Pupal melanization is associated with higher fitness in Spodoptera exigua.

Sisi Liu,Mo Wang,Xianchun Li

doi:10.1038/srep10875

Abstract

Melanism has long been thought to be a habitat adaptation with a fitness cost. Here we reported a homozygous melanic strain (SEM) of Spodoptera exigua (Hübner) (Insecta: Lepidoptera: Noctuidae) established with black pupae spontaneously occurring within a typical laboratory population (SEW). The melanization is expressed globally, and only in the pupal stage. After pupation, the melanic SEM pupae gradually accumulate melanin to become completely black within 6 hours, whereas the wild-type SEW pupae gradually turn yellow-brown. The melanic SEM strain exhibits faster development in all life stages, heavier pupa weight, more mating time, higher fecundity, and accordingly, higher net reproductive rate and population trend index. While no reproductive isolation was observed between the SEM and SEW strains, the mating times per female of the reciprocal crosses and the SEM intracrosses were significantly higher than those of the SEW intracrosses. This represents a rare case of melanization that has fitness gains, rather than costs. Analysis of the life-history traits of this case and 14 previously reported cases of insect melanism indicate that none of melanization origin, stage, space and variation type determining whether melanism will cause fitness gain or cost.

Highlights

Color changes first appear on the dorsal and ventral sides of both strains at 30 min and 1 h after pupation, respectively, with stages in both the melanic (SEM) pupae gradually blackening while SEW pupae gradually turning brown
While melanism is one of the most common conspicuous variations in insects, each melanic insect represents a unique case in terms of its origin, variation type, spatiotemporal expression pattern, and impacts on fitness
The pupal melanic mutant strain of S. exigua (SEM) we reported here was established by laboratory selection and, just like other laboratory-originated melanic Lepidoptera[4,7,13,21], has a discontinuous variation type

Summary

Introduction

All the remaining three melanic insect strains were of laboratory origin through artificial selection and had fitness costs[30] These include lower survival, lower fecundity, smaller body weight and slower development in Manduca sexta[4], lower fecundity, smaller body weight and slower development in Helicoverpa armigera[13], and lower fecundity in Bicyclus anynana[7]. It appears whether a melanic strain originates through natural or laboratory selection is a key factor determining if melanism is associated with fitness gain or cost. Our data suggest that origin of melanism is not a key factor determining if melanism is associated with fitness gain or cost

Methods

Results

Conclusion