One lays eggs, the other gives birth to alive offspring. What has happened?


Embryos. A photo of the embryos that developed inside a live-bearing female. Many different stages of development can be observed from just-fertilized embryos, to fully-hatched miniature snails. Photo: Fredrik Pleijel

One lays eggs, the other gives birth to alive offspring. What has happened?
The periwinkle snail Littorina saxatilis does not lay eggs but gives birth to fully developed youngsters. Something must have happened – and that less than 200,000 years ago.

Sometimes, evolution brings about innovations that are real "game-changers" for nature. The eye and vision are developed. Birds begin to fly. The shift to giving birth to live young instead of laying them as eggs on the ground.

Scientists call these key innovations, central "inventions" in the development of species that give organisms entirely new opportunities to live and reproduce. But how do such innovations arise?

“The problem with studying the underlying genetic mechanisms behind such major changes is that they lie so far back in time. The more time that has passed, the more difficult it is”, says researcher Anja Westram at Nord University's Faculty of Biosciences and Aquaculture.

Dozens of Mutations

But on the rocky shores in the tidal zone lives a small creature that has given researchers the opportunity to study such dramatic genetic changes, if not in real-time then at least relatively shortly after they have occurred.

The little snail Littorina saxatilis, called the rock snail in Norwegian, looks exactly the same as its close relative Littorina arcana. But while the latter reproduces in the usual snail way by laying eggs, the rock snail gives birth to fully formed snail babies.

Recently, Westram and her research colleagues were able to show how the transition from laying eggs to giving birth to live young in these snails most likely developed over the last 100,000 years. The results were recently published in the prestigious journal Science.

“One of the big questions is whether such changes occur due to a single, decisive mutation, or if there is a series of mutations. We found that there were dozens of mutations within fifty different small regions, so-called genomic windows. These were scattered throughout the entire genome of the snail”, explains Westram.

A genome is the total amount of hereditary material that the organism carries. All the DNA in the cells, from A to Z. In other words, it is not a single gene that leads to the shift from egg laying to live births. At least not in these snails.

Kvinne går på berget ved havet
Snail researcher. Anja Westram is a researcher at Nord University's Faculty of Biosciences and Aquaculture. Photo: Joost Raeymaekers

Eggs Means Limitations

The mutated genes are also not grouped together in a sort of "give-birth-to-live-young" section in the DNA.

“Instead, it is distributed over a range of different genes, which probably code for quite different things”, says Westram.

“One would think the genes were gathered in a specific region of the DNA?”

“Yes, exactly. That's why many researchers find these findings interesting. There are theories about how such major changes happen. That there is one big lucky mutation that changes a specific trait. In some cases, it might happen, but we found that there were many. Spread all over”.

The result of the shift from eggs to live young can be observed by the researchers in the tidal zone. While the egg-laying L. arcana mostly has to stay in areas where the eggs can be laid in moist cracks and crevices, L. saxatilis can spread out over larger living areas.

“These are two completely different ways of reproducing. Eggs must be laid where they can survive. These eggs are not like chicken eggs with hard shells. They are gelatinous clumps that easily dry out. They need a lot of water in the environment. If you do not lay eggs, you can carry the young inside you until they are big enough to survive in drier areas.”

Recognizing Their Fellow Species

Researchers assume that the two species have been separate species for only 100-200,000 years. The only difference between them is the way they give birth to young. That they actually are two different species, we know from the genetic differences seen when studying the DNA.

“They live side by side, but they do not mix with each other. They meet, and they sometimes mate with each other. But they do not have offspring together”, says Westram.

“How do we know that?”

“Then we would have found hybrids, crossings. But extensive studies have been done on this, and we do not find hybrids”.

“So they might have a kind of secret method of knowing that the one they mate with is of the same species?”

“Yes, I actually think so. A student of ours has studied this by observing the snails. She found that they usually did not mate with the other species, so there must be something that makes them dislike each other. A kind of cascade of various barriers. Presumably, they are not genetically compatible, so that the offspring are not born.”

The two species not only have different living areas in the tidal zone. While L. saxatilis with its live-born young is widespread both in Europe and the USA, we find the egg-laying L. arcana only along the coast of Norway and France and on the British Isles.

Strandsnegler på stein
Littorina. Littorina snails are common on the rocky shores of Europe, the UK, and the East Coast of the USA. Photo: Daria Shipilina

Challenging Science's Species Definitions

Surprisingly, the northern variants of the two species genetically resemble each other more than the northern and southern variants of L. saxatilis, which are considered the same species. When researchers place the two species in a so-called phylogenetic tree, which is supposed to show how the species have developed and the kinship between them, L. arcana ends up between the two geographically separate variants of L. saxatilis.

“Along the coast of Northern Europe, the two species live side by side, and it is assumed that they initially crossed with each other. Maybe that’s why they are genetically so similar. But if you read between the lines in our article, you will see that we are not really sure what this means. I think maybe it just shows that we still do not quite know what defines a species”, says Westram.

The work has mainly been a collaboration between the Institute of Science and Technology in Austria, the University of Gothenburg, the University of Sheffield, and Nord University. The study is funded by the British Natural Environment Research Council and the Research Council of Norway.