The ability to give birth live is one of evolution's greatest innovations. But how did it develop? Researchers have now understood this at the genetic level for the first time. To do this, they focused on the species of sea snails, some of which lay eggs, and others give birth to live offspring. Analyzes show that live birth did not appear to arise in one major evolutionary step. Instead, more and more mutations gradually accumulated, increasing the length of time that the eggs remained in the mother – until the young were eventually born alive.
Since ancient times, animals reproduce by laying eggs. However, over the course of evolution, different groups of animals have independently evolved the ability to give birth to their offspring alive. But how did the transition from egg laying to live birth occur? Was there a major mutation that caused an animal that hatched from an egg to suddenly give birth to young? Or was evolution gradual, with transitional forms between egg-laying and live birth?
Marine snails are vital
A team led by Sean Stankowski from the University of Sheffield has found an answer to this question with the help of sea ticks. “It is important to understand the evolutionary origins of key innovations, because they dramatically change the course of evolution,” says Roger Butlin, Stankowski's colleague. “For example, live birth laid the foundation for mammalian diversification. However, because most of these major evolutionary changes occurred a long time ago, there have been few opportunities to study them.”
Marine snails of the genus Littorina provided researchers with a unique example in this regard. While many members of this genus lay eggs, some species, including periwinkle (Littorina saxatilis), give birth to live offspring. The snail is widespread on the Atlantic coast of northern Europe and North America. Their snail shells can also be found on the beaches of the German North Sea. “Until now, the main focus has been on the different differences in the shells of L. saxatilis, and not on what distinguishes this species from its egg-laying relatives,” Stankowski explains. “In fact, this species of snail is an exception when it comes to its reproductive strategy.”
Genomes in comparison
Stankowski and his team analyzed the genome of Littorina saxatilis and compared it with a closely related species, which differs little from Littorina saxatilis apart from the fact that it lays eggs. “We were able to identify 50 genetic regions that together may contribute to determining whether individuals lay eggs or give birth to live young,” says Stankowski. “We don't know exactly what the individual regions do. However, by comparing gene expression patterns in egg-laying and parturient snails, we were able to link many of them to reproductive differences.
The results show that the transition to live birth occurred not in one big step, but in thousands of small steps. “The age of selection suggests that alleles for living carriers have accumulated over more than 200,000 generations,” the research team wrote. Evolution likely occurred over a period of about 100,000 years – a very short period of time on evolutionary scales.
Ready for your new living spaces
The shift to live birth may have allowed snails to explore new habitats. “Live birth likely makes reproduction possible in areas where conditions are too harsh for eggs,” the team explains. However, the offspring in the womb are protected from predators, drought and the effects of nature. “We believe that natural selection was the driving force for this shift. Eggs were encouraged to last longer, which eventually led to young ones hatching from the egg in the mother animal,” says Stankowski.
At the same time, change meant new challenges for the parent animal. “The additional investment in offspring has certainly placed new demands on the anatomy, physiology and immune system of snails. “It is likely that many of the genomic regions we identified are involved in responding to this type of challenge,” says Stankowski. In future studies, the team wants to know The function of individual modifying genes “Our goal is to understand how each genetic change gradually shaped the form and function of the snails on their way to becoming living animals.”
Source: Sean Stankowski (University of Sheffield) et al., Science, doi: 10.1126/science.adi2982
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