Identification of double-yolk trait candidate genes and hormone level analysis in the Gaoyou duck

Double-yolk eggs can occur both naturally and with artificial breeding. Although they negatively affect fertilization and hatching rates, they have positive economic benefits, as their nutritional value is higher than that of ordinary single-yolk eggs, resulting in a higher market value. The genetic mechanisms underlying the double-yolk trait, however, are unclear and require further investigation. Gaoyou ducks, which regularly produce double-yolk and multi-yolk eggs, were investigated in this study using biochemical indicators and molecular analyses to determine the regulatory mechanisms for this trait. A total of 580 female Gaoyou ducks were raised and their laying data were recorded. Of these, 36 ducks were selected as a single-yolk group and 36 as a double-yolk group based on their consistent production of such eggs. A radioimmunoassay method was used to determine the hormone levels in the venous blood of the ducks. Whole-genome resequencing was performed on 20 ducks to screen for SNPs related to the dou-ble-yolk trait and to search for candidate genes. The average double-yolk egg laying rate in the group was 1.14%, and the highest for a single day was 2.86%. The average levels of folli-cle-stimulating hormone (FSH), prolactin (PRL), estradiol (E2), and gonadotropin-releasing hormone (GnRH) in the double-yolk group were lower than in the single-yolk group. A total of ~ 370 G of raw data was obtained by whole-genome resequencing, and the average depth of individuals was 11-fold. Eleven major candidate genes (NKX2-1, HSD17B7, THOC1, HOXC10, ACTR2, ESR1, PLCB1, STMN2, NCOA7, PSMC2, and SLC26A5) for the double-yolk trait were identified by genomic data analysis. Our results provide information on the hormone levels in the Gaoyou duck and a list of candidate genes related to the double-yolk trait. These findings will help improve our understanding of the genetic mechanisms controlling the double-yolk trait and benefit future duck breeding programs.