Sexual Differentiation of the Zebra Finch Song System Parallels Genetic, Not Gonadal, Sex

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1 Hormones and Behavior 36, (1999) Article ID hbeh , available online at on Sexual Differentiation of the Zebra Finch Song System Parallels Genetic, Not Gonadal, Sex Juli Wade, David A. Swender, and Teresa L. McElhinny Department of Psychology, Department of Zoology, and Program in Neuroscience, Michigan State University, East Lansing, Michigan Received February 26, 1999; revised April 26, 1999; accepted April 28, 1999 Mechanisms regulating sexual differentiation of the zebra finch song system present an intriguing puzzle. Masculine development of brain regions and behavior can be induced in genetic females by posthatching estradiol treatment. That result is consistent with the hypothesis that estradiol, converted within the brain from testicular androgen via the aromatase enzyme, masculinizes neural structure and function. In contrast, treatment during specific stages of development with the aromatase inhibitor Fadrozole has not prevented masculine development, and the presence of testicular tissue in genetic females did not induce masculine organization of neuroanatomy or singing behavior. Fadrozole treatments in those previous studies were limited, however, and most genetic females had both ovarian and testicular tissue. The present experiments were designed to provide increased aromatase inhibition and to reliably produce genetic females with only testicular tissue. Eggs received a single injection at a later age or with higher doses of Fadrozole than had been used previously. Some embryos were exposed to Fadrozole more frequently by either injecting eggs on 2 days of development or dipping them for days in Fadrozole. Finally, in some individuals from Fadrozole-treated eggs, the left gonad was removed, leaving each genetic male and female with a single right testis. None of these treatments significantly affected development of the song system compared to appropriate control groups. These results suggest that sexual differentiation of the zebra finch song system is not regulated by embryonic aromatase activity or by gonadal secretions and instead involves events that need not be mediated by steroid hormones Academic Press Key Words: aromatase; estradiol; birdsong; sexual dimorphism. The zebra finch song system has been studied for many years, and one area that has received considerable attention is the mechanisms through which sex differences in brain and behavior develop. Normally, only males sing, and the telencephalic regions that control singing behavior are not only greater in overall volume, but contain larger and more numerous neurons in males than in females (Arnold, 1992; Gurney, 1981; Konishi and Akutagawa, 1985; Nottebohm and Arnold, 1976). In many cases, the studies have focused on gonadal steroids, which induce masculine development of brain and behavior in several vertebrate model systems (reviewed in Cooke, Hegstrom, Villeneuve, and Breedlove, 1998; Feder, 1981; Sachs and Meisel, 1988; Yahr, 1988). For example, in some mammalian species, testosterone and its metabolite estradiol masculinize both the display of sexual behavior and the morphology of regions within the preoptic area (Baum, Carroll, Cherry, and Tobet, 1990; Döhler, Coquelin, Davis, Hines, Shryne, and Gorski, 1984; Tarttelin and Gorski, 1988). Similarly, estradiol treatment in young female zebra finches (in the first few weeks after hatching) can masculinize both the anatomy of the neural song system and adult behavior patterns such that females will sing (Gurney, 1981, 1982; Konishi and Akutagawa, 1988; Pohl-Apel and Sossinka, 1984; Simpson and Vicario, 1991a, b). Thus, as in the mammalian systems, it was hypothesized that in zebra finches testicular androgens are metabolized to estradiol in the brain through the action of the aromatase enzyme. The estradiol then would act directly to induce masculine development of both neural structure and function. However, unlike the research on mammalian systems, a series of studies in the past few years has called into question the idea that testicular secretions normally regulate the sexual differentiation process in zebra finches. In particular, when zebra finch eggs are injected with 20 g of the aromatase inhibitor Fadrozole on days 3 (Gong and Arnold, 1996), 5 (Springer and X/99 $30.00 Copyright 1999 by Academic Press All rights of reproduction in any form reserved. 141

2 142 Wade, Swender, and McElhinny Wade, 1997; Wade and Arnold, 1996), or 8 (Wade, Springer, Wingfield, and Arnold, 1996), a substantial amount of functional testicular tissue develops in genetic females. Yet, unlike their treated male counterparts, these animals have feminine song systems and do not sing (behavior investigated in animals whose eggs were injected on day 5 only). These results suggest that testicular secretions are not solely responsible for masculinization of the zebra finch song system. In a couple of cases, females appeared to have no ovarian tissue, and their song systems were typical of control females (Springer and Wade, 1997). The results from those few animals provide some additional evidence that ovarian tissue does not inhibit the development of a masculine song system. However, because some ovarian tissue was present in most treated females in previous studies, we undertook a series of experiments in an effort to reliably create females with only testicular tissue. In addition, aromatase inhibition during embryonic development was in previous studies limited to a single 20- g injection in the egg. Treatments in the present study would allow the investigation of a variety of different alterations of aromatase activity in embryos. In Experiment 1, eggs were injected with Fadrozole at ages later than those previously attempted. In Experiment 2, eggs were injected with Fadrozole in higher doses or more often than in previous studies. In Experiment 3, eggs were treated daily from the day they were laid to the stage at which the 20- g Fadrozole treatment no longer sex-reverses gonads. Finally, in Experiment 4, that initial Fadrozole treatment was repeated, and the left gonad was surgically removed following hatching. This procedure created genetic males and females, each with a single right testis. MATERIALS AND METHODS Housing and Care All zebra finches were housed in communal aviaries containing approximately five breeding pairs and their young. Birds had seeds, water, cuttlebones, and grit available at all times. The food was supplemented with crumbled bread mixed with hard-boiled chicken eggs (including the shell) and fresh spinach and oranges. Females laid eggs in nest boxes, which were checked daily. New eggs were numbered with a dull pencil and catalogued. The day the eggs were found was considered day 1. In Experiment 1, the young were kept in the breeding aviaries with their parents until 60 days of age. At that time, they were removed to a holding aviary that housed other maturing birds from this experiment. After reaching sexual maturity (at approximately 105 days of age), the birds were moved into individual cages for behavioral testing (see below). In Experiments 2 4, birds remained in the breeding aviaries with their parents for 1 month. They were euthanized at that time as juveniles. Treatments Four separate experiments were conducted; sample sizes are presented with the results of each sudy. In Experiment 1, eggs were injected on day 12 or 13 with 20 g of Fadrozole in 10 l of 0.75% saline or with just the saline vehicle. In Experiment 2, eggs were injected either (a) on day 5 with 0 (vehicle control), 50, 100 or 200 g Fadrozole in 10 l saline or (b) 20 g Fadrozole in 5 l saline on both days 4 and 7. Only one animal hatched from 45 fertile eggs injected with 200 g Fadrozole, and it died within 24 h. Results presented will therefore include only the lower doses. In Experiment 3, eggs were dipped in a solution of 5 mg/ml Fadrozole in 70% ethanol or the control vehicle at C for 10 s. This treatment was administered every day for days beginning on day 1. Dips in higher doses of Fadrozole were attempted (25 and 50 mg/ml), but no birds hatched from these fertile eggs. In Experiment 4, eggs were injected with 20 g Fadrozole in 10 l saline on day 5. On days 7 10 posthatching, each individual was deeply anesthetized with Equithesin, and the left gonad (containing all of the ovarian tissue if it existed) was surgically removed in one set of males and females. In sham-treated animals, an incision was made on the animal s left side, and the gonad was exposed before the wound was sutured and sealed with collodion. In an ideal set of studies, one would like to include additional control groups (e.g., in Experiment 2 eggs injected with saline on days 4 and 7, and in Experiment 4 the removal of the left gonad in males and females that hatched from saline-treated eggs). However, given the number of eggs involved and the labor-intensive nature of these experiments, we felt that these groups were not necessary to draw relevant conclusions (see below). In eggs that received a single injection, the hole was sealed with paraffin. In eggs that received two injections, the hole was initially sealed with Tegaderm (3M), which was then peeled off to allow the needle to be inserted a second time. The egg was sealed with paraffin following the last

3 Zebra Finch Song System Development 143 injection. In all cases, the eggs hatched after approximately 15 days. Behavioral Testing Birds in Experiment 1 were allowed to acclimate in individual cages for 10 days, and then exposed to a series of tests for courtship and copulatory behaviors (details of testing conditions are identical to those described in Springer and Wade, 1997). Briefly, experimental birds were tested on 6 consecutive days for 15 min alternately with a male or female stimulus bird. Each bird thus received three tests with each type of stimulus animal. The sex of the first stimulus bird was assigned randomly. Following the last behavioral test, all experimental birds received a Silastic capsule (1.5-mm i.d., 2-mm o.d., 7 mm long) packed with crystalline testosterone propionate (TP; Steraloids). One week later, birds received behavioral tests with stimulus females on 3 consecutive days. In all of these tests, the number of audible song bouts and the number of mount attempts and cloacal contact movements (consummation of copulation, providing an indication of feminine receptivity) were recorded by an observer blind to the embryonic treatment of each individual. Tissue Collection, Preparation, and