Several genetic markers are useful to identify hybrids and backcrosses.
A few months ago, I published a scoring scheme to assess the reliability of hybrid reports (see this blog post). In short, this scheme is based on three criteria: (1) the observation of a putative hybrid with photographic evidence or a detailed description, (2) thorough morphological analyses in which the putative hybrid is compared with potential parental species, and (3) genetic analyses of the putative hybrid with reference material from potential parental species. To express the varying levels of confidence that each of these criteria provide, I weighted them differently in the final score for a putative hybrid, namely one point for an observation, two for a morphological analysis, and three for a genetic test. The final tally of these three criteria (ranging from 0 to 6 points) will indicate the level of confidence for a particular hybrid combination. I applied this scheme to the tinamous (family Tinamidae), resulting in one well-documented case and three doubtful records that require further investigation.
The goal is to apply this approach to other bird families in order to provide a better overview of the incidence and reliability of bird hybrids. A recent study in the journal Genetica summarized the literature on penguin hybrids and indicated that most hybridization studies were “based solely on morphological or nesting observations, with no genetic confirmation of hybridization.” In the context of the scoring scheme, most cases of penguin hybrids would thus receive a reliability score between 0 and 3 points. Clearly, more genetic studies are needed to determine the incidence of penguin hybrids. And the study in Genetica delivers a nice example for hybrids between Magellanic Penguins (Spheniscus magellanicus) and Humboldt Penguins (Spheniscus humboldti).
Eric Hibbets, Katelyn Schumacher and their colleagues focused on six individuals that were sampled at three colonies from the Atlantic Ocean basin (Caleta Valdés, Punta Tombo, and Cabo Dos Bahías). These birds were initially noted down as Magellanic Penguins, but were later identified as putative hybrids based on the presence of mitochondrial variants (of the COI gene) that are characteristic for Humboldt Penguins. The researchers tested this conclusion with three additional genetic markers: a set of six microsatellites, the immune gene DRβ1 and the sex-linked gene CDH1. They sequenced these markers for several reference samples from both species. The analyses revealed that “three of the four markers (COI, microsatellites, and DRß1) were informative because they provided both Magellanic and Humboldt species-specific alleles or haplotypes that could be used to trace species ancestry in hybrid individuals.”
What about the six putative hybrids? It turned out that four individuals were backcrosses with some degree of genetic introgression from Humboldt Penguins. The remaining two individuals were actually Humboldt penguins instead of hybrids. These results highlight the value of genetic analyses in hybrid detection. Morphological characters or field observations are not always reliable.
Detailed analyses revealed more admixed individuals among the reference samples. Five out of 37 penguins showed some genetic signs of past hybridization events. Interestingly, all five samples come from the Puñihuil colony, which holds a significant number of intermixed nesting sites of Magellanic and Humboldt penguins. More expeditions will probably uncover more penguin hybrids. And not just between Magellanic and Humboldt Penguins. A recent genomic study reported gene flow between several other species (see this blog post for the details). Who know what penguin hybrids will be discovered with genetic data.
Hibbets, E. M., Schumacher, K. I., Scheppler, H. B., Boersma, P. D., & Bouzat, J. L. (2020). Genetic evidence of hybridization between Magellanic (Sphensicus magellanicus) and Humboldt (Spheniscus humboldti) penguins in the wild. Genetica, 148(5), 215-228.
Featured image: Magellanic penguin (Spheniscus magellanicus) © David | Wikimedia Commons
This paper has been added to the Sphenisciformes page.