Sexual dimorphism in the regulation of meiotic process in the rabbit

Meiosis, mitosis, and apoptosis during fetal and postnatal periods were investigated in order to explore mechanisms of sexual dimorphism in initiation of germ cell meiosis. Gonads were obtained from Japanese white rabbits from 23 to 51 days postcoitum (dpc). Gonadal thin sections were stained with hematoxylin and eosin. Germ cell alkaline phosphatase and apoptosis were detected with histochemical and immunohistochemical methods, respectively. In the ovary, meiotic germ cells were initially recognized at 29 dpc and arrested after enclosure within follicles. Similarly, meiotic germ cells were recognized outside seminiferous tubules at 29 dpc, but no meiotic figures were identified in intratubular spaces. Apoptotic germ cells were not recognized in the intratubular spaces before 35 dpc, and no apoptotic figures were recognized in the ovary during the period studied. In conclusion, the initiation of meiosis in testicular interstitial tissue at the time comparable to that in the ovary indicates that germ cells of both sexes have the ability to enter meiosis during the same stage of fetal development; and it appears most likely that delayed initiation of meiosis in the intratubular space is attributable to meiosis-inhibiting substance(s) present in seminiferous tubules.     

Sex differences in the regulation of embryonic brain aromatase

Oestrogen formed from androgen by aromatization plays a critical role in the sexual differentiation of the male brain and behaviour. A question which has still to be answered is what regulates the gender-specific changes in aromatase activity forming oestrogen during sensitive periods of brain growth. Using a primary cell culture technique and sexed embryos, we have shown that in the fetal mouse brain, oestrogen formation in the male is neuronal rather than glial and aromatase activity is regionally localized, being higher in the hypothalamus than in the cortex. The aromatase activity measured from cells in culture has the same enzyme binding affinity (apparent K_m 40 nM) as intact brain samples. Neurones developing in the embryonic male brain (embryonic day (ED) 15) contain higher aromatase activity (V_max, 895 fmol/h/mg protein) than the female (V_max, 604). Although a sex difference exists at early stages of embryonic development (ED 13), the embryonic aromatase system is regulated by steroids later in fetal development. The developing aromatase-containing neuroblasts probably form processes which connect to other aromatase neurones. Immunoreactive staining with an aromatase polyclonal antibody identifies an increase in numbers of aromatase-immunoreactive hypothalamic neuronal cell bodies following testosterone treatment. Testosterone treatment also causes both stimulation of neurite growth and branching as well as functional maturation of aromatase neurones. In particular, there is an increase in aromatase activity per neurone as well as a dramatic increase in the number of neurones expressing the enzyme. Both the functional and morphological changes depend on androgen receptor stimulation for several days in vitro. This conclusion is supported by colocalization studies which reveal a high number of fetal hypothalamic aromatase neurones co-expressing androgen receptor. We conclude that testosterone influences the growth of male hypothalamic neurones containing aromatase at a sensitive period of brain development. Endogenous steroid inhibitors of aromatase, probably formed within the neuroglia, also play a role in the control of oestrogen production. An endogenous 5-reduced metabolite of testosterone, 5-androstanedione, is almost as potent in inhibiting neuronal hypothalamic aromatase activity (K_i = 23 nM) as the synthetic non-steroidal inhibitors such as the imidazole, fadrozole, and the triazoles, arimidex and letrozole. It is clear that the oestrogen-forming capacity of the male hypothalamus has the special characteristics and plasticity of regulation which could affect brain differentiation at specific steroid-sensitive stages in ontogeny.     

Sexual dimorphism and developmental patterns in the external morphology of the vaquita,Phocoena sinus

A total of 56 vaquitas (Phocoena sinus) were examined to evaluate their sexual dimorphism and isometric and/or allometric growth in 35 external characteristics. Absolute and relative (to total length) measurements and growth rates were compared between sexually immature and mature females and males. T‐tests and analysis of variance (ANOVA) and covariance (ANCOVA) were used to evaluate sexual dimorphism. Sexual dimorphism in the vaquita was detected in the total length, head region (from blowhole to tip of upper jaw), anterior section of the body (from dorsal fin to tip of upper jaw), dorsal fin and the genital and anal regions. Fluke width is relatively larger in mature males than immature males, but in females this relative metric does not change during their development. In addition, males present a higher dorsal fin. These somatic changes are probably related to the swimming capacity (speed, agility, maneuvering) during the breeding season and/or foraging activities. A linear model of growth was used to determine possible proportional changes with respect to total body length through the development of 33 external characteristics. The anterior region of the body and the flippers were relatively larger in immature individuals than in mature ones.     