Analysis Following the last set of behavioral tests in Experiment 1 or at 1 month of age (mean SE: days) in Experiments 2 4, a blood sample was taken from the wing vein of all Fadrozole-treated animals to obtain DNA for determining genetic sex (procedure completed by Zoogen, Davis, CA). The animals were then killed with an overdose of Equithesin and perfused with 0.75% saline followed by 10% phosphatebuffered formalin. Brains were removed and postfixed in formalin. A block of tissue containing the gonads was postfixed in Bouin s fluid. The syrinx of each individual was removed and weighed. In Experiment 1, the presence of a Silastic capsule that still contained some steroid was confirmed at the time of perfusion. In all four experiments, brains were sectioned frozen at 30 m and stained with thionin. In Experiment 1, the volume of RA was estimated by tracing its outline in every third section using the NIH image analysis program Image on a Power Macintosh. These areas were summed and multiplied by the sampling interval, 90 m. In Experiments 2 4, the volumes of RA and HVC were determined in the same manner. An average neuron soma size in RA was also determined using the same technique on 25 cells from each FIG. 1. Volume of RA (top) and weight of syrinx (bottom) in adult zebra finches that had received treatment on embryonic day 12 or 13 with either the aromatase inhibitor Fadrozole or the vehicle saline. Although both structures were larger in males than in females, treatment did not produce a significant effect on either measure. side of the brain (50 neurons total). Values reported are means taken from measurements on the two sides of the brain, except in the few cases in which one side could not be measured due to histological artifact. All brain measurements were obtained without knowledge of the sex or treatment of the birds. The gonads were dehydrated, cleared with xylene, embedded in paraffin, sectioned at 10 m, and stained with hematoxylin and eosin. This tissue was examined to determine whether the embryonic Fadrozole treatment altered the structure of the gonads or tissue type within the gonads. Additionally, in Experiment 4, every section of the block of tissue surrounding and including the gonad(s) was examined to determine whether the left gonad had been successfully removed. Morphological characteristics of the central and pe-

4 144 Wade, Swender, and McElhinny TABLE 1 Singing Behavior in Male Zebra Finches That Had Received Embryonic Treatment with either the Aromatase Inhibitor Fadrozole or the Control Vehicle Saline Test type Stimulus animal: Male Female Female Adult treatment: None None T Embryonic treatment Song bouts Fadrozole Saline Days of singing Fadrozole Saline Note. The number of song bouts represents the sum of those displayed on each of the 3 test days of each type. ripheral song system were analyzed by two-way ANOVA (sex treatment) using Statview (Abacus Concepts, Berkeley, CA). In Experiment 1, the frequency of behavioral displays among males (both the total number of displays and the number of days on which each type of behavior occurred during each test type), was analyzed by ANOVA using SAS (SAS Institute, Inc., Cary, NC). Effects of treatment were compared between subjects, and effects of test type (with stimulus male, with stimulus female, or with stimulus female following testosterone treatment) were assessed within subjects. Planned pairwise comparisons were then conducted when significant effects of testing conditions were found. RESULTS Experiment 1 20 g Fadrozole Injected on Day 12 or 13 Gonadal Morphology. All males from eggs injected with Fadrozole (n 9) or saline (n 13) had large, mature testes of normal morphology. All females from saline-injected eggs (n 9) had a single, well-developed ovary on the left side. Eight of the 12 genetic females from Fadrozole-treated eggs had gonads that appeared comparable histologically to those of control females. In contrast, 3 females that had received embryonic Fadrozole treatment had a few testicular tubules in or very near the ovary. These tubules appeared similar to those seen in juvenile males in the present and previous experiments, although in some cases it looked as if germ cells (or other small round portions of the tissue that stain densely with hematoxylin) were scattered throughout the tubules rather than located around their base. Finally, one genetic female from a Fadrozole-treated egg had a bit of undifferentiated tissue near the ovarian follicles, but it did not strongly resemble testicular tissue seen in other individuals. Song system morphology. RA volume (Fig. 1) was significantly greater in males than in females (F 630.3, P 0.001), but there was no effect of treatment (F 0.2, P 0.662) or sex treatment interaction (F 0.1, P 0.721). Syrinx weight (Fig. 1) was significantly greater in males than in females (F 16.7, P 0.001), but the effect of treatment (F 0.7, P 0.421) and the interaction between sex and treatment (F 0.4, P 0.514) were not statistically significant. Behavior. Only males produced audible song, and only males attempted to mount stimulus animals. Unfortunately, no stimulus male attempted to mount the females from either Fadrozole- or saline-treated eggs, so the effect of the treatment on receptivity cannot be assessed. In analyzing data on males, there was no effect of embryonic treatment on the number of song bouts displayed (F 0.9, P 0.350). However, there was a significant effect of test type (F 13.0, P 0.001) such that before testosterone treatment males preferred to sing to females than to stimulus males (F 14.0, P 0.001) (Table 1). Males also appeared to increase the number of song bouts directed to females following exogenous testosterone treatment (F 3.7, P 0.068). Similar results were obtained for the number of days on which a male produced song (main effect of treatment, F 0.3, P 0.612; overall effect of test type, F 15.9, P 0.001; male vs female stimulus animals, F 16.4, P 0.001; effect of T treatment, F 4.8, P 0.041) (Table 1). Following adult testosterone treatment, males also increased the number of days on

5 Zebra Finch Song System Development 145 FIG. 2. Photographs of cross sections through the gonads (left) and RA (right) of three juvenile zebra finches treated on embryonic day 5. (Top) Saline-treated genetic female; (Middle) genetic female from an egg that received a 100- g Fadrozole injection; (Bottom) saline-treated genetic male. Scale bars for both gonads and brains, 300 m. which they attempted to mount (main effect of test type, F 6.45, P 0.004; effect of T treatment in tests with females, F 7.62, P 0.012), but no other significant effects were detected in analyses of the frequency of mounts displayed or the number of days on which males mounted (all other comparisons of the number of mount attempts or days on which a male mounted, F 1.7, P 0.217; data not shown). Experiment 2 Higher doses of Fadrozole on Day 5 or Double Injections Gonadal morphology. All control males had two testes (n 7), and control females had a single ovary on the left side (n 7) (Fig. 2). All males from Fadrozole-treated eggs had testes comparable to those seen in control males (n: 50 g 6, 100 g 3, two injections of 20 g 5). Most Fadrozole-treated females had a testis on the right and an ovotestis (consisting of both ovarian follicles and testicular tubules) on the left (n: 50 g 6, 100 g 6, two injections of 20 g 7). In addition, three genetic females in this experiment (two given 100 g Fadrozole and one injected twice with 20 g Fadrozole) had two testes, with no obvious ovarian tissue (Fig. 2). All females had a single left oviduct, as is typical in birds. Song system morphology. RA and HVC volume and RA soma size were all significantly larger in males than in females (all F 100.7, all P 0.001), but there were no significant effects of treatment (all F 1.9, all

6 146 Wade, Swender, and McElhinny FIG. 3. Morphology of song control centers and syrinx in juvenile zebra finches that had been treated during embryonic development. Each bird received one of the following treatments: a single injection of 50 or 100 g of Fadrozole or the control vehicle (saline) or injections on both days 4 and 7 with 20 g of Fadrozole. Measures analyzed include the volumes of HVC and RA (top), neuron soma size in RA (bottom left), and syrinx weight (bottom right). P 0.157) or sex treatment interactions (all F 1.6, all P 0.220) on any of the measures of brain morphology (Fig. 3). No significant effects of sex (F 2.5, P 0.121) or treatment (F 2.4, P 0.082) on syrinx size were detected (Fig. 3). However, the sex treatment interaction was marginally significant (F 2.7, P 0.057). When the analysis of syrinx data was separated by sex, a significant effect of treatment was found in females (F 3.22, P 0.040), but not males (F 1.5, P 0.240). Specifically, females treated in ovo with 50 g Fadrozole had syrinxes that were significantly larger than those treated with saline (Fisher s PLSD, P 0.005). The treatments that induced testicular tissue to grow in genetic females to varying degrees induced an average increase in the size of the syrinx, so that the expected sex difference (male female) was present when analyzing only the tissue from control, saline-treated birds (t 4.1, P 0.002, two-tailed). Experiment 3 Dips in Fadrozole on Days 1 10 or 1 12 Gonadal morphology. All control males (n 8) had two testes, and all control females had a single left ovary (n 6). These gonads appeared very similar to those of animals in Experiment 2, in which eggs were injected. The testes in all Fadrozole-treated males (n 8) were comparable to those in control males. Of the nine Fadrozole-treated females, seven had a left ovotestis and right testis, which also looked very much like those of genetic females from eggs that had

7 Zebra Finch Song System Development 147 FIG. 4. Morphology of song control centers and syrinx in juvenile zebra finches that had been treated during embryonic development by dipping eggs daily for the first days of incubation in a warm bath containing either Fadrozole or the ethanol vehicle. Measures analyzed include the volumes of HVC and RA (top), neuron soma size in RA (bottom left), and syrinx weight (bottom right). received injections of Fadrozole. Two Fadrozoletreated females appeared to have only testicular tissue, one with two testes, and the other with a single testis and an undifferentiated ball of tissue where the second gonad should have been. All females, regardless of treatment, had a single left oviduct. Because the gonads in the first genetic females from eggs dipped in Fadrozole appeared so