Sequence Characterization of K-gene Linked Region in Various Chicken Breeds

Identification of Duplication Genotypes of the Feathering Rate Gene in Chicken by a Multiplex PCR Following Electrophoresis and/or Sanger Sequencing

Sex-linked phenotypes of late feathering (LF) and early feathering (EF) are controlled by a pair of alleles K and k⁺. Autosexing based on the feathering rate is widely used in poultry production. It is reported that a tandem duplication of 176,324 base pairs linked to the K locus is responsible for LF expression and could be used as a molecular marker to detect LF chicken. So far, there is no genotyping method that can accurately and stably identify the LF homozygote and heterozygote in all chicken breeds. In the present study, a multiplex PCR test was developed to identify EF, LF homozygote, and heterozygote according to electrophoretic bands and the relative height of the peaks by Sanger sequencing. We tested 413 chickens of six native Chinese breeds with this method. The identification was consistent with the sex and phenotype records of the chickens. Band density analysis was performed, and the results supported our genotyping using the new assay. In order to further verify the accuracy of this test in distinguishing homozygote and heterozygote males, 152 LF males were mated with EF females, and the results of the offspring's phenotypes were consistent with our expectations. Our results support tandem duplication as molecular markers of LF, and this new test is applicable to all LF chickens associated with tandem duplication.

Analysis of a genetic factors contributing to feathering phenotype in chickens

In the current study, we sought to determine whether or not the endogenous retroviral ev21 influences feathering type of chickens, and if one mutation locus in the unoccupied repeat (UR) region can be used to predict the corresponding feathering type and genotype. The distribution of ev21 as well as the mutation locus in UR and occupied site (OR) regions was detected in HY-line gray progenitor (HYGP) 4 lines, HY-line brown (HYB) and Taihang chickens (TH). Furthermore, a detection method for the genotype resulting in late feathering (LF) phenotype was developed by double PCR using C line of HYGP, C line of Dawu progenitor, commercial line of HY-line gray (HYG) males, LF males of TH and Bashang long-tail chickens (BS). Results indicated that a product of 7590 bp from the long fragment amplification was observed to be a partial segment of ev21, and was linked with the LF phenotype in HYGP but not in HYB and TH chickens. A total of 2 of 35 males and 10 of 29 females of TH LF chickens were found to be ev21 negative. HaeIII RFLP mutations of 1450 bp of UR, 1440 bp of OR, and 538 bp in the UR and OR common region were analyzed, and genotypic features at the locus correlated with the feathering type phenotype in HYGP, but exhibited no significant effects in HYB and TH chickens. The cut-off of relative intensity of 857 and 1305 bp from the double PCR for distinction between homozygous and heterozygous LF males was 1.37. In conclusion, ev21 and the HaeIII RFLP patterns within the locus in UR cannot be used for prediction of feathering type phenotypes in Chinese heritage chickens. However, the partial duplication of PRLR and SPEF2 were able to predict the LF phenotype. Therefore, the double PCR detecting products of 857 and 1305 bp described herein could be used for the accurate identification of genotypes influencing feathering type.

Endogenous viral gene ev21 is not responsible for the expression of late feathering in chickens

The late-feathering (LF) gene K on the Z chromosome is an important gene in the chicken industry, which is frequently utilized for the feather sexing, a type of autosexing, of neonatal chicks. The K gene is closely associated with the endogenous ev21 gene from an avian leukosis virus and the incomplete duplication (ID) of prolactin receptor (PRLR) and sperm flagellar protein 2 (SPEF2) genes, and ev21 has been used as a molecular marker to detect LF birds. In the present study, a comprehensive survey for the presence or absence of ev21 and ID across 1,994 birds from 52 chicken breeds, three commercial hybrid groups, and the Red Jungle Fowl revealed that almost all LF breeds have both ev21 and ID. However, only one LF breed (Ingie) has only ID and no ev21. Moreover, this study revealed that almost all early (normal)-feathering (EF) breeds lack both ev21 and ID, but only one breed (White Plymouth Rock) included EF birds with ev21 but no ID. Therefore, regarding LF expression, the results indicated that ID is responsible, but ev21 is not required. Henceforth, ID should be used as a molecular marker to detect LF birds instead of ev21. Because ev21 contains the full genome of an avian leukosis virus, there is a risk of disease development in breeds with this gene. Therefore, the Ingie breed, which has no ev21 at the K locus, represents excellent material for the establishment of new LF stocks.

Identification of candidate genes for chicken early- and late-feathering

Previous studies suggest that prolactin receptor (Prlr) is a potential causative gene for chicken early- (EF) and late-feathering (LF) phenotypes. In this study, we evaluated candidate genes for this trait and determined the expression of 3 genes, including Prlr, sperm flagellar protein 2 (Spef2), and their fusion gene, in the skins of one-day-old EF and LF chicks using RT-qPCR. Data indicated that Prlr expression in the skin did not show significant difference between EF and LF chicks, suggesting Prlr may not be a suitable candidate gene. In contrast, Spef2 expression in the skin displayed a significant difference between EF and LF chicks (P < 0.01), suggesting that Spef2 may be a good candidate gene for chicken feathering. Moreover, dPrlr/dSpef2, the fusion gene, was also a good candidate gene as it was expressed only in LF chicks. However, the expression of the fusion gene was much lower than that of Prlr Additionally, using strand-specific primers, we found that the fusion gene was transcribed in 2 directions (one from dPrlr promoter, another from dSpef2 promoter), which could result in the formation of a double strand RNA. In conclusion, both Spef2 and the fusion gene are good candidate genes for chicken feathering, but Prlr is not. The research on the function and regulation of the candidate genes will help elucidate the molecular basis of the chicken feathering trait.