A recent study, published in Molecular Ecology, shows inter-oceanic gene flow among five petrel species on a global scale.
Petrels, the narrow-winged seabirds that graciously soar across the world’s oceans, hold a peculiar contradiction. On the one hand, they disperse on a globally exploring distant waters, while, on the other hand, they display a strong instinct to return to their birth place during the breeding season (i.e. natal philopatry). In this respect, they resemble certain scientists who disperse widely, doing postdocs across the globe, only to return to the universities where they obtained their PhDs for a tenure position.
Five Species – Three Oceans
How do these contrasting behaviors in petrels translate to their genetics? To explore this question, Katherine Booth Jones and her colleagues sampled no less than 1001 petrels (genus Pterodroma) on a worldwide scale. The sampled birds represented five distinct species, distributed across three oceans (Indian, Atlantic and Pacific):
- Trindade Petrel (P. arminjoniana)
- Herald Petrel (P. heraldica)
- Kerdamec Petrel (P. neglecta)
- Murphy’s Petrel (P. ultima)
- Phoenix Petrel (P. alba)
Inter-oceanic Gene Flow
Hybridization among petrels is best studied on the Round Island in the Indian Ocean. Here, three species – Trindade, Kerdamec and Herald Petrel – hybridize extensively (read more about this case on the Procellariiformes page). However, the genetic analysis – based on microsatellites – in the present study shows that hybridization is not limited to this island. In fact, gene flow even occurrs between populations from different oceans, in other words: inter-oceanic gene flow.
This results suggests that petrels migrate between oceans. To support this suggestion, the researchers also present tracking data from two individuals that both left the Indian Ocean. One individual traveled eastwards to the Pacific Ocean, whereas another bird explored the Atlantic Ocean in the west.
The genetic data also revealed possible hybrids among three different petrel species. In birds, most three-way hybrids are known from captivity (notably falcons), but there is a well-documented case in geese. Dreyen and Gustavsson (2010) report how a hybrid between Swan Goose (Anser cygnoides) and Snow Goose (A. caerulescens) paired up with a Barnacle Goose (Branta leucopsis) and produced offspring. A three-way intergeneric hybrid.
An important lesson to take away from this petrel study is to think beyond hybrid zones. Most studies focused on narrow contact zones between two species, which has led to important insights into speciation and hybridization. However, I think it is time to expand our view and sample widely to uncover unexpected patterns of gene flow on a global scale.
Booth Jones K.A., et al. (2017). Widespread gene flow between oceans in a pelagic seabird species complex. Molecular Ecology. 26:5716-5728. https://doi.org/10.1111/mec.14330
Dreyen P. & Gustavsson C. G. (2010). Photographic documentation of a Swan Goose x Snow Goose Anser cygnoides x Anser caerulescens hybrid and its offspring with a Barnacle Goose (Branta leucopsis) – a unique three-species cross. Ornithologischer Anzeiger. 49:41-52.
This paper has been added to the Procellariiformes page.