Sexual differentiation of the zebra finch song system parallels genetic, not gonadal, sex

Emergence of sex-specific transcriptomes in a sexually dimorphic brain nucleus

We present the transcriptomic changes underlying the development of an extreme neuroanatomical sex difference. The robust nucleus of the arcopallium (RA) is a key component of the songbird vocal motor system. In zebra finch, the RA is initially monomorphic and then atrophies in females but grows up to 7-fold larger in males. Mirroring this divergence, we show here that sex-differential gene expression in the RA expands from hundreds of predominantly sex chromosome Z genes in early development to thousands of predominantly autosomal genes by the time sexual dimorphism asymptotes. Male-specific developmental processes include cell and axonal growth, synapse assembly and activity, and energy metabolism; female-specific processes include cell polarity and differentiation, transcriptional repression, and steroid hormone and immune signaling. Transcription factor binding site analyses support female-biased activation of pro-apoptotic regulatory networks. The extensive and sex-specific transcriptomic reorganization of RA provides insights into potential drivers of sexually dimorphic neurodevelopment.

Friedrich et al. demonstrate extensive transcriptomic sex differences underlying the sexually dimorphic development of vocal nucleus RA in the songbird brain. They find sex-specific gene regulation linked to distinct biological processes, developmental shifts in the relative signal from sex chromosome to autosomal genes, and evidence of female-biased pro-apoptotic regulatory networks.

Sex-limited chromosomes and non-reproductive traits

Sex chromosomes are typically viewed as having originated from a pair of autosomes, and differentiated as the sex-limited chromosome (e.g. Y) has degenerated by losing most genes through cessation of recombination. While often thought that degenerated sex-limited chromosomes primarily affect traits involved in sex determination and sex cell production, accumulating evidence suggests they also influence traits not sex-limited or directly involved in reproduction. Here, we provide an overview of the effects of sex-limited chromosomes on non-reproductive traits in XY, ZW or UV sex determination systems, and discuss evolutionary processes maintaining variation at sex-limited chromosomes and molecular mechanisms affecting non-reproductive traits.

Inhibition of TrkB limits development of the zebra finch song system

Large sexual dimorphisms exist in the zebra finch song system. Masculinization may be mediated by both estradiol and expression of one or more Z-genes (males: ZZ; females: ZW). Roles of the Z-gene tyrosine kinase B (TrkB) in HVC in masculinizing both HVC and one of its targets the robust nucleus of the arcopallium (RA), were tested using siRNA administration in juvenile males at two ages (post-hatching days 15-17 or 25-27). Birds were euthanized 10 days later. Potential interactions or additive effects with estradiol were evaluated by treating males with the estrogen synthesis inhibitor fadrozole. Females treated with estradiol were also exposed to the siRNA at the later age. Local inhibition of TrkB in males of both ages reduced the volume of HVC, an effect due to a change in cell number and not cell size. In the older males, in which the treatment spanned the period when the projection from HVC to RA grows, TrkB inhibition reduced the volume of RA and the relative number of cells within it. TrkB siRNA in HVC decreased the volume of and soma size in the RA of females, and the projection from HVC to RA in both sexes. Estradiol in females masculinized various aspects of the song system, and its effect in masculinizing the volume of RA was decreased by TrkB inhibition. However, effects of fadrozole in males were limited. The data indicate that TrkB is involved in masculinizing the song system, but for most measures it probably does not work in concert with E2.

Genetic regulation of sex differences in songbirds and lizards

Sex differences in the morphology of neural and peripheral structures related to reproduction often parallel the frequency of particular behaviours displayed by males and females. In a variety of model organisms, these sex differences are organized in development by gonadal steroids, which also act in adulthood to modulate behavioural expression and in some cases to generate parallel anatomical changes on a seasonal basis. Data collected from diverse species, however, suggest that changes in hormone availability are not sufficient to explain sex and seasonal differences in structure and function. This paper pulls together some of this literature from songbirds and lizards and considers the information in the broader context of taking a comparative approach to investigating genetic mechanisms associated with behavioural neuroendocrinology.

Masculinisation of the zebra finch song system: roles of oestradiol and the Z-chromosome gene tubulin-specific chaperone protein A

Robust sex differences in brain and behaviour exist in zebra finches. Only males sing, and forebrain song control regions are more developed in males. The factors driving these differences are not clear, although numerous experiments have shown that oestradiol (E2 ) administered to female hatchlings partially masculinises brain and behaviour. Recent studies suggest that an increased expression of Z-chromosome genes in males (ZZ; females: ZW) might also play a role. The Z-gene tubulin-specific chaperone A (TBCA) exhibits increased expression in the lateral magnocellular nucleus of the anterior nidopallium (LMAN) of juvenile males compared to females; TBCA+ cells project to the robust nucleus of the arcopallium (RA). In the present study, we investigated the role of TBCA and tested hypotheses with respect to the interactive or additive effects of E2 and TBCA. We first examined whether E2 in hatchling zebra finches modulates TBCA expression in the LMAN. It affected neither the mRNA, nor protein in either sex. We then unilaterally delivered TBCA small interfering (si)RNA to the LMAN of developing females treated with E2 or vehicle and males treated with the aromatase inhibitor, fadrozole, or its control. In both sexes, decreasing TBCA in LMAN reduced RA cell number, cell size and volume. It also decreased LMAN volume in females. Fadrozole in males increased LMAN volume and RA cell size. TBCA siRNA delivered to the LMAN also decreased the projection from this brain region to the RA, as indicated by anterograde tract tracing. The results suggest that TBCA is involved in masculinising the song system. However, because no interactions between the siRNA and hormone manipulations were detected, TBCA does not appear to modulate effects of E2 in the zebra finch song circuit.

Hormones and the neuromuscular control of courtship in the golden-collared manakin (Manacus vitellinus)

Many animals engage in spectacular courtship displays, likely recruiting specialized neural, hormonal and muscular systems to facilitate these performances. Male golden-collared manakins (Manacus vitellinus) of Panamanian rainforests perform physically elaborate courtship displays that include novel forms of visual and acoustic signaling. We study the behavioral neuroendocrinology of this male's courtship, combining field behavioral observations with anatomical, biochemical and molecular laboratory-based studies. Seasonally, male courtship is activated by testosterone with little correspondence between testosterone levels and display intensity. Females prefer males whose displays are exceptionally frequent, fast and accurate. The activation of androgen receptors (AR) is crucial for optimal display performance, with AR expressed at elevated levels in several neuromuscular tissues. Apparently, courtship enlists an elaborate androgen-dependent network that includes spinal motoneurons, skeletal muscles and somatosensory systems. This work highlights the value of studying non-traditional species to illuminate physiological adaptations and, hopefully, stimulates future research on other species with complex behaviors.

17β-estradiol regulates the sexually dimorphic expression of BDNF and TrkB proteins in the song system of juvenile zebra finches

Mature brain derived neurotrophic factor (BDNF) plays critical roles in development of brain structure and function, including neurogenesis, axon growth, cell survival and processes associated with learning. Expression of this peptide is regulated by estradiol (E2). The zebra finch song system is sexually dimorphic - only males sing and the brain regions controlling song are larger and have more cells in males compared to females. Masculinization of this system is partially mediated by E2, and earlier work suggests that BDNF with its high affinity receptor TrkB may also influence this development. The present study evaluated expression of multiple forms of both BDNF and TrkB in the developing song system in juvenile males and females treated with E2 or a vehicle control. Using immunohistochemistry and Western blot analysis, BDNF was detected across the song nuclei of 25-day-old birds. Westerns allowed the pro- and mature forms of BDNF to be individually identified, and proBDNF to be quantified. Several statistically significant effects of sex existed in both the estimated total number of BDNF+ cells and relative concentration of proBDNF, varying across the regions and methodologies. E2 modulated BDNF expression, although the specific nature of the regulation depended on brain region, sex and the technique used. Similarly, TrkB (both truncated and full-length isoforms) was detected by Western blot in the song system of juveniles of both sexes, and expression was regulated by E2. In the context of earlier research on these molecules in the developing song system, this work provides a critical step in describing specific forms of BDNF and TrkB, and how they can be mediated by sex and E2. As individual isoforms of each can have opposing effects on mechanisms, such as cell survival, it will now be important to investigate in depth their specific functions in song system maturation.

