Dipartimento di Biologia, Università degli Studi di Milano, Milano, Italy.
PLoS One. 2012;7(4):e35140. doi: 10.1371/journal.pone.0035140. Epub 2012 Apr 10.
In diverse taxa, photoperiodic responses that cause seasonal physiological and behavioural shifts are controlled by genes, including the vertebrate Clock orthologues, that encode for circadian oscillator mechanisms. While the genetic network behind circadian rhythms is well described, relatively few reports exist of the phenological consequences of and selection on Clock genes in the wild. Here, we investigated variation in breeding phenology in relation to Clock genetic diversity in a long-distance migratory bird, the barn swallow (Hirundo rustica).
METHODOLOGY/PRINCIPAL FINDINGS: In a sample of 922 adult barn swallows from a single population breeding in Italy we found one very common (Q(7)) and three rare (Q(5), Q(6), Q(8)) length variants of a functionally significant polyglutamine repeat. Rare (2.9%) Q(7)/Q(8) heterozygous females, but not males, bred significantly later than common (91.5%) Q(7)/Q(7) females, consistent with the expectation that 'long' alleles cause late breeding, as observed in a resident population of another bird species. Because breeding date depends on arrival date from migration, present results suggest that the association between breeding date and Clock might be mediated by migration phenology. In addition, fecundity selection appears to be operating against Q(7)/Q(8) because late migrating/breeding swallows have fewer clutches per season, and late breeding has additional negative selection effects via reduced offspring longevity. Genotype frequencies varied marginally non-significantly with age, as Q(7)/Q(8) frequency showed a 4-fold reduction in old individuals. This result suggests negative viability selection against Q(7)/Q(8), possibly mediated by costs of late breeding.
CONCLUSIONS/SIGNIFICANCE: This is the first study of migratory birds showing an association between breeding phenology and Clock genotype and suggesting that negative selection occurs on a phenologically deviant genotype. Low polymorphism at Clock may constrain microevolutionary phenological response to changing climate, and may thus contribute to the decline of barn swallow populations.
在不同的分类群中，导致季节性生理和行为变化的光周期反应受基因控制，包括脊椎动物生物钟同源基因，这些基因编码昼夜节律振荡器机制。虽然昼夜节律背后的遗传网络已得到充分描述，但关于野生环境中生物钟基因的物候后果及选择的报道相对较少。在此，我们研究了一种长距离候鸟家燕（Hirundo rustica）的繁殖物候与生物钟基因多样性的关系。
方法/主要发现：在来自意大利一个繁殖种群的922只成年家燕样本中，我们发现了一个功能重要的多聚谷氨酰胺重复序列的一种非常常见的（Q(7)）和三种罕见的（Q(5)、Q(6)、Q(8)）长度变体。罕见的（2.9%）Q(7)/Q(8)杂合子雌性而非雄性，繁殖时间显著晚于常见的（91.5%）Q(7)/Q(7)雌性，这与“长”等位基因导致繁殖延迟的预期一致，正如在另一种鸟类的留居种群中所观察到的那样。由于繁殖日期取决于从迁徙地到达的日期，目前的结果表明繁殖日期与生物钟之间的关联可能由迁徙物候介导。此外，繁殖力选择似乎不利于Q(7)/Q(8)，因为迁徙/繁殖晚的家燕每个季节的窝数较少，并且繁殖晚通过降低后代寿命还有额外的负选择效应。基因型频率随年龄略有变化但不显著，因为Q(7)/Q(8)频率在老年个体中降低了4倍。这一结果表明对Q(7)/Q(8)存在负生存力选择，可能由繁殖晚的代价介导。
结论/意义：这是第一项关于候鸟的研究，显示了繁殖物候与生物钟基因型之间的关联，并表明对物候异常的基因型存在负选择。生物钟的低多态性可能限制了对气候变化的微进化物候反应，从而可能导致家燕种群数量下降。
