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Relationships between prolactin, LH and et al. Distribution and regulation by oestrogen of fully broody behaviour in bantam hens. The and gonadotropin releasing hormone receptor mRNAs in the male role of hypothalamic vasoactive intestinal polypeptide in the main- chicken. A chicken gonadotropin-releasing hormone receptor that immunohistochemistry. Identification of Sharp, P. Takahashi, T. Arginine vasotocin binding component in the uterus shell gland of Sherwood, N. Identity of gonadotropin-releasing hormone in passerine birds: Takahashi, J. Multiple redundant circadian comparison of GnRH in song sparrow Melospiza melodia and oscillators within the isolated avian pineal gland.

A, starling Sturnus vulgaris with five vertebrate GnRHs. Shimada, K. Arginine vasotocin Evidence for alternative splicing of the chicken vasoactive intestinal AVT release in relation to uterine contractility in the hen. Tanabe, Y. Differences in the endocrine mechanism of ovulation Shimizu, M. Identification of a novel of the domestic fowl and the Japanese quail. Follett Ed. Bangor: University College of North Wales. Shimizu, T. Cholera toxin Tanaka, K.

Vis Neurosci. Sibley, C. Distribution and Taxonomy of Tobari, Y. Identification of gonadotropin-inhibitory hormone in the Silver, R. Coexpression of opsin- and VIP-like-immu- and brain distribution. Cell Tissue Toth, M.


Binding of prostaglandin Res. F2a to membranes of shell gland muscle of laying hens: correlations Small, T. Mura, E. Photoperiod-independent hypothalamic Tsutsui, K. Hypothalamic LPXRF-amide peptides regulation of luteinizing hormone secretion in a free-living Sonoran in vertebrates: identification, localization and hypophysiotropic desert bird, the Rufous-winged Sparrow Aimophila carpalis. Brain activity. Freeman Ed. Demonstration, localization, and development of and Biochemistry of the Domestic Fowl pp.

London, UK: galanin receptors in the quail oviduct. Academic Press. Devel- Stevenson, T. Effects of social cues on effect of ovarian sex steroids on galanin receptor induction. Kikuchi, M. A novel avian hypothalamic peptide Stevenson, T. Cloning of gonadotropin-releasing hormone I complemen- , e Identification, localization and mediating seasonal changes in reproduction. In regulation of passerine gonadotropin-releasing hormone-I messenger press. Stokes, T. The telen- Ubuka, T. Melatonin induces the expression of gonadotropin-inhibitory canaria, in stereotaxic coordinates. USA, , e Ubuka, T.

Wilson, F. Thyroid hormone acts centrally to Ubuka, T. Gonadotropin-inhibitory hormone neurons interact directly arborea for vernal and autumnal components of seasonality. Endocrinology, , e, Erratum in: Endocri- Wilson, S. Effect of photoperiod on the nology , The control of reproductive physiology Wilson, S. Variations in plasma LH levels during and behavior by gonadotropin-inhibitory hormone. Environmental and endocrine control of repro- Developmental changes in galanin in lumbosacral sympathetic duction: an ecological approach.

Wada Eds. Developmental Tokyo, and Springer-Verlag. Control of testicular cycles in the song sparrow, Coturnix japonica hypothalamoehypophysial system. Endo- Melospiza melodia: interaction of photoperiod and an endogenous crinol. Control of seasonal reproduction Gonadotropin-inhibitory hormone inhibits gonadal development and in temperate-zone birds. New York, male quail. NY: Karger.

Distribution of a novel avian Wingfield, J. Effect gonadotropin-inhibitory hormone in the quail brain. Underwood, H. Rhythms, 5, e Melatonin J. Schneider Eds. Cam- rhythms in the eyes, pineal bodies, and blood of Japanese quail bridge, MA: M. Coturnix coturnix japonica. Communication Underwood, H. Circadian organi- in vertebrate aggression and reproduction: the role of hormones. In zation and the role of the pineal in birds.

Neil Eds. Van Gils, J. Evidence that cells expressing Balthazart, J.

Fant du det du lette etter?

USA, 86, e Yamada, S. Immunohistochemical Van Tienhoven, A. Mammalian luteinizing localization of vasoactive intestinal polypeptide VIP -containing hormone-releasing hormone induces ovulation in the domestic fowl. Wada, Y. Phototransduction molecules Yamamoto, K. Chicken pineal Cry in the pigeon deep brain.

