Studies On The Mechanism Of Melanic Mutation Of The Beet Armyworm, Spodoptera Exigua (Hübner)

The melanic mutant strain of Spodoptera exigua (Hiibner), SEM, was derived from yellow wild strain, SEW. The pupae of SEM were black. In order to identify the mechanism of melanic mutant of S. exigua, genetic regulation on pupae body color, puparial exuviae, phenoloxidase activity, dopamine andβ-alanine of SEW and SEM were investigated.Genetic regulation on pupae body color of SEM was investigated by crosses and test crosses. Genetic heredity experiments showed that the inheritance of the melanic characteristics was incomplete dominance and autosomal and followed the Mendalian law.Apart from body color of puparial cuticle, there were microstructure differences which were found out by scanning electron microscopy. SEM had denser, more packed structure of the puparial exuviae than that of SEW.The phenoloxidase activity of SEM and SEW under different conditions was determined. The results showed that the optimum pH value was 6.2 and the optimum substrate concentration was 50mmol/L for both SEW and SEM. These results indicated that there was no quality difference in phenoloxidase between SEM and SEW. The phenoloxidase activities of SEW and SEM were measured, respectively. The results showed that the whole body phenoloxidase activity of SEM was higher than that of SEW since 5th instar. The highest phenoloxidase activity appeared in the pre-pupa stage when the whole body phenoloxidase activity of SEM was 216.73% of that of SEW. The results suggested that there was difference in quantity of phenoloxidasebetween SEM and SEW. The haemolymph phenoloxidase activity was higher than cuticular phenoloxidase activity in pre-pupa and pupae. The haemolymph phenoloxidase activity of SEW was lower than that of SEM whereas cuticular phenoloxidase activity of SEW was higher than that of SEM.Microinjection experiments were performed to determine if the phenotype could be induced by pigment precursor. When SEW pupae were injected 1.2mg of dopamine, the resulting phenotype resembled the SEM rather than the SEW. An intermediate color was obtained when 0.3 to 1.2mg of dopamine was injected, and no effect was observed when 0.3mg or less of dopamine was injected. Whenβ-alanine (0.22mg/insect) was injected into pupae of SEM, the resulting puparial phenotype was similar to that of SEW. Injection of 0.06mg had no effect on the puparial color of SEM, and injection of 0.06 to 0.22mg ofβ-alanine gave an intermediate phenotype. The phenotype was partially maintained when β-alanine was co-injected with dopamine. The results indicated that higher level of dopamine, or lower level ofβ-alanine, caused the black puparial cuticle of SEM. The phenotype other than genetype was changed when SEM and SEW were reared with feed mixed withβ-alanine and dopamine, respectively.Reversed phase HPLC analysis of dopamine in whole body showed that the concentration of dopamine in SEW was 80.82% of that in SEM. The concentration of dopamine of SEM reared with feed mixed withβ-alanine was 551.46±36.15μg/g, which indicated thatβ-alanine did not affect the beginning concentration of dopamine in SEM.