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Endogenous migration programmes & genetics of migration

Willow warblers (Phylloscopus trochilus) in Sweden belong to two different subspecies; one breeding in the south (Ph. t. trochilus) migrating to wintering areas in West Africa, and the other breeding in the north (Ph. t. acredula) migrating to wintering areas in Southeast Africa. These differences in migratory behavior are thought to be controlled by endogenous programs that encode directional and timing information. Even though these programs have been shown to have a strong genetic component, virtually nothing is known the specific genes underlying them.

In this project, we investigate the molecular genetics behind the migration programs using willow warblers and a hybrid zone between the two subspecies as a model system for studies on how selection in the wild may evolve different migration routes.

We use high-throughput genomic assays to identify genetic differences between the subspecies that potentially could be shaping their different migratory programs. The shuffling of the subspecies’ genomes that occurs in hybrids allows these genetic differences to be more independently associated with particular phenotypes. Stable isotopes levels in feathers moulted on the wintering ground offer a proxy for the migratory strategy used by each individual and could be used to associate particular genes or chromosome regions with differences in migratory strategies across a population.

Another focus of the project is the temporal and spatial dynamics of the hybrid zone. Differences in arrival time could generate assortative mating between the subspecies and limit gene flow between them. Alternatively, reduced gene flow could be due to reduced fitness of hybrids. For example, ecological selection against hybrids may act on the expression of intermediate migratory programs. The intermediate programs are likely to bring hybrids to geographic barriers that they cannot successfully cross.

What is the relative importance of these two processes in maintaining the hybrid zone? Does it vary between years and in different locations in the zone? In addition, is gene flow between the subspecies symmetrical and does it also vary in time and space?

This project is financially supported by project grants from the Swedish Research council.

Well suited for studies on migratory genetics

Map

The willow warbler is exceptionally well suited for studies of migratory genetics for several reasons. First, the migratory divide is sharp - about 100 km wide running across Scandinavia at 62° N. This map of Europe and Africa shows the breeding range and migration routes of willow warblers of the two subspecies Ph. t. trochilus (south of Sweden) and Ph. t. acredula (north of Sweden).

Diverged as a result of selection

Willow warbler in hand
Photo: Keith Larson

Second, the migratory direction of adults can be inferred from their stable isotope signatures in their feathers replaced in Africa and carried to the breeding ground. And third, most of the genetic variation in the willow warbler is shared across the migratory divide suggesting that the subspecies have recently diverged as a result of selection, leaving time too short for differentiation in neutral markers. Hence, most (if not all) of the detected genetic differences will be linked to genes under selection.

Stable Isotope analysis

Willow warbler feather
Photo: Keith Larson

Stable isotopes are different types of energetically stable atoms of the same chemical element, for example carbon and nitrogen, but with different number of neutrons. Since ratios of naturally occurring stable isotopes often vary in systematic ways across large geographical areas it is possible to tell where an animal has been through analyses of the occurence of stable isotopes in for example body tissue or feathers. As for the willow warblers, their feathers are moulted in the winter quarters and the observed isotopic signatures reflect the food eaten in different parts of Africa during their growth.

The hybrid zone is fixed in central Sweden

WW habitat
Photo: Keith Larson

Recently we studied how the differential migration in the two subspecies of willow warblers breeding around the Baltic Sea evolved after the last glaciations. We are also using the breeding bird counts of Sweden to study hybrid zone dynamics in Scandinavia. We have found interesting variations in the breeding bird density across and outside the hybrid zone, and indications that the hybrid zone is currently fixed to a habitat and climate gradient present in central Sweden. The picture shows a typical habitat for willow warblers in northern Sweden.

Further studies needed

Investigation of the stability of the hybrid zone over time, and what effect the zone has on mate choice, migration phenology, individual fitness and hybridization is needed. The orientation and physiological adaptations of the endogenous program of migrating willow warblers and other songbirds will be studied and we will use stable isotope techniques to establish connectivity between the breeding and wintering grounds for each subspecies. Furthermore, we conduct orientation experiments to determine if there are different directional preferences between the two subspecies.
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Centre for Animal Movement Research
Evolutionary Ecology, Department of Biology
Ecology building S-223 62 Lund Sweden