Warren, D. Influence of light on ovulation in Neurosci, Lett. Yamamoto, N. Wechsung, E. A possible role of prosta- Migration of GnRH-immunoreactive neurons from the glandins in the regulation of ovum transport and oviposition in the olfactory placode to the brain: a study using avian embryonic domestic hen? Prostaglandins, 12, e Neither retinal nor pineal photoreceptors mediate Yamamura, T. Plasma and pituitary luteinizing endfeet in Japanese quail. T 3 implantation mimics photoperiodically reduced Wilson, F.

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The photoperiodic control circuit encasement of nerve terminals by glial processes in the median in euthyroid American tree sparrows Spizella arborea is already eminence of Japanese quail. Yasuo, S. The reciprocal switching of two thyroid hormone- Tissue-specific alternative splicing of turkey pre- activating and -inactivating enzyme genes is involved in the photo- provasoactive intestinal peptide messenger ribonucleic acid, its periodic gonadal response of Japanese quail. Endocrinology, , regulation, and correlation with prolactin secretion.

Endocrinology, e Youngren, O. Regulation Circadian clock genes and photoperiodism: comprehensive of prolactin secretion by dopamine and vasoactive intestinal peptide analysis of clock gene expression in the mediobasal hypothalamus, at the level of the pituitary in the turkey. Neuroendocrinology, 68, the suprachiasmatic nucleus, and the pineal gland of Japanese quail e Yin, H. A novel G protein- Dopaminergic control of prolactin secretion in the turkey. Matsuda, Y. Molecular analysis of avian circadian Yuan, H. Identification of the suprachiasmatic nucleus in birds. Yuan, H. Signals, 1, e Hirunagi, K.

Light-induced hormone conver- Zann, R. The sion of T4 to T3 regulates photoperiodic response of gonads in birds. Emu, 95, e Chapter 2. Hurley and H. Adult testicular function gland Kuenzel, The production and secretion of is modulated by a myriad of external factors and orchestrated by these hormones is presumably regulated also by the numerous hormones that together enable birds to adapt to and inhibitary neuropeptide, gonadotropin-inhibiting hormone breed in diverse habitats worldwide.

These factors have generated GnIH Tsutsui et al. The mechanisms of action of a wide range of avian reproductive strategies, which has further GnIH are not fully elucidated, but mounting evidence shaped testicular structure and function. This chapter describes the suggests that this neuropeptide acts at multiple levels: mechanisms that control avian exocrine and endocrine testicular hypothalamus, anterior pituitary gland, and potentially the functions.

It also discusses testicular Several studies have also demonstrated an important dysfunction and the potential effects of anthropogenic distur- role for thyroid hormones in regulating the seasonality of bances on testis function. Details of this biology. Recent research has focused on how photic information is transduced into changes in GnRH cell function. One current model 1. The the testes to initiate or terminate reproduction. In vertebrates, the primary neural system responsible The regulation of seasonal reproduction in birds may for regulating reproduction consists of hypothalamic also involve a central role for brain-derived vasoactive neurons that secrete gonadotropin-releasing hormone intestinal peptide VIP.

This 3. They contain interstitial tissue and seminiferous tubules, which are the site of spermatogenesis and, in developed testes, make up most of the testicular mass. Birds differ from mammals in that Section 3. An extreme case of avian testicular synthesis by day two of incubation, but around day five asymmetry is found in coucals Centropus sp.

Sexual asymmetry is discussed below Section 7. Leydig Cells chos Koba et al.

Avian Sexual Differentiation beyond the Gonads: Role of Hormones and Direct Genetic Effects

As demon- and then rapidly increases Scheib, Current A. Cells in these species show morphological changes with Z chromosome two in males, one in females or the that generally parallel their androgen secretion. For presence of the W chromosome in females. Similar changes are 4. Testicular Androgen Hormone Synthesis et al. Photomanipulations of captive birds also support this conclusion.

Depending on the presence of cofactors degeneration. The testes of rounded by a basal lamina consisting of fibroblasts, immature birds, however, can produce more AND than T, myoepithelial cells, and connective tissue. Spermatogenesis Sertoli cells are elongated cells that extend from the basal lamina of seminiferous tubules to the lumen. Adjacent As in mammals, spermatogenesis in birds is defined as the Sertoli cells are joined by tight junctions, thereby dividing formation of mature spermatozoa from spermatogonia and the seminiferous epithelium into basal and distal adlumi- it includes three sequential steps: spermatocytogenesis, nal regions, which form an effective bloodetestis barrier spermatidogenesis, and spermiogenesis.

