高清福利片

Our earliest vertebrate (animals with backbones) ancestors laid eggs, but over millions of years of evolution, some species began to give birth to live young.

There is a traditional dichotomy in vertebrate reproduction: species either lay eggs or have live births. However, as is often the case in biology, things aren鈥檛 as simple as they first appear, and there are a handful of vertebrate animals that do both.

One of these is the three-toed skink (Saiphos equalis).听听suggests the egg-laying听S. equalis听may currently be in the process of transitioning from egg-laying to giving live birth.

高清福利片ing them gives us a unique opportunity to watch evolution in action.

Eggs to babies, and back again?

There are two main reproductive strategies in vertebrates.

Animals that lay eggs are called 鈥渙viparous鈥�. For instance, many fish species spawn eggs that are fertilised externally. In other oviparous species, including birds and some lizards and snakes, eggs are fertilised inside the mother, an eggshell is added, and then eggs are laid.

Depending on the species, much or all of the nutrition needed to grow a healthy baby is supplied in the egg yolk.

In contrast, 鈥渧iviparous鈥� animals carry embryos internally until they are fully developed. The embryos can rely entirely on yolk for nutrition, or the parents can provide supplementary nutrition, sometimes via a placenta (as in humans).

There is strong evidence that听egg-laying is ancestral to live birth, meaning it came first. Many physiological changes were necessary for live birth to have evolved from egg-laying. With this transition, some structures were lost, including the hard outer eggshell. Other mechanisms were gained to ensure embryonic survival within the parent, including the supply of adequate oxygen and water during development.

The evolution of live birth has occurred frequently,听including at least 121 times in independent groups of reptiles.

Evolutionary 鈥渞eversals鈥� to egg-laying are much rarer, probably because听听would be exceptionally difficult.

Why not both?

Despite the vast differences between egg-laying and live birth, some species can do both. This phenomenon called 鈥渂imodal reproduction鈥� is exceptionally rare. There are听more than 6500 species of lizards worldwide, but only three exhibit bimodal reproduction.

We鈥檙e lucky enough to have two of these in Australia.听听at the University of Sydney studies the bimodally reproductive three-toed skink, in the hope of understanding how live birth evolved.

In northern NSW, the three-toed skink听. Even though they reproduce differently,听听has shown these lizards are a single species.

Even the egg-laying members of the species are odd, as the eggs are retained within the mother for a relatively long time. After being laid, ordinary skink eggs are听incubated for at least 35 days听before they hatch, but some three-toed skink eggs听hatch in as few as five days听after being laid.

One female even听laid eggs and gave birth to a live baby in the same litter.

The genetics behind different reproductive modes

Most aspects of an animal鈥檚 development are controlled by its genes, but not every gene is always active. Genes can be expressed (switched on) to different degrees, and gene expression can stop when not needed.

An egg-laying skink uterus undergoes听only a couple of genetic changes听between being empty and holding an egg.

A live-bearing skink uterus is different. It undergoes听thousands of genetic changes听to help support the developing baby, including genes that probably help provide oxygen and water, and regulate the mother鈥檚 immune system to keep the baby safe from听immunological attack.

Unexpected similarities between the egg-laying and the live-bearing

听measured changes in gene expression between egg-laying and live-birth in the three-toed skink. We investigated how the expression of all genes in the uterus differed between when the uterus was empty and when it held an egg or embryo.

As expected, live-bearing听S. equalis, undergo thousands of genetic changes during pregnancy to produce a healthy baby.

But surprisingly, when we looked at the uterus of the egg-laying听S. equalis, we found these also undergo thousands of genetic changes, many of which are similar to those in their live-bearing counterparts.

Some of the most important genetic changes in gene expression in egg-laying听S. equalis听allow embryos to develop within the mother for a long time. These genes also seem to allow the uterus to remodel to accommodate a growing embryo, and drive the same kinds of functions required for the embryonic development in live-birthing three-toed skinks.

Are 鈥榬eversals鈥� to egg-laying easier than previously thought?

Our findings are important because they demonstrate that egg-laying three-toed skinks are an evolutionary intermediate between 鈥渢rue鈥� egg-laying and live birth.

We now know that uterine gene expression in egg-laying听S. equalis听mirrors live-bearing skinks much more closely than true egg-laying skinks. These results may explain why it鈥檚 possible for a female three-toed skink to听.

The similarities in gene expression between egg-laying and live-bearing three-toed skink uteri might also mean 鈥渞eversals鈥� from live birth back to egg-laying could be be easier than previously thought. However, this may be restricted to species in which live-birth has evolved recently, such as the three-toed skink.

This article was written by Dr Camilla Whittington, in the School of Life and Evironmental Sciences,听and Charles Foster, a Postdoctoral Research Associate at University of Sydney. The article was first published in The Conversation as: