Evolution in Action, Skink Style!

Lizard Moving From Eggs to Live Birth

Yellow bellied three toed skinks are an interesting bunch.  They’re apparently in a little bit of a transitional period with regard to the final stages of their reproductive cycle.  Skinks living in the coastlands of New South Wales are currently laying eggs in order to reproduce … while their counterparts located in the mountains are giving birth to live young.  This difference is shedding some of light on the evolutionary processes in play during the transition from egg laying to live births.

Evolutionary records shows that nearly a hundred reptile lineages have independently made the transition from egg-laying to live birth in the past, and today about 20 percent of all living snakes and lizards give birth to live young only.

Of course I’m a transitional species! Do I look like I’m done to you??  I can’t even touch my nose!
(Photograph courtesy Rebecca A. Pyles, National Geographic)

“By studying differences among populations that are in different stages of this process, you can begin to put together what looks like the transition from one [birth style] to the other.”

The last quote was from James Stewart, a biologist at East Tennessee State University who co-authored the study on the animals.  It goes into some detail about potential problems encountered during the evolutionary transition, like the method by which nutrients like calcium are efficiently passed from the mother to the developing embryo.  In mammals, it’s done by way of the placenta.  In egg laying animals, it’s done through the use of the yolk plus the contents of the shell (which is high in calcium).  While the live-bearing skink has neither of these mechanisms, researchers looked for alternative methods of transfer:

Stewart and colleagues, who have studied skinks for years, decided to look for clues to the nutrient problem in the structure and chemistry of the yellow-bellied three-toed skink’s uterus.

“Now we can see that the uterus secretes calcium that becomes incorporated into the embryo—it’s basically the early stages of the evolution of a placenta in reptiles,” Stewart explained.

The conclusion they drew from observing both types of skinks and the fossil record showing the frequency with which they occur is that these types of transitions are apparently a lot more common – and a lot more simple – than they had previously thought.

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4 Responses to Evolution in Action, Skink Style!

  1. Richard says:

    Nice story! I generally really enjoy the science-related links you post here.

    • Yeah, they’re not as frequent as I’d like, but I do get around to them every so often. It’s obviously easier to complain about politics than try to decipher a paper from evolutionary biology. I actually have one about astrophysics, but it hasn’t gone through the queue yet … you can probably say more about than I could: http://phys.org/news/2013-06-international-team-big-theory.html

      • Richard says:

        Ah yes, the lithium problem. Good choice. I’m not the best one to comment there since I don’t know a hell of a lot about stellar evolution, but I know this has been a bit of a thorn in BBN, and I could believe that data quality could in principle lead to misidentification of lines.

        What surprises me a bit is that people let things like data quality get in the way for observations of the most metal-poor stars in the universe — but then, getting multiple hours on Keck to really burn in a single object is probably quite a challenge. Lucky Keck time has been getting cheaper while the world prepares for GMT, TMT, ELT and other frickin’ huge telescopes.

      • Richard says:

        Oh yeah, and it also makes sense that Martin Asplund’s team here at RSAA figures prominently in their discovery. His postdocs all live around here and have been giving me rides to and from work!

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