Data from: Genetic loci inherited from hens lacking maternal behaviour both inhibit and paradoxically promote this behaviour

Background: A major step towards the success of chickens as a domesticated species was the separation between maternal care and reproduction. Artificial incubation replaced the natural maternal behaviour of incubation and, thus, in certain breeds, it became possible to breed chickens with persistent egg production and no incubation behaviour; a typical example is the White Leghorn strain. Conversely, some strains, such as the Silkie breed, are prized for their maternal behaviour and their willingness to incubate eggs. This is often colloquially known as broodiness. Results: Using an F2 linkage mapping approach and a cross between White Leghorn and Silkie chicken breeds, we have mapped, for the first time, genetic loci that affect maternal behaviour on chromosomes 1, 5, 8, 13, 18 and 19 and linkage group E22C19W28. Paradoxically, heterozygous and White Leghorn homozygous genotypes were associated with an increased incidence of incubation behaviour, which exceeded that of the Silkie homozygotes for most loci. In such cases, it is likely that the loci involved are associated with increased egg production. Increased egg production increases the probability of incubation behaviour occurring because egg laying must precede incubation. For the loci on chromosomes 8 and 1, alleles from the Silkie breed promote incubation behaviour and influence maternal behaviour (these explain 12 and 26 % of the phenotypic difference between the two founder breeds, respectively). Conclusions: The over-dominant locus on chromosome 5 coincides with the strongest selective sweep reported in chickens and together with the loci on chromosomes 1 and 8, they include genes of the thyrotrophic axis. This suggests that thyroid hormones may play a critical role in the loss of incubation behaviour and the improved egg laying behaviour of the White Leghorn breed. Our findings support the view that loss of maternal incubation behaviour in the White Leghorn breed is the result of selection for fertility and egg laying persistency and against maternal incubation behaviour.

背景：鸡作为驯化物种取得成功的关键一步，在于母性照料与繁殖过程的分离。人工孵化取代了自然的孵卵母性行为，由此，部分鸡品种得以选育出持续产蛋且无孵卵行为的类群；来亨鸡（White Leghorn）品系便是典型代表。与之相反，丝羽乌骨鸡（Silkie）等品系则因出色的母性行为与孵卵意愿而备受青睐，这类行为通常被俗称为就巢性（broodiness）。
结果：本研究采用F2连锁定位（F2 linkage mapping）方法，以来亨鸡与丝羽乌骨鸡的杂交后代为实验材料，首次在1、5、8、13、18、19号染色体及连锁群E22C19W28上定位到影响母性行为的遗传基因座（genetic loci）。令人意外的是，杂合基因型与来亨鸡纯合基因型与孵卵行为发生率升高相关，在多数基因座上，该发生率甚至高于丝羽乌骨鸡纯合基因型。此类情况表明，相关基因座可能与产蛋量提升有关：产蛋量增加会提升孵卵行为发生的概率，因为孵卵行为必然发生于产蛋之后。针对1号和8号染色体上的基因座，丝羽乌骨鸡的等位基因可促进孵卵行为并影响母性行为，二者分别可解释两个创始品种间12%与26%的表型差异（phenotypic difference）。
结论：5号染色体上的超显性基因座（over-dominant locus）与已报道的鸡最强选择性清除（selective sweep）区域重合，该基因座与1号、8号染色体上的基因座均包含促甲状腺激素轴（thyrotrophic axis）相关基因。这提示甲状腺激素可能在来亨鸡丢失孵卵行为、提升产蛋性能的过程中发挥关键作用。本研究结果支持如下观点：来亨鸡母性孵卵行为的丢失，是人类针对繁殖力与持续产蛋性能进行选择、同时淘汰母性孵卵行为的结果。