Sexually dimorphic expression of a laryngeal-specific, androgen-regulated myosin heavy chain gene during Xenopus laevis development.

Masculinization of the larynx in Xenopus laevis frogs is essential for the performance of male courtship song. During postmetamorphic (PM) development, the initially female-like phenotype of laryngeal muscle (slow and fast twitch fibers) is converted to the masculine form (entirely fast twitch) under the influence of androgenic steroids. To explore the molecular basis of androgen-directed masculinization, we have isolated cDNA clones encoding portions of a new Xenopus myosin heavy chain (MHC) gene. We have detected expression of this gene only in laryngeal muscle and specifically in males. All adult male laryngeal muscle fibers express the laryngeal myosin (LM). Adult female laryngeal muscle expresses LM only in some fibers. Expression of LM during PM development was examined using Northern blots and in situ hybridization. Males express higher levels of LM than females throughout PM development and attain adult levels by PM3. In females, LM expression peaks transiently at PM2. Treatment of juvenile female frogs with the androgen dihydrotestosterone masculinizes LM expression. Thus, LM appears to be a male-specific, testosterone-regulated MHC isoform in Xenopus laevis. The LM gene will permit analysis of androgen-directed sexual differentiation in this highly sexually dimorphic tissue.     

The sexually dimorphic larynx of Xenopus laevis: development and androgen regulation

The aims of this study were to characterize sexual dimorphism in the larynx of adult Xenopus laevis and to determine how sex differences arise during postmetamorphic development. The larger male larynx is a result of greater cell numbers in both cartilage and muscle. The dilator laryngis muscle of the male larynx has 6-7 times more muscle fibers than that of the female. At metamorphosis, the larynx is sexually monomorphic and feminine in phenotype. The DNA content of the male larynx doubles during the first 6 months following metamorphosis; there is no net DNA increase in the female larynx during this time. Both sexes experience a marked increase in laryngeal DNA content and mass between 6 months and adulthood. The number of muscle fibers in the male larynx increases at an average rate of 150 fibers a day during the first 10 months of postmetamorphic development. There is no net change in fiber numbers in the female larynx from metamorphosis to adulthood. Administration of the antiandrogen Flutamide to metamorphic frogs prevents the net addition of laryngeal muscle fibers in males. Thus, we propose that addition of postmetamorphic laryngeal muscle fibers in males is dependent upon the presence of circulating androgens. Exogenous testosterone administration results in an increase in laryngeal mass, DNA content, and cellular proliferation in juvenile frogs. Using [3H]thymidine injections to probe ongoing, as well as testosterone-induced, cell proliferation, we conclude that cellular proliferation is regulated differently in males and females during development. Thus androgen-induced proliferation is one cellular mechanism responsible for the sexual dimorphism observed in adults.     

Neuroeffectors for vocalization in Xenopus laevis: hormonal regulation of sexual dimorphism

South African clawed frogs use sex-specific vocalizations during courtship. In the male, vocalizations are under the control of gonadal androgen. Though females have moderate levels of circulating androgen, they do not give male-typical mate calls. Both muscles of the vocal organ and neurons of the central nervous system (CNS) vocal pathway are sexually dimorphic and androgen-sensitive. Recent studies suggest that the failure of androgen to masculinize adult females results from a male-specific, androgen-regulated developmental program. At metamorphosis the larynx is sexually monomorphic and feminine in morphology, muscle fiber number and androgen receptor content. During the next six months, under the influence of increasing androgen titers and high receptor levels, myoblasts proliferate in the male and muscle fibers increase at an average rate of 100/day. Females have much lower hormone levels, receptor values decline and they display no net addition of fibers. At metamorphosis, both males and females have approximately 4000 muscle fibers. By adulthood, males have eight times the female fiber number. In the CNS, adult laryngeal motor neurons are more numerous with larger somata and dendritic trees in males than in females. Certain connections of neurons in the vocal pathway are also less robust in females. Unlike the periphery, motor neuron number does not appear to be established by androgen-induced proliferation. Our current hypothesis is that androgen acts at the level of laryngeal muscle to produce more muscle fibers and thus provide more target for motor neurons in the male. This process could regulate cell number by ontogenetic cell death. In the CNS, androgen-target neurons become capable of accumulating hormone shortly before metamorphosis. Androgen receptor in laryngeal motor neurons may permit the dendritic growth characteristic of males by increasing sensitivity to afferent stimuli. Such a process could account for the observed differences in CNS vocal "circuitry" in X. laevis and thus behavioral differences between the sexes.     

Androgen directs sexual differentiation of laryngeal innervation in developing Xenopus laevis

In adult Xenopus laevis, innervation of the vocal organ is more robust in males than in females. This sex difference originates during tadpole development; at stage 56, when the gonads first differentiate, the number of axons entering the larynx is the same in the sexes, but by stage 62, innervation is greater in males. To determine if androgen secretion establishes sex differences in axon number, we treated tadpoles with antiandrogen or androgen beginning at stage 48 or 54 and counted laryngeal nerve axons at stage 62 using electron microscopy. When male tadpoles were treated with the antiandrogen hydroxyflutamide, axon numbers were reduced to female-typical values; axon numbers in females were unaffected by antiandrogen treatment. When female tadpoles were treated with the androgen DHT (dihydrotestosterone), axon numbers were increased to male-like values. These findings suggest that endogenous androgen secretion during late tadpole stages in males is required for the sexual differentiation of laryngeal innervation observed from stage 62 on. Because androgen treatment and laryngeal innervation affect myogenesis in postmetamorphic frogs, numbers of laryngeal dilator muscle fibers were determined for hormonally manipulated tadpoles. At stage 62, vehicle-treated males had more laryngeal axons than females; laryngeal muscle fiber numbers did not, however, differ in the sexes. Both male and female tadpoles, treated from stage 54 with DHT, had more muscle fibers at stage 62 than vehicle-treated controls. Thus, while endogenous androgen secretion during late tadpole stages is subthreshold for the establishment of masculinized muscle fiber numbers, laryngeal myogenesis is androgen sensitive at this time and can be increased by suprathreshold provision of exogenous DHT. A subgroup of tadpoles, DHT treated from stage 54 to 62, was allowed to survive, untreated, until postmetamorphic stage 2 (PM2: 5 months after metamorphosis is complete). Androgen treatment between tadpole stages 54 and 62 does not prevent the ontogenetic decrease in axon numbers characteristic of laryngeal development. In addition, the elevation in stage 62 axon numbers produced by DHT-treatment at late tadpole stages was not associated with elevated numbers of laryngeal muscle fibers at PM2. Juvenile males normally maintain elevated axon numbers (relative to final adult values) through PM2 and the presence of these additional axons may result from-rather than contribute directly to—laryngeal muscle fiber addition. 1994 John Wiley & Sons, Inc.     

Temporal constraints on androgen directed laryngeal masculinization in Xenopus laevis

Temporal constraints on androgen regulated masculinization of three sexually dimorphic laryngeal properties--tension, fiber type, and fiber recruitment--were examined in Xenopus laevis frogs. Endocrine state was manipulated at PM0 when the larynx is similar in males and females, at PM2 when the larynx begins sexual differentiation, and at PM6 when sexual differentiation is complete. Removing the testes in developing males (PM0 or PM2) completely arrests laryngeal masculinization. Masculinization resumes when testosterone is replaced later in development (PM2 or PM6, respectively). Thus, testicular secretions, in particular androgens, are required for laryngeal masculinization. The ability of androgens to masculinize tension, fiber type, and fiber recruitment in developing and adult larynges was also determined. Five weeks of testosterone treatment in PM0 or PM2 males and females completely masculinizes laryngeal tension and fiber type, but only partially masculinizes fiber recruitment. However, fiber recruitment can be fully masculinized in PM6 males castrated at PM2. We conclude that androgen induced masculinization of tension and fiber type are not temporally constrained but that androgen induced masculinization of fiber recruitment is. Prolonged androgen treatment can override the temporal constraints on masculinization of the larynx. Testosterone treatment for more than 6 months fully masculinizes fiber recruitment in developing (PM0 or PM2) females. In addition, prolonged treatment (greater than 9 months) completely masculinizes tension, fiber type, and fiber recruitment in adult females; these properties were not fully masculinized by shorter (1-3 months) treatments in adult females. Testosterone induced masculinization in females is maintained for up to 8 months following testosterone removal; thus androgen effects are long lasting and possibly permanent.     