Hormonal organization and activation: evolutionary implications and questions

Comparative endocrinology is a fascinating field of science in part because it addresses both ultimate and proximate causation. Research on sexual dimorphism and sexual differentiation has excellent potential for this kind of integration. Vertebrate comparative endocrinologists have made many important discoveries about the role of genes and sex steroid hormones in the organization and activation of sexually differentiated behavior, brain function, anatomy and physiology. In addition to taxonomically general principles and conserved features, there is also striking diversity in sexual differentiation processes. Much of the evolutionary basis of this diversity (its phylogenetic history and adaptive functions) is not well understood. A set of questions is raised to illustrate this point, with an emphasis on mechanisms of sexual dimorphism in body size and ornamentation, sexual differentiation of avian behavior, particularly in Japanese quail and zebra finches, and the puzzle of the phylogenetic distribution of vertebrate sex determining mechanisms. Applying a comparative approach grounded in established phylogenies and concepts from evolutionary developmental biology such as developmental modules holds promise for generating and testing new hypotheses and eventually answering some of these questions.

A lumpers versus splitters approach to sexual differentiation of the brain

Over 50 years of rigorous empirical attention to the study of sexual differentiation of the brain has produced sufficient data to reveal fundamental guiding principles, but has also required the generation of new hypotheses to explain non-conforming observations. An early emphasis on the powerful impact and essential role of gonadal steroids is now complemented by an appreciation for genetic contributions to sex differences in the brain. The organizing effects of early steroid hormones on reproductively relevant brain regions and endpoints are largely dependent upon neuronal aromatization of androgens to estrogens. The effect of estradiol is mediated via estrogen receptors (ER). The presence or absence of ER can restrict hormone action to select cells and either prevent or invoke cell death. Alternatively, ER activation can initiate signaling cascades that induce cell-to-cell communication and thereby transduce organizational steroid effects to large numbers of cells. However, the specific details by which cell death and cell-to-cell communication are achieved appear to be locally, even cellularly, unique and specific to that particular subpopulation. As the field moves forward the increasingly specific and detailed elucidation of mechanism challenges us to generate new guiding principles in order to gain a holistic understanding of how the brain develops in males and females.

Sex- and age-related differences in ribosomal proteins L17 and L37, as well as androgen receptor protein, in the song control system of zebra finches

The zebra finch song system is sexually dimorphic--only males sing, and the morphology of forebrain regions controlling the learning and production of this song is greatly enhanced in males compared to females. Masculinization appears to involve effects of steroid hormones as well as other factors, perhaps including the expression of sex chromosome genes (males: ZZ, females: ZW). The present study investigated three proteins--two encoded by Z-linked genes, ribosomal proteins L17 and L37 (RPL17 and RPL37), including their co-localization with androgen receptor (AR), from post-hatching day 25 to adulthood. Extensive co-expression of AR with the ribosomal proteins was detected in the three song nuclei investigated (HVC, robust nucleus of the arcopallium (RA), and Area X) across these ages. In general, more cells expressed each of these proteins in males compared to females, and the sex differences increased as animals matured. Specific patterns differed across regions and between RPL17 and RPL37, which suggest potential roles of one or both of these proteins in the incorporation and/or differentiation of song system cells.

