Accidental Breakthrough: An Unexpected Laboratory Hybrid of a Critically Endangered Fish Species

Above: Russian sturgeon (top); American paddlefish (bottom). Sources: Wikipedia and Wikipedia

Introduction:

Sturgeon refers to the fish species in the family Acipenseridae. They are among the group commonly called primitive fish or living fossils, as they have existed on Earth for over 100 million years. Until recently, sturgeon species were not in danger of extinction as food supplies and habitat were plentiful, and they had no natural predators.

However, sturgeon – more accurately, female sturgeons – are the world’s primary source of caviar, the prized luxury food that consists of sturgeon roe (or eggs). Thanks to the demand for and profitability of caviar, sturgeon have been heavily overfished. Sturgeon also grow slowly and don’t reproduce very often, so the population decline caused by overfishing is not being offset by newly-hatched sturgeon. Further, more and more areas of sturgeon habitat are being destroyed and polluted – which also affects sturgeons’ food supply. Thus, most sturgeon species are listed as critically endangered – on the brink of extinction.

Scientists around the world are researching methods to conserve the remaining sturgeon population and facilitate sturgeon reproduction. One such group of scientists, based in Hungary, also studies the American paddlefish, another living fossil. It is also overfished, but far less so than sturgeon, and is not critically endangered.

These scientists recently made history by creating the first surviving hybrid of Russian sturgeon and American paddlefish – by accident, to boot. Their work was published recently in Genes, an open-access peer-reviewed journal.

Hybridization and research overview:

Hybridization, or combining the genetic material of two species to create a third, is not unusual among plants and animals. Mules, likely the most common hybrid, are the result of humans breeding male donkeys and female horses – with the first mule being born thousands of years ago. Interestingly, mules themselves are sterile, meaning they cannot produce offspring.

Several different hybrids of wild cat species have been documented in nature, and many more in captivity. Sometimes the hybrid offspring are fertile (meaning they can produce offspring), sometimes they are sterile. Lifespan of these hybrids can vary from a few days to approaching that of normal, non-hybrid individuals of their parents’ species.

What all these hybrids have in common is genetic similarity. Successful hybrids are generally born of two parents who are in the same taxonomic genus or family.

This makes the Hungarian scientists’ work all that much more groundbreaking as Russian sturgeon (Acipenser gueldenstaedtii) and American paddlefish (Polyodon spathula) are from different taxonomic families. Until this specific experiment, in fact, such phylogenetically distant species were assumed to not be capable of producing hybrid offspring.

The scientists themselves only combined the sturgeon and paddlefish DNA as a negative control, or to demonstrate that this hybridization would not work.

Unexpectedly, it did work.

Depending of the proportion of sturgeon DNA that is expressed in their genes, the hybrids look more like sturgeon or more like paddlefish, as shown in Figure 4 from the article:

Observed survivability rates of the hybrid offspring are similar to non-hybrid sturgeon, and surviving hybrids seem to mature and develop much like non-hybrid sturgeon.

Implications:

First and foremost, this is a scientific milestone in the realm of hybridization. This is the first documented hybrid of sturgeon with paddlefish. Second, Russian sturgeon and American paddlefish are now the two most phylogenetically distant species documented to have produced viable hybrids.

Second, unlike some hybrids with short lifespans or stunted growth, the sturgeon-paddlefish hybrids are surviving and developing similarly to non-hybrid sturgeon. This suggests that they could mature normally and live a normal sturgeon lifespan (which is several decades).

A hybrid sturgeon that is able to live normally has immense potential to help stave off the looming extinction of wild sturgeon species. Much of the population decline of sturgeon is due to overfishing and the caviar trade. Thus, aquacultured hybrid sturgeon could potentially replace wild sturgeon as a source for food and caviar. This could result in wild sturgeon no longer being overfished.

To get to this point, the long-term viability of these new hybrids needs to be better known. Also needing further study is their ability to reproduce normally and produce roe (that is so prized as caviar, and the root of sturgeon overfishing). Both of these issues require many years of careful study and observation, particularly as most non-hybrid sturgeon don’t spawn (reproduce) for the first time until at least age 12.

Finally, until now, the hybrids have spent their entire lives in controlled, laboratory conditions. How would they do in aquaculture conditions? We don’t yet know.

We also don’t know whether similar strategies could be used in the conservation of other critically endangered fish species.

But stay tuned, a pure accident has gotten us to this point, who knows how far deliberate study can take us.