New findings on hybridization and backcrossing among birds-of-paradise

When feathers look greener elsewhere

Thanks to their colorful plumage and rhythmic jumps, birds-of-paradise are the stars of nature documentaries and social media feeds. They have also been part of natural history collections for centuries. However, for a long time, male specimens whose breast feathers match one species and whose tail feathers match another species, for instance, have posed a mystery to scientists. An international team led by the Swedish Museum of Natural History and the Museum für Naturkunde Berlin has now compared the genomes of almost all species of birds-of-paradise using historical and modern specimens to investigate the evolutionary consequences of these hybrids in more detail.

To attract females, males in many species have developed extraordinary behaviours and looks. The females in such species can be very picky ('female preference') and this form of sexual selection can drive the evolution of extravagant male traits and even lead to the formation of new species. New species are formed when individuals of two populations do not successfully produce offspring any longer and this is exactly what happens when female preference starts to differ between populations. Sexual selection is therefore generally considered as an important evolutionary process that prevents females from one species to mate with males from another species or population ('hybridization'). However, on rare occasions, individuals are observed in nature that have a weird combination of characters from different species and this has puzzled biologists for a very long time: Especially if these individuals have features that are only found in species where females radically differ in preference.

An international collaboration of researchers, led by Dr. Mozes Blom (Museum für Naturkunde Berlin) and Dr. Martin Irestedt (Swedish Museum of Natural History), wanted to investigate this natural phenomenon further and focused on a group of iconic birds from Australasia. Birds-of-Paradise (family: Paradisaeidae) are widely known for their extravagant feathers and behaviours. Males try to impress females at dancing stages ('leks') and females subsequently choose the sexiest male. Their elegant outfits and rhythmic hops have made it into many nature documentaries and have even gone viral on social media. Yet, Natural History Museums have been collecting birds-of-Paradise for centuries and male birds are occasionally reported where, for example, breast feathers can match one species and tail fathers match another ('hybrids'). The international team of scientists used a combination of both museum and modern specimens, and compared the genomes of nearly all species within the Bird-of-Paradise family to further investigate the evolutionary consequences of hybridization between species. 

They have now published their findings in two international scientific journals and their research yielded some surprising new insights. First, published in the journal Evolution Letters, they showed that these odd-looking males are indeed the outcome of hybridization between wildly different looking species. The study, led by their PhD student (Filip Thörn), demonstrated that there are not only hybrids between different species but that some of the hybrids surprisingly are also the outcome of multiple rounds of backcrossing between species. This suggests that hybrid individuals are not necessarily infertile, as is the case for example when crossing a donkey and a horse ('mule').

In fact, with their second study, published in the journal iScience, the researchers demonstrated that hybridization between different species has been a recurring process throughout the evolutionary history of the family. They could conclude this because they repeatedly found genetic material from one species in the genome of other species which suggests that hybrid offspring were able to reproduce, and genetic material was transferred between species as a consequence.

"Our two studies shed new light on the evolutionary consequences of hybridization in organismal groups with strong forms of sexual selection", says Dr. Mozes Blom. "Our findings not only confirm that it is indeed still possible to produce fertile offspring between species, where the males differ so radically in traits that are a key indicator for female preference, it has actually also resulted in the sharing of genes between species. There is a growing body of work which indicates that hybridization may lead to phenotypic diversification and our findings surprisingly suggest that this may still hold true for species with extreme forms of sexual selection. However, many questions remain, in particular regarding the functional significance of genes that have been shared between species and why females occasionally make 'mistakes' in the first place and mate with a male that is very obviously not of her own species". 

Publications
Filip Thörn, André E R Soares, Ingo A Müller, Martin Päckert, Sylke Frahnert, Hein van Grouw, Pepijn Kamminga, Valentina Peona, Alexander Suh, Mozes P K Blom, Martin Irestedt, Contemporary intergeneric hybridization and backcrossing among birds-of-paradise, Evolution Letters, 2024.
DOI: https://doi.org/10.1093/evlett/qrae023

Mozes P K Blom, Valentina Peona, Stefan Prost, Knud A Jønsson, Alexander Suh, Martin Irestedt, Hybridization in birds-of-Paradise: Widespread ancestral gene flow despite strong sexual selection in a lek-mating system, iScience, 2024. 
DOI: https://doi.org/10.1016/j.isci.2024.110300