The evolution of live birth took place gradually
Once upon a time, all animals laid eggs. Now researchers are on the trail of live birth: the genetic analysis of a sea snail sheds new light on the causes.
Which came first: hen or egg? It's a popular question and not easy to answer. However, it can be said that egg-laying (oviparity) is deeply rooted in evolution and emerged before the first animals ventured onto land. Since then, however, there have been numerous independent transitions to live birth (viviparity) throughout the animal kingdom. Many insects, fish, reptiles and above all mammals now give birth to their offspring alive. The oldest evidence of live birth to date is the 380-million-year-old fossil of an armoured fish. However, the genetic changes required to drive this particular evolutionary process were previously unclear. An international research team led by Sean Stankowski from the Institute of Science and Technology Austria (ISTA) has now used the example of a marine snail to analyse which genetic regions contribute to whether individuals lay eggs or give birth to live young. The results were published in the journal "Science".
The fact that the scientists investigated the phenomenon using the marine snail Littorina saxatilisq as an example is due to the fact that the development of saltwater animals into livebearers took place over a period of just 100,000 years - a very short period in evolutionary terms. Mammals, on the other hand, have been giving birth to their young alive for around 140 million years. In addition, live birth is the only known feature that distinguishes L. saxatilis from its egg-laying relatives. This makes it easy to investigate the genetic cause of the difference. The team focussed on around 50 genetic changes that span the entire snail genome.
We suspect that natural selection was the driving force behind the transition. In the womb, the young are better protected from dehydration, predators and physical damage.
"We don't know exactly what the individual regions do," Stankowski said, according to an ISTA press release. "However, by comparing gene expression patterns in egg-laying and viviparous snails, we were able to link many of them to reproductive differences." Viviparity has evolved gradually through the accumulation of many mutations that have arisen over the past 100,000 years. This has enabled the snail to spread to new areas and habitats where conditions would be too inhospitable for eggs. "We suspect that natural selection was the driving force behind this transition," explained Stankowski. In the womb, the young are better protected from dehydration, predators and physical damage,
It has long been debated whether the accumulation of many small genetic changes or rather large leaps are more decisive for evolution, writes Kathryn Elmer in an accompanying article. The fact that a single genetic mutation in snails is not responsible for them giving birth to their young is consistent with recent findings from genome studies in organisms as diverse as humans and microbes: Changes to relatively small genetic regions can have large effects on phenotypes.
Spectrum of Science
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Header image: © Daria Shipilina (detail) Littorina snails are common on the rocky coasts of Europe, the United Kingdom and the east coast of the USA
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