Isolation of blue-green eggshell pigmentation-related genes from Putian duck through RNA-seq

The diversity of avian eggshell colour plays important biological roles in ensuring successful reproduction. Eggshell colour is also an important trait in poultry, but the mechanisms underlying it are poorly understood in ducks. This study aimed to provide insights into the mechanism of blue-green eggshell colour generation.

Here, white-shelled ducks (HBR) and blue-green-shelled ducks (HQR) were selected from Putian black ducks, and white-shelled ducks (BBR) were selected from Putian white ducks. Transcriptional changes in the shell gland were analysed using RNA-sequencing on the Illumina HiSeq 2500. Twenty-seven individual cDNA libraries were sequenced and generated an average of 7.35 million reads per library; 70.6% were mapped to the duck reference genome, yielding an average of 13,794 genes detected, which accounted for approximately 86.39% of all 15,967 annotated duck genes. A total of 899 differentially expressed genes (DEGs) were detected between the HQR and BBR groups, and 373 DEGs were detected between the HQR and HBR groups. We analysed the DEGs in the HQR-vs-BBR and HQR-vs-HBR comparisons. None of these DEGs were directly involved in the eggshell pigmentation process in HQR-vs-HBR, while UDP-glucuronosyltransferase 2A2 (UGT2A2) and UDP-glucuronosyltransferase 1-1-like (UGT1-1-like), which participate in biliverdin breakdown, were two of the DEGs in HQR-vs-BBR. In the RT-qPCR results, delta-aminolevulinic acid synthase 1 (ALAS1) and EPRS glutamyl-prolyl-tRNA synthetase were significantly upregulated in the HBR group compared with the HQR and BBR groups (P < 0.05). Haem oxygenase (HMOX1) was significantly downregulated in BBR compared with HQR and HBR (P < 0.05). Biliverdin reductase A (BLVRA), GUSB glucuronidase beta, cytochrome c-type haem lyase, protohaem IX farnesyltransferase and UGT2A2 were significantly upregulated in HBR and BBR compared with HQR (P < 0.05).

We conducted a comparative transcriptome analysis of the shell glands of Putian white ducks and Putian black ducks. None of the differentially regulated pathways were directly involved in the eggshell pigmentation process in the HQR-vs-HBR comparison, while 2 DEGs related to biliverdin breakdown were found in HQR-vs-BBR. Based on the RT-qPCR results, we can speculate that both HQR and HBR can produce biliverdin, but HBR cannot accumulate it. Compared with HQR, BBR produced less biliverdin and did not accumulate it.