The peculiar genomes of Falcons

Falcons are the odd ball out when it comes to avian genomics.

Falcons are amazing birds. Take the Peregrine Falcon (Falco peregrinus), for instance. The large raptor can reach speeds over 320 km/h during a hunting dive, making it the fastest bird on this planet. From a hybrid point of view, falcons are also fascinating. Several hybrids have been reported in the wild, but most crosses are known from captivity. Falconers have combined certain species, aiming to create the perfect hunting bird. In the United Arab Emirates, for example, mixes of 7/8 Gyrfalcon (Falco rusticolus) and 1/8 Saker Falcon (Falco cherrug) are quite common. For an overview of hybridization in this bird group, you can check the Falconiformes page.

Clearly, there are plenty of good reasons to study falcons. But a recent review in the journal Ecology and Evolution adds another reason to this long list: genomics. It turns out that falcons are the odd ball out when it comes to bird genomes.

Peregrine_Falcon

A Peregrine Falcon © Mosharaf Hossain | Wikimedia Commons

 

Some Taxonomy

Before we dive into the genomics of falcons, let’s see where they belong on the avian Tree of Life. Surprisingly, falcons are not closely related to other birds of prey. Instead, the closest relatives of falcons are parrots and songbirds, from which they diverged about 60 million years ago. More on this topic in this blog post.

Within the falcon family, you can find about 40 species. These can be divided into three main groups based partly on differences in hunting strategies. We already met the Peregrine Falcon which catches its prey in swooping dives. Other members of this first group of large and mid-sized falcons include Gyrfalcon and Sakerfalcon. The second group holds birds of the subgenus Hypothriorchis, better known as hobbies. These smaller falcons, such as the Sooty Falcon (Falco concolor), catch small birds and insects in flight. Finally, there are the kestrels, which often hover over grasslands, scanning the ground for unsuspecting prey.

Apart from these three main groups, there are several smaller groups, namely “1–2 species of merlins, two atypical African kestrels (Dissodectes), two Southern American falcons, the single New World Kestrel and two unplaced clades (“Red‐Footed Group” and “Southern Group”) each of which contains two species.” A wide range of falcons.

Common_kestrel_in_flight

A Common Kestrel in flight © Вых Пыхманн | Wikimedia Commons

 

Microdeletions

Now for some peculiar genomic falcon facts. So far, the genomes of five falcon species have been sequenced: Peregrine Falcon, Saker Falcon, Gyrfalcon, Common Kestrel (Falco tinnunculus) and Prairi Falcon (Falco mexicanus). Although these genomes have the typical length of an avian genome – about 1,200,000,000 nucleotides – they contain more and longer protein-coding genes compared to your average bird genome. These longer sequences cannot be explained by more repetitive DNA (such as transposable elements). Instead, some researchers think that this pattern is due to a lack of microdeletions, small DNA sequences (less than 30 nucleotides long) that have been removed from the genome. Indeed, the Peregrine Falcon had the lowest rate of microdeletions compared to seven other birds (see here).

Eurasian_Hobby

Eurasian Hobby © Imran Shah | Wikimedia Commons

 

NUMTs

Mitochondrial genes can be found on – you guessed it – mitochondrial DNA, the small circular genomes in these power-producing organelles. However, several mitochondrial genes have been transfered to the nuclear genomes, giving rise to so-called NUMTs (nuclear mitochondrial DNA segment). These NUMTs take up a big part of falcon genomes. In fact, more than 90 percent of the Peregrine and Saker Falcon mitochondrial genome can be found in the nuclear genome, including one big insertion that represents 70 percent of the mtDNA (see here). These insertions might hold the ancestral DNA sequence of the falcon mtDNA and could be very useful in evolutionary studies.

African_Pygmy_Falcon_Female

African Pygmy Falcon © Sumeet Moghe |Wikimedia Commons

 

Fusions

Finally, the falcon karyotype, the number of chromosomes you can count under a light microscope. In birds, chromosomes can be divided into a few huge macrochromosomes and several tiny microchromosomes. Most birds have 40 pairs of chromosomes (so 80 in total, mostly denoted as 2N = 80). Falcons, however, show karyotypes of only 2N = 40 to 2N = 52. These atypical chromosome counts are probably the outcome of fusions between microchromosomes into macrochromosomes.

These peculiar features – long genes, many NUMTs and atypic karyotypes – raise numerous questions. Hence, plenty of reasons to study falcons…

Karyotype-of-chicken

The karyotype of a Chicken. From: Li et al. (2015) BioMed Research International

 

References

Kapusta, A., Suh, A., & Feschotte, C. (2017). Dynamics of genome size evolution in birds and mammals. Proceedings of the National Academy of Sciences114(8), E1460-E1469.

Nacer, D. F., & do Amaral, F. R. (2017). Striking pseudogenization in avian phylogenetics: numts are large and common in falcons. Molecular Phylogenetics and Evolution115, 1-6.

Wilcox, J. J., Boissinot, S., & Idaghdour, Y. (2019). Falcon genomics in the context of conservation, speciation, and human culture. Ecology and Evolution, 9(24): 14523-14537.

One thought on “The peculiar genomes of Falcons

  1. Only just seen this – interesting article, but because Falco subbuteo is the type species of the genus, the hobbies are subgenus Falco, not Hypothriorchis as given above.

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