Immunological larval polyphenism in the map butterfly Araschnia levana reveals the photoperiodic modulation of immunity.

Arne Baudach,Kwang-Zin Lee,Andreas Vilcinskas,Heiko Vogel

doi:10.1002/ece3.4047

Abstract

The bivoltine European map butterfly (Araschnia levana) displays seasonal polyphenism characterized by the formation of two remarkably distinct dorsal wing phenotypes: The spring generation (A. levana levana) is predominantly orange with black spots and develops from diapause pupae, whereas the summer generation (A. levana prorsa) has black, white, and orange bands and develops from subitaneous pupae. The choice between spring or summer imagoes is regulated by the photoperiod during larval and prepupal development, but polyphenism in the larvae has not been investigated before. Recently, it has been found that the prepupae of A. levana display differences in immunity‐related gene expression, so we tested whether larvae destined to become spring (short‐day) or summer (long‐day) morphs also display differences in innate immunity. We measured larval survival following the injection of a bacterial entomopathogen (Pseudomonas entomophila), the antimicrobial activity in their hemolymph and the induced expression of selected genes encoding antimicrobial peptides (AMPs). Larvae of the short‐day generation died significantly later, exhibited higher antibacterial activity in the hemolymph, and displayed higher induced expression levels of AMPs than those of the long‐day generation. Our study expands the seasonal polyphenism of A. levana beyond the morphologically distinct spring and summer imagoes to include immunological larval polyphenism that reveals the photoperiodic modulation of immunity. This may reflect life‐history traits that manifest as trade‐offs between immunity and fecundity.

Highlights

Polyphenism is defined as the environmentally induced development of distinct alternative morphs encoded by the same genome (Shapiro, 1976) and plays an important role in evolutionary theory.Seasonally changing environmental parameters can in some species result in seasonally occurring phenotypes
Most previous studies have focused on morphological polyphenism in the adult, with little attention paid to nonmorphological traits and no reports looking at the larval and pupal stages of development
Seasonal polyphenism can be considered an adaptation to consistent and predictable variations in the environment such that each seasonal phenotype achieves the greatest fitness in the environmental condition under which the phenotype evolved (Brakefield, 1996; Nijhout, 2003)

Summary

| INTRODUCTION

Polyphenism is defined as the environmentally induced development of distinct alternative morphs encoded by the same genome (Shapiro, 1976) and plays an important role in evolutionary theory. A recent transcriptomic analysis indicated that the phenotypic differences in the map butterfly correlate with strong differential gene expression in a number of different gene families (Vilcinskas & Vogel, 2016). Among these were genes related to cuticle formation (overwintering) and nutrient reservoir activity (accelerated metamorphosis) which seems intuitive, and genes that regulate innate immunity—a finding which is more difficult to interpret. For the first time, polyphenic innate immunity in A. levana larvae and characterize the different immunity- related phenotypes induced by two priming regimes representing summer and autumn day lengths. We discuss our findings in the context of life-history theory and in relation to the well-studied adult polyphenism

| MATERIALS AND METHODS

Findings

| DISCUSSION