Characterization of the hypothalamo-pituitary-gonadal axis in low and high egg producing turkey hens

Abstract

Variation in egg production exists in commercial turkey hens, with low egg producing hens (LEPH) costing more per egg produced than high egg producing hens (HEPH). Egg production correlates with ovulation frequency, which is governed by the hypothalamic-pituitary-gonadal (HPG) axis. Ovulation is stimulated by a preovulatory surge (PS) of progesterone and luteinizing hormone, triggered by gonadotropin releasing hormone release and inhibited by gonadotropin inhibiting hormone. Differences between LEPH and HEPH were characterized by determining HPG axis plasma hormone profiles and mRNA levels for key genes, both outside and inside of the PS (n = 3 per group). Data were analyzed with a 2-way ANOVA using the mixed models procedure of SAS. In the HPG axis, plasma progesterone levels were not affected by egg production level but were elevated during the PS. In contrast, plasma estradiol levels were higher in HEPH than in LEPH but were not associated with the PS. LEPH exhibited decreased gene expression associated with ovulation stimulation and increased gene expression associated with ovulation inhibition in the hypothalamus and pituitary. In ovarian follicle cells, LEPH displayed decreased gene expression associated with progesterone, androgen, and estradiol production in the F1 follicle granulosa cells, F5 theca interna cells, and small white follicle cells, respectively. Different degrees of stimulation and inhibition within all tissues of the HPG axis were noted between LEPH and HEPH turkey hens, with HEPH showing higher expression of genes related to ovulation and steroidogenesis.

Keywords: egg production; gene expression; hypothalamo–pituitary–gonadal axis; ovulation; steroid hormone.

Figure 1

Production calculations showing (A) egg laid per day (EPD), (B) average clutch length, and (C) average pause length in low egg producing hens (LEPH) and high egg producing hens (HEPH). Significance is denoted with an asterisk (*P ≤ 0.05, **P ≤ 0.01, and ***P ≤ 0.001).

Figure 2

Plasma progesterone and estradiol hormone profiles in low egg producing hens (LEPH) and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). Significant differences in steroid plasma concentrations are denoted with an asterisk (*P ≤ 0.05, **P ≤ 0.01, and ***P ≤ 0.001). Solid lines indicate a significant difference between LEPH and HEPH for a given condition, whereas dotted lines indicate a significant difference between basal and surge for a given egg production level.

Figure 3

Hypothalamic gene expression of gonadotropin releasing hormone (GNRH), gonadotropin inhibitory hormone (GNIH), progesterone receptor (PGR), and estrogen receptors 1 and 2 (ESR1 and ESR2) in low egg producing hens (LEPH) and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to LEPH basal expression for each gene. Significant expression differences denoted with an asterisk (*P ≤ 0.05, **P ≤ 0.01, and ***P ≤ 0.001). Solid lines indicate a significant difference between LEPH and HEPH for a given condition, whereas dotted lines indicate a significant difference between basal and surge for a given egg production level.

Figure 4

Pituitary gene expression of gonadotropin releasing hormone receptor (GNRHR), gonadotropin inhibitory hormone receptors 1 and 2 (GNIHR1 and GNIHR2), luteinizing hormone beta-subunit (LHB), follicle stimulating hormone beta-subunit (FSHB), common alpha-subunit (CGA), progesterone receptor (PGR), and estrogen receptor 1 (ESR1) in low egg producing hens (LEPH) and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to LEPH basal expression for each gene. Significant expression differences denoted with an asterisk (*P ≤ 0.05, **P ≤ 0.01, and ***P ≤ 0.001). Solid lines indicate a significant difference between LEPH and HEPH for a given condition, whereas dotted lines indicate a significant difference between basal and surge for a given egg production level.

Figure 5

F1 follicle gene expression follicle stimulating hormone receptor (FSHR), luteinizing hormone receptor (LHCGR), steroidogenic acute regulatory protein (STAR), cholesterol side chain cleavage enzyme (CYP11A1), 3β-hydroxysteroid dehydrogenase (HSD3B1), 17, 20-lyase (CYP17A1), 17β-hydroxysteroid dehydrogenase (HSD17B1), and aromatase (CYP19A1) in the (A) granulosa, (B) theca interna, and (C) theca externa from low egg producing hens (LEPH) and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to LEPH basal expression for each gene. Significant expression differences denoted with an asterisk (*P ≤ 0.05, **P ≤ 0.01, and ***P ≤ 0.001). Solid lines indicate a significant difference between LEPH and HEPH for a given condition, whereas dotted lines indicate a significant difference between basal and surge for a given egg production level.

Figure 6

F5 follicle gene expression of follicle stimulating hormone receptor (FSHR), luteinizing hormone receptor (LHCGR), steroidogenic acute regulatory protein (STAR), cholesterol side chain cleavage enzyme (CYP11A1), 3β-hydroxysteroid dehydrogenase (HSD3B1), 17, 20-lyase (CYP17A1), 17β-hydroxysteroid dehydrogenase (HSD17B1), and aromatase (CYP19A1) in the (A) granulosa, (B) theca interna, and (C) theca externa from low egg producing hens (LEPH) and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to LEPH basal expression for each gene. Significant expression differences denoted with an asterisk (*P ≤ 0.05, **P ≤ 0.01, and ***P ≤ 0.001). Solid lines indicate a significant difference between LEPH and HEPH for a given condition, whereas dotted lines indicate a significant difference between basal and surge for a given egg production level.

Figure 7

Small white follicle gene expression of follicle stimulating hormone receptor (FSHR), luteinizing hormone receptor (LHCGR), 17, 20-lyase (CYP17A1), 17β-hydroxysteroid dehydrogenase (HSD17B1), and aromatase (CYP19A1) in low egg producing hens (LEPH) and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to LEPH basal expression for each gene. Significant expression differences denoted with an asterisk (*P ≤ 0.05, **P ≤ 0.01, and ***P ≤ 0.001). Solid lines indicate a significant difference between LEPH and HEPH for a given condition, whereas dotted lines indicate a significant difference between basal and surge for a given egg production level.