Genetic changes for a new home

Researchers have identified genetic changes that enable herring to reproduce successfully in the low-salinity Baltic Sea.

• Adaptation to low salinity: Four genetic changes ensure that fertilization and embryonic development can occur even in the brackish waters of the Baltic Sea.
• Reproduction ensured: The osmotic pressure caused by the brackish water strains eggs, sperm, and embryos. In Baltic Sea herring, a structurally altered ion channel regulates the salt balance of sperm, while the egg membranes have been strengthened by two genetic adaptations, and a hatching enzyme has been multiplied which degrades the reinforced egg membrane.
• Insights into evolution: Natural selection has altered several genes simultaneously, thereby enabling the herring’s adaptation to a new habitat.

How do species adapt to a new environment? This question has puzzled evolutionary biologists for more than a century. Using the herring as an example, researchers have now shown how the fish’s reproduction genetically adapted within a few thousand years to a new habitat – the Baltic Sea with its low-salinity. The Baltic Sea was formed approximately 8,000 years ago following the last Ice Age; it is connected to the Atlantic Ocean only via narrow straits and receives large amounts of freshwater from rivers. As a result, the salinity in some parts of the Baltic Sea is only two to three per mille, whereas in the Atlantic Ocean it is about 35 per mille – that is, roughly ten times higher.

This difference has far-reaching consequences for aquatic life. Cells are surrounded by membranes through which water and dissolved substances can pass only in a controlled manner with the help of special proteins in the membrane, known as ion channels or transporters. If the salt concentrations inside and outside the cell differ, water moves across the membrane until equilibrium is reached – a process called osmosis. As a result, cells can either take in water and swell or lose water and shrink.

For fish, which release eggs and sperm directly into the water, osmosis is particularly delicate. Eggs and sperm are exposed to environmental conditions without protection. At very low salinity, water penetrates cells, causing them to swell and, in extreme cases, burst. In very high-salinity water, the opposite occurs: cells lose water, making fertilization of eggs more difficult.

In search of the genetic causes

When herring migrated from the Atlantic to the Baltic Sea, eggs, sperm, and embryos had to adapt to the brackish water. An international research consortium investigated how this adaptation occurred. Researchers from the universities of Uppsala, Bergen, Bonn, and Tokyo, as well as the Max Planck Institute for Multidisciplinary Sciences (MPI-NAT) in Göttingen (Germany), analyzed the genomes and proteins of Atlantic and Baltic herring.

At MPI-NAT, a team from the Bioanalytical Mass Spectrometry research group, led by Henning Urlaub in close collaboration with Emeritus Director Benjamin Kaupp, used high-resolution mass spectrometry to examine the protein composition of sperm, eggs, and various tissues. “Using mass spectrometry, we were able to show that the proteins in Baltic Sea herring – which have been genetically altered by evolutionary pressure – are indeed present in sperm and egg cells, allowing us to formulate plausible hypotheses about their biological function,” says Kaupp. Only then were the researchers able to understand how evolution leads to specific adaptations to a changing environment at the molecular level.

A special ion channel protects sperm

Many cells possess ion channels, known as VRACs, through which ions and other dissolved substances can leave or enter the cell, thereby counteracting osmotic pressure. “We have discovered a new form of a VRAC channel gene that occurs only in sperm,” explains Kaupp. “This ion channel, along with changes in a total of four genes – one in sperm and three in eggs – enables herring to reproduce in the low-salinity waters of the Baltic Sea.” This newly discovered variant of the VRAC ion channel helps sperm regulate their salt balance and remain functional even under drastically altered salinity conditions. Only herring and closely related fish species possess two different variants of this channel protein: one in body cells and one that occurs specifically in sperm. Other vertebrate species, including humans, have only one form of this gene, which is active in both sperm and other cells. The evolution of a sperm-specific VRAC channel is an innovation that happened million of years ago in the herring lineage. It is possible that it allows this group to better tolerate variation in salinity during reproduction.

The eggshell as a protective shield

Two further genetic adaptations concern the egg coat. Changes in the blueprint of a protein in the egg coat and of an enzyme that creates cross-links between proteins in the egg coat make the eggs more resilient. This provides them with better protection against swelling in brackish water. However, this reinforced egg coat creates a new problem: The larvae must break through it again when hatching.

The researchers also found evidence of a genetic solution for this problem: Baltic Sea herring possess about 20 additional copies of a gene that provides the blueprint for a “hatching enzyme.” This enzyme helps the embryo break down the reinforced egg membrane during hatching. The combination of a modified ion channel, a more stable eggshell, and more hatching enzyme indicates that the selective pressure was particularly strong.

“The study contributes to our understanding of how biological diversity arises at the molecular level,” says Kaupp. “It shows how strong natural selection has led to changes in multiple genes that, together, enable successful reproduction in a new environment.” Similar mechanisms could also play an important role in other fish species and, more generally, in species with external fertilization. (vl)