Some studies indicate that Sertoli cells multiply e. More recently, Bakst, Akuffo, Trefil, and Brillard 4. As a result of these cell divisions, and glutamate, that are secreted into the accessory gland each Ad spermatogonium can generate 32 spermatids. The structure of mature sperm in passerines is In passerines, the distal end of the vas deferens forms relatively conserved, but differs in several respects from the seminal glomus, a specialized sperm storage accessory that in nonpasserines for details see Jamieson, The glomus in passerines may have considerably from one species to another e.

The dog Canis lupus familiaris : 42 days Ghosal et al. Thus, the glomus may accumulate a shorter and perhaps more consistent duration of sper- sperm during the night for use during the day in the course matogenesis than in mammals chicken: 12e13 days De of multiple copulations. Sperm Maturation, Transport, and 4. Sperm in nisms. In Japanese quail, this apoptosis in the avian testes. In Two anterior pituitary hormones, LH and FSH, play domestic ducks, plasma LH and T follow the same general a primary role in the control of testicular endocrine and time course of changes during constant light exposure exocrine functions.

In sectomy. This surgery causes testicular involution in the several species, seasonal changes in plasma LH parallel chicken Brown et al. Madekurozwa et al. Receptors for these mediators in Sertoli The effects of in-vivo GTH treatment on testicular cells and seminiferous tubules presumably influence aspects function were studied by Brown et al. Repeated injections of purified chicken LH to young steroid production and those in epididymal tissue may males of this species induce differentiation of the inter- regulate sperm maturation and seminal fluid composition. These injections also The expression of various chemical messengers and stimulate spermatogonial division and differentiation of their receptors by testes is not entirely surprising as testes Sertoli cells, but whether these effects are direct or result are sites of extensive cell proliferation, differentiation, from increased androgen secretion has not been deter- steroid synthesis, and apoptosis.

These complex processes mined. The testicular effects of LH in Japanese quail are presumably require the participation of numerous regula- specific as they are not replicated by treatment with purified tory hormones and growth factors, particularly as birds chicken FSH fractions. The specificity of LH action on often exhibit large seasonal variation in testis size. The androgen production in vivo and in vitro was confirmed by following sections summarize avian research on the direct measuring the effects of purified chicken, turkey, and regulation of steroidogenesis and spermatogenesis by ostrich LH in several avian species chicken, Japanese LH, FSH, and some other mediators, and they outline quail, mallard duck, and turkey Chase, In these potentially promising future avenues for research on this species, LH generally induces a dose-related increase in topic.

The presence of 3b-HSD in Leydig cells and the The production and maturation of sperm in mammals generally hydrophobic nature of steroids suggest that LH primarily depends on FSH acting on Sertoli cell receptors stimulates androgen production and not just release. Duck D D Yang et al. Abbreviations for life stages are as follows: E, embryo; J, juvenile; A, adult.

Saxena, ; Kawashima, Tsutsui, Sax- Goldsmith, It was found that testes contain wise show a correlation between plasma FSH and testicular specific, high affinity nanomolar range binding sites for development. Following Ishii, ; Kawashima et al. In white-crowned photostimulation, plasma FSH increases gradually for sparrows that are transferred from short to long days to approximately four weeks, concurrently with testicular stimulate gonadal development, FSH binding capacity development and the initiation of spermatogenesis.

Similar observations quail Follett, , photoinduced plasma FSH decreases have been made in Indian weaver birds Tsutsui et al.

Research on suggesting that early but not late stages of spermatogenesis Japanese quail suggests that photoinduced increases in require elevated plasma FSH. Further, plasma FSH is often gonadal FSH binding capacity result from upregulation of higher than baseline in birds with regressed testes. Further research is needed to fully Ishii, The mechanism mediating this putative understand the significance of this observation. In this species, plasma FSH increases in parallel Zeller, The gene coding for avian testicular FSH with testis development following photostimulation with receptors has been sequenced and cloned, and the putative photoperiods of various durations.

