The ability to give live birth is one of the great innovations of evolution. But how did it develop? Researchers have now understood this for the first time at the genetic level. To do this, they focused on a genus of sea snails, some species of which lay eggs, while others give birth to their offspring live. The analyzes show that live birth apparently did not arise in one major evolutionary step. Instead, more and more mutations gradually accumulated, which increased the length of time the eggs remained in the mother - until the young were finally born alive.
Since ancient times, animals have reproduced by laying eggs. However, over the course of evolution, different groups of animals have independently developed the ability to give live birth to their offspring. But how did the transition from egg-laying to vivipary, i.e. live birth, take place? Was there a major mutation that caused an animal that itself had hatched from an egg to suddenly give birth to its young? Or was development gradual with transitional forms between egg laying and live birth?
Viviparous sea snail
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 Stankowski’s colleague Roger Butlin. “For example, live birth laid the foundation for the diversification of mammals. However, because most of these major evolutionary changes occurred a long time ago, there have been few opportunities to study them.”
Sea snails of the genus Littorina offered the researchers a unique example in this regard. While many members of this genus lay eggs, some species, including the rock periwinkle (Littorina saxatilis), give birth to their offspring live. The snail is widespread on the Atlantic coast of Northern Europe and North America. Their snail shells can also be found on German North Sea beaches. “So far, the main focus has been on the different shell variations of L. saxatilis and not on what distinguishes the species from its egg-laying relatives,” explains Stankowski. “In fact, this snail species is an exceptional case when it comes to its reproductive strategy.”
Genomes in comparison
Stankowski and his team analyzed the genome of Littorina saxatilis and compared it with that of closely related species, which differ little from Littorina saxatilis apart from the fact that they lay eggs. “We were able to identify 50 genomic regions that probably contribute together to whether individuals lay eggs or give birth to live young,” reports Stankowski. “We don’t know exactly what the individual regions do. However, by comparing gene expression patterns in egg-laying and viviparous snails, we were able to link many of them to reproductive differences.”
The results show that the transition to live birth did not take place in a single, large step, but in thousands of small ones. “The age of the selection processes suggests that the alleles specific to live-bearers have accumulated over more than 200,000 generations,” the research team writes. Development probably took place over a period of around 100,000 years - a very short period of time on evolutionary scales.
Ready for new living spaces
The switch to live birth probably allowed the snails to explore new habitats. “It is likely that live birth makes reproduction possible in areas where conditions would be too harsh for eggs,” the team explains. In the womb, however, the offspring are protected from predators, dehydration and the influences of nature. “We suspect that natural selection was the driving force for this transition. A longer residence time for the eggs was encouraged, which led to the young eventually hatching from the egg in the mother animal,” says Stankowski.
At the same time, the change meant new challenges for the mother animal. “The additional investment in the offspring certainly led to new demands on the anatomy, physiology and immune system of the snails. “It is likely that many of the genomic regions we identified are involved in responding to these types of challenges,” says Stankowski. In future studies, the team wants to find out what function the individual modified genes have. “Our goal is to understand how each genetic change gradually shaped the form and function of the snails on their way to becoming viviparous animals.”
Source: Sean Stankowski (University of Sheffield) et al., Science, doi: 10.1126/science.adi2982