Independent evolution of intermediate bill widths in a seabird clade

Abstract

Interspecific introgression can occur between species that evolve rapidly within an adaptive radiation. Pachyptila petrels differ in bill size and are characterised by incomplete reproductive isolation, leading to interspecific gene flow. Salvin’s prion (Pachyptila salvini), whose bill width is intermediate between broad-billed (P. vittata) and Antarctic (P. desolata) prions, evolved through homoploid hybrid speciation. MacGillivray’s prion (P. macgillivrayi), known from a single population on St Paul (Indian Ocean), has a bill width intermediate between salvini and vittata and could also be the product of interspecies introgression or hybrid speciation. Recently, another prion population phenotypically similar to macgillivrayi was discovered on Gough (Atlantic Ocean), where it breeds 3 months later than vittata. The similarity in bill width between the medium-billed birds on Gough and macgillivrayi suggest that they could be closely related. In this study, we used genetic and morphological data to infer the phylogenetic position and evolutionary history of P. macgillivrayi and the Gough medium-billed prion relative other Pachyptila taxa, to determine whether species with medium bill widths evolved through common ancestry or convergence. We found that Gough medium-billed prions belong to the same evolutionary lineage as macgillivrayi, representing a new population of MacGillivray’s prion that originated through a colonisation event from St Paul. We show that macgillivrayi’s medium bill width evolved through divergence (genetic drift) and independently from that of salvini, which evolved through hybridisation (gene flow). This represents the independent convergence towards a similarly medium-billed phenotype. The newly discovered MacGillivray’s prion population on Gough is of utmost conservation relevance, as the relict macgillivrayi population in the Indian Ocean is very small.

At-sea behavioural ecology of the endangered MacGillivray’s prion from Saint Paul Island: combining tracking and stable isotopes

“…Its specific status has been debated (Penhallurick & Wink 2004, Rheindt & Austin 2005, as it has been considered as a subspecies of broadbilled prion (Harper 1980) or Salvin's prion (Roux & Martinez 1987), while its bill measurements are closer to those of Salvin's prion (Bretagnolle et al 1990). Recent works suggests that the MacGillivray's prion is best treated as a separate species (Masello et al 2022). The MacGillivray's prion was previously considered as an endemic species to Amsterdam and Saint Paul Island until its recent discovery at Gough Island in the Atlantic Ocean where approximately 600 000 -1 000 000 pairs bred in 2000/01 (Ryan et al 2014, Dilley et al 2015, Jones et al 2021.…”

“…Although the behavioural ecology of four prion species (the slender-billed P. belcheri, Antarctic P. desolata, MacGillivray's P. macgillivrayi, and broad-billed P. vittata) has been studied recently (Cherel et al 2002, Quillfeldt et al 2015a, Jones et al 2020, there are many knowledge gaps remaining. This limits our understanding of mechanisms allowing the coexistence of these abundant seabird species issued from a recent adaptive radiation (Masello et al 2022).…”

Genomic analyses of fairy and fulmar prions (Procellariidae: Pachyptila spp.) reveals parallel evolution of bill morphology, and multiple species

“…These species are closely-related, with molecular dating estimating that they diverged within the last 6 million years [12]. Genetic studies have clarified some of the relationships between prion species [7,[12][13][14][15]. For example, Salvin's prion appears to be a hybrid between broad-billed prion and Antarctic prion [12].…”

“…Several genetic studies have included fairy prion samples, but these had very limited geographical sampling [7,[13][14][15]23], with at most three geographic regions sampled (Australia, mainland New Zealand and the Antipodes Islands [15]). Only one genetic study [15] has included fulmar prions with the single fulmar prion specimen they included nested within their six fairy prion specimens in their mitochondrial DNA (mtDNA) cytochrome c oxidase subunit I (COI) phylogeny.…”