Clock gene variation in Tachycineta swallows

Many animals use photoperiod cues to synchronize reproduction with environmental conditions and thereby improve their reproductive success. The circadian clock, which creates endogenous behavioral and physiological rhythms typically entrained to photoperiod, is well characterized at the molecular level. Recent work provided evidence for an association between Clock poly‐Q length polymorphism and latitude and, within a population, an association with the date of laying and the length of the incubation period. Despite relatively high overall breeding synchrony, the timing of clutch initiation has a large impact on the fitness of swallows in the genus Tachycineta. We compared length polymorphism in the Clock poly‐Q region among five populations from five different Tachycineta species that breed across a hemisphere‐wide latitudinal gradient (Fig. 1). Clock poly‐Q variation was not associated with latitude; however, there was an association between Clock poly‐Q allele diversity and the degree of clutch size decline within breeding seasons. We did not find evidence for an association between Clock poly‐Q variation and date of clutch initiation in for any of the five Tachycineta species, nor did we found a relationship between incubation duration and Clock genotype. Thus, there is no general association between latitude, breeding phenology, and Clock polymorphism in this clade of closely related birds. 1 Photos of Tachycineta swallows that were used in this study: A) T. bicolor from Ithaca, New York, B) T. leucorrhoa from Chascomús, Argentina, C) T. albilinea from Hill Bank, Belize, D) T. meyeni from Puerto Varas, Chile, and E) T. thalassina from Mono Lake, California, Photographers: B: Valentina Ferretti; A, C‐E: David Winkler. We investigated the circadian Clock gene, Clock, in Tachycineta swallow and found relatively low variation in this gene across Tachycineta species. This variation was not associated within population with clutch initiation date or incubation duration. However, there was a relationship between Clock allele diversity and the degree of clutch size decline within breeding seasons among populations.