Bacterial Protection of Beetle-Fungus Mutualism

Abstract

A major theme in the history of life is the formation of beneficial associations between symbiotic microbes and plants and animals (1). The pervasiveness of these associations in every ecosystem illustrates how the acquisition of a microbe's physiological capacity confers substantial fitness benefits to hosts (1). However, dependence on mutualistic microbes becomes a liability if antagonistic microbes attack or out-compete beneficial ones (2). Therefore, mechanisms to preserve beneficial microbes must be a widespread, although poorly understood, component of host-microbe mutualisms. Here, we show that a beetle uses a bacterium to protect its fungal food source from a competitor fungus.Southern pine beetles (SPB, Dendroctonus frontalis) engage in a beneficial symbiosis with the fungus Entomocorticium sp. A (EsA), which provides nourishment for their developing larvae. Adult SPBs carry EsA in a specialized storage compartment called a mycangium (Fig. 1A); excavate ovipositional galleries within the inner bark and phloem of host pine trees; and inoculate these galleries with EsA (3,4). The success of the SPB-EsA mutualism is challenged by an antagonistic fungus, Ophiostoma minus (Om), which can out-compete EsA and thereby disrupt SPB larval development (3,4). Our results indicate that successful maintenance of the SPB-EsA mutualism is likely mediated by an actinomycetous bacterium that produces antibiotics which selectively inhibit Om.The presence of previously unknown actinomycetes within the SPB-EsA mutualisms was established by scanning electron microscopy (SEM) and enrichment culture isolations. SEM revealed unexpected and profuse growth of actinomycetes within the galleries of SPBs, as well as inside the mycangia (Fig. 1B, Fig S1A). Isolations from 110 beetle individuals yielded 846 colony forming units (CFUs) of actinomycetes, including at least one CFU from each of 92 individuals. Out of 164 actinomycete CFUs selected to be transferred to pure culture, 99 isolates had a red morphotype while 65 isolates had a white morphotype. DNA sequence analyses confirmed the visual morphotype distinction, and within each of the two morphotypes there was complete 16S rDNA sequence identity. The two morphotypes form a monophyletic clade closely related toStreptomyces thermosacchari. We also successfully isolated the same red morphotype from 5 of 10 mycangia sampled.

Diversity and Antimicrobial Activity of Actinomycetes from Fecal Sample of Rhinoceros Beetle Larvae

“…이들 곤충의 표피나 장에는 다양한 미생물이 공생하고 있 어 곤충의 생장과 대사활동에 큰 영향을 주고 있다 (Scott et al, 2008;Kikuchi, 2009;Seipke et al, 2012). 특히, 곤충들의 체내 혹은 체외에 서식하면서 다양한 생리활성 물질을 생산하는 방선 균과 숙주와의 상호관계에 대해서 최근 활발하게 연구되고 있다 (O'Brien and Wright, 2011;Seipke et al, 2011).…”

Fungal extracellular ribotoxins as insecticidal agents

“…In this regard, fungi constitute a rich source of nitrogen and phosphorous for arthropods and therefore share a long evolutionary history with them. Thus, not only fungi are under constant attack by fungivorous animals such as collembolan, mites and insects (Boddy and Jones, 2008;Ruess and Lussenhop, 2005) but also display frequent mutualistic relationships (Scott et al, 2008), as can be exemplified by fungus farming ants (Currie et al, 1999). Underlying these relationships there is a complex interaction network involving preying, defense, and feeding.…”

Cellulose-degrading bacteria associated with the invasive woodwasp Sirex noctilio

“…Streptomyces isolates also grew on corn stover at pH 5.6 and 9.0, which suggest an ability to grow in a diversity of insect-associated environments, such as frass and galleries. The capillary liquids of fir, spruce and poplar are near pH 6.0 (Schmidt, 1986), and colonization of frass and insect galleries by other bacteria has been shown (Dillon et al, 2002;Scott et al, 2008). If bacteria colonize woody substrates ingested by S. noctilio larvae, utilization of predigested materials or enzymes would benefit development, similar to the hypothesized model for the fungal mutualist A. areolatum (Kukor and Martin, 1983).…”

“…Cellulosedegrading Streptomyces spp. have been found to be associated with other wood-feeding insects, such as termites (Pasti and Belli, 1985;Watanabe et al, 2003), bark beetles (Scott et al, 2008) and woodboring beetles (Vasanthakumar et al, 2008). A combination of enzymes is required to convert cellulose into glucose monomers (Ghose, 1977), and the Streptomyces isolates sequenced in this study contain genes that putatively encode for each of these enzymes.…”