Sexual dimorphism in proconsul

One of the more important sources of variability in primate species is sexual dimorphism. Most Primates heavier than five kilos bodyweight are sexually dimorphic, both in body size and in shape of certain hard tissues. Despite these facts, most of the fossil Primates from East African Miocene deposits were originally perceived as being monomorphic, a perception which has propogated through the literature. Re-examination ofProconsul from various sites in Western Kenya results in the view that it was as dimorphic in its splanchonocranium and in bodyweight as chimpanzees and gorillas. The clearest evidence comes from Rusing Island, where adequate samples are known of two morphs, traditionally identified as two species, but more likely to represent two sexes of a single species,P. nyanzae. Co-occurrence of the two morphs is 100% at the various Rusinga sites. Less complete samples have been collected from the Tinderet sites os Koru and Songhor, yet what is available shows that similar patterns of dimorphism characterise the speciesP. africanus andP. major, and that the co-occurrence of the two morphs in each species is 100%. The identification of fossils taking into consideration the role of sexual dimorphism clarifies many of the old debates in which individual specimens frequently shifted between different species, mainly on the basis of metric rather than morphologic evidence. Consequently, the distribution of the species ofProconsul is rather different after accounting for dimorphism, than it was before.     

昆虫的雌雄二型现象

对发生雌雄二型现象的昆虫类群、生态因子及进化进行了概括总结;还特别介绍了长足虻科昆虫雌雄二型的相关方面;并简要讨论了雌雄二型与性选择的关系。     

Sexual dimorphism inSebastes

Synopsis Sexual dimorphism and factors that may cause it were investigated in 34 species of the genusSebastes. Sexual dimorphism in standard length and morphometric characters are fairly common in rockfish. In many species males are shorter than females. However in males head length, width of orbit, interorbital width, length of upper jaw, longest pectoral fin ray and longest dorsal spine tend to be larger at a specified size than in females. Water-column species tend to be more dimorphic than demersal species. We suggest that the observed differences in dimorphism in standard length may be related to differences in mating and territorial behavior. Dimorphisms in morphometric measurements may be related to compensation in feeding ability for reduced standard length of males, mating and territorial behavior.     

Analysis of spatiotemporal regulation of aromatase in the brain using transgenic mice

Brain aromatase is widely distributed in the vertebrates, from fish to mammals, and plays important roles in functional reproductive behavior through production of estrogen as a neurosteroid. It is expressed only in the nerve cells of specific brain regions with a transient peak in the neonatal period when sexual behavior becomes organized, and therefore provides a good model system to study regulatory mechanism of cell-specific, brain region-specific, and developmental stage-specific expression.

To elucidate spatiotemporal regulation of brain aromatase, we prepared transgenic mice carrying a reporter gene under the promoter of brain-specific exon 1f of the mouse aromatase gene. The reporter transgene carrying a 6.5 kb upstream region of the brain-specific promoter accurately reproduced the spatiotemporal expression patterns of aromatase in mouse brain, whereas transgenes carrying smaller fragments of the promoter showed ambiguous or inconsistent expression patterns.

The binding sites of Aro-AI, Aro-AII, and Aro-B for nuclear factors were also identified in the proximal region of the exon 1f brain-specific promoter. Introduction of a mutation into the Aro-AII site in the reporter transgene carrying −6.5 kb promoter region of exon 1f caused complete alteration of the spatiotemporal expression pattern of the reporter gene in the transgenic mice.

These results indicate that the −6.5 kb promoter region of exon 1f is the minimal essential element for brain-specific regulation, with both proximal and distal promoter regions required for accurate spatiotemporal expression of aromatase in the mouse brain.     

