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When individuals from two different species interbreed, hybrid organisms may emerge that display characteristics from both genetic lineages. While hybridization is a common natural phenomenon, it is often seen as a challenge in biodiversity conservation.

Now, a published in the Proceedings of the National Academy of Sciences reveals that hybridization between the Mediterranean shearwater and the Balearic shearwater—a seabird at critical risk of extinction—has been decisive in maintaining genetic diversity and facilitating the survival of the most threatened seabird in Europe.

With doctoral student Guillem Izquierdo-Arànega as the main driving force, the research was led by professors Julio Rozas, Marta Riutort and Jacob González-Solís from the Faculty of Biology and the Biodiversity Research Institute of the University of Barcelona (IRBio), along with Joan Ferrer Obiol from the University of Milan (Italy). Cristian Cuevas-Caballé, recently awarded his Ph.D. at the UB, is also a co-author of the article, along with other experts.

The new research emphasizes that hybridization should be considered in conservation and management plans, especially at a time when human pressure on biodiversity is accelerating the decline and extinction of life forms across the planet. In the specific case of shearwaters, there is also an urgent need to reduce the accidental bycatch of seabirds in fishing gear to prevent the future extinction of these iconic species.

The Balearic shearwater: An extinction foretold?

The Balearic shearwater (Puffinus mauretanicus), a taxon endemic to the Balearic Islands, is critically endangered according to the International Union for Conservation of Nature (IUCN). Its breeding area is limited to the Balearic Islands, it has a low reproductive rate, and it suffers high additional mortality due to accidental bycatch in longline fisheries. Several simulations suggest that it could become extinct in the coming decades if urgent action is not taken.

The new study investigates the dynamics of hybridization between the Balearic shearwater and the Mediterranean shearwater (Puffinus yelkouan) and its effect on the genomic diversity of the Balearic shearwater—a poorly studied issue that has limited conservation strategies.

These two genetically close lineages have undergone recurrent episodes of divergence and hybridization during the Pleistocene glacial cycles, creating a geographical gradient of hybridization and genetic differentiation from the Eastern Mediterranean to the Pityusic Islands (Ibiza and Formentera). The study also finds that the Menorca population is genetically closer to the yelkouan lineage than to the mauretanicus.

Although hybridization was once seen as a problem in conservation management due to the potential impact of non-adaptive genetic variants, this view is increasingly being challenged. The new study offers a benchmark example of the evolutionary importance of hybridization in protecting a critically endangered flagship species.

"Hybrids can introduce genetic diversity and adaptive potential, thereby benefiting small and threatened populations. Using simulations to predict the population dynamics of the Balearic shearwater, we show the value of preserving hybrid populations to avoid problems stemming from inbreeding (increased homozygosity, reduced genetic diversity, etc.).

Hybridization may counteract these effects through natural genetic rescue, helping prevent a population from reaching extinction thresholds," explain researchers Izquierdo-Arànega and Ferrer Obiol. "The evolutionary history of these shearwaters also exemplifies how hybridization can be critical for maintaining genetic diversity in threatened taxa."

Avoiding bycatch in fishing equipment

Actions focused solely on preserving genetic diversity are not enough to ensure the survival of the Balearic shearwater, warn experts. Beyond conservation measures, current demographic projections suggest the species could become extinct within the next hundred years.

As Professor González-Solís, head of the UB-IRBio Seabird Ecology Group, explains, "Longline fishing is a highly selective technique, but it occasionally catches individuals of all three endemic shearwater species in the Mediterranean, amounting to thousands of birds each year. This phenomenon has been known for decades, and there are effective measures that can completely eliminate this accidental bycatch—all that's missing is the political will to implement them."

Earth BioGenome Project

In this context, it's important to highlight that the genome of the Balearic shearwater—the first obtained under the Catalan Initiative for the Earth BioGenome Project (CBP)—was sequenced and annotated in the Rozas and Riutort laboratories. This initiative, which began within the Catalan Society of Biology, a branch of the Institute for Catalan Studies (IEC), is supported by the IEC through the Biogenome–IEC project. It is co-chaired by Marta Riutort (UB-IRBio) and forms a network of experts with the mission to sequence reference genomes of eukaryotic species found in Catalan-speaking regions.

As Riutort explains, "This study shows that reference genomes have become a crucial tool in biodiversity conservation efforts. Having a well-annotated genome of the Balearic shearwater was key for conducting analyses on hybridization with its sister taxon and identifying the affected genomic regions, as well as for conducting population-level genomic studies that reveal the past history of the two lineages and allow us to model their future under different protection scenarios.

"At the CBP, this encourages us to continue our efforts to increase the number of reference genomes and, most importantly, to promote this kind of research, which should contribute decisively to improving the state of our environment."