Fig. 1: Male and female chickens with complete androgen receptor knockouts lack male and female sexual ornaments, respectively, and are infertile due to impaired gametogenesis.

a–d Depict the phenotypic differences observed between homozygous AR^−/− males (a1, a2) and AR^−/− females (c1, c2) compared to AR^+/+ males (b1, b2) and AR^+/+ females (d1, d2) at adulthood, being 20 weeks old. The AR^−/− males and females lacked typical sexual characteristics such as comb, earlaps and wattles. However, there were notable differences in body size, plumage (presence of sickle tail feathers in males), and leg ornaments (presence of spurs in males) between AR^−/− males and females. “3” depicts the differences of the gonads and “4” their morphology (Haematoxylin stained paraffin (a4, b4) and cryostat (c4, d4) sections). The testicles of AR^−/− males (a3) were significantly smaller compared to those of AR^+/+ roosters (b3) (see Fig. 2e). Morphologically, AR^−/− testicles lacked the typical organization, especially the seminiferous tubules (ST, arrows) with lumen (a4) compared to AR^+/+ testicles (b4). The mean diameters of tubules were 493, 464 and 398 µm for three AR^+/+ and were 268, 208 and 158 µm for three AR^+/+ males, respectively. In addition, the seminiferous tubules of the AR^−/− testicles lacked mature sperm (insert in A4) compared to the AR^+/+ testicles (insert in b4). In a4 and b4, the small squares indicate the enlarged areas of the inserts. The ovaries of AR^−/− hens contained smaller follicles of varying sizes (c3, c4) but lacked the late-stage hierarchical follicles (HF) seen in AR^+/+ hens (d3, d4), and they lacked the ovulated oocytes (not depicted). Histological staining was performed for gonads of three individuals per group. Scale bar represents 1 cm in a3–d3 and 200 µm in a4–d4, and represents 50 µm for the large inserts in a4 and b4.