From gene networks underlying sex determination and gonadal differentiation to the development of neural networks regulating sociosexual behavior

Abstract

Genes are not expressed in isolation any more than social behavior has meaning outside of society. Both are in dynamic flux with the immediate environment that the gene/individual finds itself, which in turn establishes the timing, pattern, and conditions of expression. This means that complex behaviors and their genetic underpinnings should be viewed as a cumulative process, or as the result of experiences up to that point in time and, at the same time, as setting the stage for what will follow. The evidence indicates that as experiences accumulate throughout life, early experiences shape how genes/individuals will respond to later experiences, whereas later experiences modify the effects of these earlier experiences. A method of graphically representing and analyzing change in gene and neural networks is presented. Results from several animal model systems will be described to illustrate these methods. First, we will consider the phenomenon of temperature-dependent sex determination in reptiles. We will illustrate how the experience of a particular temperature during a sensitive period of embryogenesis sculpts not only the patterns of expression of genes involved in sex determination and gonadal differentiation but also the morphological, physiological, neuroendocrine, and behavioral traits of the adult phenotype. The second model system concerns the effects of the sex ratio in the litter in rats, and the genotype ratio in the litter of transgenic mice, on the nature and frequency of maternal care and how this in turn influences the patterns of activation of identified neural circuits subserving the offspring's sociosexual behavior when it is an adult.

Fig. 1

Incubation temperature during the temperature-sensitive period (TSP) determines the sex in the red-eared slider turtle (T. scripta). Gonad development can be divided into three periods: formation of the bipotential gonad, sex determination, and sexual (gonadal) differentiation. The gonad of turtle embryos is bipotential throughout the TSP, during which time incubation at 26 °C (male-producing temperature or MPT) leads to testis development and incubation at 31 °C (female-producing temperature or FPT) leads to ovary development. At MPT, primordial germ cells (PGCs) migrate into the medullary region (∼embryonic stage 18), are enclosed in sex cords (∼stage 19), and enter mitotic arrest. At FPT, PGCs proliferate in the rapidly growing cortex, shown at the top. Compared to the relatively short period of sex determination in mice (10.5 days), in T. scripta, this process may take up to 3.5 weeks in males and 1.5 weeks in females.

Fig. 2

Although the triggers differ, the same genes are involved in the process of gonadal differentiation in species that exhibit genotypic sex determination (mammals and birds) and those that exhibit temperature-dependent sex determination. Illustrated is the abundance of various genes as measured by quantitative PCR during the periods of sex determination (embryonic Stage 17) and gonadal differentiation (Stages 19–23) of the red-eared slider turtle (T. scripta) embryos while incubating at a male-producing temperature (MPT) or a female-producing temperature (FPT) after standardization using the percent maximum method. Abbreviations: winged-helix/forkhead transcription factor (FoxL2); dosage-sensitive sex-reversal adrenal hypoplasia congenital critical region on the X chromosome (Dax1); DM-related transcription factor one (Dmrt1); Müllerian inhibiting substance (Mis); steroidogenic factor one (Sf1); SRY-related HMG box nine (Sox9); wingless-related integration site 4 (Wnt4). The symbols > ♂ and > ♀ indicate that expression levels are higher at the MPT or FPT, respectively.

Fig. 3

Corticosterone levels in male and female leopard geckos (E. macularius) from different incubation temperature after being placed in a brightly lit room, a mild stressor (Tf females vs. Tf males, p<0.05; Tm females vs. Tm males, p<0.01).

Fig. 4

Incubation temperature influences neurotransmitter levels in sexually inexperienced male leopard geckos (E. macularius). Panel A: TH-ir cell counts are significantly different in the ventral tegmental area between castrated, androgen-implanted Tf and Tm male leopard geckos. Panel B: Dopamine levels in the nucleus accumbens of intact Tf and Tm male geckos exposed to a stimulus receptive female across a wire-mesh barrier.