Laryngeal muscle and motor neuron plasticity in Xenopus laevis: Testicular masculinization of a developing neuromuscular system

In Xenopus laevis, the sexual differentiation of the neuromuscular system responsible for courtship song is controlled by testicular androgen secretion. To explore the sensitivity of this system to androgenic stimulation, male and female frogs were gonadectectomized and given testis transplants at seven different developmental stages between the end of metamorphosis and adulthood, grown to sexual maturity, and the laryngeal muscle fibers and motor axons were counted. Muscle fiber and axon numbers in males were not affected by the testicular transplant at any stage. In females, testicular transplants at all developmental stages increased muscle fiber numbers in adulthood. Values attained were, however, significantly less than those of adult intact or testis-transplanted males. Testis transplantation increased laryngeal axon numbers in females to levels equivalent to those of intact males; this effect was obtained at every stage of postmetamorphic development including adulthood. To further explore androgen regulation in adults, males and females were gonadectomized and implanted with silicone tubes containing testosterone propionate for 1.5–3 years and laryngeal muscle fibers and axon numbers compared to those of gonadectomized or sham-operated adult controls. Neither treatment with exogenous androgen nor gonadectomy had any effect on laryngeal muscle fiber or axon number in either males or females; values did not differ from those of sham-operated controls. We conclude that testicular secretions can induce laryngeal muscle fiber and axon addition in females throughout postmetamorphic life. This degree of plasticity, exhibited after the period when adult values are normally attained, stands in contrast to the effects of administration of synthetic androgen and suggests that the degree of plasticity in adult females may be underestimated if exogenous hormones rather than testicular transplants are provided. © 1993 John Wiley & Sons, Inc.     

Development of functional sex differences in the larynx of Xenopus laevis

Three laryngeal properties associated with the production of masculine song--laryngeal muscle tension, fiber twitch type, and fiber recruitment--are markedly sexually dimorphic in adult Xenopus laevis frogs. To elucidate the pattern of sexual differentiation, tension and fiber recruitment in male and female larynges and fiber twitch type in male larynges were examined throughout postmetamorphic development. Masculinization of male laryngeal properties begins early in postmetamorphic development and continues until adulthood. In contrast, tension and fiber recruitment in females do not change after the end of metamorphosis. Laryngeal muscle tension and fiber type are gradually and progressively masculinized; the temporal pattern of masculinization is very similar for these properties. Fiber recruitment, on the other hand, appears to masculinize in a stepwise manner. Masculinization of all three properties is highly correlated with larynx weight in males. We have used this relation to divide postmetamorphic development into seven stages associated with key events in sexual differentiation. This staging scheme provides an important experimental tool for studying the hormonal regulation of sexual differentiation, the subject of the accompanying paper.     

Testicular masculinization of vocal behavior in juvenile female Xenopus laevis reveals sensitive periods for song duration,rate, and frequency spectra

Summary In Xenopus laevis, adult males but not females produce courtship songs comprised of rapid trills. Two experiments were conducted to determine whether male-typical singing could be induced in females. At 6 different juvenile stages, male and female frogs were gonadectomized and implanted with testes, grown to sexual maturity, and tested for vocal behavior. All frogs with functional testicular implants sang; females sang as much as males. The frequency spectra of the clicks within trills were fully masculinized in females implanted at PM0, PM1, and PM2. There were deficiencies in song quality in females implanted late in juvenile life. Females receiving testis implants at PM3, PM4, and PM5 did not produce clicks with masculine spectral qualities. In a concurrent experiment, adult males and females were gonadectomized and implanted with testes or silicone tubes containing testosterone proprionate. When tested for vocal behavior 10 to 15 months after implantation, 8/10 androgen-treated males, 3/12 androgen-treated females, 5/5 testes-implanted males, and 2/4 testes-implanted females sang. The females that did sing spent much less time singing than males. The click rates of females were uniformly slower than males and no female produced clicks with a masculine frequency spectrum. Thus, testicular secretions can induce male-typical singing in females until late in juvenile development. However, females exhibit a progressive decline in vocal potential with increasing age, culminating in an almost complete loss of singing ability by adulthood.Abbreviations FFT fast Fourier transform - ICI inter-click interval - PM post-metamorphic - TP testosterone proprionate     