similar to those in the injection studies, treatments were increased from 10 to 12 dips. The additional two dips did not alter the gonadal histology. Song system morphology. Results on the neural song system were comparable to those in Experiment 2, in which eggs were treated by injection (Fig. 4). RA and HVC volumes and the soma size of neurons in RA were all larger in males than in females (all F 67.55, all P 0.001). There was no effect of treatment on any of those variables (all F 0.95, all P 0.339) nor any significant sex treatment interactions (all F 2.38, all P 0.135). Data on the syrinx were also similar, in that there were no significant effects of sex (F 0.11, P 0.112) or treatment (F 0.66, P 0.424). The sex x treatment interaction was also not statistically significant (F 0.01, P 0.947). Experiment 4 Fadrozole Treatment with Surgical Removal of Left Gonad Gonadal and song system morphology. All three measures of the neural song system (RA and HVC volume, and RA soma size) were significantly larger in males than in females (all F 66.6, all P 0.001), but no significant effect of treatment (all F 1.0, all P 0.181) or sex treatment interaction (all F 0.3, all P 0.485)

8 148 Wade, Swender, and McElhinny FIG. 5. Morphology of song control centers and syrinx in juvenile zebra finches that had been treated on embryonic day 5 with 20 g of Fadrozole. After hatching, each individual either had its left gonad removed (Gonadex) or received a sham surgery. Measures analyzed include the volumes of HVC and RA (top), neuron soma size in RA (bottom left), and syrinx weight (bottom right). existed (Fig. 5). There was also no significant effect of sex, treatment, or interaction on syrinx weight (all F 1.3, all P 0.266) (Fig. 5). While no ovarian tissue was detected in any individual and the left gonad was completely removed in almost all animals, a very small bit of tissue remained on the left side of one female that contained what appeared to be a couple of testicular tubules. It does not change the significance of any of the results to analyze the data without those obtained from that individual (including that genetic female, n: sham male 8, sham female 11, gonadectomized male 5, gonadectomized female 7). DISCUSSION This study documents that the presence of testicular tissue in genetic females, even in the absence of ovarian tissue, does not induce masculine development of the song system. Further, inhibiting aromatase activity during most of embryonic development does not prevent masculinization of the song system. The data also indicate that the critical period for altering gonadal development in genetic females ends by approximately embryonic day 12. In combination with several other studies that altered the hormonal environment in zebra finch embryos (Gong and Arnold, 1996; Springer and Wade, 1997; Wade and Arnold, 1996; Wade et al., 1996), the present results strongly suggest both that the gonads are not the source of sexual differentiation and that embryonic aromatase activity does not play a substantial role in masculinizing the neural song system. These conclusions are important because they indicate that mechanisms regulating sexual differentiation of the song system are different than many other

9 Zebra Finch Song System Development 149 vertebrate courtship and copulatory systems studied to date, in which both structures and the capacity to display the behaviors are organized in one way or another by gonadal steroids. For example, in mammalian species including rats, ferrets and gerbils, portions of the preoptic area and the capacity to display copulatory behaviors are masculinized by neonatal exposure to testosterone or its metabolite estradiol (Arnold and Schlinger, 1993; Baum et al., 1990; Cooke et al., 1998; Döhler et al., 1984; Holman and Rice, 1996; Rhees, Shryne, and Gorski, 1990a; Rhees, Shryne, and Gorski, 1990b; Tarttelin and Gorski, 1988; Tobet, Zahniser, and Baum, 1986; Ulibarri and Yahr, 1988; Yahr, 1988). Similarly, in the African frog, Xenopus laevis, the neuromuscular structures required for the production of courtship vocalizations are masculinized by early testicular androgen (reviewed in Kelley, 1992). In avian species, including the zebra finch and Japanese quail, early estradiol exposure prevents the development of masculine copulatory behaviors (Adkins- Regan, 1983; Adkins-Regan and Ascenzi, 1987). However, the anatomy of a sexually dimorphic region of the preoptic area in quail is masculinized by testosterone in adulthood and is not affected by an embryonic estradiol treatment that feminizes copulation (Panzica, Viglietti-Panzica, Calacagni, Anselmetti, Schumacher, and Balthazart, 1987). To date, sexually dimorphic anatomy has not been identified in the zebra finch preoptic area. While previous studies (Gong and Arnold, 1996; Springer and Wade, 1997; Wade and Arnold, 1996; Wade et al., 1996) provided strong evidence in support of the idea that, unlike these other vertebrate models, sexual differentiation of the zebra finch song system does not involve testicular secretions or the aromatization of androgens, the interpretation of data in those studies was complicated by the following factors: (1) Most genetic females possessed some ovarian in addition to testicular tissue, and it was possible that some agent secreted by the