Abstract
Following chicken domestication, diversified chicken breeds were developed by both natural and artificial selection, which led to the accumulation of abundant genetic and phenotypic variations, making chickens an ideal genetic research model. To better understand the genetic structure of chicken breeds under different selection pressures, we genotyped various chicken populations with specific selection targets, including indigenous, commercial, gamecock, and wild ancestral chickens, using the 600K SNP array. We analyzed the population structure, genetic relationships, run of homozygosity (ROH), effective population number (Ne), and other genetic parameters. The wild ancestral population, red junglefowl (RJF), possessed the highest diversity, in comparison with all other domesticated populations, which was supported by linkage disequilibrium decay (LD), effective population number, and ROH analyses. The gamecock breeds, which were subjected to stronger male-biased selection for fighting-related traits, also presented higher variation than the commercial and indigenous breeds. Admixture analysis also indicated that game breed is a relatively independent branch of Chinese local breeds. Following intense selection for reproductive and productive traits, the commercial lines showed the least diversity. We also observed that the European local chickens had lower genetic variation than the Chinese local breeds, which could be attributed to the shorter history of the European breed. ROH were present in a breed specific manner and 191 ROH island were detected on four groups (commercial, local, game and wild chickens). These ROH islands were involved in egg production, growth and silky feathers and other traits. Moreover, we estimated the effective sex ratio of these breeds to demonstrate the change in the ratio of the two sexes. We found that commercial chickens had a greater sex imbalance between females and males. The commercial lines showed the highest female-to-male ratios. Interestingly, RJF comprised a greater proportion of males than females. Our results show the population genetics of chickens under selection pressures, and can aid in the development of better conservation strategies for different chicken breeds.
Highlights
Abundant phenotypic and genotypic variations make the chicken an ideal model species for genetic studies
1,254 chickens were randomly selected from 15 chicken populations, including five Chinese local breeds (Beijing You [BY], n = 56; Hongshan [HS], n = 96; Shouguang [SG], n = 109; Silkie [SK], n = 89; Tibet [TB], n = 41), one European local breed (Houdan [HD], n = 86), three commercial breeds (Rhode Island Red [RIR], n = 478; White Leghorn [WL], n = 230; Cornish [Cor], n = 10), five gamecock breeds (Luxi Game [LX-G], n = 10; Henan Game [HN-G], n = 11; Xishuangbanna Game [XS-G], n = 10; Zhangzhou Game [Zhangzhou gamecock (ZZ-G)], n = 11; Turfan Game [TurG], n = 10), and the wild ancestral population (RJF, n = 7)
The results showed that red junglefowl (RJF), Tibetan, Shouguang, and Silkies were further away from other breeds, which may be associated with their geographic location, selection targets, and production performance, among other factors
Summary
Abundant phenotypic and genotypic variations make the chicken an ideal model species for genetic studies. Following domestication in Southeast Asia (Liu et al, 2006; Storey et al, 2012; Miao et al, 2013), many breeds, including indigenous, commercial, and cockfighting chickens, were developed by artificial selection for different purposes. These diversified chicken breeds secured abundant genetic variants, such as various feather colors and comb types (Gunnarsson et al, 2007; Wright et al, 2009; Bed’hom et al, 2012; Dorshorst et al, 2015), which have played a pivotal role in the conservation and sustainable utilization of these genetic resources. Such studies have already been conducted in many domesticated mammals and poultry, including chicken breeds, and cattle and pig populations (Maiorano et al, 2018; Zhang Z. et al, 2018; Elbeltagy et al, 2019)
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