Trends towards revealing the genetic architecture of sheep tail patterning: Promising genes and investigatory pathways

Abstract

Different sheep breeds have evolved after initial domestication, generating various tail phenotypic patterns. The phenotypic diversity of sheep tail patterns offers ideal materials for comparative analysis of its genetic basis. Evolutionary biologists, animal geneticists, breeders, and producers have been curious to clearly understand the underlying genetics behind phenotypic differences in sheep tails. Understanding the causal gene(s) and mutation (s) underlying these differences will help probe an evolutionary riddle, improve animal production performance, promote animal welfare, and provide lessons that help comprehend human diseases related to fat deposition (i.e., obesity). Historically, fat tails have served as an adaptive response to aridification and climate change. However, the fat tail is currently associated with compromised mating and animal locomotion, fat distribution in the animal body, increased raising costs, reduced consumer preference, and other animal welfare issues such as tail docking. The developing genomic approaches provide unprecedented opportunities to determine causal variants underlying phenotypic differences among populations. In the last decade, researchers have performed several genomic investigations to assess the genomic causality underlying phenotypic variations in sheep tails. Various genes have been suggested with the prominence of several potentially significant causatives, including the BMP2 and PDGFD genes associated with the fat tail phenotype and the TBXT gene linked with the caudal vertebrae number and tail length. Although the potential genes related to sheep tail characteristics have been revealed, the causal variant(s) and mutation(s) of these high-ranking candidate genes are still elusive and need further investigation. The review discusses the potential genes, sheds light on a knowledge gap, and provides possible investigative approaches that could help determine the specific genomic causatives of sheep tail patterns. Besides, characterizing and revealing the genetic determinism of sheep tails will help solve issues compromising sheep breeding and welfare in the future.

Transcriptome Profiling of Developing Ovine Fat Tail Tissue Reveals an Important Role for MTFP1 in Regulation of Adipogenesis

“…Among these 183 enriched genes, 17 genes have been reported in previous tail phenotype related studies in sheep ( Wang et al, 2014 ; Li et al, 2018 ; Ma et al, 2018 ; Bakhtiarizadeh et al, 2019 ; Kalds et al, 2021 ; Wang et al, 2021 ; Zhang et al, 2021 ). Expression of these genes fluctuated from the E60 to Fat stages and was significantly higher at E70 than E70_SFK ( Supplementary Figures S6A,S6B ).…”

“…Based on tail morphology, domestic sheep ( Ovis aries ) can be assigned to three biological categories: fat-tail sheep, thin-sheep and fat-rump sheep ( Kalds et al, 2021 ). Among mammals, only sheep deposit extensive fat tissue during development of the tail ( Al-Rehaimi et al, 1989 ).…”

“…Among various population genetics and transcriptome studies targeting the fat-tail trait, several genes (for example, PDGFD , BMP2 and TBXT ) have been identified as promising candidate genes ( Kalds et al, 2021 ) and almost all of these studies were conducted in adult sheep. Based on resequencing data or SNP data from various sheep breeds worldwide with different tail types, the PDGFD gene has been identified as the most important candidate gene associated with tail morphology ( Wei et al, 2015 ; Pan et al, 2019 ; Dong et al, 2020 ; Li et al, 2020 ); however, the strongest signals identified within PDGFD gene are different.…”

“…A substantial body of research work has been published on sheep fat-tail biology and development ( Kalds et al, 2021 ; Pourlis, 2011 ); however, to-date the genetic architecture of this complex trait has not been investigated. In addition, it is important to keep in mind that prior to birth, substantial fat already exists in tail tissue and that the regulatory mechanisms governing mammalian tail development remain poorly understood.…”

Long divergent haplotypes introgressed from wild sheep are associated with distinct morphological and adaptive characteristics in domestic sheep

“…hypoxia responses and high-attitude adaptation (Yang, et al 2016; Hu, et al 2019; Lv, et al 2021), resistance to pneumonia (Cao, et al 2020) and reproduction (Liu, et al 2016). It would be a reasonable expectation that traits resulting from human selection (Xu and Li 2017; Li, Yang, Shen, et al 2020) were only indirectly influenced by wild introgression, such as the different wool (Jackson, et al 2020) and tail types (Kalds, et al 2021). However, the absence of horns, a typical domestic feature, corresponding to RXFP2 haplotypes is also detected in “horned” Asiatic mouflon.…”