By contrast, quail exposed to 12 hours of light daily to study the regulation of these receptors. The role of FSH also has been studied through 5. Gonadotropin-inhibiting Hormone hormone-administration experiments e. Administration of chicken purified FSH to hypophysecto- The discovery of the hypothalamic inhibitory neuropeptide mized Japanese quail stimulates Sertoli cells and germ cell GnIH Tsutsui et al. As shown recently, avian testes also contain GnIH is revealed by the fact that they are not observed following and its receptors Bentley et al.

Hormones and Reproduction of Vertebrates, Volume 4: Birds | NHBS Academic & Professional Books

A this volume. By contrast, other investigations found no evidence for an obligatory role of the pineal gland or its secretions in the regulation of seasonal reproduction 5. Inhibin and activin are dimeric glyco- related to the activity of the reproductive system, sug- proteins that share a common a subunit but differ with gesting inhibitory effects of pineal gland secretions on respect to their b subunit; inhibin has either a single bA or this activity. These structural similarities Takahashi, This observation is consistent with an have made separating the biological effects and responses inhibitory effect of MEL on the reproductive system but of the two hormones to FSH at the testis level challenging.

Melatonin may also affect gonadal tissue directly. In the Japanese quail, the number of testicular MEL binding sites is photoperiodically regu- 5. Melatonin MEL lated; birds have fewer such sites when exposed to short Melatonin MEL in birds is produced primarily by the rather than long days Pang et al. Prolactin PRL induction of photorefractoriness in relatively photo- refractory species Dawson et al.

This influence presumably effects in this species Suescun et al. Prolactin results in part from central effects of PRL. Maximal circulating PRL concentrations are reached toward the end of the reproductive season, coinciding with 5. During a prolonged stressful situation, plasma T and testicular involution but does not cause photo- LH levels are thought to decline as a result of elevated refractoriness. This evidence comes, in particular, from plasma CORT. Supporting this hypothesis, CORT admin- studies on the European starling, a species that responds to istration blocks photoperiod-induced testicular develop- chronically long days by completely losing sensitivity to ment in the male Japanese quail Deviche, Massa, Bottoni, this stimulus absolute photorefractoriness.

Further, plasma ulative. Testes express glucocorticoid receptors Table 2. As in other species, PRL in the Japanese ductive hormones. In the white-crowned Wingfield et al. Contrary to the situation in the European plasma T decreases within one hour and fifteen minutes, starling, however, PRL may play a critical role in the respectively, of capture and handling.

In contrast to these studies, other research lead to the development of probes that can be used to has found a positive relation between plasma T and short- address the specific role of GH on avian testicular function. This increase in roosters is not seen in castrated birds, indicating that T is presumably secreted by testes and 5.

In the latter study, selected for divergence in adrenocortical activity. An Avian Androgen-binding Protein? Studies in laboratory rodents found that anabolic whether avian seminiferous tubules contain ABP. Similar mechanisms as in mammals may, therefore, produce this protein is unknown. Corticosterone-binding operate in birds. In many In many avian species, testes are maximally developed species, increasing photoperiod from short winter days during the breeding season, which is preceded and followed stimulates testicular recrudescence Hamner, ; Daw- by a period of reproductive quiescence during which testes son et al.

Little work has been done on the during the year Table 2. The dramatic morphological and physiological Birds can respond to even small photoperiodic changes changes associated with seasonal in- activation of the avian around the critical day length, defined as the shortest reproductive system require time.

Males must, therefore, be photoperiod that stimulates testicular development. The able to anticipate the start and end of the optimal time for duration of the critical day length varies intra- and inter- breeding to maximize their chances of successful repro- specifically. For example, the critical day length in starlings duction. The specific cues used to predict seasonal envi- is 12 hours light L and birds kept under a 12L : 12 dark D ronmental changes vary among species.

This section light schedule show normal testicular cycling Dawson, discusses abiotic cues used to determine the annual cycle of However, a minute shift in photoperiod testicular development, with particular emphasis on birds increasing to Thus, the directionality, rather than only the magnitude of a change in photoperiod, is important for 6. Abiotic Factors photoperiodic regulation of testicular development Daw- son, The same applies to equatorial species, in which 6.