Co-localization of sorting nexin 2 and androgen receptor in the song system of juvenile zebra finches

Mechanisms regulating sexual differentiation of the zebra finch song system appear to include both genetic and hormonal factors. Sorting Nexin 2 (SNX2), which is involved in trafficking proteins between cellular membranes, and androgen receptor (AR) mRNA are both increased in song control nuclei of juvenile males compared to females. Here, in situ hybridization for SNX2 and immunohistochemistry for AR were used to evaluate these sexual dimorphisms in more detail. Estimates of the total number of HVC cells expressing SNX2 and AR, individually as well as together, were greater in 25-day-old males compared to females. The densities of these types of cells were generally also increased in males compared to females in HVC and Area X (or the equivalent portion of the medial striatum in females). On average, more than half of the AR+ cells co-expressed SNX2 in both brain regions. The potential, therefore, exists for both AR and SNX2 to be involved in masculinization of these two brain regions. One possibility is that they, either separately or in conjunction, enhance the action of trophic factors within the brain.

Neural expression and post-transcriptional dosage compensation of the steroid metabolic enzyme 17beta-HSD type 4

Background

Steroids affect many tissues, including the brain. In the zebra finch, the estrogenic steroid estradiol (E₂) is especially effective at promoting growth of the neural circuit specialized for song. In this species, only the males sing and they have a much larger and more interconnected song circuit than females. Thus, it was surprising that the gene for 17β-hydroxysteroid dehydrogenase type 4 (HSD17B4), an enzyme that converts E₂ to a less potent estrogen, had been mapped to the Z sex chromosome. As a consequence, it was likely that HSD17B4 was differentially expressed in males (ZZ) and females (ZW) because dosage compensation of Z chromosome genes is incomplete in birds. If a higher abundance of HSD17B4 mRNA in males than females was translated into functional enzyme in the brain, then contrary to expectation, males could produce less E₂ in their brains than females.

Results

Here, we used molecular and biochemical techniques to confirm the HSD17B4 Z chromosome location in the zebra finch and to determine that HSD17B4 mRNA and activity were detectable in the early developing and adult brain. As expected, HSD17B4 mRNA expression levels were higher in males compared to females. This provides further evidence of the incomplete Z chromosome inactivation mechanisms in birds. We detected HSD17B4 mRNA in regions that suggested a role for this enzyme in the early organization and adult function of song nuclei. We did not, however, detect significant sex differences in HSD17B4 activity levels in the adult brain.

Conclusions

Our results demonstrate that the HSD17B4 gene is expressed and active in the zebra finch brain as an E₂ metabolizing enzyme, but that dosage compensation of this Z-linked gene may occur via post-transcriptional mechanisms.

Neuropeptidomic analysis of the embryonic Japanese quail diencephalon

Background

Endogenous peptides such as neuropeptides are involved in numerous biological processes in the fully developed brain but very little is known about their role in brain development. Japanese quail is a commonly used bird model for studying sexual dimorphic brain development, especially adult male copulatory behavior in relation to manipulations of the embryonic endocrine system. This study uses a label-free liquid chromatography mass spectrometry approach to analyze the influence of age (embryonic days 12 vs 17), sex and embryonic day 3 ethinylestradiol exposure on the expression of multiple endogenous peptides in the developing diencephalon.

Results

We identified a total of 65 peptides whereof 38 were sufficiently present in all groups for statistical analysis. Age was the most defining variable in the data and sex had the least impact. Most identified peptides were more highly expressed in embryonic day 17. The top candidates for EE₂ exposure and sex effects were neuropeptide K (downregulated by EE₂ in males and females), gastrin-releasing peptide (more highly expressed in control and EE₂ exposed males) and gonadotropin-inhibiting hormone related protein 2 (more highly expressed in control males and displaying interaction effects between age and sex). We also report a new potential secretogranin-2 derived neuropeptide and previously unknown phosphorylations in the C-terminal flanking protachykinin 1 neuropeptide.