Fig. 5

Incubation temperature modifies the abundance of cytochrome oxidase in limbic nuclei subserving sociosexual behavior in the adult female leopard gecko (E. macularius). Illustrated are means of cytochrome oxidase abundance relative to background in each nucleus of each of four groups. Eggs were incubated at one of two temperatures (all-female or 26 °C and male-biased or 32.5 °C) but the hatchlings were raised at identical temperatures. At one year of age females were allowed to bread for one reproductive season (experienced) or remained inexperienced (inexperienced). Note that in both sexually inexperienced and experienced groups (columns) comparison of the two different incubation temperatures results in a significant percentage increase in most, but not all, nuclei relative to overall brain activity. This in turn results in significant differences in the overall circuit. Within each incubation temperature (rows), however, adult sexual experience modifies the effect of embryonic incubation temperature in only the POA (all-female) or the POA and VMH (male-biased) nuclei. The effect of experience on the overall circuit is not significant at the all-female temperature, and only marginally significant at the male-biased temperature. Values are average cytochrome oxidase abundance in identified cell nuclei relative to background. Data derived from Crews et al. (1997). Brain nuclei: ventromedial hypothalamus (VMH); anterior hypothalamus (AH); nucleus sphericus (NS); preoptic area (POA); periventricular preoptic area (PP); and septum (SEP). Bottom row reveals effect of embryonic temperature; peaks above the plane indicate values are greater at the male-biased incubation temperature. Right column reveals effect of adult experience; peaks above plane indicate values are greater in sexually experienced individuals. Asterisk indicates significant differences in particular nuclei.

Fig. 6

Litter composition influences how the limbic landscape changes after experience in adult male rats. Litters were constituted at birth to be either female-biased (FB, 2 males: 6 females) or male-biased (MB, 6 males: 2 females). As adults, one male from each litter type was given sexual experience consisting of ten 30-min encounters with a hormonally primed stimulus female, whereas one male sibling remained sexually inexperienced. Values are average cytochrome oxidase abundance in identified cell nuclei relative to background. Note that in both inexperienced and experienced groups (columns) comparison of the two litter compositions there is a significant increase in most, but not all, nuclei relative to overall brain activity and, further, that the functional landscape of the overall circuit varies depending upon adult experience. The effect of experience within each litter composition (rows) similarly results in dramatically different functional landscapes. Brain nuclei abbreviations: posterior medial bed nucleus of the stria terminalis (BNSTpm); medial preoptic area (MPOA); ventromedial hypothalamus (VMH); anterior paraventricular nucleus (PVNa); medial amygdala (meAMY); and central amygdala (ceAMY). Bottom row reveals effect of litter composition and valleys below the plane indicate values are greater in the MB litters. Right column reveals effect of adult experience and peaks above plane indicate values are greater in sexually experienced individuals whereas valleys indicate values greater in sexually inexperienced individuals.

Fig. 7

Both sex ratio and genotype ratio influences how the limbic landscape changes after experience in adulthood in female mice. Litters were (1) same sex, same genotype litters (all individuals being female and having a WT genotype—FW/FW); (2) same sex, mixed genotype litters (all females, but with half having a WT genotype and the other half having a knockout genotype—FW/FK); or (3) mixed sex, same genotype litters (all individuals having a WT genotype, but half being female and the other half being male—FW/MW). All mice were produced by mating of females heterozygous for the estrogen receptor α gene, producing the knockout (ERKO) and male and female wild-type mice (MW and FW, respectively). Values are average cytochrome oxidase abundance in identified cell nuclei relative to background. For sake of comparison landscape maps for FW/FW litter are double plotted (top row). Note the Effect of KO and the Effect of male (bottom left and middle column) functional landscapes. This difference is highlighted in the right column, the Differences in Types of Sib, plotted in mirror images to illustrate differences in specific nuclei. Brain nuclei abbreviations: central medial preoptic area (C MPO); rostral medial preoptic area (RMPO); corticomedial amygdala (coAMY); central amygdala (ceAMY); bed nucleus of the stria terminalis (BNST); main bed nucleus of the stria terminalis (BNSTma); and paraventricular nucleus (PVN). Asterisks indicate significant differences in specific nuclei.