Egg Laying Or Live Birth: How Evolution Chooses | Quanta Magazine

At the time of that publication, scientists thought that live birth might have evolved among the reptilian ancestors of ichthyosaurs only after they moved from the land to the sea. But the discovery of a 248-million-year-old fossil changed that. In a paper published in PLOS ONE in 2014, researchers describe the fossil of an ichthyosaur that died while giving birth. Amazingly, the fossil captured the precise moment when the newborn emerged from its mother’s pelvis headfirst. That position is telling: Most viviparous marine reptiles are born tail first so that they can continue to draw oxygen from their mother during labor. The headfirst birth position indicates the ichthyosaur inherited live birth from an even more ancient land ancestor. Land reptiles may therefore have been giving birth to live young for at least 250 million years, though the oldest fossil of live birth on dry land doesn’t date nearly that far back.

Eggs, Babies or Both

Live birth or egg laying might seem like a definitive either-or choice for a species, but surprisingly, that’s not always the case. Whittington and her team study the Australian three-toed skink (Saiphos equalis), a lizard with the remarkable distinction of being able to both lay eggs and give birth to live young. A couple of other lizard species have been known to do both, usually in different settings, but in Whittington’s laboratory, the researchers observed a three-toed skink produce a litter that consisted of three eggs and one live baby. “We were absolutely flabbergasted,” Whittington said.

Recently in Molecular Ecology, Whittington and her team describe the differences in gene expression — which genes are switched on or off — between a lizard mother that lays eggs and one that gives birth to live young. Within a single species, there are thousands of such differences between a female with an egg and one without. That’s because certain genes get switched on when it’s time for the uterus to house an egg. The same goes for a uterus that’s sheltering an embryo. Crucially, the specific genes that get switched on in these cases are very different.

What is the genetic toolkit that enabled live birth?

Camilla Whittington, University of Sydney

But in three-toed skinks, a lot of the genes that switch on when a mother makes an egg also get switched on in mothers with embryos. The finding implies that this lizard is in a transitional state between egg laying and live bearing.

Which way the lizard is evolving is impossible to say and may still be undetermined. “Evolution is a random process rather than being directed,” Whittington said. “With environmental changes, it could change the direction of selection and push it back the other way.”

The idea that the skink could be moving away from live bearing and back to egg laying is a relatively new development in the field. “Twenty years ago we thought it was difficult or impossible for egg laying to re-evolve,” Whittington said. But a growing body of research since then has shown that it may be quite common. Recent analyses of genetic relationships between species revealed that certain egg layers are deeply nested within an evolutionary tree of live-bearing neighbors.

Related:

1. Why Sex? Biologists Find New Explanations.

2. Cell by Cell, Scientists Map the Genetic Steps as Eggs Become Animals

3. Why Evolution Reversed These Insects’ Sex Organs

4. Did the Chicken Come First or Is It Turtles All the Way Down?

Whittington’s work is driven by a desire to understand what different live-bearing species have in common. “What is the genetic toolkit that enabled live birth?” she asked. “Are there fundamental rules about viviparity? Does it use the same genetic instructions when it evolved? Do [different species] have the same problems?”

The three-toed skink is not the only remarkable creature she studies as she searches for answers. Sea horses are the only known animals in which the males become pregnant: A female transfers her egg into her mate’s pouch for fertilization and development. Whittington’s work with sea horses has revealed that the males activate the same genes that females of many other species do to bear live young, which Whittington argues is remarkable. “We’re talking about different sexes. We’re talking about completely different tissues. We’re talking about this trait having evolved in completely different species and millions of years apart,” she said. “It’s like having these amazing naturally replicating evolution experiments that have been running for millions of years.”