Photophase small changes in both photoperiod i. Precipitation mating system Section 6. To our knowledge, no avian species has been iden- regions varies seasonally. In these regions, a seasonal tified that relies exclusively on nonphotic cues to time increase in photophase, and rising temperatures, are often. From M. Rowe, unpublished data. TABLE 2. Turkey Meleagris gallopavo Starling Sturnus vulgaris Canary Serinus canaria 50 Bentley et al. Redheaded bunting Emberiza bruniceps 45 Kumar et al. Siberian hamster Phodopus sungorus 5. Vole Microtus agrestis 1. By contrast, in regions such as the Sonoran Desert, combine to increase plant and insect food resources.

Birds in this region have moderately developed Huston, By contrast, comparative studies using testes in preparation for the breeding season and regressed several subspecies of white-crowned sparrows that breed at testes outside this season Perfito et al. By contrast, different latitudes found that temperature does not influ- in the arid center of the continent, where rains are unpre- ence testicular development in high-latitude breeders with dictable, zebra finches breed opportunistically, maintaining short, predictable breeding seasons. However, lower-lati- semideveloped, yet functional, testes year-round, even tude breeders with more flexible breeding seasons than when their body condition deteriorates as a result of poor arctic breeders show a positive association between environmental conditions Perfito et al.

Sil- Small, ; Deviche et al. Conversely, In both zebra finches and Aimophila sparrows, it warmer temperatures can advance testicular development remains unclear whether reproductive responses to precip- in southern-breeding great tits Silverin et al. Sug- Research comparing species inhabiting environments gesting direct effects, testicular regression in captive differing in seasonal temperature patterns, combined with rufous-winged sparrows is delayed in birds that receive food captive studies manipulating ambient temperatures and and water ad libitum and are exposed to artificial rain under photoperiod, should help untangle the role and mechanism fall-like photoperiod, compared to control sparrows that are of action of temperature on testicular function.

By contrast, other research suggests that rain 6. Food influences testicular function indirectly, by acting as a predictive cue for future increase in biomass availability The type, amount, quality, and predictability of food Zann et al. In environ- rains fell and breeding, suggesting that birds use rain to time ments where food availability is relatively predictable and reproduction so that young hatch when grass seeds are first breeding flexible, food supplementation can advance or ripening Zann et al.

Temperature altitude and presumably food availability Perfito et al. Similarly, food restriction at the onset of the provide a secondary cue that birds can use to predict future breeding season can delay seasonal testicular regression breeding conditions. These obser- changes seasonally in parallel with the photophase e.

As a result, repro- tion and development. For example, in chickens, high duction in these birds is not thought to be tightly regulated ambient temperatures impair testicular function and by photoperiod Hahn, Despite this, food alone does not artificial long days can increase testicular size even during act as the primary cue Hahn et al.

Testicular regression in the Japanese quail is et al. However, exposure to long photoperiod and ad- Wingfield et al. Opportunistic breeders breed less libitum food availability induce maximal testicular devel- seasonally, respond minimally to day length, and primarily opment Perfito et al. Perfito et al. These birds typically inhabit envi- ronments where the cues used to time reproduction are largely unpredictable. They must be capable of rapidly 6. Life History developing their gonads when conditions become favorable Perfito et al. Breeding strategy The degree to which a given species exhibits absolute or Seasonal breeders are often categorized based on the relative photorefractoriness varies within and between development of a refractory state at the end of their species.

For example, within cardueline finches, common breeding period rather than by the cues used to stimulate redpolls Carduelis flammea breed seasonally, pine siskins reproductive development. A combination of studies on free-ranging 6. Another study on cardueline finches found that red e. The closely related canary Serinus canaria , breeds flex- The seasonal breeders that are most easily defined are ibly in the wild Leitner et al. In these species, has generated genetic lines with characteristics of absolute transfer from short to long i. Studies on increasing or previously stimulatory day lengths Hamner, cardueline finches illustrate the diversity and complexity ; Nicholls et al.

Termination of absolute pho- that need to be considered in order to understand mecha- torefractoriness requires exposure to short days and coin- nisms that regulate avian reproductive cycles. Migratory vs. Other birds do Follett, and sparrows of the genus Aimophila not migrate, which may extend their breeding season and Deviche et al. Still other species onset and intensity of the monsoon varies considerably exhibit nomadic behavior with large home ranges and from one year to another. Similarly to the rufous-winged unpredictable annual movements Hahn ; Hahn et al.