Sexual size dimorphism in anurans

Several hypotheses have been proposed to explain the direction and extent of sexual size dimorphism in anurans (in which males are usually smaller than females) as a result of sexual selection. Here, we present an analysis to test the hypothesis that sexual dimorphism in anurans is largely a function of differences between the sexes in life-history strategies. Morphological and demographic data for anurans were collected from the literature, and the mean size and age in each sex were calculated for 51 populations, across 30 species and eight genera. Comparisons across 14 Rana species, eight Bufo species and across the genera showed a highly significant relationship between size dimorphism, measured using the female-male size ratio, and mean female-male age difference. A comparison of a subset of 17 of these species for which phylogenetic information was available, using the method of independent contrasts, yielded a similar result. These results indicate that most of the variation in size dimorphism in the anura can be explained in terms of differences in the age structure between the sexes in breeding populations. If sexual selection has an effect on size dimorphism in anurans, it is likely to be only a secondary one.     

Studies on the role of catecholamines in the regulation of the developmental pattern of hypothalamic aromatase

Experiments were conducted to study the regulation of the developmental pattern of aromatase in the forebrain of the perinatal rat. Two experimental designs were used: aromatase measured in primary cultures of fetal hypothalamic cells and in cell-free preparations of forebrain tissue excised at varying ages. In cultured cells, aromatase decreased logarithmically at a slow rate (t1/2 = 7.8 days). Norepinephrine caused a pronounced dose (4 × 10⁻⁶ M) and time-dependent (2–6 days) drop in aromatase without affecting the levels of 5-reductase or substance P. In isolated tissue, aromatase activity was compared with the concentrations of norepinephrine and dopamine in the forebrain of males vs females at different perinatal ages and in discrete forebrain areas at postnatal day 4. In no case was a sex difference in catecholamines seen. An overall developmental decline in aromatase was associated with developmental increases in catecholamine levels. Acute treatment with the beta-agonist, isoproterenol, had no effect on brain aromatase activity.     

Sexual dimorphism in immune defense

ABSTRACT Sex differences in parasitism are common and may depend on sex differences in intensity of sexual competition, immunosuppression, or exposure to parasites. We used a large data set on the mass of two immune defense organs in birds (the bursa of Fabricius and the spleen) to test for consistent sex differences in immune defense. Males had a relative spleen mass that was consistently smaller, but more variable, than that of females across species of birds. A sex difference in the size of the spleen was not present among juveniles but was large and statistically significant among adults. The suppression of spleen mass in adult males increased with a measure of intensity of sexual selection: the frequency of extra-pair paternity. These findings suggest that sexdifferences in parasitism may arise as a consequence of sex differences in immune function, as mediated by sexual selection.     

Sexual dimorphism in primate teeth

A new measure of sexual dimorphism in tooth size (Relative Male Tooth Size) is estimated for a sample of 39 primate species and is used to test predictions concerning the adaptive significance of sexual dimorphism in primate tooth size. It is concluded that both sexual selection and predator defence are important selective forces producing marked sexual dimorphism in the teeth used for fighting.     

Sexual dimorphism in the growth of the cranium

The major sexual dimorphisms in body size appear at puberty but, by then, 95% of the growth of the cranium is completed. As sexual dimorphism in the cranium is as great as for other parts of the body, this suggests that it must appear at an earlier age, and that cranium/body size ratios for the two sexes will vary during growth. Results from a longitudinal study of Montreal children are used to investigate this phenomenon. The effect is expressed quantitatively by proportional growth and growth velocity curves, based on the final size of boys, which show that the dimorphism indeed makes an early appearance. The data are also analyzed on an age scale relative to the ages of peak growth velocity in stature, derived from the individual growth curves. This shows that although there is a minor pubertal spurt in growth for the external cranial dimensions of boys, it contributes relatively little to the final dimorphism in cranial size. To summarize this aspect of growth, an index of cephalization is calculated: head length × head width/stature. Cross-sectional standards for the change of the mean index with age show a linear decline for boys and girls until puberty, with a constant difference between them. After puberty, the index becomes equal in the two sexes. Individual development curves for the index are however not linear.     