ovarian tissue inhibited masculine development; (2) aromatase activity was inhibited for a limited duration during embryonic development, by a single injection of Fadrozole in ovo; and (3) the amount and duration of that inhibition was unknown. The present set of experiments provides some closure on the first two concerns, and while we still have not found a way to adequately address the third issue, it is less critical in light of the new data. First, while we were not successful in finding a treatment that reliably induced two testes to develop in genetic females, four such individuals were produced in the present study. Two genetic females with only testicular tissue were also identified in a previous study in our laboratory (Springer and Wade, 1997). In none of these cases did the presence of testicular tissue in the apparent absence of ovarian tissue induce masculinization of the neural song system. Furthermore, surgical removal of the left gonad, which contains all of the ovarian tissue, did not alter sexual differentiation of the song system in either males or females. Although one could argue that sexual differentiation may have been triggered before the left gonad could be surgically removed, the results from that gonadectomy experiment combined with the data from the six genetic females with only testicular tissue strongly suggest that ovarian tissue does not inhibit masculine development of the song system. The second concern, that aromatase activity was inhibited for only a limited period of embryonic development, was more easily resolved. Even dipping eggs daily in Fadrozole for the first days of incubation did not inhibit masculine neural development. The compound clearly entered the eggs, since the gonads of all genetic females contained testicular tissue, and there is no reason to suspect that the exposure would not be similar on each day of treatment. Thus, it seems unlikely that aromatase activity during the first two-thirds of embryonic development is important for masculine development of the neural song system. The syrinx, in contrast, may have been affected by gonadal secretions in the present experiments, since in some cases sex differences were eliminated following Fadrozole treatment. This avian tissue is sensitive to both estrogens, which during ontogeny prevent masculine development, and androgens, which increase its size in adult birds (Adkins-Regan, 1981; Luine, Nottebohm, Harding, and McEwen, 1980; Taber, 1964). Sex differences were present in the adults in Experiment 1 and in control birds in Experiment 2. No effects of treatment existed in any of the four experiments. These results are similar to those obtained in previous studies (Springer and Wade, 1997; Wade and Arnold, 1996). The testicular tissue in adult genetic females in Experiment 1 was so minimal that it is unlikely its secretions would increase syrinx size over control females, and in any case the high level of exogenous testosterone present at the time of perfusion would have eliminated any effect due to endogenous androgen secretions (as in Springer and Wade, 1997). Unexpectedly, though, the effect of sex in Experiment 2 was lost when all groups were considered, primarily due to an increase in syrinx size in one group of Fadrozole-treated females. It is possible that the gonads in those individuals were secreting more

10 150 Wade, Swender, and McElhinny androgen than those in other groups. It is unlikely, though, that less estradiol was present in the masculinized females, as all of those females had some ovarian tissue and received the lower of the two doses of Fadrozole (50 g) given in single injections. Expected sex differences were also not present in Experiments 3 and 4, in which eggs were repeatedly dipped in Fadrozole or the left gonad was surgically removed. Those results could reflect treatment, such that the sexual differentiation process was altered by the increased aromatase inhibition during embryonic development (Experiment 3) and the complete absence of ovarian tissue in all individuals for at least 3 weeks prior to perfusion (Experiment 4). However, an alternative explanation exists. The birds in all groups in these two experiments appeared to develop a bit more slowly than those in Experiments 1 and 2, presumably because of the harsher nature of the treatments. It is possible that the sex difference in the syrinx, which is not as dramatic as the neural dimorphisms, may simply not have developed by the time of perfusion. In hindsight, we should have weighed the birds or obtained some other indication of growth in all of these studies, but unfortunately that information is not available. It is possible that other potential long-term effects of treatment were missed in Experiments 2 4, because the animals were perfused as juveniles rather than as adults. However, there was no hint of a reduction in the degree of sexual dimorphism in the song control nuclei, which one would predict if the paths of sexual differentiation were altered by treatment. In addition, data on both gonadal and song system morphology in the present study were very similar to those seen in other studies (Springer and Wade, 1997; Wade and Arnold, 1996; Wade et al., 1996) in which birds were sacrificed as adults. Thus, while we cannot rule out the possibility of effects