Cassida rubiginosa, sometimes known as the thistle tortoise beetle, passes bacterial symbionts onto its offspring—a process known as vertical transmission—by harboring some of the bacteria in its reproductive tract so it can deposited on eggs in the form of a protected caplet. A new review published in the Journal of Insect Science examines existing research to explore the diverse relationships between holometabolous insects and their gut bacteria. (Photo by Laura Parsons, University of Idaho, PSES, Bugwood.org

Melissa Mayer

By Melissa Mayer

Gut bacteria are exceptionally tiny, but the roles they play for their insect hosts are huge. And understanding those roles may even offer insight into some of the biggest questions—like how symbionts evolved into their roles in the first place. In a paper published August 20 in the Journal of Insect Science, Clemson University M.S. candidate Roy Kucuk reviews the literature to reveal the diverse relationships between holometabolous insects and their gut bacteria.

The Whole Holometabolan

Roy Kucuk

Holometabolans (insects that undergo a complete metamorphosis with distinct larval and pupal life stages) comprise a ginormous clade, covering 11 living orders and about 850,000 species. Those orders include the four largest: Coleoptera (beetles), Hymenoptera (bees, ants, and wasps), Diptera (true flies), and Lepidoptera (butterflies and moths). So, it probably goes without saying that the group is strikingly diverse, and that broad range of insects is an ideal window through which to view the varied relationships between hosts and their gut symbionts.

All that diversity also presents unique challenges. Kucuk says experimentally manipulating those bacterial communities is tricky. “An entire bacterial gut community may be correlated with healthy development through experimental antibiotic treatment,” he explains, “but we still have our work cut out for us in terms of showing which bacteria are essential. We may find that no species in particular is necessary or that a range of taxa may do a similar job or that a collection of individuals greatly improves things for their host.”

The goal of the review—to compile the available knowledge about gut bacteria among holometabolans—meant looking at those gut bacteria in terms of what they do for their hosts, where they localize, and how they are transmitted.

Work It

Gut bacteria perform such diverse roles that Kucuk calls them “Swiss-Army-knife-like services.” Some bacteria help the host access nutrients or remove toxins from food sources. They can enhance the host’s immune response and even defend the host directly—a phenomenon the author describes as akin to having a bodyguard. For Kucuk, one of the most surprising roles involves weevils, whose symbiont synthesizes a vital component of the weevil’s exoskeleton—like “a personal armor smith”—rendering it so hard that 19th century entomologists struggled to pin these beetles when preparing collections.

When it comes to where gut bacteria take up residence, a lot depends on how each species has adapted its guts to its diet. These adaptations can include compartmentalized gut structures, unique internal membranes, and even whole organs (bacteriomes) made of specialized cells (bacteriocytes) that house bacteria and essentially provide them room service in exchange for services for the host.

Pass It On

That setup also plays a role in passing on those gut bacteria, which can happen vertically (mother to offspring) as well as horizontally (environment to host). Again, diversity is front and center. When it comes to vertical transmission, some insects (like Cassida rubiginosa, sometimes known as the thistle tortoise beetle) pass the bacteria directly into the eggs and top the eggs off with a caplet of bacteria. Some, like tsetse flies (genus Glossina), pass symbionts to larvae via milk glands. Others smear the eggs with bacteria-rich excrement. In the case of some dung beetles (family Scarabaeidae), the mother lays the egg on a pedestal of her own excrement within a brood ball of dung.

Horizontal transmission strategies are just as diverse. For example, Aedes aegypti mosquito larvae, which only need beneficial gut bacteria during that single stage of life, take in those bacteria directly from the environment. Other insects acquire beneficial bacteria thanks to a shared habitat, whether through defecation or regurgitation on food sources or through a colony’s “social stomach,” whereby members regurgitate and exchange liquids between adults or from adults to larvae.

The Big Questions

The evolutionary success of holometabolans played a role in their selection for the review, and it’s impossible to explore host–symbiont relationships without questioning how those relationships evolved in the first place. Figuring out the intricacies could yield insight into those bigger questions—and ourselves.

“Many of the problems insects face are analogous to our own,” explains Kucuk. “By testing for the presence of bacteria that aid insects, we might end up finding allies of our own. There’s the existentialist value too—we have new territory to explore and the tools to do so, and we should do it for the sake of solidifying our own authenticity.”

And that’s no small thing.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.