Sexual dimorphism inDryopithecus africanus

Among the numerous specimens presently classified withinDryopithecus africanus only one can be identified as a male of this species. Poor sampling is not the reason for the unequal numbers of male and female specimens. Rather, the males have been classified elsewhere, specifically withinDryopithecus nyanzae and Kenyapithecus africanus. The specimens to be transferred from these two taxa are proved to be males ofD. africanus. The newly transferred males are compared with the females to show the cranial dimorphism of the species.     

Sexual dimorphism in the baboon facial skeleton

Baboons exhibit marked sexual dimorphism in many aspects of their morphology. Dimorphism is especially pronounced in the face. We use finite-element analysis to investigate the ontogeny of sexual dimorphism in a cross-sectional sample of baboon (Papio sp.) faces. This method provides detailed quantitative information about size and shape changes at anatomical landmarks in the face during growth. Allometric results suggest that sexual dimorphism in facial size and shape is produced by ontogenetic scaling: males and females share a common ontogenetic trajectory. Analyses of growth in time, which complement allometric analyses, show that female growth slows much earlier than male growth, accounting for the differences between sexes. Local size and local shape follow similar patterns of growth, but changes in these variables are slower in females. Local and global facial size are much more dimorphic than local and global facial shape.     

Sexual dimorphism and growth hormone regulation of a hybrid gene in transgenic mice.

The sexually dimorphic expression of the urinary protein genes of mice (Mup genes) in the liver is mediated by the different male and female temporal patterns of circulating GH. Normal females were induced to male levels when GH was administered by injection to mimic the male GH pattern, showing that expression at the male level does not require a male sex steroid status in addition to intermittent GH. Two Mup-alpha 2u-globulin hybrid transgenes with different Mup gene promoters showed sexually dimorphic expression, and their expression in females increased to male levels upon testosterone treatment. GH-deficient (lit/lit) mice did not express these transgenes, and GH-deficient females did not respond to testosterone treatment, showing that GH was required for induction. Both normal and GH-deficient females were induced to male levels when GH was administered by injection. This is the first report of a transgene responsive to GH. A transgene consisting of a Mup promoter fused to a Herpes simplex virus thymidine kinase reporter sequence also showed sexual dimorphism, although to a lesser degree. It was expressed at the same level in normal females and GH-deficient mice of both sexes and was induced when GH-deficient mice were treated with GH. We propose that this transgene has a basal constitutive expression, possibly due to the absence of any rodent DNA downstream of the promoter. Since expression of the transgene was significantly induced by GH, the GH response is due at least in part to sequences in the promoter region.     

Sexual dimorphism in the face of Australopithecus africanus

Recently discovered crania of Australopithecus africanus from Sterkfontein Member 4 and Makapansgat enlarge the size range of the species and encourage a reappraisal of both the degree and pattern of sexual dimorphism. Resampling methodology (bootstrapping) is used here to establish that A. africanus has a greater craniofacial size range than chimpanzees or modern humans, a range which is best attributed to a moderately high degree of sexual dimorphism. Compared to other fossil hominins, this variation is similar to that of Homo habilis (sensu lato) but less than that of A. boisei. The finding of moderately high dimorphism is corroborated by a CV-based estimate and ratios between those specimens considered to be male and those considered to be female. Inferences about the pattern of craniofacial dimorphism in the A. africanus face currently rely on the relationship of morphology and size. Larger specimens, particularly Stw 505, show prominent superciliary eminences and glabellar regions, but in features related in part to canine size, such as the curvature of the infraorbital surface, large and small specimens of A. africanus are similar. In this respect, the pattern resembles that of modern humans more so than chimpanzees or lowland gorillas. A. africanus may also show novel patterns of sexual dimorphism when compared to extant hominines, such as in the form of the anterior pillar. However, males of the species do not exhibit characteristics of more derived hominins, such as A. robustus. Am J Phys Anthropol 108:97–127, 1999. © 1999 Wiley-Liss, Inc.