Investigating egg weight and expression of growth differentiation factor 9 gene in preovulatory ovarian follicles of Potchefstroom Koekoek chicken genotype

Abstract

Poultry eggs are an essential source of animal protein, and their consumption has expanded drastically in recent decades. The study aimed to determine the relationship between egg weight and egg quality traits, ovarian follicles morphology, and mRNA expression levels of the growth differentiation factor 9 gene in pre-ovulatory ovarian follicles of the Potchefstroom Koekoek chicken genotype. A total of 300 eggs were collected to measure the egg weight (EW) and egg quality traits viz. egg length (EL), egg width (EWD), egg shape index (SI), shell weight (SW), egg yolk weight (YW), albumen weight (AW), shell surface area (SSA), unit surface shell weight (USSW), shell ratio (SR), albumen ratio (AR), yolk ratio (YR), yolk/albumen (Y/A) and egg volume (EV). Six chickens aged 30 weeks were slaughtered to harvest ovarian follicles for morphological examination, and three chickens were slaughtered to harvest follicles for evaluating the gene expression levels. Pearson’s correlation, Student’s T-test and Analysis of variance (ANOVA) were used for data analysis. The correlation results showed that EW had a highly positive significant association (P < 0.01) with EWD, YW, SSA, AW, AR and EV, a negatively high statistically significant relationship (P < 0.01) with SR, YR and Y/A. The findings further revealed that EW had a positive significant association (P < 0.05) with EL and a negatively high statistically significant association (P < 0.05) with USSW. The findings also showed that EW had no statistically significant association (P > 0.05) with SW and SI. The Student’s T-test results revealed that the numbers of large yellow follicles were significantly different (P < 0.05) from those of small yellow follicles. The ANOVA findings showed that there was a significant difference (P < 0.05) in the average weight of the large yellow follicles. The Quantitative real-time PCR findings indicated that there were significant differences (P < 0.05) in the mRNA expression levels of GDF9 gene in preovulatory ovarian follicles of Potchefstroom Koekoek chicken genotype. The mRNA expression was most abundant in F1 and F4. The results also showed significant differences (P < 0.05) in the GDF9 mRNA expression between F2 and F3, with significant amounts in F2 than F3. In conclusion, the results indicate that the improvement of EWD, YW, SSA, AW, AR, EV and EL might enhance egg weight in Potchefstroom Koekoek chicken breed. The average weight of the large yellow follicles ranges from 1.15g to 10.28g in Potchefstroom Koekoek at 30 weeks of age. The GDF9 gene expression findings suggest that GDF9 gene play an important role in the growth and development of ovarian follicles of the Potchefstroom Koekoek chicken genotype. The findings might potentially aid with GDF9 gene expression levels for selection during breeding to improve Potchefstroom Koekoek chicken eggs. However, more research on the expression of the GDF9 gene in additional South African indigenous chicken breeds is needed.