on the neural song system developing after 1 month of age, it does not seem very likely. Some questions are likely to remain unanswered. We do not know the extent of Fadrozole s ability to inhibit aromatase in ovo, and despite several attempts (unpublished results), we have so far been unable to detect the production of androgen in the gonads of hatchling males or females (untreated birds). We do not, therefore, have a good method for determining whether the testes in genetic females were secreting gonadal steroids during development in a manner comparable to that occurring in males. However, based on assays of plasma steroids, it is not clear what compound one would measure to determine that secretions are masculine. That is, no consistent evidence of sex differences in circulating androgens or estrogens exists in the first few weeks after hatching (Adkins-Regan, Abdelnabi, Mobarak, and Ottinger, 1990; Hutchison, Wingfield, and Hutchison, 1984; Schlinger and Arnold, 1992). We do know that in adulthood the testes in genetic females are functional. Testicular tissue in females treated with 20 g Fadrozole on embryonic day 5 or 8 secretes androgens in the range of control males and produces sperm (Springer and Wade, 1997; Wade and Arnold, 1996; Wade et al., 1996). Implanting gonads from genetic males into hatchling females would provide an additional means of assessing the effects of testicular tissue. Positive results, such as those obtained in X. laevis in which the implants induced masculine development of singing behavior in genetic females (Watson and Kelley, 1992), would be very informative. However, negative results would be difficult to interpret for the same reasons as data obtained from individuals with testes induced by Fadrozole administration. One could also imagine numerous variations of treatments with Fadrozole, involving different times, doses, and/or routes of administration, for example, that would allow assessment of different patterns of aromatase inhibition and might produce more genetic females with only testicular tissue. However, in addition to the Fadrozole treatments in the present set of experiments, inhibiting aromatase activity on embryonic day 3, 5, or 8 or for the entire first month after hatching has not prevented normal masculine development of the song system (Gong and Arnold, 1996; Springer and Wade, 1997; Wade and Arnold, 1994; Wade and Arnold, 1996; Wade et al., 1996). Thus, we suggest that, despite a few remaining questions, the evidence overwhelmingly supports the idea that sexual differentiation of the zebra finch song system parallels genetic, rather than gonadal, sex. It therefore seems most plausible that sexual differentiation of the neural song system is normally regulated by a cascade of proteins produced through some sexually dimorphic genetic mechanism(s) rather than through the action of gonadal steroids. The challenge now is to determine that sequence of events and the direct consequences of each step. For example, it is currently unclear not only how many events are involved, but whether they influence development in a masculine or feminine direction or whether both processes require particular mechanisms to be actively engaged.

11 Zebra Finch Song System Development 151 ACKNOWLEDGMENTS We thank Camilla Peabody and the dedicated undergraduate students who helped with animal care, behavioral testing, and histology. Dr. Rick DeShon helped with the statistical analysis of behavioral data. Fadrozole was generously supplied by Novartis Pharmaceuticals. The work was supported by the NIH (MH55488). REFERENCES Adkins-Regan, E. (1981). Early organizational effects of hormones: An evolutionary perspective. In N. T. Adler (Ed.), Neuroendocrinology of Reproduction, pp Plenum, New York. Adkins-Regan, E. (1983). Sex steroids and the differentiation and activation of avian reproductive behaviour. In J. Balthazart, E. Prove, and R. Gilles (Eds.), Hormones and Behaviour in Higher Vertebrates, pp Springer-Verlag, Berlin/Heidelberg. Adkins-Regan, E., Abdelnabi, M., Mobarak, M., and Ottinger, M. A. (1990). Sex steroid levels in developing and adult male and female zebra finches (Poephila guttata). Gen. Comp. Endocrinol. 78, Adkins-Regan, E., and Ascenzi, M. (1987). Social and sexual behaviour of male and female zebra finches treated with oestradiol during the nestling period. Anim. Behav. 35, Arnold, A. P. (1992). Developmental plasticity in neural circuits controlling birdsong: Sexual differentiation and the neural basis of learning. J. Neurbiol. 23, Arnold, A. P., and Schlinger, B. A. (1993). Sexual differentiation of brain and behavior: The zebra finch is not just a flying rat. Brain Behav. Evol. 42, Baum, M. J., Carroll, R. S., Cherry, J. A., and Tobet, S. A. (1990). Steroidal control of behavioural, neuroendocrine and brain sexual differentiation: Studies in a carnivore, the ferret. J. Neuroendocrinol. 2, Cooke, B., Hegstrom, C. D., Villeneuve, L. S., and Breedlove, S. M. (1998). Sexual differentiation of the vertebrate brain: Principles and mechanisms. Front. Neuroendocrinol. 