Genetic parameters and weighted single-step genome-wide association study for supernumerary teats in Holstein cattle

Supernumerary teats (SNT) are a common epidermal abnormality of udders in mammals. The SNT negatively affect machine milking ability, udder health, and animal welfare and sometimes act as reservoirs for undesirable bacteria, resulting in economic losses on calves and lactating cows due to the cost of SNT removal surgery, early culling, and low milk yield. This study aimed to analyze the incidence and genetic parameter of SNT and detect SNT-related genes in Chinese Holstein cattle. In this study, the incidence of SNT was recorded in 4,670 Chinese Holstein cattle (born between 2008 and 2017) from 2 farms, including 734 genotyped cows with 114,485 SNPs. The SNT had a total frequency of 9.8% and estimated heritability of 0.22 (SE = 0.07), which were obtained using a threshold model in the studied Chinese Holstein population. Furthermore, we calculated approximate genetic correlations between SNT and the following indicator traits: 12 milk production, 28 body conformation, 5 fertility and reproduction, 5 health, and 9 longevity. Generally, the estimated correlations, such as 305-d milk yield for third parity (-0.55; SE = 0.02) and age at first calving in heifer (0.19; SE = 0.03), were low to moderate. A single-step GWAS was implemented, and 10 genes associated with SNT located in BTA4 were identified. The region (112.70-112.90 Mb) on BTA4 showed the highest genetic variance for SNT. The quantitative trait loci on BTA4 was mapped into the RARRES2 gene, which was previously shown to affect adipogenesis and hormone secretion. The WIF1 gene, which was located in BTA5, was also considered as a candidate gene for SNT. Overall, these findings provide useful information for breeders who are interested in reducing SNT.

Size of supernumerary teats in sheep correlates with complexity of the anatomy and microenvironment

Supernumerary nipples or teats (polythelia) are congenital accessory structures that may develop at any location along the milk line and have been implicated in the pathogenesis of mastitis. We describe the anatomy and histology of 27 spontaneously occurring supernumerary teats from 16 sheep, delineating two groups of teats - simple and anatomically complex - according to the complexity of the anatomy and microenvironment. Anatomically complex supernumerary teats exhibited significantly increased length and barrel diameter compared with simple supernumerary teats. A teat canal and/or teat cistern was present in anatomically complex teats, with smooth muscle fibres forming a variably well-organised encircling teat sphincter. Complex supernumerary teats also exhibited immune cell infiltrates similar to those of normal teats, including lymphoid follicle-like structures at the folds of the teat cistern-teat canal junction, and macrophages that infiltrated the peri-cisternal glandular tissue. One complex supernumerary teat exhibited teat end hyperkeratosis. These anatomical and histological features allow inference that supernumerary teats may be susceptible to bacterial ingress through the teat canal and we hypothesise that this may be more likely in those teats with less well-organised encircling smooth muscle. The teat cistern of anatomically complex teats may also constitute a focus of milk accumulation and thus a possible nidus for bacterial infection, potentially predisposing to mastitis. We suggest that size of the supernumerary teat, and relationship to the main teats, particularly in the case of 'cluster teats', should be considerations if surgical removal is contemplated.

Heritability and genome-wide association mapping for supernumerary teats in French Alpine and Saanen dairy goats

This paper reports a quantitative genetics and genomic analysis of undesired presence of supernumerary teats (SNT) in goats. Supernumerary teats are a problem in goat breeding as they can considerably impede machine milking efficiency, leading to increased milking time and injury. This phenotype has routinely been recorded for the past 15 yr in French Alpine and Saanen goats. Around 4% of the females had been assigned the SNT phenotype and consequently could not be included in the breeding program as elite animals. The heritability of this binary trait, estimated by applying linear logistic polygenic models to 32,908 Alpine and 23,217 Saanen females, was 0.40 and 0.44, respectively. A genome-wide association study was implemented using a daughter design composed of 810 Saanen goats sired by 9 artificial insemination bucks and 1,185 Alpine goats sired by 11 bucks, genotyped with the goatSNP50 chip (Illumina Inc., San Diego, CA). This association study was based on logistic polygenic models, one with separately taken single nucleotide polymorphisms and the other with haplotypes as fixed effects. The 2 breeds were analyzed together and separately. No region was found to be significant at the genome level, but 17 regions on 10 chromosomes were significant at the chromosome level. These signals were always only slightly above the chromosome significance threshold and only a few of them overlapped across analyses. No evidence of segregation of a major gene in our Saanen and Alpine populations was observed, suggesting that SNT presence is inherited in a polygenic fashion. This conclusion regarding SNT determinism agrees with recent association analyses in cattle, and one locus was even found in an orthologous region. The possibility of applying markers-based selection on the SNT trait is therefore unlikely, but, as this trait is heritable and routinely recorded, it could be managed by attributing a dedicated estimated breeding value.