Few things wreck a picnic faster than the discovery of a poppy seed tucked into the warm folds behind your knee that turns out to be a tick. If left undisturbed, it will slowly gorge on your blood until it resembles a flattened jelly bean – with legs. For picnickers and parents, it's a revolting transformation fraught with dangers, because ticks are major vectors of a range of debilitating illnesses, including Lyme disease; yet, for biologists, the transformation is nothing short of remarkable.
Ticks can endure weeks of starvation while awaiting the chance arrival of new hosts, but even after tapping into a vein, all they get is a diet of blood, upon which they would surely starve. Although blood is rich in proteins and lipids, it lacks other key nutrients required for survival. However, a fascinating new study – by Olivier Duron from the CNRS in Montpellier, France, and his team of colleagues from across the country – shows how ticks overcome this nutritional deficiency.
Ticks are arachnids, but they aren't the only creepy-crawlies that suck. Several insect families are exclusive blood feeders and others live on a diet of plant sap; but their survival doesn't depend on this alone. Typically, these beasts rely on microbial lodgers, known as mutualists, to supply their missing dietary requirements. To test whether a similar symbiosis was at play in ticks, Duron and his colleagues treated ticks with antibiotics to kill off potential bacterial symbionts. The response was dramatic. Male juvenile ticks fed with antibiotics were 10 times less likely to reach adulthood than their untreated counterparts and the ticks that did survive were smaller and deformed. For females, it was even worse. Of 120 treated females, not a single one reached adulthood. So, bacteria matter, but what do they provide for their hosts?
To answer this question, the team dissected ticks and discovered that they were all colonized with a strain from the bacterial genus Francisella. These bacteria are usually vertebrate pathogens, but in ticks they showed the hallmarks of long-term mutualists. First, they were extremely numerous and showed a preference for certain tissues. In both males and females, the bacteria accumulated in the Malpighian tubules, a site from which they could alter tick nutrition. More importantly, the bacteria also accumulated in the ovaries of pregnant females, thereby ensuring a stable route of transmission from mothers to offspring. Indeed, all tested tick eggs carried Francisella. Second, consistent with other intracellular bacterial symbionts, the Francisella genomes were so degraded that these bacteria could never live outside of their hosts. However, despite these losses, the genes for biosynthesis of B vitamins were conspicuously intact.
To test whether Francisella provides ticks with the B vitamins essential for their survival, the team fed the antibiotic-treated ticks with a B vitamin supplement and this solved everything! The ticks on drugs and vitamins were as healthy as wild-type ticks.
This and countless other studies make clear the numerous essential roles that bacteria play in animal well-being. In addition, this study highlights the massive evolutionary flexibility of some bacterial groups. Within Francisella, there are both mutualists and pathogens that are nasty enough to be used as bioweapons. Interestingly, some other tick species harbor nutritional symbionts from another genus, Coxiella, that also contain species used as bioweapons. Remarkably, both were tamed by ticks – which is exciting news for biologists, but probably not enough to calm panicked parents after a day in the woods.