Conclusions

This study is the first larger study on endogenous peptides in the developing brain and implies a previously unknown role for a number of neuropeptides in middle to late avian embryogenesis. It demonstrates the power of label-free liquid chromatography mass spectrometry to analyze the expression of multiple endogenous peptides and the potential to detect new putative peptide candidates in a developmental model.

Effects of estradiol on incorporation of new cells in the developing zebra finch song system: potential relationship to expression of ribosomal proteins L17 and L37

Mechanisms regulating masculinization of the zebra finch song system are unclear; both estradiol and sex-specific genes may be important. This study was designed to investigate relationships between estrogen and ribosomal proteins (RPL17 and RPL37; sex-linked genes) that exhibit greater expression in song control nuclei in juvenile males than females. Four studies on zebra finches were conducted using bromodeoxyuridine (BrdU) injections on posthatching days 6-10 with immunohistochemistry for the ribosomal proteins and the neuronal marker HuC/D at day 25. Volumes of brain regions were also assessed in Nissl-stained tissue. Most BrdU+ cells expressed RPL17 and RPL37. The density and percentage of cells co-expressing BrdU and HuC/D was greatest in Area X. The density of BrdU+ cells in Area X (or its equivalent) and the percentage of these cells that were neurons were greater in males than females. In RA and HVC, total BrdU+ cells were increased in males. A variety of effects of estradiol were also detected, including inducing an Area X in females with a masculine total number of BrdU+ cells, and increasing the volume and percentage of new neurons in the HVC of females. The same manipulation in males decreased the density of BrdU+ cells in Area X, total number of BrdU+ cells in RA, and density of new neurons in HVC and RA. These data are consistent with the idea that RPL17, RPL37, and estradiol might all influence sexual differentiation, perhaps with the hormone and proteins interacting, such that an appropriate balance is required for normal development.

Brain aromatase: new lessons from non-mammalian model systems

This review highlights recent studies of the anatomical and functional implications of brain aromatase (estrogen synthase) expression in two vertebrate lineages, teleost fishes and songbirds, that show remarkably high levels of adult brain aromatase activity, protein and gene expression compared to other vertebrate groups. Teleosts and birds have proven to be important neuroethological models for investigating how local estrogen synthesis leads to changes in neural phenotypes that translate into behavior. Region-specific patterns of aromatase expression, and thus estrogen synthesis, include the vocal and auditory circuits that figure prominently into the life history adaptations of vocalizing teleosts and songbirds. Thus, by targeting, for example, vocal motor circuits without inappropriate steroid exposure to other steroid-dependent circuits, such as those involved in either copulatory or spawning behaviors, the neuroendocrine system can achieve temporal and spatial specificity in its modulation of neural circuits that lead to the performance of any one behavior.

Testosterone in Females: Mediator of Adaptive Traits, Constraint on Sexual Dimorphism, or Both?

When selection on males and females differs, the sexes may diverge in phenotype. Hormones serve as a proximate regulator of sex differences by mediating sex-biased trait expression. To integrate these perspectives, we consider how suites of traits mediated by the same hormone in both sexes might respond to selection. In male birds, plasma testosterone (T) varies seasonally and among species according to mating system. When elevated experimentally, it is known to enhance some components of fitness and to decrease others. We report that female T also varies seasonally and co-varies with male T. Female T is higher in relation to male T in sexually monomorphic species and is higher absolutely in females of species with socially monogamous mating systems, which suggests adaptation. We also consider the effect of experimentally elevated T on females and whether traits are sensitive to altered T. We hypothesize that sensitive traits could become subject to selection after a natural change in T and that traits with opposing fitness consequences in males and females could constrain dimorphism. Results from birds, including the dark-eyed junco (Junco hyemalis), reveal many sensitive traits, some of which appear costly and may help to account for observed levels of sexual dimorphism.

