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Abstract

Timing is essential for the survival and successful reproduction of animals across all taxa and environments. The circadian clock gene, Clock has been identified as candidate gene for circadian and circannual processes. Clock sequence is extremely conserved across species except for one highly variable exonic region that shows lengths variation in a poly Glutamine (poly-Q) repeat motif. Several studies report increasing repeat lengths associated with higher breeding latitude. A link between Clock gene variability and migratory behaviour has been suggested. Here we capitalise on the trans-equatorial distribution range of the Saxicola species complex in addition to variation in migratory phenotype. First, we (i) characterise Clock gene variability across a wide latitudinal gradient and second (ii) analyse this variation in Clock genotype in the context of seasonal timing (migration, moulting). Lowest levels of Clock gene variability are found in African stonechat populations that experience a constant 12:12 light/dark cycle throughout the year, thus lacking seasonal light-entrainment. In a circannual context of Clock involvement this suggests that the lack of light-entrainment might have selected for a conserved Clock genotype. Populations of migrants showed higher levels of Clock gene variability including longest alleles detected in Siberian long-distance migrants, supporting an association of longer alleles with higher breeding latitudes. Clock gene variability in the context of migratory timing detected a strong signal in autumn migration, but not in spring, with long alleles associated with later timing. This is the first study reporting a correlation between Clock gene variability and migratory timing within a closely related species complex.