Clock gene polymorphism and scheduling of migration: a geolocator study of the barn swallow Hirundo rustica

Abstract

Circannual rhythms often rely on endogenous seasonal photoperiodic timers involving 'clock' genes, and Clock gene polymorphism has been associated to variation in phenology in some bird species. In the long-distance migratory barn swallow Hirundo rustica, individuals bearing the rare Clock allele with the largest number of C-terminal polyglutamine repeats found in this species (Q8) show a delayed reproduction and moult later. We explored the association between Clock polymorphism and migration scheduling, as gauged by light-level geolocators, in two barn swallow populations (Switzerland; Po Plain, Italy). Genetic polymorphism was low: 91% of the 64 individuals tracked year-round were Q7/Q7 homozygotes. We compared the phenology of the rare genotypes with the phenotypic distribution of Q7/Q7 homozygotes within each population. In Switzerland, compared to Q7/Q7, two Q6/Q7 males departed earlier from the wintering grounds and arrived earlier to their colony in spring, while a single Q7/Q8 female was delayed for both phenophases. On the other hand, in the Po Plain, three Q6/Q7 individuals had a similar phenology compared to Q7/Q7. The Swiss data are suggestive for a role of genetic polymorphism at a candidate phenological gene in shaping migration traits, and support the idea that Clock polymorphism underlies phenological variation in birds.

Figure 1. Frequency distribution of phenotypic values of the Q₇/Q₇ individuals and phenology of the Q₆/Q₇ and Q₇/Q₈ individuals in the Swiss population.

(a) Departure from the breeding colony; (b) arrival to the wintering area; (c) departure from the wintering area; (d) arrival to the breeding colony. The horizontal boxes above histograms show the mean and 95% non-parametric bootstrap confidence interval (CI) of the Q₇/Q₇ phenotypic distribution, while the horizontal lines show the standard deviation. Dots show the phenotypic values of the rare genotypes (open dot = Q₆/Q₇; filled dot = Q₇/Q₈). Histograms and dots refer to actual data, while asterisks above dots denote that the Q₆/Q₇ or Q₇/Q₈ values were outside the 95% CI of a given phenological trait for centred values (removing any between-year and between-sex variation in phenology; see Supplementary Table S2 and Methods for details).

Figure 2. Arrival date predicts onset of reproduction among female barn swallows equipped with geolocators (day 1 = January 1^st).

The fitted regression equation is: first egg laying date = 0.88 (0.12 s.e.m.) × arrival date + 30.48 (12.51 s.e.m.) (R² = 0.80). The filled dot denotes the single Q₇/Q₈ female from the Swiss population.