ELSEVIER Brain Research 663 (1994) 163-167 BRAIN RESEARCH Short communication The bisexual brain: sex behavior differences and sex differences in parthenogenetic and sexual lizards Matthew S. Rand *, David Crews Institute of Reproductive Biology, Department of Zoology, University of Texas at Austin, Austin, TX 78712, USA Accepted 2 August 1994 Abstract The parthenogenetic lizard Cnemidophorus uniparens alternates in the display of male-like and female-like sexual behavior, providing a unique opportunity for determining the neuronal circuits subserving gender-typical sexual behavior within a single sex. Here we report a 6-fold greater [14C]2-fluoro-2-deoxyglucose uptake in the medial preoptic area of C. uniparens displaying male-like behavior in comparison with C. uniparens displaying female-like receptivity. The ventromedial nucleus of the hypothalamus showed greater 2DG accumulation in receptive C. uniparens than in courting C. uniparens. When a related sexual species (C. inornatus) was compared to the unisexual species, the anterior hypothalamus in C. inornatus males exhibited significantly greater activity. Keywords: 2-Deoxyglucose; Anterior hypothalamus; Medial preoptic area; Reptile; Ventromedial hypothalamus Female-typical and male-typical sex behavior are known to be integrated by specific hypothalamic nuclei in the vertebrate brain [6,18,22]. The ventromedial nucleus of the hypothalamus (VMH) and the medial preoptic area (mPOA) are involved in sexual receptiv- ity in females and both the mPOA and anterior hy- pothalamus (AH) play an important role in the regula- tion of copulatory behavior in males [6,18,19,22]. The majority of this work has been done on mammals or other gonochoristic species (gonadally separate male and female individuals). Many species of whiptail lizards, genus Cnemidophorus, are also gonochoristic. However, approximately one third of the species in this genus are unisexual, consisting of females only and reproducing via parthenogenesis [5]. Cnemidophorus uniparens is parthenogenetic and originally evolved through hybridization between C. inornatus (the maternal ancestor) and another Cnemi- dophorus species [9,15]. Individual C. uniparens exhibit male-like (heterotypical) and female-like (homotypical) pseudosexual behavior identical to the mating behavior of male and female members of the ancestral species, * Corresponding author. Fax: (1) (512) 471-6078 or 471-9651. Bit- net address: zoom666@utxvm or crews@bull.zo.utexas.edu. 0006-8993/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0006-8993(94)00925-2 C. inornatus [7,17]. The aims of this study were to determine: (1) if specific regions in the brains of parthenogenetic and gonochoristic whiptail lizards ex- hibit sexually dimorphic metabolic activity, as mea- sured by the accumulation of [14C]2-fluoro-2-de- oxyglucose (2DG) in the brain during mating behavior, and (2) if these dimorphisms complement previous findings based on implant, lesion and morphometric studies in the brains of the same species. Experimental animals were housed individually in partitioned glass aquaria for at least 2 months prior to testing. All lizards used in this study were gonadec- tomized under cold anesthesia at least two months prior to testing. Lizards that were tested for male-like behavior or were used as male-like stimulus animals received a subcutaneous implant (1-2 months prior to testing) of crystalline testosterone packed into a 10 mm length of Silastic® tubing (i.d. 1.47 mm, o.d. 1.96 mm). Receptivity was induced by a single subcutaneous in- jection of estradiol benzoate (1.0 /zg in 10 /zl steroid suspension vehicle NIH) 24-72 h prior to testing. The stimulus animals were introduced into the experimen- tal animal's home tank. Each test animal received an intraperitoneal injec- tion of 0.8 ~Ci []4C]2-fluoro-2-deoxyglucose suspended in 10 Izl saline (American Radiolabeled Chemicals,