The most precious fluid in the world isn't black. It's red. According to some estimates, there are over 14,000 species of insects and other crawlers that feed on blood. Every class and order seems to have its own blood loving family member. The most famous blood sucking arachnids, eight-legged animals such as mites, spiders and scorpions, are the ticks. This month, South African researchers announced in PLoS ONE the rediscovery of a rare tick. This strange creature has the potential to answer some of the evolutionary questions that still surround these parasitic blood suckers.
Ticks come in two varieties. There are soft ticks (Argasidae) and hard ticks (Ixodidae). Hard ticks get their name from the large and tough shield that they carry on their backs. Soft ticks lack this shield and have a leathery 'skin' instead. Another difference between soft and hard ticks is the location of their mouth parts. Hard ticks have their mouth parts on the front their body, giving the impression of a 'head', while soft ticks carry their mouth parts on their belly.
While these differences are enough to warrant a separation of ticks into two distinct families, there are still many uncertainties about how and when these families evolved. Some researchers argue that ticks evolved from mite-like creatures as much as 400 million years ago, in the Devonian. These dawn ticks could perhaps have parasitized the first amphibians. Others suggest a much later date for the evolution of ticks, around 100 million years ago, in the Cretaceous. It isn't even clear whether blood-feeding evolved once, in the common ancestor of all ticks, or twice, in hard and soft ticks independently.
How to solve this tick conundrum? Enter a third family of ticks, the Nuttalliellidae. The only species of tick in this family, Nuttalliella namaqua, is a strange creature that combines features of both soft and hard ticks. It carries a small pseudo-shield and has its 'head' on the front like hard ticks do, but it has the same leathery skin as soft ticks. The hybrid nature of this 'soft hard tick' makes it perfect for illuminating the some of the dark corners in the evolutionary history of ticks, were it not for one problem: it was last seen in 1981, in South Africa.
Searching for this little parasite was difficult because no one knew on what kind of animal it fed. Up till now, biologists had collected a mere 21 specimens. These were found under rocks, in dusty meerkat skins in museums and inside the nests of the striped swallow. Mammals and birds thus seemed likely hosts, but attempts to let these ticks feed on pigeons, rabbits and rodents all failed.
It's not that biologists didn't try to find Nuttalliella. Daniel de Klerk, one of the co-authors of the PLoS ONE study, has been searching for it for almost thirty years. He even lost a finger tip due to a snake bite during one of his expeditions. This month, he and colleagues from the University of Pretoria were finally rewarded for their perseverance. They discovered thirty ticks, including many living ones, not far from where were this species was first found in 1930. Ben Mans, lead author of the study, describes the find:
"We found them by pure luck. We were driving along a dirt road and I saw a patch of green grass and just stopped. Daniel started to pick at rocks in a crevice, looked through them with a magnifying glass, and suddenly he called out that he found something. And that was it. It felt quite amazing to know that we have found a tick that we had little chance of finding. It was one of those moments that makes science really worthwhile."
The team soon knew that these ticks were Nuttalliella:
"They are really small and blend in well with their environment, so they can be quite difficult to see. However, their legs are distinctly reddish in color and their pseudo-scutum [pseudo-shield] was clearly visible when you zoomed in with a camera. So we knew that these were probably the correct ticks. Later in the day we identified them using a stereo microscope and our hunch was confirmed."
Some of the females had recently fed and still carried the blood of their previous hosts within their guts. After dissecting the tick and removing her guts, the team saw that the blood cells of the tick's victims had a nucleus, indicating that the blood came from a reptile or a bird (mammals have blood cells without a nucleus). DNA analysis of the blood revealed that the blood came from several species of lizards, including some species of girdled lizards. Live feeding experiments later confirmed that Nuttalliella happily feeds on lizards, solving the mystery of the missing host. However, Mans says that few ticks feed on lizards exclusively, and it is quite possible that Nuttalliella is a generalist.
Even more interesting than the identity of its host, was the nature of this little tick itself. The old museum specimens of Nuttalliella are too old for DNA extraction, but isolating DNA from fresh ticks is no problem. Based on this DNA, Mans and his colleagues found that Nuttalliella belong at the base of the family tree of ticks. This means that hard and soft ticks share a more recent ancestor than all the three families do together. Such a placement is ideal for inferring what properties the ancestral tick must have had. For example, since all three families of ticks feed on blood, the ancestral tick probably did so too.
Since Nuttalliella only occurs in South Africa, Mans suggests that the origin of ticks also lies in this region, around 270 million years ago. At this time Africa was still part of the supercontinent Gondwanaland, which Antarctica, South America and Australia. As Mans writes in his paper, these were turbulent times. Many ancient lineages, such as those that would lead up to modern lizards, mammals and birds were founded in this time. Vertebrate evolution was in full swing. And then disaster struck. Around 250 million years ago, most species on this earth went extinct. Never before and never since did so many species die out.
This upheaval must have affected these ancient ticks. Many of their hosts went extinct, after all. The soft ticks and hard ticks eventually spread over the world and diversified into many different species, perhaps as vertebrate life recovered from 'the Great Dying'. The ancestor of Nuttalliella straggled along in Africa however, without spawning a large family. Perhaps this ancestor had difficulty switching hosts after the Permian extinction, but this is speculation. Mans says that "nobody has yet seriously considered why species richness differs between various tick lineages." And considering the difficulty with which Nutalliella was found, it remains to be seen whether this lineage was really unlucky, or whether more family members are hiding under rocks, somewhere in Africa.
Tick anatomy from ticktexas.com
Other pictures from reference.
Mans BJ, de Klerk D, Pienaar R, & Latif AA (2011). Nuttalliella namaqua: A Living Fossil and Closest Relative to the Ancestral Tick Lineage: Implications for the Evolution of Blood-Feeding in Ticks. PloS one, 6 (8) PMID: 21858204