Species Differences in the Regulation of Tyrosine Hydroxylase in Cnemidophorus Whiptail Lizards Sarah C. Woolley,* David Crews Section of Integrative Biology, Patterson 141, 2400 Speedway, University of Texas at Austin, Austin, Texas 78712 Received 30 July 2003; accepted 5 January 2004 ABSTRACT: Evolution of behavioral phenotype involves changes in the underlying neural substrates. Cnemidophorus whiptail lizards enable the study of be- havioral and neural evolution because ancestral species involved in producing unisexual, hybrid species still ex- ist. Catecholaminergic systems modulate the expression of social behaviors in a number of vertebrates, including whiptails, and therefore we investigated how changes in catecholamine production correlated with evolutionary changes in behavioral phenotype by measuring the size and number of catecholamine producing (tyrosine hy- droxylase-immunoreactive, or TH-ir) cells across the reproductive cycle in females from two related whiptail species. Cnemidophorus uniparens is a triploid, parthe- nogenetic species that arose from hybridization events involving the diploid, sexual species C. inornatus. Prior to ovulation, females from both species display female- like receptive behaviors. However, after ovulation, only parthenogenetic individuals display malelike mounting behavior. In all nuclei measured, we found larger TH-ir cells in the parthenogen, a difference consistent with species differences in ploidy. In contrast, species differ- ences in the number of TH-ir cells were nucleus specific. In the preoptic area and anterior hypothalamus, parthe- nogens had fewer TH-ir cells than females of the sexual species. Reproductive state only affected TH-ir cell num- ber in the substantia nigra pars compacta (SNpc), and C. uniparens individuals had more TH-ir cells after ovula- tion than when previtellogenic. Thus, species differences over the reproductive cycle in the SNpc are correlated with species differences in behavior, and it appears that the process of speciation may have produced a novel neural and behavioral phenotype in the parthenogen. © 2004 Wiley Periodicals, Inc. J Neurobiol 60: 360 –368, 2004 Keywords: dopamine; lizard; ploidy; reproduction; ty- rosine hydroxylase; parthenogenetic INTRODUCTION Neural and behavioral evolution are often difficult to study because ancestral species no longer exist. Cne- midophorus lizards enable the study of evolutionary processes because new species arise through multiple hybridizations. Cnemidophorus uniparens, for exam- ple, is a triploid parthenogen that arose through two hybridization events, both involving the diploid, sex- ual species C. inornatus (Wright, 1993). Though fe- males of both species show identical patterns of ste- roid hormone secretion across the reproductive cycle, there are considerable behavioral differences between the two. Females of both species display female-like receptive behaviors during vitellogenesis when estro- gen levels are rising. However, following ovulation when there is a surge of progesterone, females of the ancestral, sexual species become sexually unreceptive while the parthenogens display male-like copulatory behaviors (Moore and Crews, 1986; Moore et al., 1985a,b; reviewed in Crews and Sakata, 2000). By investigating the neural correlates of this behavioral * Present address: WM Keck Center for Integrative Neuro- science, 513 Parnassus, Box 0444, University of California, San Francisco, San Francisco, CA 94143-0444. Correspondence to: S. C. Woolley (scwoolley@phy.ucsf.edu). Contract grant sponsor: NIMH; contract grant numbers: T32 18837 (S.C.W.) and 41770 (D.C.). © 2004 Wiley Periodicals, Inc. Published online 15 June 2004 in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/neu.20044 360