Sex differences in the motor nucleus of cranial nerve IX-X in Xenopus laevis: a quantitative Golgi study

In the clawed frog (Xenopus laevis), motor neurons in cranial nerve nucleus IX-X control contraction of laryngeal muscles responsible for sexually dimorphic vocal behaviors. We examined sex differences in dendritic arbors of n.IX-X cells using the Golgi-Cox method. Three morphological classes of somal types (ovoid, triangular, and elongate) are present in similar frequencies in n.IX-X of both males and females. The male n.IX-X neuron is a more complex and hypertrophied version of the female n.IX-X cell. The number of primary dendrites is the same for both sexes, but males have more total dendritic segments. The overall dendritic length of male n.IX-X neurons is two to three times that of the female. Males have longer dendritic segments between all branch points. Male and female frogs differ in levels of circulating androgens; neurons of n.IX-X are targets for androgenic steroids. To determine if androgen can affect dendritic morphology in adult females, we examined Golgi-impregnated cells in n.IX-X from ovariectomized females treated with testosterone for 1 month. The total number of dendritic segments was reduced by androgen treatment due to reduction in the number of higher order dendritic segments; the number of primary dendritic segments was unchanged. Androgen treatment may induce resorption of higher order dendritic branches. The overall dendritic length of androgen-treated female n.IX-X neurons was unchanged, and dendritic segments were longer. Thus, although androgen can alter dendrites of n.IX-X cells in adult females, this short-term treatment does not produce a masculine dendritic architecture.     

Sexual differentiation and hormonal regulation of the laryngeal synapse in Xenopus laevis

In Xenopus laevis frogs, sex differences in adult laryngeal synapses contribute to sex differences in vocal behavior. This study explores the development of sex differences in types of neuromuscular synapses and the development and hormone regulation of sex differences in transmitter release. Synapses in the juvenile larynx have characteristics not found in adults: juvenile muscle fibers can produce subthreshold or suprathreshold potentials in response to the same strength of nerve stimulation and can also produce multiple spikes to a single nerve stimulus. Juvenile laryngeal muscle also contains the same synapse types (I, II, and III) as are found in adult laryngeal muscle. The distribution of laryngeal synapse types in juveniles is less sexually dimorphic than the distribution in adults. Analysis of quantal content indicates that laryngeal synapses characteristically release low amounts of transmitter prior to sexual differentiation. Quantal content values from male and female juveniles are similar to values for adult males and are lower than values for adult females. When juveniles are gonadectomized and treated with exogenous estrogen, quantal content values increase significantly, suggesting that this hormone may increase transmitter release at laryngeal synapses during development. Gonadectomy alone does not affect quantal content of laryngeal synapses in either sex. Androgen treatment decreases quantal content in juvenile females but not males; the effect is opposite to and smaller than that of estrogen. Thus, muscle fiber responses to nerve stimulation and transmitter release are not sexually dimorphic in juvenile larynges. Transmitter release is strengthened, or feminized, by the administration of estradiol, an ovarian steroid hormone. © 1995 John Wiley & Sons, Inc.     

Sexual differentiation of identified motor terminals in Drosophila larvae

In Drosophila, we have found that some of the motor terminals in wandering third-instar larvae are sexually differentiated. In three out of the four body-wall muscle fibers that we examined, we found female terminals that produced a larger synaptic response than their male counterparts. The single motor terminal that innervates muscle fiber 5 produces an EPSP that is 69% larger in females than in males. This is due to greater release of transmitter from female than male synaptic terminals because the amplitude of spontaneous miniature EPSPs was similar in male and female muscle fibers. This sexual difference exists throughout the third-instar: it is seen in both early (foraging) and late (wandering) third-instar larvae. The sexual differentiation appears to be neuron specific and not muscle specific because the same axon produces Is terminals on muscle fibers 2 and 4, and both terminals produce larger EPSCs in females than males. Whereas, the Ib terminals innervating muscle fibers 2 and 4 are not sexually differentiated. The differences in transmitter release are not due to differences in the size of the motor terminals. For the terminal on muscle fiber 5 and the Is terminal on muscle fiber 4, there were no differences in terminal length, the number of branches, or the number of synaptic boutons in males compared to females. These sexual differences in neuromuscular synaptic physiology may be related to male-female differences in locomotion.     