Enhanced gene expression in the forebrain of hatchling and juvenile male zebra finches

The molecular mechanisms regulating sexual differentiation of the brain are largely unknown, although progress is being made, particularly in some mammalian systems. To uncover more of the key factors, a screen was conducted for genes involved in sexually dimorphic development of the neural song system in zebra finches. cDNA microarrays were initially used to compare gene expression in the telencephalons of hatchling and juvenile males and females. Then, real-time quantitative polymerase chain reaction (PCR) was employed to confirm sex differences, and the brain regions expressing the cDNAs of interest were localized using in situ hybridization. Several genes, including those likely to encode two ribosomal proteins (RPL17 and RPL37), SCAMP1, ZNF216, and a COBW-domain containing protein, showed enhanced expression in the telencephalon of males compared to females. In several cases, expression in the song control nuclei specifically was detected only in males. Interestingly, the sequences of some of these cDNAs shared substantial homology with regions of the chicken Z chromosome (male birds are ZZ, females ZW). Thus, we have identified genes likely to be involved in masculinization of the structure and/or function of the song circuit, some of which could be initial triggers for the sexual differentiation process.

Sexual differentiation of the zebra finch song system

The song system of zebra finches (Taeniopygia gutatta) is highly sexually dimorphic. Only males sing, and the brain regions and muscles controlling song are much larger in males than in females. Development of the song system is highly sensitive to steroid hormones. However, unlike similar sexually dimorphic systems in other animal models, masculinization of song system structure and function is most likely not induced by testosterone secreted from the testes. Instead, sex-specific development of the neural song system appears to be regulated by factors intrinsic to the brain, probably by the expression of sex chromosome gene(s) that influence the levels of estradiol synthesized in the brain and/or the responses of brain tissue to estradiol. However, the existing data are complex and in some cases contradictory. More work is required to identify the critical genes and their relationships with steroid hormones.

Sexually dimorphic neurocalcin expression in the developing zebra finch telencephalon

Differential display RT-PCR was used on RNA isolated from the zebra finch telencephalon to identify gene products potentially involved in its development, including the sexually dimorphic nuclei responsible for song learning and production. A cDNA identified only in juvenile females was cloned and sequenced. It shares homology with neurocalcin, a calcium binding protein. Northern blots indicated three neurocalcin species. A 10.6 kb transcript was present in males and most females throughout development and in adulthood. Smaller 6.2 and 3.3 kb species were detected almost exclusively in females and primarily between posthatching days 18-25. In situ hybridization, using a probe that identified all three mRNA species, indicated a broad distribution in the telencephalon of both sexes, with particularly high levels in the song nucleus RA. Across regions examined, neurocalcin expression was enhanced in females compared to males, probably reflecting the presence of the two smaller transcripts. However, within RA, neurocalcin expression was statistically equivalent between the sexes. These data indicate that calcium signaling via neurocalcin may be involved in telencephalic development, but suggest that sexually dimorphic expression of this gene exists on a level too general to specifically regulate masculine or feminine development of song control regions. Neurocalcin might: broadly influence functional differentiation, including areas that are not morphologically distinct between the sexes; be a benign consequence of general dimorphisms, such as those due to sex chromosomes; or involve a compensatory mechanism, which allows function of the juvenile female telencephalon to equal that of males, despite fundamental physiological differences.

Antiandrogen blocks estrogen-induced masculinization of the song system in female zebra finches

Song behavior and the neural song system that serves it are sexually dimorphic in zebra finches. In this species, males sing and females normally do not. The sex differences in the song system include sex differences in the proportion of neurons that express androgen receptors, which is higher in specific brain regions of males. Estradiol (E2) administered in early development profoundly masculinizes the song system of females, including the proportion of neurons expressing androgen receptors. We examined whether or not the expression of these androgen receptors was causally related to the E2-induced masculinization of this system by co-administering Flutamide, which blocks androgen action at the receptor, along with E2 at hatching. E2 alone had its usual masculinizing effect on the female song system, measured in adulthood: increasing the size of song nuclei, the size of neurons in HVC, RA, and 1MAN, and the number of neurons in HVC. E2's masculinizing action, however, was significantly diminished on all measures by co-administering Flutamide. Indeed, females receiving both E2 and Flutamide were never significantly more masculine than controls on any measure. Flutamide alone had no effect. Our results strongly suggest that the activation of androgen receptors is necessary for the E2-induced masculinization of the song system in females.