19, Döhler, K. D., Coquelin, A., Davis, F., Hines, M., Shryne, J. E., and Gorski, R. A. (1984). Pre- and postnatal influence of testosterone propionate and diethylstilbestrol on differentiation of the sexually dimorphic nucleus of the preoptic area in male and female rats. Brain Res. 302, Feder, H. (1981). Perinatal hormones and their role in the development of sexually dimorphic behaviors. In N. T. Adler (Ed.), Neuroendocrinology of Reproduction, pp Plenum, New York. Gong, A., and Arnold, A. P. (1996). Pre-hatching inhibition of aromatase activity masculinizes syringeal and gonadal tissue but not the song system in zebra finch females. Soc. Neurosci. Abstr. 22, 756. Gurney, M. E. (1981). Hormonal control of cell form and number in the zebra finch song system. J. Neurosci. 1, Gurney, M. E. (1982). Behavioral correlates of sexual differentiation in the zebra finch song system. Brain Res. 231, Holman, S. D., and Rice, A. (1996). Androgenic effects on hypothalamic asymmetry in a sexually differentiated nucleus related to vocal behavior in Mongolian gerbils. Horm. Behav. 30, Hutchison, J. B., Wingfield, J. C., and Hutchison, R. E. (1984). Sex differences in plasma concentrations of steroids during the sensitive period for brain differentiation in the zebra finch. J. Endocrinol. 103, Kelley, D. B. (1992). Opening and closing a hormone-regulated period for the development of courtship song. Dev. Psychobiol. 662, Konishi, M., and Akutagawa, E. (1985). Neuronal growth, atrophy and death in a sexually dimorphic song nucleus in the zebra finch brain. Nature 315, Konishi, M., and Akutagawa, E. (1988). A critical period for estrogen action on neurons of the song control system in the zebra finch. Proc. Natl. Acad. Sci. USA 85, Luine, V., F. Nottebohm, Harding, C., and McEwen, B. S. (1980). Androgen affects cholinergic enzymes in syringeal motor neurons and muscle. Brain Res. 192, Nottebohm, F., and Arnold, A. P. (1976). Sexual dimorphism in vocal control areas of the songbird brain. Science 194, Panzica, G. C., Viglietti-Panzica, C., Calacagni, M., Anselmetti, G. C., Schumacher, M., and Balthazart, J. (1987). Sexual differentiation and hormonal control of the sexually dimorphic medial preoptic nucleus in the quail. Brain Res. 416, Pohl-Apel, G., and Sossinka, R. (1984). Hormonal determination of song capacity in females of the zebra finch: Critical phase of treatment. Z. Tierpsychol. 64, Rhees, R. W., Shryne, J. E., and Gorski, R. A. (1990a). Onset of the hormone-sensitive perinatal period for sexual differentiation of the sexually dimorphic nucleus of the preoptic area in female rats. J. Neurobiol. 21, Rhees, R. W., Shryne, J. E., and Gorski, R. A. (1990b). Termination of the hormone-sensitive period for differentiation of the sexually dimorphic nucleus of the preoptic area in the male and female rats. Dev. Brain Res. 52, Sachs, B. D., and Meisel, R. L. (1988). The physiology of male sexual behavior. In E. Knobil and J. Neill (Eds.), The Physiology of Reproduction, pp Raven Press, New York. Schlinger, B. A., and Arnold, A. P. (1992). Plasma sex steroids and tissue aromatization in hatchling zebra finches: Implications for the sexual differentiation of singing behavior. Endocrinology 130, Simpson, H. B., and Vicario, D. S. (1991a). Early estrogen treatment alone causes female zebra finches to produce learned, male-like vocalizations. J. Neurobiol. 22, Simpson, H. B., and Vicario, D. S. (1991b). Early estrogen treatment of female zebra finches masculinizes the brain pathway for learned vocalizations. J. Neurobiol. 22, Springer, M. L., and Wade, J. (1997). The effects of testicular tissue and prehatching inhibition of estrogen synthesis on the development of courtship and copulatory behavior in zebra finches. Horm. Behav. 32, Taber, E. (1964). Intersexuality in birds. In C. N. Armstrong and A. J. Marshall (Eds.), Intersexuality in Vertebrates Including Man, pp Academic Press, London. Tarttelin, M. F., and Gorski, R. A. (1988). Postnatal influence of diethylstilbestrol on the differentiation of the sexually dimorphic nucleus in the rat is as effective as perinatal treatment. Brain Res. 456, Tobet, S. A., Zahniser, D. J., and Baum, M. J. (1986). Differentiation in male ferrets of a sexually dimorphic nucleus of the preoptic/ anterior hypothalamic area requires prenatal estrogen. Neuroendocrinology 44, Ulibarri, C., and Yahr, P. (1988). Role of neonatal androgens in

12 152 Wade, Swender, and McElhinny sexual differentiation of brain structure, scent marking, and gonadotropin secretion in gerbils. Behav. Neur. Biol. 49, Wade, J., and Arnold, A. P. (1994). Posthatching inhibition of aromatase activity does not alter sexual differentiation of the zebra finch song system. Brain Res. 639, Wade, J., and Arnold, A. P. (1996). Functional testicular tissue does not masculinize development of the zebra finch song system. Proc. Natl. Acad. Sci. USA 93, Wade, J., Springer, M. L., Wingfield, J. C., and Arnold, A. P. (1996). Neither testicular androgens nor embryonic aromatase activity alters morphology of the neural song system in zebra finches. Biol. Reprod. 55, Watson, J. T., and Kelley, D. B. (1992) Testicular masculinization of vocal behavior in juvenile female Xenopus laevis reveals sensitive periods for song duration, rate and frequency spectra. J. Comp. Physiol. A 171, Yahr, P. (1988). Sexual differentiation of behavior in the context of developmental psychobiology. In E. M. Blass (Ed.), Handbook of Behavioral Neurobiology, Vol. 9, pp Plenum, New York.

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