Effects of size at metamorphosis on stonefly fecundity, longevity, and reproductive success

Many organisms with complex life cycles show considerable variation in size and timing at metamorphosis. Adult males of Megarcyssignata (Plecoptera: Perlodidae) are significantly smaller than females and emerge before females (protandry) from two western Colorado streams. During summer 1992 stoneflies from a trout stream emerged earlier in the season and at larger sizes than those from a colder fishless stream, and size at metamorphosis did not change over the emergence period in either stream. We performed two experiments to determine whether variation in size at metamorphosis affected the fecundity, reproductive success and longevity of individuals of this stonefly species and if total lifetime fecundity was affected by the number of matings. In the first experiment, total lifetime fecundity (eggs oviposited) was determined for adult females held in small plastic cages in the field. Males were removed after one copulation, or pairs were left together for life and allowed to multiply mate. Most copulations occurred in the first few days of the experiment. Females in treatments allowing multiple matings had significantly lower total lifetime fecundity and shorter adult longevity than females that only mated once. Multiple matings also reduced longevity of males. Fecundity increased significantly with female body mass at emergence, but only for females that mated once. While multiple matings eliminated the fecundity advantage of large female body size, number of matings did not affect the significant positive relationship between body mass at metamorphosis and longevity of males or females. In a second experiment designed to determine if body mass at emergence affected male mating success, we placed one large and one small male Megarcys in an observation arena containing one female and recorded which male obtained the first mating. The large and the small male had equal probabilities of copulating with the female. Copulations usually lasted all night, and the unmated male made frequent, but unsuccessful attempts to take over the copulating female. Our data suggest that selection pressures determining body size at metamorphosis may operate independently on males and females, resulting in evolution of sexual size dimorphism, protandry, and mating early in the adult stage. We emphasize the importance of interpreting the fitness consequences of larval growth and development on the timing of and size at metamorphosis in the context of the complete life cycle. Received: 1 July 1997 / Accepted: 12 November 1997     

Differential expression of the Ca2+-binding protein parvalbumin during myogenesis in Xenopus laevis

We have used immunocytological techniques to examine the developmental expression of the Ca2+-binding protein parvalbumin in Xenopus laevis embryos. Western blot experiments show that at least three different forms of parvalbumin are expressed during embryogenesis; the tadpole tail expresses one form, adult brain expresses another, mylohyoid muscle expresses both, and gastrocnemius and sartorius muscles express these two plus a third form. Parvalbumin (PV) is first detectable by immunofluorescence at stages 24-25 of development, a time when myotomal muscles are differentiating and contractile activity occurs spontaneously in embryos. At metamorphosis, PV is expressed in developing limb muscles. While the majority of skeletal muscle fibers express high levels of PV in both embryos and adults, a second fiber type has no detectable PV. The arrangement of PV-containing fibers is stereotyped in each muscle group examined. Histochemical staining of tadpole muscles indicate that PV-containing fibers correspond to fast-twitch skeletal muscles, whereas those without PV correspond to slow-twitch muscles. During tail resorption at metamorphosis, PV appears to be extruded from dying tail muscle cells and taken up by phagocytic cells.     

Male wrens with large testes breed early

To display to females, male wrens, Troglodytes troglodytes, use cock nests built on their territories. Nest building starts about a month before the first females begin egg laying. The timing of nest building is highly variable between males with up to 8 weeks separating the earliest from the latest males to initiate this display activity. Males that weigh more before the breeding season initiate nest building earlier than lighter males. We measured testis size in male wrens in the prebreeding period during which seasonal testicular recrudescence is occurring. The initiation of nest building was predicted by male age and by testis size. This suggests that variation in the start of courtship activity may reflect variation between males in the rate of testicular recrudescence. Variation in male prebreeding body mass was influenced by variation in testis size, which suggests that the relation reported earlier between body mass and timing of nest construction could be explained, in part, by variation in testis size. Alternatively, if body mass reflects body condition then both the extent of testes recrudescence and the timing of courtship activity could be condition-dependent traits. Copyright 2000 The Association for the Study of Animal Behaviour.     

