Symbiosis, or the interaction of two unlike species, is common and often necessary for survival. In insects, symbiotic microbes often facilitate and promote growth of hosts. Before the symbiotic collaboration can begin, however, hosts must acquire their symbionts. Sometimes, they are passed on from parents to offspring, other times hosts must find their symbionts themselves – or face dire consequences if they fail to do so in a timely fashion, a new Frontiers in Microbiology study has shown.
"We show that for the leaffooted bug, Leptoglossus zonatus, acquisition of the obligate bacterial symbiont Caballeronia by ingestion is time-restricted," said first author Liam Sullivan, a PhD student studying insect science at The University of Arizona. "If these bugs cannot acquire Caballeronia in time they begin to suffer from reduced survivorship, reduced adult weight, and increased development time to adulthood."
"Leaffooted bugs are unusual among insects to rely on an environmental source of bacteria for an obligate symbiont," added co-author Dr Molly Hunter, professor of entomology at The University of Arizona. "Environments where dryness, radiation, and soil pH influence symbiont acquisition make finding the symbiont a game of roulette."
Race against the clock
Leaffooted bugs undergo five juvenile stages, called instars, before reaching adulthood. Nymphs hatch in the tree canopy, where their soil-living bacterial symbionts are scarce or can't be found at all. After hatching, a race against the clock begins. The journey from canopy to ground is a dangerous undertaking for nymphs measuring just a few millimeters. Risk of predation and the energetic demand are high. However, failing to acquire the symbiont is also lethal in the species.
For their study, the team tested how badly delayed symbiont acquisition impacted bugs by using leaffooted bugs that hatched on cowpea plants and were fed peanuts in the lab. Upon reaching the second instar, nymphs were fed Caballeronia either on the day of their molt ('day zero') or every four days after. A control group was never presented with the symbiont.
"When leaffooted bugs reach the second instar, a timer starts for symbiont acquisition. As it becomes ever more critical that bugs find the symbiont, it becomes less likely, not more, that if they do find the symbiont, ingestion will result in symbiont colonization and survival," Sullivan pointed out.
Ingesting the symbiont on day zero or four days later meant similar survival rates – around 86% and 89%, respectively. Survival rates declined after eight days (63%) and plummeted after. Of the control group, just 29% survived.
Nymphs that received their symbionts on day zero or four days later, developed into adults after 22 and 24 days, respectively. An eight-day delay meant nymphs developed fully after 27 days. The few surviving bugs in the control group took 57% longer to develop than the zero-day group. Many bugs exposed to the symbiont after 16 days or more failed at acquiring it altogether. Delayed acquisition also meant restricted development of bugs' midgut symbiotic organ, a specialized organ where microbial symbiotic partners live.
Beating the odds
So why did this – for the bug so dangerous and energetically costly – symbiosis evolve this way? Leaffooted bugs are just one species in an order of thousands of insects that associate with Caballeronia, and this relationship may predate the diversification into groups and families.
"It's hard to know how the ancestors of these families started down the path of acquiring their nutritional symbiont from the environment, but there can be clear benefits, for example, bugs could acquire a symbiont already adapted to the local environment. This could confer greater fitness," said Hunter. "Our results highlight the potential costs. Yet these families have diversified and are successful which means that the benefits often outweigh the costs."
Since the study was done in the lab, evidence is limited if delayed symbiont acquisition is a problem in the wild. Preliminary data from the field shows that some individuals fail to find the symbiont in time and die at greater rates, suggesting that this is a real risk for these bugs. Work like this could inform management of leaffooted bugs, which, like many Caballeronia associated species, are agricultural pests, the team pointed out.
"As biologists, we know that herbivores' lives are hard and many die young," concluded Sullivan. "Acquiring one's symbiont in time for it to be beneficial is yet another existential challenge."