Post-hatching hormonal modulation of a sexually dimorphic neuromuscular system controlling song in zebra finches

Sexual dimorphisms are present throughout the zebra finch song system, from forebrain centers to the tracheosyringeal portion of the hypoglossal nucleus (nXIIts) to the muscles of the syrinx (vocal organ). In females, gonadal steroids administered during development can partially masculinize the telencephalic areas, and in adulthood can increase the size of syrinx muscles. In the present study, two experiments were designed to investigate the role of early androgen and estrogen in the development of nXIIts and the ventralis and dorsalis muscles of the syrinx. In experiment one, males and females were treated with testosterone, estradiol, dihydrotestosterone, the anti-androgen flutamide, or a vehicle control for 21 days after hatching. At day 60, nXIIts volume, motoneuron soma size and number were assessed, as well as syrinx weight and the size of ventralis and dorsalis fibers. In experiment two, animals were administered either the estrogen synthesis inhibitor, fadrozole, or vehicle, and the syrinx measurements were taken at day 60. Male-biased sex differences were detected on all measures in both experiments, and several right-side biases were detected. In females, dihydrotestosterone masculinized soma size in nXIIts and testosterone slightly increased syrinx weight. E2 feminized the syrinx of males. However, flutamide did not prevent masculine development of either structure, and fadrozole did not inhibit feminine syrinx development. These results are consistent with the idea that, as in the forebrain, steroid hormones can stimulate aspects of sexual differentiation, but they may not be direct triggers for the process.

Posthatch oral estrogen exposure impairs adult reproductive performance of zebra finch in a sex-specific manner

We determined whether short-term, posthatch oral exposure to estradiol benzoate (EB) or the industrial surfactant octylphenol (OP) could impair the reproductive performance of zebra finches. If so, naturally occurring phytoestrogens and xenoestrogens might influence reproduction in wild populations. Chicks were given oral administration of 10 or 100 nmol EB per gram of body mass (earlier work showed the latter to be the minimum oral dose required to maximally masculinize female song nuclei) or an equimolar amount of OP daily from 5 through 11 days of age. Canola oil was used as a vehicle and control. Reproductive testing was done either in individual pair cages or in communal cages that permitted self-selection of mates, N = 10 pairs per group. Pairs consisted of EB-treated males and females, EB-treated males paired with canola-treated females, vice versa, and canola-treated males and females. Posthatch EB treatment produced sex-specific impairments in reproduction that, in some instances, were additive when both sexes were treated. Egg production was reduced and egg breakage was increased in 100 nmol/g EB-treated male and female pairs. The incidence of missing eggs was increased in 10 nmol/g EB-treated male and female pairs. Candled fertility was reduced in both groups containing 100 nmol/g EB-treated males. The number of hatched chicks was severely reduced in all EB-treated groups. No adverse effects of OP treatment were detected. These significant treatment effects (all P < 0.05) show that posthatch EB treatment profoundly disrupts the reproductive performance of zebra finches, suggesting that exposure to estrogens in the wild could impair the reproductive performance of wild populations.

