Complex plant quality-microbiota-population interactions modulate the response of a specialist herbivore to the defence of its host plant

Abstract

Many specialist herbivores have evolved strategies to cope with plant defences, with gut microbiota potentially participating to such adaptations.In this study, we assessed whether the history of plant use (population origin) and microbiota may interact with plant defence adaptation.We tested whether microbiota enhance the performance of Melitaea cinxia larvae on their host plant, Plantago lanceolata and increase their ability to cope the defensive compounds, iridoid glycosides (IGs).The gut microbiota were significantly affected by both larval population origin and host plant IG level. Contrary to our prediction, impoverishing the microbiota with antibiotic treatment did not reduce larval performance.As expected for this specialized insect herbivore, sequestration of one of IGs was higher in larvae fed with plants producing higher concentration of IGs. These larvae also showed metabolic signature of intoxication (i.e. decrease in Lysine levels). However, intoxication on highly defended plants was only observed when larvae with a history of poorly defended plants were simultaneously treated with antibiotics.Our results suggest that both adaptation and microbiota contribute to the metabolic response of herbivores to plant defence though complex interactions. Read the free Plain Language Summary for this article on the Journal blog.

Keywords: Lepidoptera; herbivore; microbiota; plant defence; trophic interactions.

FIGURE 1

Experimental design. (a) Larval families were obtained from parents collected as larvae in Åland islands. Males and females that were mated came from different habitat patches belonging to different population networks. Five and four families were obtained from individuals collected in Eckerö and Sund, respectively. (b) Each larvae represent a group of 20 individuals that were daily fed with a leaf of Plantago lanceolata collected from a plant genotype selected to produce either high (H2, H3, H9) or low (L4, L6, L7) levels of plant defence (iridoid glycosides or IGs). (c) The larval performance and survivorship of larvae were recorded. Prokaryotic and eukaryotic microbiota of plant leaves and larval gut were investigated through metabarcoding and qPCR. The metabolism linked to plant defence and the insect response was investigated through ¹H‐NMR and LC‐ESI MS

FIGURE 2

Variation in microbiota. (a) The nonmetric multidimensional scaling represents similarities (Bray–Curtis distances) in the bacterial communities across treatment groups. Abundance data are shown for the three most important bacterial OTUs namely (b) Cedecea sp.—Otu0002, (c) Erwinia sp.—Otu0003 and (d) an unclassified Enterobacteriaceae—Otu0005. (e) The non‐metric multidimensional scaling represents similarities (Bray–Curtis distances) in the fungal communities across treatment groups. Abundance data are shown for the two most important bacterial OTUs namely (f) an unclassified Ustilaginaceae—Otu0001, (g) Cladosporium sp.—Otu0005. Plant selection lines that contain high‐IG are referred as ‘H’ while those containing low‐IG are referred as ‘L’. Each dot represents a pool of three individuals from the same petri dish. Different point shapes represent pools of individuals fed with the same plant genotype. Different point colours represent pools of individuals that come from different families. Statistical significances from Tukey post hoc tests are reported for p < 0.05 ‘*’, p < 0.01 ‘**’ and p < 0.001 ‘***’

FIGURE 3

Survival and performance of the larvae across the treatment groups. (a) The survival rate, (b) the dry mass and (c) the development time to L3 have been measured either on average for the whole population (survival) for pools of three individuals (dry mass) or individually (development time). Plant selection lines that contain high‐IG are referred as ‘H’ while those containing low‐IG are referred as ‘L’. Different point shapes represent individuals fed with the same plant genotype. Different point colours represent individuals that comes from different families. Statistical significances from Tukey post hoc tests are reported for p < 0.01 ‘**’

FIGURE 4

Iridoid glycosides and lysine variations in larvae across the treatment groups. (a) Aucubin and (b) catalpol were measured with LC‐ESI‐MS while (c) lysine was measured with ¹H‐NMR across the treatment group. Plant selection lines that contain high‐IG are referred as ‘H’ while those containing low‐IG are referred as ‘L’. Different point shapes represent pools of individuals fed with the same plant genotype. Different point colours represent pools of individuals that come from different families. Statistical significances from Tukey post hoc tests are reported for p < 0.05 ‘*’ and p < 0.01 ‘**’