Abstract
Melanism has been found in a wide range of species, but the molecular mechanisms involved remain largely elusive. In this study, we studied the molecular mechanisms of the pupal melanism in Spodoptera exigua. The full length cDNA sequences of tyrosine hydroxylase (TH) and dopa decarboxylase (DDC), two key enzymes in the biosynthesis pathway of melanin, were cloned, and their temporal expression patterns in the integument were compared during the larval-pupal metamorphosis process of the S. exigua wild type (SEW) and melanic mutant (SEM) strains. No amino acid change in the protein sequence of TH and DDC was found between the two strains. Both DDC and TH were significantly over-expressed in the integument of the SEM strain at late-prepupa and 0 h pupa, respectively, compared with those of the SEW strain. Feeding 5th instar larvae of SEM with diets incorporated with 1 mg/g of the DDC inhibitor L-α-Methyl-DOPA and 0.75 mg/g of the TH inhibitor 3-iodo-tyrosine (3-IT) resulted in 20% pupae with partially-rescued phenotype and 68.2% of pupae with partially- or fully-rescued phenotype, respectively. These results indicate that overexpressions of TH and DDC are involved in the pupal melanization of S. exigua.
Highlights
The full-length cDNA sequences of TH and dopa decarboxylase (DDC) from S. exigua were obtained by RT-PCR cloning of a partial fragment, followed by cloning of their 5’ and 3’ ends by 5’ and 3’ rapid amplification of cDNA ends (RACE)
Sequence analysis shows that S. exigua TH (SeTH) shares 97% amino acid identity with TH from Mythimna separata, 94% from Papilio xuthus, M. sexta and B. mori, 91% from Heliconius melpomene malleti and Samia cynthia ricini
All the melanic mutant phenotypes, whether occurred locally or globally in space/time and associated with or without a fitness cost, are concentration of one or both of the two insect melanins—Dopa melanin and Dopamine melanin20,28,29—on a particular body surface at a particular developmental time. This can only happen if the precursors of one or both of the two melanins, Dopa and Dopamine respectively, are overproduced, or less of them are channeled to synthesize the yellow and/or colorless pigments in the melanic insect strains
Summary
Studies of the black larval mutant of M. sexta establish a causal link between JH (juvenile hormone) deficiency, higher DDC (dopa decarboxylase) expression, and the melanic phenotype[11,23,24]. We detected overexpression of TH and DDC in the integuments of SEM prepupae and puape, and found that feeding larvae of the mutant SEM strain with inhibitors of TH and DDC led to conversion of the melanic pupae to wild type (brown) or intermediate (dark brown) color pupae These results suggest that overexpression of the two melanin precursor-producing genes are responsible for the pupal melanic color morph in the SEM strain
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