Therefore, migrants must balance the energetic Hau, ; Hahn et al. However, in these species constraints associated with migration with the need to there appears to be an annual cycle of testicular recrudes- develop their reproductive system so as to minimize the cence and involution that is minimally influenced by impact on reproductive success and body condition Bau- photoperiod Hau et al.

Testicular Nomadic species such as crossbills and zebra finches, development in migratory species appears closely tied to which travel widely in search of food resources, rely changes in photoperiod, particularly on wintering grounds heavily on nonphotic factors to time reproductive devel- e. Species exhibiting this distance migrants may differ with respect to their respon- breeding strategy constitute particularly interesting models siveness to small changes in day length.

This difference to investigate the role of secondary cues in the timing of may in turn enable long-distance migrants to initiate reproduction. Replicating the energetic In many migratory species, males reach breeding demands of migration in captive birds is difficult and limits grounds before females, which provides time to complete our ability to study how migration impacts testicular testicular recrudescence and establish territories before functions.

Identi- brief breeding seasons, testicular growth often begins fying factors that influence the flexibility of the annual during migration and, as a result, testes can become fully testicular cycle should enable us to better predict how developed shortly after arrival on breeding grounds Bau- environmental perturbations such as global climate change chinger et al. Some species even produce sperm and Section 9.

Male Reproductive System - Hormonal Function and Regulation (sperm synthesis and maturation)

Brood parasitism ization Quay, ; In the during the breeding period Small et al. The latter species in temperate regions typically Wingfield, a. In species such as the rufous- 6. Many investigations on this breed until the summer monsoon, when day length is topic have emphasized effects of auditory and pairing cues decreasing Small et al. By contrast, little work has been done on 7. Linking Testosterone T and Sperm the effects of song on testicular function of other males. In Quality one of the few studies on this subject, exposure of rufous- Testosterone in vertebrates is essential for spermatogenesis, winged sparrows to conspecific song increased the effects but the negative feedback effect of T on the hypothalamus of long-day exposure on testicular growth Small et al.

No such effect has, however, been observed in the functions.

An species, is influenced by the presence of a mate. The impor- may reflect a treatment-mediated increase in copulation tance of the pair bond is also demonstrated in crossbills, rate that in turn results in reduced residual ejaculate which can develop their testes and breed in winter provided volumes Kast et al. Alternately or in addition, the that they receive proper food and are exposed to mates negative action of T on the hypothalamoepituitary gland Hahn et al.

Similarly, captive male ptarmigans axis may decrease testicular function, resulting in smaller Lagopus sp. In contrast to the above studies, Penfold free-ranging conspecifics Stokkan et al. The role of such factors should be considered number of sperm and the percentage of sperm exhibiting during studies, particularly those using captive birds from normal morphological characteristics, and T administration wild populations, on the environmental control of in some circumstances stimulates testicular functions e.

Deviche et al. Future studies should aim at eluci- dating the mechanisms mediating this increase and address 7. Work on this subject largely stems from three basic char- 7. Correlates of Testicular Size acteristics of birds: 7. Body size and age 1 The dependence of measurable behaviors on numerous Most testicular tissue is devoted to spermatogenesis and physiology-based processes such as territoriality and testis size is, therefore, often used to estimate sperm courtship displays; production Section 3 Moller, Large testes produce 2 Breeding structures and behaviors that are easily more sperm than small testes in the house sparrow Birk- quantifiable as potential costs of reproduction; head et al.

However, little empirical information resulting from numerous physiological adaptations concerning sperm production in wild birds is available with Section 6. Inter- and intra- selection pressures and are considered to be energetically specific comparisons suggest allometric associations costly. These between testis size and T profiles, particularly through observations have prompted researchers to correct testis comparative studies in avian taxa whose life-history traits sizes for variations in body mass, most often by using differ.

Seasonally maximal plasma T levels are thought to regression residual methods. These subject have provided largely consistent results, but intra- studies have primarily compared after-hatch-year males specific comparisons attempting to relate plasma T to with older males two or more years of age , as it is often testicular size have generated mixed results. For example, difficult to age birds, especially passerines, that are older a positive relation between these variables was reported for than two years.

Age-dependent differences in mean and were confounded by strong latitudinal variation, with maximal testis size may account for lower plasma T in tropical species having lower plasma T and smaller testes younger adult males compared with older males Sorenson, than temperate or high-latitude breeders Section 6. Similar latitudinal trends of et al. The length of the breeding season and, therefore, moen et al.

