Low variability and absence of phenotypic correlates of Clock gene variation in a great tit Parus major population

Abstract

Studies of a range of taxa, including birds, have revealed latitudinal clines in allele length at the conserved Clock locus, a gene with known influences on behaviour and physiology. Such clines might reflect adaptation to seasonal variation, a suggestion supported by a recent within-population analysis of blue tits Cyanistes caeruleus, which found associations between Clock genotype and timing of breeding in females. To test the generality of this pattern, we sequenced the polymorphic poly-Q locus of the Clock gene in 521 female great tits Parus major, which were selected based on possession of extreme breeding phenotypes. In total, we identified five alleles with one allele accounting for 96% of allelic diversity in the sample set. Overall variability at the poly-Q locus was very low, and the spatial distribution of Clock alleles across Wytham was highly homogenous. Our data further provide no evidence for a connection between Clock genotype and reproductive timing phenotype in female great tits; further, we found no effect of Clock genotype on reproductive success. Hence, these results are in contrast to the pattern found for the sympatric blue tit population inhabiting the same woodlands, suggesting that phenotypic effects of Clock are not general in passerine birds

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.…”

“…Genetic variation in the non-migratory Blue Tit was also higher compared to the migratory Bluethroat [9]. Yet, a non-migratory Great Tit population showed lower variation than both [16]. Therefore, we cannot assume any general relationship between migratory patterns and genetic variation in the Clock poly-Q locus based on the available data.…”

“…This polymorphism is associated with both breeding latitude and breeding phenology in Blue Tits, suggesting that this locus might be involved in local adaptation to seasonal environments, possibly through the response to photoperiod [9], [15]. However, this association has not been detected in several other bird species [9], [16], leaving the question regarding the generality of this relationship unresolved.…”

“…Longer tandem repeats are more likely to be more polymorphic, therefore the lower variation observed for Barn Swallows may be related to their relatively shorter Clock poly-Q repeats. However, despite long Clock poly-Q repeats in a Great Tit population (Q 11–15 ), polymorphism was relatively low [16]. Liedvogel and Sheldon [16] suggested that this might be due to the fact that CAG-repeat variation in this locus is tightly regulated and that selection in Great Tit acts in favor of the Clock poly-Q 14 allele.…”

“…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]. Thus, further comparisons are required to assess patterns of association between Clock variation and life-history traits.…”