed that dopamine melanin synthesis is the primary pathway in the elytra of H. axyridis ( Chen et al, 2019b ; Xiao et al, 2020 ). Meanwhile, the aspartate- β -alanine-NBAD pathway being able to regulate the number and size of melanin spots in elytra has been reported in H. axyridis ( Zhang et al, 2020 ; Zhang et al, 2021 ).…”

“…In our previous study, suppression of the expression level of dopa decarboxylase ( DDC ) also resulted in a melanin-absent phenotype, which is the crucial gene in dopamine melanin synthesis. Even though the melanin disappeared in the elytra in H. axyridis , we can easily discern the characteristic of color pattern because of the original melanin area transforming into a light-yellow color when HaDDC was suppressed ( Xiao et al, 2020 ; Chen et al, 2019b ). However, we did not discern the color pattern in H. axyridis when HaPnr was suppressed in the fourth instar larvae or one-day-old pupae due to the whole elytra represent the same yellow color ( Figures 4F , 5D ).…”

“…In addition, it also works as an ideal model organism to study insect phenotype variations because of more than 200 different elytral color patterns in H. axyridis ( Gautier et al, 2018 ; Ando and Niimi, 2019 ). There are four common color patterns of elytra that have been widely studied in H. axyridis , which include: succinea , axyridis , spectabilis , and conspicua ( Ando et al, 2018 ; Gautier et al, 2018 ; Xiao et al, 2020 ). This striking intraspecific variation has prompted the investigation of its genetic, biochemical, and evolutionary meaning ( Bezzerides et al, 2007 ; Ando et al, 2018 ).…”