The Tell-Tale Toad: hero or villain?
The Toad Who Knew
Fun fact: pregnancy tests didn’t always come in a neat package you could pick up from a local store. Back in the olden days (1930s), pregnancy tests generally comprised of injecting lab animals like mice with urine of a potentially pregnant person. If ovulation was discovered after a dissection, the person was pregnant! This all changed in the 1950s when Lancelot Hogben, science stalwart, discovered that if you injected a female Xenopus laevis (also known as the African Clawed Toad, African Clawed Frog, Xenopus, or the platanna) with the urine or serum of the patient, the frog would produce eggs within 24 hours! This is because the human chorionic gonadotropin (HCG) hormone, found in the urine of pregnant people, induces oocyte production in the toads. A multi-use, fast-acting pregnancy test for the modern individual, without needing to dissect anything!
Xenopus, meaning ‘strange-foot’ in Latin, serves as an exceptionally useful model organism, and the toad is found as a lab animal and pet worldwide. It has a long and storied history in science, its tale being scattered with accolades through the ages. With a killer combination of experimental tractability and similar evolutionary relationship with humans, and the fact that it is the only vertebrate that allows cell-free DNA manipulation from extracts from its eggs, this toad is ubiquitous in science. It has been launched into space to investigate reproduction in zero gravity in 1992. It’s perfect for the study of apoptosis mechanisms in vertebrates. They’ve been used to describe the development of the visual system. The genome sequence has been fully characterised and published online. It was the first vertebrate to be successfully cloned, for which Sir John Gurdon was awarded the 2012 Nobel Prize in Physiology or Medicine.
The toad’s usefulness as a lab animal is not fully a cause for celebration. The African clawed toad is a voracious predator, capable of adapting to any number of habitats. Although they’re native to Sub-Saharan Africa, they are found in North America, South America, and Europe (including a feral colony in South Wales). Despite being a fully aquatic species, they can travel short distances to other ponds. They are able to survive being frozen. They use their clawed toes to tear food apart, and their appetites have devastated local frog populations by eating their young. They’re long-lived and hardy, and one of the few amphibians that is able to survive in high salinity water. They’ve been documented to live up to 20-30 years in captivity.
Furthermore, the toad might be a vector (or even initial source) for Batrachochytrium dendrobatidis (Bd). This is a chytrid fungus that has been suggested as the cause for a huge, worldwide decline in amphibian populations. Bd can be introduced to amphibians through water exposure. The development of the Chytridiomycosis disease is ultimately capable of causing cardiac arrest for the individual, depending on infection intensity. Unlike many other species, Xenopus seems to be immune to its effects, although it still is infected with the disease. This makes it a very dangerous carrier for the disease. For that reason it’s thought to be a possible originator, or at the very least vector, of the disease.
African clawed toads have helped so much of human science progress and develop, from pregnancy tests to cloning. But they’re also a cautionary tale. They prove that when humans meddle in established ecosystems, whether intentionally or accidentally, it can have vast and unforeseen consequences.
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