Adaptation to a pelagic lifestyle in Procellariiformes
Our selection inference uncovered 20 genes evolving under positive selection in Procellariiformes, being therefore candidates to be actively involved in the adaptation of the order to a pelagic lifestyle. Indeed, the GO’s Enrichment analysis of these genes reveals biological processes related to striated muscle cell differentiation, response to starvation, and nutrient reservoir activity, that may be related to the high energy expenditure during the vast distances they cover in the open ocean, while visual related genes could be related with underwater vision to fish and night vision (Hayes, Martin, & Brooke, 1991; Martin & De, 1991; Mitkus, Nevitt, Danielsen, & Kelber, 2016). Positive selection of genes related to natriuresis also makes sense for Procellariiformes since this biological process plays a key role to maintain the osmotic equilibrium in a sodium-rich environment like the ocean (Gutiérrez, 2014; “Water and Salt Balance in Seabirds,” 2001), which Procellariiformes perform thanks to the development of salt glands (modified nasal glands engaged in secretion of salts). Olfactory receptors, also found here among enriched GOs of positively selected genes, showed signature of adaptive evolution in shearwaters (C. Silva et al., 2020), and are crucial to Procellariiformes for navigation (Gagliardo et al., 2013; Padget, Dell’Ariccia, Gagliardo, González-Solís, & Guilford, 2017; Pollonara et al., 2015), partner recognition and mating (Bonadonna & Nevitt, 2004; Hoover et al., 2018; Strandh et al., 2012), finding their own burrows (Bonadonna & Bretagnolle, 2002) or foraging (Bastos et al., 2020; Nevitt, 2008; Nevitt, Veit, & Kareiva, 1995; Yung, Sin, Cloutier, Nevitt, & Edwards, 2019).
We also inferred genes with intensified natural selection in Procellariiformes, for which the GOs annotations (Table S8) are similar and coherent to those in the candidate set of genes with positive selection in all tubenoses, or, in other words, related to the adaptation of the order to a pelagic lifestyle. For example, molecular functions such as sensory perception of sound, smell and chemical stimulus, neurological system process, camera-type eye development are related with oceanic navigation. On the other hand, functions such as homeostatic process, renal system, renal response, chloride transport and regulation of ion transport point to the need of maintaining osmotic equilibrium. We also found intensified natural selection in genes participating in functions related to immune response (like inflammatory response, defense response, response to wounding, wound healing, positive regulation of phagocytosis, etc.), accompanied by a relaxation of natural selection in regulators of blood constituents, induction of bacterial agglutination, regulation of antigen processing and presentation, viral budding via host ESCRT complex or macrophage antigen processing and presentation. As Procellariiformes exposure to parasites is high (Khan et al., 2019) and their life-history traits favour parasite maintenance within populations (McCoy et al., 2016), the tuning between the intensification and relaxation of natural selection in multiple biological processes and molecular functions related with immune response would have emerged following an arms race-like model. For example, as many parasites of tubenoses are blood-feeding, the intensified natural selection on the thrombin-activated receptor signaling pathway (GO:0070493), may be an evolutionary response to counter the anticoagulant activity that most blood-feeding parasites present (Bensaoud et al., 2018).
Among the gene families expanded in the branch of Procellariiformes, the one encoding olfactory receptors is remarkable as it is coherent not only with the finding of a gene with positive selection in all Procellariiformes with the same functional annotation (g16276.t1 in theP. mauretanicus reference annotation, Table 4) but also with 3 olfactory receptors genes with intensification selection in the same branch (g14377.t1, g16276.t1 and g17936.t1 in P. mauretanicusreference annotation, Table S8). This triple evidence highlights the importance of how the adaptation to a pelagic life resulted in the enhancement of the olfactory function in Procellariiformes, as we discussed already in the paragraphs above. Moreover, Silva et al. 2020 obtained similar results of positive selection in olfactory genes inC. borealis . Physiologically, tubenoses have one of the largest olfactory bulb to brain size (OB) ratio of all birds (Cobb, 1968)