Zebra finch sexual differentiation: the aromatization hypothesis revisited

Zebra finches have emerged as an outstanding model system for the investigation of the mechanisms regulating brain and behavior. Their song system has proven especially useful, as the function of discrete anatomical regions have been identified, and striking parallels exist between the morphology of these regions and the level of their function in males and females. That is, the structures are substantially more developed in males, who sing, compared to females, who do not. These parallels extend from higher (telencephalic) centers to the brainstem motor nucleus that innervates the muscles of the vocal organ. Other dimorphic aspects of reproduction in the zebra finch, such as copulatory behaviors and sexual partner preference, however, are not associated with known sex differences in anatomy. In many species, sex differences in neural and peripheral structures and behavior are regulated by secretions from the gonads, which of course are sexually dimorphic themselves. In birds, sex differences at all of these levels (gonad, brain, and behavior) can be mediated by steroid hormones. However, it is not entirely clear that gonadal secretions normally participate at all of the levels. This paper reviews the evidence relating to the role of gonadal steroids in the sexual differentiation of reproductive behaviors and the central and peripheral structures known to regulate them in zebra finches, with a focus on estradiol, which has been most extensively studied in the masculinization of song system morphology and function.

Masculinized sexual partner preference in female zebra finches with sex-reversed gonads

Previous research in the zebra finch, a socially monogamous pair-bonding species, suggests that the preference for opposite-sex partners may arise in part through the organizing actions of sex steroids. To further investigate this process, zebra finch eggs were injected with 20 microg fadrozole, a potent estrogen synthesis inhibitor, or with the saline vehicle on embryonic day 5. As adults they were given two-choice sexual partner preference tests followed by group aviary tests. Fadrozole females had masculinized beak color and had testes or ovotestes instead of ovaries. Males were not affected by fadrozole; they did not differ from controls on any measure. In contrast, sexual partner preference was substantially masculinized in fadrozole females in the group aviary tests. Untreated males given a choice between fadrozole and untreated females preferred the untreated females, but this was equally the case when they were given a choice between saline-treated and untreated females. These results suggest that males do not specifically avoid females with testes and that male avoidance is unlikely to explain why fadrozole-treated females pair with other females. The present data add to the evidence that actions of gonadal steroids during development contribute to adult sex differences in partner preference in this pair-bonding species. Furthermore, because fadrozole-treated females do not produce audible song, the mechanisms regulating partner preference and song system development are dissociated.

Spatial ability of XY sex-reversed female mice

Perinatal gonadal hormones significantly affect subsequent sex differences in reproductive and non-reproductive behaviors in rodents. However, the influence of the sex chromosomes on these behaviors has been largely ignored. To assess the influence of the non-pseudoautosomal region of the Y chromosome, C57BL/JEi male and female mice and mice from the C57BL/6JEi-Y(POS) consomic strain were given behavioral tests known to distinguish males from females. The C57BL/6JEi-Y(POS) strain contains sex-reversed XY-females which, when compared to their XX-female siblings, allow assessment of the influence of the Y chromosome in a female phenotype. XX-females and XY-females did not differ on open-field activity, the Lashley maze, or active avoidance learning, but XY-females were significantly better than XX-females on the Morris hidden platform spatial maze. These findings suggest that males may have both a genetic and a hormonal mechanism to ensure visuospatial superiority.

TrkB-like immunoreactivity in the song system of developing zebra finches

The neural song system in zebra finches develops for approximately the first 2 months after hatching. During that time, male-biased sexual dimorphisms emerge in the volume of song control nuclei as well as in the number and size of neurons within them. Brain derived neurotrophic factor (BDNF) has been documented in song control nuclei at various stages of development. Its high affinity receptor (tyrosine kinase B; trkB) is also in the song system, at least at around I month of age. The present study was designed to more completely describe the timing and potential location of BDNF action by investigating trkB expression during sexual differentiation of the song control nuclei. The pattern of immunoreactivity to a trkB antibody was examined in male and female zebra finches at post-hatching days 3-60. Labeling in somata and neuropil appeared to define the telencephalic components of the motor pathway (high vocal center and robust nucleus of the archistriatum) for song production in males from days 30 to 60, and in females on days 45 and 60 (high vocal center). These results are consistent with the hypothesis that the receptor, and its ligand BDNF, play a role in processes related to song learning in both sexes, including perhaps the motor component exhibited by developing males.