Overexpression of tyrosine hydroxylase accounts for pupal melanic mutation in Spodoptera exigua

Melanization is a marked phenotypic variation that is caused by overproduction of melanin. Tyrosine hydroxylase (TH, EC 1.14.16.2), which converts tyrosine to Dopa, is the rate-limiting enzyme in the melanin biosynthesis pathway of insects. Dopa and its decarboxylated product, Dopamine, are further processed by a series of phenoloxidases (POs) and co-factors to form melanin in the epidermal cells during cuticle development. To determine whether TH is responsible for a spontaneously occurring pupal melanic mutation in the beet armyworm, Spodoptera exigua (Hübner), we cloned the TH genes from both the wild type and pupal melanic strains of S. exigua and quantified its expression in the two strains. No differences in the amino acid sequence were found between the wild type and melanic TH alleles. Quantitative RT-PCR showed that the melanic strain expressed 5.36- and 5.22-folds more TH in the integuments at the early prepupal and 0-h pupal stages, respectively, than did the wild type strain. Administration of the specific TH enzyme inhibitor 3-iodo-L-tyrosine (3-IT) to the 5^th instar larvae of the melanic strain showed that inhibition of TH reverted the melanic pupal cuticle back to the wild type pupal cuticle in a dose-dependent manner. These results demonstrate that over-expression of TH contributes to the pupal melanic mutation in the beet armyworm.