Seasonal plasticity in the copulatory neuromuscular system of green anole lizards: a role for testosterone in muscle but not motoneuron morphology

The copulatory system of green anoles is highly sexually dimorphic. Males possess bilateral copulatory organs called hemipenes, each independently controlled by two muscles: the transversus penis (TPN) and retractor penis magnus (RPM). The TPN everts the hemipene through the cloaca and the RPM retracts it. Adult females do not possess hemipenes or either of these two muscles. The spinal nucleus projecting to the TPN and RPM contains more and larger motoneurons in males than females. Because anoles breed seasonally, two experiments were designed to test whether adult copulatory morphology varies with environmental condition, and if so, whether the effect is mediated by testicular androgens. Three groups of adult males were used in each experiment: males from breeding environmental conditions with reproductive testes (BS); males in breeding conditions with regressed testes (BS-X); and males in nonbreeding conditions with regressed testes (NBS). Experiment 1 compared gonadally intact males and Experiment 2 compared castrated males treated with either testosterone (T) or an empty implant. In both experiments, copulatory and control motoneurons appeared smaller in NBS males, but T did not affect their size. In contrast, while hemipene and RPM muscle fiber size were not plastic across season in gonadally intact males, T in castrated males significantly increased both measures under BS and BS-X, but not NBS, conditions. These results demonstrate that neuron soma size might change on a general level and environmental cues can mediate T-induced changes in peripheral structures, suggesting that plasticity across copulatory system components is regulated by different mechanisms.     

Rates of development in male and female Wood Frogs and patterns of parasitism by lung nematodes

Researchers are becoming interested in testing whether investment in growth and/or development trades off against investment in parasite defence. We tested this idea by examining relations between development of Wood Frogs (Rana sylvatica) and susceptibility to lung nematodes (Rhabdias ranae). Male and female frogs reared in outdoor mesocosms were the same length and mass at metamorphosis. However, males metamorphosed sooner than females. Lung nematodes were no more likely to penetrate male versus female metamorphs following controlled exposures, but males had higher intensities of adult female worms and the largest worms per host were, on average, of larger size in male metamorphs. Males that took longer to metamorphose carried higher numbers of worms in their lungs than males that metamorphosed early. In comparison, females that developed faster harboured more worms in their lungs than females that took longer to reach metamorphosis. Our results suggest that variation in susceptibility to lung nematodes is influenced by host sex and possibly also by sex-specific relations with developmental rate. Further, male hosts might prove to be a more important source of infective stages of worms than female hosts.     

Testosterone-induced development of the rat levator ani muscle

The perinatal development of the levator ani (LA) muscle in male and female rats was investigated by measuring the total number of muscle units (MU) (i.e., mononucleate cells, clustered or independent myotubes, and muscle fibers) in transverse semithin sections of the entire muscle and the MU cross-sectional area in 22-day-old fetuses (F22), 1-day-old (D1 = day of birth), 3-day-old (D3), and 6-day-old (D6) newborns. Male muscle contained 350 +/- 64 MU on F22, twice that of the female. The number of MU increased markedly in males from F22, but changed little in females; the number of MU in males was 760% that of females on D6. The MU cross-sectional area was greater in males on F22 (120.8 micron(s)2 +/- 7.5) and D1 (155.2 micron(s)2 +/- 64.8) than in females (F22: 89.2 micron(s) +/- 14.2, D1: 64.1 micron(s)2 +/- 19.7) and dropped to about 30 X micron(s)2 in both sexes on D6. Female rats given a single injection of testosterone propionate (TP) before D7 showed a significant increase in the number of fibers, but no increase in cross-sectional area. TP given after D7 had no effect on the fiber number, but increased the average cross-sectional area. The increase in fiber number induced by postnatal TP treatment was a permanent effect, still quantifiable in 15-month-old females. We conclude that the sexual dimorphism of the rat LA muscle is principally due to a dramatic increase in the MU number in male muscles during the perinatal period, rather than to involution of the fibers in female muscles as it is widely accepted. This increase seems to be, at least partly, under the control of testosterone.     

