Avian eggshell color is an interesting genetic trait. Here, we report that the blue eggshell color of the domestic duck is caused by two cis-regulatory G to A transitions upstream of ABCG2, which encodes an efflux transporter. The juxtaposed blue eggshell allele A-A exhibited higher promoter activity and stronger nuclear protein binding capacity than the white eggshell allele G-G. Transcription factor analysis suggested differential binding capability of CTCF between blue eggshell and white eggshell alleles. Knockdown of CTCF expression significantly decreased the promoter activity of the blue eggshell but not the white eggshell allele. DNA methylation analysis revealed similar high methylation of the region upstream of the CTCF binding sites in both blue-eggshelled and white-eggshelled ducks. However, DNA methylation levels downstream of the binding sites were decreased and 35% lower in blue-eggshelled ducks than in white-eggshelled ducks. Consistent with the in vitro regulatory pattern of causative sites, ABCG2 exhibited higher expression in uteruses of blue-eggshelled ducks and also showed polarized distribution in their endometrial epithelial cells, distributing at the apical surface of endometrial epithelial cells and with orientation toward the uterine cavity, where the eggshell is pigmented. In conclusion, our results suggest that two cis-regulatory SNPs upstream of ABCG2 are the causative mutations for blue eggshells in ducks. The blue eggshell variant up-regulated ABCG2 expression through recruiting CTCF binding, which may function as a barrier element to shield the downstream region from high methylation levels present upstream. ABCG2 was identified as the only candidate causative gene for blue eggshells; it may function as an efflux transporter of biliverdin to the uterine cavity.
Background: In mammal herbivores, the digestion of fiber usually occurs predominantly in either the foregut or in the hindgut. However, how both gut regions function synergistically in the digestion of fiber and other nutrients has rarely been reported in wild mammals. This requires an integrative study of host anatomy, physiology and gut microbiome. Colobine monkeys (Colobinae) are folivorous, with fiber fermentation primarily occurring in the foregut, with residual fermentation in the hindgut. For the few colobine species that live in temperate regions obtaining energy from fiber during winter is critical but the mechanisms enabling this remain unclear. Results: We studied microbial and morphological digestive adaptations of golden snub-nosed monkeys (GSMs), Rhinopithecus roxellana, a temperate forest colobine from central China. We tested for synergistic foregut and hindgut fiber digestion in a species that experiences high thermal energy demands while restricted to a fibrous, low-energy winter diet. We found that the GSM’s colon has a significantly greater volume than that of other foregut fermenting colobines, and both gut regions of GSMs are dominated by microbial taxa producing enzymes to enable active digestion of complex carbohydrates. The microbiomes of the fore- and hindgut differed significantly in composition and abundance. Although the expression of microbial gene functions for fiber digestion were higher in the foregut than in the hindgut, our microbiome analysis in conjunction with that for morphology, enzyme activity and fiber-protein digestion, suggests complementary fiber and protein metabolism in both gut regions. Conclusions: Our results support that both the GSM fore- and hindgut facilitate fiber digestion, with an enlarged colon consistent as an adaptation to accommodate high throughput of fiber-rich food during winter.
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