Abstract
Genome reduction has been widely studied in obligate intracellular bacterial mutualists of insects because they have, in comparison to closely-related, nonhost-associated bacteria, extremely small genomes. Pentatomid stinkbugs also maintain bacterial symbionts, yet they are extracellular, residing within host-derived crypts, and are transmitted to offspring outside of the host's tissues, which exposes them to the external environment. In this review, we explore how the biphasic lifestyle of stinkbug symbionts (e.g. on the surfaces of eggs in various matrices during transmission and inside host-derived tissues during much of the host's life), in contrast with the solely intracellular lifestyle of many insect endosymbionts, may impact their genome's architecture, size and content. Furthermore, we demonstrate how additional stinkbug symbiont genomes are needed to more fully explore these questions and the potential value of the stinkbug-symbiont system in understanding genome evolution and reduction in the absence of intracellularity.
Highlights
Insects spanning many species-rich groups, including aphids, cicadas, cockroaches, and whiteflies, maintain mutualisms with intracellular bacteria that are essential to their growth and development (Moran et al, 2008)
Symbiont transmission and genome reduction if similar characteristics of an obligate host-bacterial mutualism would be observed in insects that harbor symbionts in specialized tissues but transmit them to offspring by alternative modes of inheritance (Hosokawa et al, 2005, 2012a,b; Prado and Almeida, 2009a; Bansal et al, 2014; Bistolas et al, 2014)
The last few years have greatly increased the amount of knowledge on primary extracellular gut symbionts of pentatomid stinkbugs
Summary
Insects spanning many species-rich groups, including aphids, cicadas, cockroaches, and whiteflies, maintain mutualisms with intracellular bacteria that are essential to their growth and development (Moran et al, 2008). Typical characteristics of these bacterial partners are that they are not cultivable by classical methods, maternally transmitted to offspring (with one reported exception; Watanabe et al, 2014), and have highly reduced, A+T% biased genomes (Moran and Bennett, 2014). Phytophagous stinkbugs are useful exemplars in that they harbor bacterial symbionts in modified sections of their midgut, called crypts or caeca, and employ a variety of extracellular intergenerational transmission strategies. In this mini-review, we will detail what is known about the genomic architecture and content of stinkbug bacterial symbionts in the context of alternative intergenerational transmission strategies
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