Clock gene variation in Tachycineta swallows

Abstract

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.Figure 1Photos 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.

Clock Gene Variation Is Associated with Breeding Phenology and Maybe under Directional Selection in the Migratory Barn Swallow

“…A covariation was detected in resident blue tits [32] but not e.g. in an analysis of migratory Tachycineta bicolor swallows where the effects of age on breeding date could be controlled for [35]. All European barn swallow breeding populations are migratory [41], [42].…”

“…A recent study of five distant barn swallow ( Hirundo rustica ) populations disclosed very low polymorphism at the Clock poly Q region [34]. Finally, a comparison among five Tachycineta swallow species found considerable variation in polymorphism at Clock poly Q but no evidence of latitudinal variation across species [35]. In addition, no evidence was found for an association between laying date and Clock genotype at the within-species level [35].…”

“…Based on the evidence for blue tits [32] (but see [35]), we expected individuals with longer Clock alleles to breed relatively late. However, we expected this effect to be manifest only among females, as breeding dates are believed to be constrained by female physiological disposition to egg laying [32], [40].…”

Low Variation in the Polymorphic Clock Gene Poly-Q Region Despite Population Genetic Structure across Barn Swallow (Hirundo rustica) Populations

“…We identified three Clock poly-Q length variants in Barn Swallows, however the variation was extremely low with one allele (Q 7 – corresponding to 7 poly-Q repeats) accounting for 98% of the total alleles (and two other very rare alleles – Q 6 and Q 8 ), and a mean observed heterozygosity of only 0.030 (range: H  = 0.000–0.082). This variation is much lower than similar values reported for the Blue Tit (9 alleles; Q 9 -Q 17 ; H  = 0.489), Bluethroat (9 alleles; Q 10 -Q 16 ; H  = 0.213) [9] or even a low-variability Great Tit population (5 alleles; Q 11 -Q 15 ; H  = 0.077) [16] and five species of Tachycineta swallows (2–4 alleles; Q 5 -Q 9 ; H  = 0.047–0.472) [14]. This low variation in Clock poly-Q in Barn Swallows was also reflected in the genetic differentiation analysis that showed no differentiation among populations based on this locus, while similar analyses based on mtDNA haplotypes or microsatellite loci showed significant and correlated differentiation between these populations (Fig.…”

“…Studies on various taxa have reported an association between Clock genotype and breeding latitude [9], [12], [13]. In birds, latitudinal variation in Clock genotypes have been detected in the Blue Tit ( Cyanistes caeruleus ) but not for the Bluethroat ( Luscinia svecica ) [9] or Tachycineta swallows [14], therefore the generality of this pattern in birds and its possible relationship with adaptive phenotypic differentiation remains unclear. Additional within-population analyses showed an association between Clock genotype and breeding phenology (clutch initiation date and incubation duration) in a Blue Tit population [15] but not in a sympatric Great Tit ( Parus major ) population, in which Clock poly-Q variation was low [16].…”