Animals reproduce in two different ways: they either lay eggs or give birth to live offspring. A research team from the Institute of Science and Technology Austria (ISTA), the University of Sheffield and the University of Gothenburg has now examined sea snails and their evolutionarily young transition from egg laying to live birth and shed new light on the genetic changes that allow organisms to make such changes. The results were published in the journal Science.
The snail or the egg?
One thing in advance: the egg arrived first – egg laying is deeply rooted in evolution and emerged long before the first animals dared to venture onto land. However, the course of evolution shows us that numerous independent transitions have occurred throughout the animal kingdom. Over time, many insects, fish, reptiles, and mammals became viviparous animals. It has not yet been clarified how many genetic changes are necessary to drive these special evolutionary processes.
Using the example of a sea snail, an international research team led by ISTA postdoctoral fellow Sean Stankowski has now discovered genetic changes that support the transition to live birth (vivipary). The reason this phenomenon was analyzed specifically in marine snails is that the evolution of saltwater animals into livebearers occurred over a period of just 100,000 years – a very short period in evolutionary terms. The mollusk could therefore provide a unique opportunity to discover the genetic basis of live birth.
“Almost all mammals give birth live – a function that has accompanied their evolution for around 140 million years. However, our new study now shows that live births in marine snails developed completely independently and only recently”, explains Stankowski. The team is focusing here on about 50 genetic changes that are spread throughout the snail's genome and that caused the switch to vivipary as a mode of reproduction.
A species with hundreds of names
According to a 2015 report by The Guardian, the sea snail Littorina saxatilis is the most misidentified creature in the world. Although it is widespread along the North Atlantic coast, scientists have described it as a new species or subspecies more than a hundred times over the centuries. All this confusion is probably due to the many variations in shells and habitats of this species. Furthermore, L. saxatilis has a unique mode of reproduction that has evolved over time. Unlike their relatives, sea snails, with whom they share their habitat, do not lay eggs, but practice live birth. “The researchers primarily looked at the different shell variations of L. saxatilis rather than what distinguishes the species from its egg-laying relatives. In fact, this species of snail is an exceptional case when it comes to its reproductive strategy,” says Stankowski.
Lose the egg step by step
Things got exciting when Stankowski used genetic sequences to derive the evolutionary family tree of L. saxatilis and other related egg-laying Littorina species. Analysis of it showed that although live birth is the only feature that distinguishes it from its egg-laying relatives, L. saxatilis does not appear to form a distinct evolutionary group. This discrepancy between reproductive strategy and ancestry ultimately allowed Stankowski and his colleagues to separate the genetic basis of live birth from other genetic changes throughout the snail genome. “We were able to identify 50 genomic regions that probably contribute together for individuals to lay eggs or give birth to live offspring,” explains the postdoctoral fellow. “We don’t know exactly what each region does. However, by comparing gene expression patterns in egg-laying and viviparous snails, we were able to link many of them to reproductive differences.” Overall, the results suggest that viviparia evolved gradually through the accumulation of many mutations that emerged over the last 100,000 years.
Born alive: advantages and disadvantages
The current study shows that the evolution of live births has allowed snails to expand into new areas and habitats where egg layers cannot survive or reproduce. However, the exact benefits remain a mystery. “We suspect that natural selection was the driving force for this transition. Longer egg residence time was favored, which led to the eggs hatching in the animal mother. The eggs were likely more susceptible to dehydration, physical damage and predation. “, explains Stankowski. In viviparous women, the offspring are protected from the influences of nature until they can support themselves, he adds. Although one problem has been eliminated, it does not mean that others have arisen as a result. “Additional investment in offspring has certainly led to new demands on the snails' anatomy, physiology and immune system. It is likely that many of the genetic regions we identified are involved in responding to these types of challenges.”
Record the function of individual genes
Although the work gives us new insights into the transition from eggs to living offspring, many questions still remain unanswered. “Most genetic innovations are very old and entangled on an evolutionary scale. This makes it difficult to investigate its origin,” said the biologist. “While these snails have allowed us to do just that, it’s just the beginning of what they can teach us about the origins of novelty.” In the next step, the researchers want to map the function of each mutation. “Our goal is to understand how each genetic change gradually shaped the form and function of snails on their way to viviparous animals,” concluded Stankowski.
Sean Stankowski is a postdoctoral fellow in Nicholas Barton's group at the Austrian Institute of Science and Technology (ISTA). He started this project at the University of Sheffield (UK) and led a team of collaborators at ISTA, the University of Sheffield and the University of Gothenburg (Sweden), among others.
Publication:
Stankowski, S., et al., 2024. The genetic basis of the recent transition to viviparity in marine snails. Science. DOI: https://doi.org/10.1126/science.adi2982
Project financing:
This project was funded by the National Environmental Research Council (NERC) with project number NE/P001610/1 and by the European Research Council (ERC) with project number ERC-2015-AdG693030-BARRIERS.
Media contact: Andreas Rothe [email protected] +43 664 8832 6510