Life-history traits and mating preference of a melanic strain of Spodoptera exigua

Melanism is one of the most marked phenotypic variations that naturally occur in a wide range of organisms. In this study, we established a homozygous melanic mutant strain (called SEM) with black pupae spontaneously occurring within a wild- type population (called SEW) of Spodoptera exigua. The melanization is globally expressed in the pupal stage, but not in the egg, larval, or adult stage. After pupation, the melanic SEM pupae gradually turn black, while the SEW pupae gradually turn yellow-brown, indicating that melanin might be overproduced in the SEM strain. Life-table experiments reveal that the melanism is associated with faster development in all life stages, heavier pupa weight, higher mating rate and fecundity, more mating time, and accordingly higher net reproductive rate and population trend index. Single pair inbreeding and reciprocal crosses show significantly higher mating rate in the inter-strain crosses than in the intra-strain crosses. The reciprocal crosses also show that the body colors of F1 pupae are dark-brown for the SEW female×SEM male cross and light brown for the SEM female×SEW male cross, respectively. These results suggest that the pupal melanism is controlled by a male-linked semi-dominant gene and is associated with fitness advantages in S. exigua.

 Keyword: Spodoptera exigua; pupal melanism mutation; life-history traits; fitness