Abstract
Low and high egg producing hens exhibit gene expression differences related to ovarian steroidogenesis. High egg producing hens display increased expression of genes involved in progesterone and estradiol production, in the granulosa layer of the largest follicle (F1G) and small white follicles (SWF), respectively, whereas low egg producing hens display increased expression of genes related to progesterone and androgen production in the granulosa (F5G) and theca interna layer (F5I) of the fifth largest follicle, respectively. Transcriptome analysis was performed on F1G, F5G, F5I, and SWF samples from low and high egg producing hens to identify novel regulators of ovarian steroidogenesis. In total, 12,221 differentially expressed genes (DEGs) were identified between low and high egg producing hens across the four cell types examined. Pathway analysis implied differential regulation of the hypothalamo-pituitary-thyroid (HPT) axis, particularly thyroid hormone transporters and thyroid hormone receptors, and of estradiol signaling in low and high egg producing hens. The HPT axis showed up-regulation in high egg producing hens in less mature follicles but up-regulation in low egg producing hens in more mature follicles. Estradiol signaling exclusively exhibited up-regulation in high egg producing hens. Treatment of SWF cells from low and high egg producing hens with thyroid hormone in vitro decreased estradiol production in cells from high egg producing hens to the levels seen in cells from low egg producing hens, whereas thyroid hormone treatment did not impact estradiol production in cells from low egg producing hens. Transcriptome analysis of the major cell types involved in steroidogenesis inferred the involvement of the HPT axis and estradiol signaling in the regulation of differential steroid hormone production seen among hens with different egg production levels.
Highlights
Avian species display an ovarian hierarchy, with follicles in all stages of development present in the ovary at any given time during egg production (Johnson, 2015a)
All animal procedures were approved by the Institutional Animal Care and Use Committee at Beltsville Agricultural Research Center (BARC) and at the University of Maryland
Egg records were used classify hens as low egg producing hens or high egg producing hens, using cutoffs based on previous studies examining average flock egg production and egg production distribution (Brady et al, 2020a)
Summary
Avian species display an ovarian hierarchy, with follicles in all stages of development present in the ovary at any given time during egg production (Johnson, 2015a). Throughout follicle maturation, steroid production shifts from estradiol production in less mature follicles, to androgen production in follicles in the middle stages of development, to progesterone production in more mature follicles (Porter et al, 1991). Along with this shift, a change in gonadotropin sensitivity is seen as well, shifting from follicle stimulating hormone (FSH) responsiveness in less mature follicles to luteinizing hormone (LH) responsiveness in more mature follicles (Johnson and Woods, 2009)
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