Sex differences in neuromuscular androgen receptor expression and sociosexual behavior in a sex changing fish

Abstract

Androgen signaling, via receptor binding, is critical for regulating the physiological and morphological foundations of male-typical reproductive behavior in vertebrates. Muscles essential for male courtship behavior and copulation are highly sensitive to androgens. Differences in the distribution and density of the androgen receptor (AR) are important for maintaining dimorphic musculature and thus may provide for anatomical identification of sexually selected traits. In Lythrypnus dalli, a bi-directional hermaphroditic teleost fish, both sexes produce agonistic approach displays, but reproductive behavior is sexually dimorphic. The male-specific courtship behavior is characterized by rapid jerky movements (involving dorsal fin erection) towards a female or around their nest. Activation of the supracarinalis muscle is involved in dorsal fin contributions to both agonistic and sociosexual behavior in other fishes, suggesting that differences in goby sexual behavior may be reflected in sexual dimorphism in AR signaling in this muscle. We examined sex differences in the local distribution of AR in supracarinalis muscle and spinal cord. Our results demonstrate that males do express more AR in the supracarinalis muscle relative to females, but there was no sex difference in the number of spinal motoneurons expressing AR. Interestingly, AR expression in the supracarinalis muscle was also related to rates of sociosexual behavior in males, providing evidence that sexual selection may influence muscle androgenic sensitivity to enhance display vigor. Sex differences in the distribution and number of cells expressing AR in the supracarinalis muscle may underlie the expression of dimorphic behaviors in L. dalli.

Fig 1. Location and size of the supracarinalis muscle in L. dalli.

Illustration of where the supracarinalis muscle is located (shown in red) and relative area sectioned in this study to assess receptor expression (A). Hemotoxylin and eosin stained cross section showing the exact location of the supracarinalis muscle relative to other muscles and peripheral tissues (B). Mean (±SEM) relative supracarinalis muscles size (muscle area (μm²) / standard length (mm) in male (n = 10) and female (n = 7) L. dalli (C).

Fig 2. Immunolocalization of androgen receptor (AR) expression between sexes in different muscle types.

Representative AR staining within the supracarinalis muscle of males (A) and females (B), and the epaxial and hypaxial muscles of males (n = 12) (C/E) and females (n = 8) (D/F) respectively. The mean (±SEM) number of AR positive cells within the supracarinalis, epaxial, and hypaxial muscles in males and females (G). All images were captured under a 40x objective. Scale bar = 50 μm *** denotes a significant difference at p < 0.001.

Fig 3. Immunolocalization of spinal cord androgen receptor (AR) expression between the sexes and differences in expression between the sexes in the ventral and dorsal horn of the spinal cord.

Representative AR staining in the dorsal and ventral horn of male (n = 12) (A) and female (n = 8) (B) L. dalli. Mean (±SEM) AR staining intensity within the dorsal and ventral horns of the spinal cord in males and females (C). DH = dorsal horn, VH = ventral horns.

Fig 4. The relationship between the number of androgen receptor (AR) expressing cells in the supracarinalis muscle and the rate of jerk (A) and approach (B) behaviors in male L. dalli (n = 12).