Message 1 of 16 (594712) 12-04-2010 7:17 PM

quote:

---

Evolution has been caught in the act, according to scientists who are decoding how a species of Australian lizard is abandoning egg-laying in favor of live birth.

Along the warm coastal lowlands of New South Wales (map), the yellow-bellied three-toed skink lays eggs to reproduce. But individuals of the same species living in the state's higher, colder mountains are almost all giving birth to live young.

Only two other modern reptiles—another skink species and a European lizard—use both types of reproduction. (Related: "Virgin Birth Expected at Christmas—By Komodo Dragon.")

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.

(See "Oldest Live-Birth Fossil Found; Fish Had Umbilical Cord.")

But modern reptiles that have live young provide only a single snapshot on a long evolutionary time line, said study co-author James Stewart, a biologist at East Tennessee State University. The dual behavior of the yellow-bellied three-toed skink therefore offers scientists a rare opportunity.

"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."
Eggs-to-Baby Switch Creates Nutrient Problem

One of the mysteries of how reptiles switch from eggs to live babies is how the young get their nourishment before birth.

In mammals a highly specialized placenta connects the fetus to the uterus wall, allowing the baby to take up oxygen and nutrients from the mother's blood and pass back waste. (See related pictures of "extreme" animals in the womb.)

In egg-laying species, the embryo gets nourishment from the yolk, but calcium absorbed from the porous shell is also an important nutrient source.

Some fish and reptiles, meanwhile, use a mix of both birthing styles. The mother forms eggs, but then retains them inside her body until the very last stages of embryonic development. (Related: "Dinosaur Eggs Discovered Inside Mother—A First.")

The shells of these eggs thin dramatically so that the embryos can breathe, until live babies are born covered with only thin membranes—all that remains of the shells.

This adaptation presents a potential nourishment problem: A thinner shell has less calcium, which could cause deficiencies for the young reptiles.

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.
Evolutionary Transition Surprisingly Simple

Both birthing styles come with evolutionary tradeoffs: Eggs are more vulnerable to external threats, such as extreme weather and predators, but internal fetuses can be more taxing for the mother.

(Related: "Human Sperm Gene Traced to Dawn of Animal Evolution.")

For the skinks, moms in balmier climates may opt to conserve their own bodies' resources by depositing eggs on the ground for the final week or so of development. Moms in harsh mountain climates, by contrast, might find that it's more efficient to protect their young by keeping them longer inside their bodies.

In general, the results suggest the move from egg-laying to live birth in reptiles is fairly common—at least in historic terms—because it's relatively easy to make the switch, Stewart said.

"We tend to think of this as a very complex transition," he said, "but it's looking like it might be much simpler in some cases than we thought."

---

Message 2 of 16 (594724) 12-04-2010 7:59 PM

Reply to: Message 1 by Taz 12-04-2010 7:17 PM

Edited by Larni, : clarity

Edited by Adminnemooseus, : Hide and Off-topic banner.

Message 3 of 16 (594725) 12-04-2010 8:19 PM

Reply to: Message 1 by Taz 12-04-2010 7:17 PM

Edited by Panda, : No reason given.

Edited by Adminnemooseus, : Hide and off-topic banner.

Message 4 of 16 (594727) 12-04-2010 8:38 PM

Reply to: Message 1 by Taz 12-04-2010 7:17 PM

Edited by Adminnemooseus, : Hide and off-topic banner.

Message 5 of 16 (594728) 12-04-2010 8:47 PM

Reply to: Message 4 by subbie 12-04-2010 8:38 PM

Edited by Adminnemooseus, : Hide and off-topic banner.

Message 6 of 16 (594729) 12-04-2010 9:07 PM

Edited by Adminnemooseus, : Hide and off-topic banner.

What if Eleanor Roosevelt had wings? -- Monty Python

You can't build a Time Machine without Weird Optics -- S. Valley

Message 7 of 16 (594734) 12-04-2010 10:00 PM

Reply to: Message 1 by Taz 12-04-2010 7:17 PM

Edited by Bluejay, : No reason given.

Darwin loves you.

Message 8 of 16 (594735) 12-04-2010 10:08 PM

Reply to: Message 7 by Blue Jay 12-04-2010 10:00 PM

Damn, I was shooting for 6 straight hide with off-topic banners.

There's going to be some heavy slaps done if the above trend continues.

NO REPLIES TO THIS MESSAGE - JUST DO REAL ON-TOPIC MESSAGES.

Adminnemooseus

Message 9 of 16 (594767) 12-05-2010 8:30 AM

Reply to: Message 1 by Taz 12-04-2010 7:17 PM

Evolution in action: Transition from egg-laying to live birth in lizards. Message 1

What would be interesting to know is if future research can identify the mutations that make the difference between the slightly "egg retaining" mothers and their viviparous cousins. Also, it would be interesting to know if egg- eating predators have been identified in the "viviparous" area, as that would provide an obvious strong selection reason behind the transition.

Here's the abstract. They were mainly researching the nutrition problem.

Message 10 of 16 (594780) 12-05-2010 9:32 AM

Reply to: Message 9 by bluegenes 12-05-2010 8:30 AM

I read that as "the neutrino problem" and was suitably confused

Bloody physicists...

Message 13 of 16 (595151) 12-06-2010 11:06 PM

Reply to: Message 9 by bluegenes 12-05-2010 8:30 AM

Or if the difference in the ecology\temperature\climate makes a difference.

Enjoy