Genome assemblies of several birds-of-paradise reveal the genes that have driven the evolution in this bird group.
WTF, Evolution?! is a website that gathers the crazy outcomes of the evolutionary process. No purposeful design, just evolution tinkering with the available material and producing what is needed to survive and reproduce, no matter how wacky the end result. When we turn to the avian diversity, there are many species with unusual features or behaviors. Think of the knob-billed duck or the “moon-walking” manakins. Another great candidate is the bird family Paradisaeidae, better known as the birds-of-paradise. This diverse assemblage of passerines – 41 species in 16 genera – displays a bewildering array of extravagant plumage and courtship displays. A recent GigaScience paper explored the genomes of a few birds-of-paradise to pinpoint the genetic basis of this diversity.
Stefan Prost and his colleagues studied the genomes of five birds-of-paradise, representing the five major lineages in this family: the paradise-crow (Lycocorax pyrrhopterus), the paradise riflebird (Ptiloris paradiseus), the Huon astrapia (Astrapia rothschildi), the King of Saxony bird-of-paradise (Pteridophora alberti) and the red bird-of-paradise (Paradisaea rubra).
Next, they scanned these genomes for genes under positive selection and rapidly evolving gene families. The result is a list of candidate genes that could explain the diversity of birds-of-paradise. As I have written before, we should be careful with making up just-so-stories based on a handful of candidate genes. But we are of course free to speculate a bit.
Plumage Color and Vision
Because of the diversity in plumage, the researchers expected genes related to coloration and feather structure to be under positive selection. Indeed, they found a few of these genes under positive selection (although they turned out non-significant after correcting for multiple testing). These genes include ADAMTS20 and ATP7B, which are both involved in the production of melanin, an important pigment.
Colors have to be perceived, so you might also expect the find genes with roles in eye function and eye development to be under selection. Three genes (GNB1, ATP6AP2 and MYOC) popped up as significant when testing for positive selection. There might thus be some co-evolution between coloration and vision. For example Lawes’s parotia (Parotia lawesii) modifies the color of its plumage by altering the angle of light reflection. Birds need a visual system that is able to process these color changes. However, vision is used for a wide range of other activities, such as foraging and detecting predators. Finding evidence for co-evolution of coloration and vision is thus tricky.
Smelling and Startling
Let’s look at the gene families that expanded in the birds-of-paradise. A first family that increased in size holds the olfactory receptors, which are important in smelling. The branch leading to the core bird-of-paradise (i.e. excluding the paradise-crow) showed an increase of 5 genes, with another addition of 6 genes in the Huon astrapia. These genes could serve several functions, ranging from species recognition to foraging. Clearly, more research is needed here.
A second gene family that expanded is involved in the startle response, a behavioral trait that allows birds to react quickly to a stimulus. This behavior could be important in the context of leks where several males gather to woo the females. During this mating display, the males are exposed and have to react quickly to incoming predators.
The researchers also explored the amount of transposable elements (TEs) in the bird-of-paradise genomes. TEs are short stretches of DNA that jump around the genome using copy/paste or cut/paste mechanisms. This search revealed the activity a particular class of TEs, namely retroviral LTRs. Interestingly, this activity coincides with the radiation of birds-of-paradise about 24 million years ago. Could it be that the evolution of this bird family was partly driven by transposable elements? Possibly, but let’s not jump to conclusions yet.
So, what is the recipe for a radiation of birds-of-paradise? Just select your favorite genes involved in coloration, vision, smell and startling response. Mix them in a big pot and add a dash of transposable elements. And let it boil for a few million years.
Prost, S., Armstrong, E,.E., Nylander, J., Thomas, G.W.C., Suh, A., Petersen, B., Dalen, L., Benz, B.W., Blom, M.P.K., Palkopoulou, E., Ericson, P.G.P. & Irestedt. M. (2019) Comparative analyses identify genomic features potentially involved in the evolution of birds-of-paradise. GigaScience