Abstract
The volatile alkylpyrazines methyl‐ and methoxypyrazines (MPs) present in the reflex bleeds of coccinellid beetles such as the harlequin ladybird beetle Harmonia axyridis are important semiochemicals that function in antipredatory defense behavior. Pyrazines have also been coadapted from a primarily defensive role into pheromones that function in intraspecific communication, attraction, and aggregation behavior. However, the biosynthesis of MPs in ladybird beetles is poorly understood. Here, we tested the hypothesis that MPs could be produced by microbial symbionts in H. axyridis, which generates four different MPs. The evaluation of tissue‐specific MP production showed that MP concentrations were highest in the gut tissue and hemolymph of the beetles rather than the fat body tissue as the presumed site of MP biosynthesis. Furthermore, manipulation of gut microbiota by antibiotic‐containing diets resulted in a lower MP content in adult beetles. The analysis of the bacterial community of the digestive tract revealed the presence of bacteria of the genera Serratia and Lactococcus which are reportedly able to produce MPs. In line with the known diet‐dependent production of MP in H. axyridis, we determined that the presence or relative abundance of some of the potential MP producers (Enterococcus and Staphylococcus) is also diet‐dependent. We hypothesize a potential role of the microbiota in MP production in H. axyridis as a possible example for outsourcing the synthesis of ecologically important semiochemicals to its gut bacteria.
Highlights
The volatile alkylpyrazines, methyl‐ and methoxypyr‐ azines (MPs) are heterocyclic aromatic, nitrogen‐containing com‐ pounds, which produce odors of diverse biological significance
In the predacious subfamily Coccinellinae (Coleoptera: Coccinellidae) containing the two‐spotted ladybeetle Adalia bipunctata, the seven‐pointed ladybird Coccinella septempunc‐ tata, the convergent ladybird Hippodamia convergens as well as the harlequin ladybird Harmonia axyridis, have evolved to use pyrazine in several inter‐ and intraspecific interactions, as allomones or phero‐ mones, which function as deterrents or attractants (Guilford, Nicol, Rothschild, & Moore, 1987, Rizzi, 1988, Woolfson & Rothschild, 1990, Rothschild & Moore, 1987, Moore et al, 1990, Rowe & Guilford, 1999, Siddall & Marples, 2008, Verheggen, Vogel, & Vilcinskas, 2017; Supporting information Table S2)
The three main groups of MPs found in insects are 2‐isobu‐ tyl‐3‐methoxypyrazine (IBMP), 2‐isopropyl‐3‐methoxypyrazine (IPMP), and 2‐sec‐butyl‐3‐methoxypyrazine (SBMP)
Summary
The volatile alkylpyrazines, methyl‐ and methoxypyr‐ azines (MPs) are heterocyclic aromatic, nitrogen‐containing com‐ pounds, which produce odors of diverse biological significance. The three main groups of MPs found in insects are 2‐isobu‐ tyl‐3‐methoxypyrazine (IBMP), 2‐isopropyl‐3‐methoxypyrazine (IPMP), and 2‐sec‐butyl‐3‐methoxypyrazine (SBMP) They are pres‐ ent in diverse species of Orthoptera, Hemiptera, Lepidoptera, and Coleoptera (Moore et al, 1990; Supporting information Table S1). The importance of MPs in the ecology of Coccinellinae has received considerable attention, little is known about the site and mechanism of synthesis/acquisition in these beetles, leading to the question as to how the insects acquire these multi‐purpose semiochemicals. Since, (a) pyrazines are a major group of volatile compounds released by bacteria, and (b) H. axyridis MP concentration in adults and larvae is determined by their dietary components (Kögel et al, 2012), we hypothesize that a combination of diets and gut microbiota affects MP concentrations in the beetles. We test this hypothesis by characterizing the gut microbiota of H. axyridis fed on different diets and its effect on the concentration of MPs
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