Snail giving birth to live young reveals that evolutionary leaps occur gradually

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Egg-laying was the preferred choice of snails for millions of years, but sometime in the last 100,000 years, a group of sea snails abandoned eggs in favor of live births. The change occurred in the blink of an eye in evolutionary terms, and scientists have now discovered that it was driven by around 50 genetic changes. The rare opportunity to study the genetic architecture of an evolutionary change has revealed that the secret is not in taking one giant leap, but in the accumulation of many gradual changes.

That Littorina saxatilis Being a living snail is the only obvious trait that separates it from its egg-laying neighbors because it exhibits a wide diversity of shell types and habitats. This may explain why it is one of the most misidentified creatures on the planet, having been given new species and subspecies names more than 100 times according to The Guardian.

To get a better idea of ​​what sets these sea snails apart on a genetic level, a team of researchers led by Austrian Institute of Science and Technology (ISTA) postdoc Sean Stankowski examined the family tree of L. saxatilis and their relatives using whole genome sequences. In doing so, it was revealed that The ability to give birth to live young was related to 50 genetic changes spread throughout the snail’s genome and its identification was made easier due to the degree of interbreeding between snails in L. saxatilisdistribution range, which can lead to gene exchange between species.

“In this case, it is what gives us the opportunity to find the genes involved in the evolution of life, because the genetic background has been mixed with all this mixing,” Professor Roger Butlin from the University of Sheffield’s School of Biosciences, told IFLScience. “That’s why the genes responsible for life stand out in the background.”

two ecotypes of Littorina saxatilis

Both are Littorina saxatilis, but the larger snail is an ecotype that is adapted to defend against crab attacks, while the smaller one is a second ecotype that is adapted to live in areas with strong wave exposure.

Image credit: Sophie Webster.

Searching through the wealth of genetic information revealed that living baby sea snails emerged gradually over the past 100,000 years as a series of mutations accumulated. However, we still do not know which of those incremental changes were instrumental in the surprising shift in reproductive strategy.

“I don’t think we can say exactly what was necessary specifically for the snails’ life trait at this time,” Butlin continued. “All 50 occur together in all living snails, so it seems that many of them are necessary – together – for the production of life, but we think that some of them are probably responsible for other things that are associated with the production of life , such as the shift to year-round breeding instead of only breeding in one season.”

As for why they made this move, it’s possible that the transition from laying eggs to living could benefit these snails by making their young less susceptible to drying out, being crushed, or being predated before they had a chance to hatch. But that doesn’t mean it hasn’t generated some costs for parents.

“Additional investment in hatchlings would almost certainly have placed new demands on the snails’ anatomy, physiology and immune system,” Stankowski said in a statement. statement. “Many of the genomic regions we identified are likely involved in responding to these types of challenges.”

Diversity of Littorina snail shells

The wide range of shell types and habitats of Littorina snails may be a result of their way of life allowing them to adapt to a wide range of environmental conditions. This has led to the evolution of many “ecotypes” that vary in size, shape and behavior.

Image credit: Fredrik Pleijel.

Understand how big changes like this occur in evolution It is something we rarely have the opportunity to investigate, as it occurs very slowly and in a wide range of species, many of which are extinct. The University of Sheffield team says this rare example of studying the genetic architecture of evolutionary change demonstrates that breakthrough innovations occur through incremental steps – no giant genetic leap is needed to establish surprising new functions.

Not only does this teach us about some of the big evolutionary changes of the past, like how feathers led to flight, but it can also give us insight into how future changes may occur in response to things like climate change, and what species are It is unlikely to adapt the thermal regulation necessary for survival.

“By discovering and studying the recent evolutionary change in the way sea snails give birth, we can now understand these important changes and apply our methods to many other evolutionary changes,” Butlin said in another statement.

“Our results will change the way biologists view major evolutionary transitions, shifting the focus from large leaps in evolution toward understanding the progressive benefits of small evolutionary steps. “They will also help others analyze the genetic and historical bases of other adaptive traits, which is important when many organisms are forced to adapt rapidly to a changing world.”

The study is published in the journal. Science.

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