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

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.

Longer life span is associated with elevated immune activity in a seasonally polyphenic butterfly

“…This raises the question about how well can we extrapolate the laboratory‐based results to natural conditions. This aspect is also relevant for interpreting the studies on P. napi (Prasai & Karlsson, ) and A. levana (Baudach et al., ) that reported differences in the immunity between two different developmental pathways relying on laboratory measurements. One of the most significant results in the present study is the good correlation in measured traits between the laboratory and wild grown individuals.…”

“…A recent study investigating the differences in induced immunity in different developmental pathways of A. levana is in good correlation with our predictions. It showed that also in the case of artificial immune challenge, the larvae entering the diapause had higher survival and higher induced antibacterial activity than directly developing ones (Baudach et al., ).…”

“…Between‐generation differences in the immunological traits have been nearly neglected in insect studies. However, two earlier studies have found some evidence of better immune capacity in the overwintering generation (Baudach, Lee, Vogel, & Vilcinskas, ; Prasai & Karlsson, ). Unfortunately, both were relying on artificial laboratory settings only, so that the results cannot necessarily be extrapolated to the field conditions (see van Zweden, Heinze, Boomsma, & d'Ettorre, ; Rivera, Rodriguez‐Saona, Alborn, & Koppenhöfer, ; Gall, Scoles, Magori, Mason, & Brayton, , for cautionary examples).…”

“…Both of these immunological indices are well‐studied markers in insect ecological immunity, which gives us good grounds to relate our results to those of other studies. Focussing on constitutive rather than induced defences (compare Baudach et al., , for a recent work on the same species) was motivated by our intention to interpret the results in the context of putative trade‐offs between immunity and larval growth. In addition to controlled laboratory trials, in which the different developmental pathways were artificially induced, we sampled the larvae representing either direct (short) or diapause (long) developmental pathway also in the field.…”

Seasonal phenotype‐specific expression of microRNAs during metamorphosis in the European map butterfly Araschnia levana

“…One of the genes among those differentially expressed in prepupae according to the day length during larval development encodes an A. levana ortholog of the silkworm ( Bombyx mori ) diapause bioclock protein which is known to be responsible for measuring the duration of diapause (Isobe et al, 2006). Moreover, a recent investigation has shown that photoperiodic modulation of phenotypes involve changes in life‐history traits of A. levana , featuring stronger innate immune responses upon challenge with the bacterial entomopathogen Pseudomonas entomophila in short‐day larvae (resulting in spring imagoes) in comparison to long‐day larvae (resulting in spring imagoes; Baudach, Lee, Vogel, & Vilcinskas, 2018).…”