Zoologger: The first reptile with a real placenta
Zoologger is our weekly column highlighting amazing animals – and sometimes other organisms – from around the world
October 6, 2011
Species: Trachylepis ivensii
Habitat: Angola, Zambia and the Democratic Republic of the Congobeing very hard to find
In evolution, as in life, some things are easier than others. It seems quite simple to evolve complex eyes, which appeared dozens of times.
Likewise, for some groups of animals, it is easy to stop laying eggs and start giving birth to live young. Backbone animals have developed a live birth no less than 132 times, and today a fifth of lizards and snakes give birth. Human mothers may disagree, but live birth is clearly not that difficult.
What is difficult, however, is nurturing the unborn young as mammals do. A female mammal allows each embryo to burrow deep into the wall of her uterus, where it draws its nutrients directly from her blood. It was long thought that this intimate arrangement evolved only once, in mammals.
Not so. It now appears to have evolved at least twice: once in mammals, and once in an obscure African lizard called Trachylepis ivensii.
These lizards don’t look like rule breakers. T.ivensii are quite typical skinks, one of approximately 1200 species. Adult females are 9 to 14 centimeters long, plus the tail. They are rarely seen: only a few specimens have been collected. “We don’t know much about them,” says Daniel Blackburn from Trinity College in Hartford, Connecticut.
Blackburn began to study T.ivensii seriously after his colleague Alexander Fleming from the University of Stellenbosch, South Africa, found nine women preserved in a museum. Blackburn and Flemming worked together to dissect them and find out how their young developed.
They focused on the oviducts – tubes that run from the ovaries outwards. Living reptiles release eggs into the oviducts, where they develop into babies before being born. The question is, how were the developing embryos nourished while they were in the oviduct?
All living reptiles have a basic placenta, but unlike its mammalian counterpart, the embryo does not receive much nourishment this way. It cannot: although nestled against the wall of the oviduct, the embryo remains inside a remnant of eggshell that acts as a barrier. Instead, it is fed by a large yolk.
A few rare reptiles, including T.ivensii, break this rule. Their eggs are small, with little yolk, so they must receive plenty of food from their mother via the placenta. But only T.ivensii allows the embryo to implant in the wall of the oviduct. “It’s unprecedented,” says Blackburn.
Blackburn and Flemming found 42 preserved embryos in the females. More advanced embryos had lost their shells and attached themselves to the wall of the oviduct.
It works like this. Cells outside the embryo send out extensions that burrow between the cells of the oviduct wall and then swell into pimples. These pimples then produce more embryonic cells, which spread under the outer layer of the wall. Eventually, the cells of the original wall are eliminated and the oviduct is lined with cells from the embryo.
This arrangement means that the embryonic cells are pressed against the mother’s blood vessels, where they can absorb a lot of nutrients. That’s why eggs don’t need a lot of yolk.
But the arrangement has its problems, says Blackburn. An embryo in close contact with its mother’s blood risks being attacked by its immune system. Male embryos could also be “feminized” by his sex hormones. This could explain why large-scale placental feeding has evolved so infrequently.
The only reptiles that come close to T.ivensii belong to a South American skink called Mabuya. Their placentas are complex and transfer a lot of nutrientsand there is evidence that embryonic cells are capable of invade the wall of the oviduct in a limited way. Another African species, Eumecia anchietaetoo feeds its young entirely through its placenta.
Neither shows as much intimacy as T.ivensii, however, and Blackburn also says their placentas are “fundamentally different.” This may mean that placental feeding evolved in skinks not once, but three times. Obviously, in terms of evolution, these reptiles have the gift.
Journal reference: Morphology JournalDOI: 10.1002/jmor.11011
Read previous Zoologger columns: The fearsome jaws of a mini movie monster, the stealthy centipede wears living camouflage, the Dozy hamsters reverse the aging process, the most beautiful bird in the world murders chicks, the architect mouse builds a food mansion, the fish amphibian that mates with itself, the smartest insect in the world, the most athletic ape in the canopy, The monkey that really gets brotherly love, Bullied lunatics develop a bully’s brain, How the monkeys dumb frogs talk to each other, No brains, but at least he’s got personality.
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