Skip to main content
ARS Home » Pacific West Area » Dubois, Idaho » Range Sheep Production Efficiency Research » Research » Publications at this Location » Publication #149637
Title: SEXUAL DIFFERENTIATION OF PARTNER PREFERENCE: THE RAM MODEL

Author
item ROSELLI, CHARLES - OREGON HLTH & SCI UNIV
item LARKIN, K. - OREGON HLTH & SCI UNIV
item SCHRUNK, JESSICA - OREGON STATE UNIV
item Stellflug, John
item RESKO, JOHN - OREGON HLTH & SCI UNIV
item STORMSHAK, FRED - OREGON STATE UNIV

Submitted to: Neuroendocrinology Journal
Publication Type: Proceedings
Publication Acceptance Date: 5/6/2003
Publication Date: 7/16/2003
Citation: Workshop on the Neuroendocrinology of Development. 2003. Abstract. p. 17.

Interpretive Summary: Domestic rams display a natural variation in sexual partner preference, such that 6-8% prefer male sexual partners (male-oriented rams, MORs) in contrast to the majority of rams that prefer female sexual partners (female-oriented rams, FORs). We have experimentally compared MORs and FORs under controlled conditions in an effort to relate circulating hormone levels, nervous system structure, and gene expression to behavior. Systemic concentrations of androgen do not appear to be causally related to the expression of male-oriented sexual preference in adult rams. However, male-oriented partner preference is associated with low levels of aromatase and estrogen receptors in the brain. In mammals, differentiation of male-typical brain function depends predominantly on exposure to testosterone or its estrogenic metabolites during a critical perinatal developmental period. The medial preoptic area (MPOA), in particular the sexually dimorphic nucleus (SDN), appears to be involved in the expression of male sexual behavior and mate preferences. The presence of a SDN that is larger in males than in females is considered a morphological hallmark of a masculinized brain in several species. We identified a cell group in the caudal MPOA of the sheep that is significantly larger and contains two to four times as many neurons in rams than in ewes. This sexual dimorphism, together with its neuroanatomic position, strongly suggests that this nucleus constitutes the ovine analogue of the SDN, the oSDN. Quantitative morphological comparisons demonstrate that the volume of the oSDN is larger in FORs than in MORs and significantly larger in MORs than ewes in the luteal phase of their cycle. The oSDN in FORs also contains more neurons and is more elongated than in MORs and ewes, however, cell densities in the oSDN do not differ. In situ hybridization histochemistry revealed that neurons of the oSDN express high levels of aromatase mRNA and that expression in FORs is greater than in MORs and ewes. These results demonstrate that the volume of the oSDN corresponds to the type of sexual partner preference exhibited by rams. These data further suggest that the brains of MORs are incompletely masculinized and support the hypothesis that naturally occurring variations in sexual partner preference are determined by underlying biological mechanisms. Supported by NIH grant RR14270.

Technical Abstract: Domestic rams display a natural variation in sexual partner preference, such that 6-8% prefer male sexual partners (male-oriented rams, MORs) in contrast to the majority of rams that prefer female sexual partners (female-oriented rams, FORs). We have experimentally compared MORs and FORs under controlled conditions in an effort to relate circulating hormone levels, nervous system structure, and gene expression to behavior. Systemic concentrations of androgen do not appear to be causally related to the expression of male-oriented sexual preference in adult rams. However, male-oriented partner preference is associated with low levels of aromatase and estrogen receptors in the brain. In mammals, differentiation of male-typical brain function depends predominantly on exposure to testosterone or its estrogenic metabolites during a critical perinatal developmental period. The medial preoptic area (MPOA), in particular the sexually dimorphic nucleus (SDN), appears to be involved in the expression of male sexual behavior and mate preferences. The presence of a SDN that is larger in males than in females is considered a morphological hallmark of a masculinized brain in several species. We identified a cell group in the caudal MPOA of the sheep that is significantly larger and contains two to four times as many neurons in rams than in ewes. This sexual dimorphism, together with its neuroanatomic position, strongly suggests that this nucleus constitutes the ovine analogue of the SDN, the oSDN. Quantitative morphological comparisons demonstrate that the volume of the oSDN is larger in FORs than in MORs and significantly larger in MORs than ewes in the luteal phase of their cycle. The oSDN in FORs also contains more neurons and is more elongated than in MORs and ewes, however, cell densities in the oSDN do not differ. In situ hybridization histochemistry revealed that neurons of the oSDN express high levels of aromatase mRNA and that expression in FORs is greater than in MORs and ewes. These results demonstrate that the volume of the oSDN corresponds to the type of sexual partner preference exhibited by rams. These data further suggest that the brains of MORs are incompletely masculinized and support the hypothesis that naturally occurring variations in sexual partner preference are determined by underlying biological mechanisms. Supported by NIH grant RR14270.