Analysis of the sternotrachealis muscle fibers in some Anseriformes: histochemistry and sex differences

Histochemical characteristics and sizes of the fibers of the sternotrachealis (ST) muscle have been investigated in some Anseriformes (mallard, Pekin duck, Muscovy duck, and goose) of both sexes. A sexual dimorphism has been shown in the muscle of the species examined. In the mallard and Pekin duck, the male ST muscle shows type IIIA fibers in addition to the type I, IIA, and IIB fibers observed also in the female. In the Muscovy duck, the male muscle has only type I and IIA fibers, whereas the female muscle presents type I fibers and both types IIA and IIB fibers. Moreover, the mean frequencies for each fiber type were significantly different between males and females. In the goose, both male and female muscles present only type I and IIA fibers. In all the species examined, the mean areas of each fiber type are significantly different between male and female, being always larger in the male muscles. The anatomical sexual dimorphism observed in the ST muscle is discussed in relation to function.     

Sexually differentiated,androgen-regulated,larynx-specific myosin heavy-chain isoforms in <Emphasis Type="Italic">Xenopus tropicalis</Emphasis>; comparison to <Emphasis Type="Italic">Xenopus laevis</Emphasis>

We have shown that the sarcoplasmic myosin heavy-chain (MyHC) isoform xtMyHC-101d is highly and specifically expressed in the larynx of the aquatic anuran, Xenopus tropicalis. In male larynges, the predominant MyHC isoform is xtMyHC-101d, while in females, another isoform, xtMyHC-270c, predominates. The X. tropicalis genome has been sequenced in its entirety, and xtMyHC-101d is part of a specific array of xtMyHC genes expressed otherwise in embryonic muscles (Nasipak and Kelley, Dev Genes Evol, in press, 2008). The administration of the androgen dihydrotestosterone increases the expression of xtMyHC-101d in juvenile larynges of both sexes. Using ATPase histochemistry, we found that in adults, X. tropicalis male laryngeal muscle contains only fast-twitch fibers, while the female laryngeal muscle contains a mix of fast- and slow-twitch fibers. Juvenile larynges are female-like in fiber type composition (44% slow twitch, 56% fast twitch); androgen treatment increases the percentage of fast-twitch fibers to 86%. xtMyHC-101d predominates in larynges of dihydrotestosterone-treated juveniles but not in larynges of untreated juveniles. We compared the larynx-specific expression of xtMyHC genes in X. tropicalis to the MyHC gene expressed in X. laevis larynx (xlMyHC-LM) by sequencing the entire xlMyHC-LM gene. The androgen-regulated xtMyHC that predominates in the male larynx of X. tropicalis is not the gene phylogenetically most similar to xlMyHC-LM at the nucleotide level but is instead a similar isoform found in the same MyHC array and expressed in the embryonic muscle.     

Testis mass of the spinifex hopping mouse and its impact on fertility potential

Captive-bred Australian spinifex hopping mice Notomys alexis have very small testes regardless of their age. Compared with other rodents of similar body mass, these animals also produce and store comparatively low numbers of spermatozoa. In the present study, we thus ask the following questions: (1) what is the testis weight of sexually mature spinifex hopping mice in the natural environment and does this change at different times of reproductive activity of the population and (2) what is the fertility potential of adult sexually mature males? The results show that wild-caught individuals, like the captive-bred animals, invariably have very small testes, regardless of whether females in the population are, or are not, reproducing. Spermatogenesis continues at times when females are reproductively inactive, thus suggesting males may remain potentially able to inseminate females as soon as they enter oestrus. In spite of their very small testes and low epididymal sperm stores, an adult male can fertilize the ovulated oocytes of at least two females within a period of a few days. Thus, although sperm reserves are likely to be heavily depleted after the first ejaculation, males rapidly return to maximum fertility, which is no doubt due, at least in part, to the short sperm epididymal transit time that occurs in this species.