Superb fairy-wrens Malurus cyaneus potentially the period during which plasma T is elevated, are thought to endure intensive sexual selection. This hypothesis is supported by intraspecific few, and this topic warrants further investigation. It is tunities, the defense of females against their extrapair currently unclear whether positive correlations between forays, and the allocation of energy towards paternal care of testicular size and latitude reflect genetic differences offspring Ketterson et al.

Silverin et al. Evolutionary Explanations for Testicular Size Asymmetry 7. Theories of Sperm Competition Variation in size between the left and right testes has also Two seminal papers Parker, a, b; no pun intended drawn the attention of researchers. This of competition between the sperm of the mated males. This hypothesis is supported by studies showing that hemi- hypothesis, initially developed based on insect studies, has castration causes compensatory growth of the remaining become a predominant theme in avian research.

Sperm testis, regardless of which testis is removed Farner, Morton, competition may be especially prominent in birds. In situations of factors, such as GTHs. Within an individual, however, the sperm competition, males can gain more fertilizations by two testes may be differentially sensitive to GTHs, but the 1 producing greater quantities of sperm per ejaculate cellular basis of this potential difference has not been often inferred from studies of testicular sizedsee Section researched.

The compensation hypothesis predicts that 7. Few engage in this compensatory growth. Accordingly, within data exist on whether an increase in testicular size is a population the degree of testicular asymmetry should associated with increased sperm production. Further, this reflect overall male quality, with lower quality males having relation does not need to be linear because large increases less symmetric testes. Sperm length, in particular, may be an important thought to be associated with survival Birkhead et al. However, there is A flight efficiency by reducing paired testicular mass and, comparative study of 16 shorebird species revealed that therefore, wing loading.

Further, some predominantly terrestrial between sperm length and mating system, but rather birds chicken Hocking, , but also highly aerial a positive and a negative correlation between sperm length species e. Several models have sperm storage i.

Hormones and Reproduction of Vertebrates: Birds, Volume 4

The mediobasal hypothalamus MBH is thought to be the regulatory center for seasonal reproduction in birds. Collectively, these phenomena support the notion that the MBH plays central roles in regulating seasonal reproduction. In order to identify the components of the photoperiodic signaling cascade, genes differentially expressed in the MBH between LD and SD conditions were explored by subtractive hybridization. Expression of the DIO2 gene is significantly induced by a 1-h light pulse during the photoinducible phase Yoshimura et al.

These results suggest that local activation of TH in the MBH plays a key role in the regulation of seasonal reproduction. Chronic intraventricular administration of T 3 in quail stimulates testicular development even in SD conditions Yoshimura et al. These results confirm that the local concentration of T 3 at the MBH is increased by induction and repression of DIO2 and DIO3 , respectively, and facilitates photoperiodic gonadal development. In vertebrates, gonadal development is regulated by the HPG axis.

Gonadotropin-releasing hormone GnRH is secreted from the hypothalamus and stimulates the release of gonadotropins, such as luteinizing hormone and follicle-stimulating hormone, from the anterior pituitary gland. These hormones act on the testis and ovary to facilitate gonadal development and the production of steroid hormones, including androgen, estrogen and progesterone. Therefore, seasonal breeders activate the HPG axis only in the breeding season. TH is involved in neural development and plasticity in the central nervous system Bernal , raising the possibility that locally activated T 3 in the MBH promotes the development of testis via modification of neuroendocrine pathways.

Electron microscopy revealed that terminal morphology of GnRH neurons varied according to the photoperiod. GnRH nerve terminals are wrapped with endfeet of glial processes in SD conditions, whereas in LD conditions, the terminals contact the basal lamina of capillary vessels and allow GnRH secretion into the bloodstream Yamamura et al. It is possible that these morphological changes induced by MBH-localized T 3 allow gonadal development only in LD conditions. In order to elucidate the signaling pathway between light stimulus and DIO2 induction, the regulatory network of genes activated in the MBH upon light stimulus was analyzed using a chicken genome microarray.

Intracerebroventricular administration of TSH increases the testis size to the same degree as seen in the LD condition Nakao et al. These results demonstrate that PT-TSH is a master regulatory factor of the photoperiodic response downstream of photoreception. In addition to photoreceptors in the retina, nonmammalian vertebrates have extraretinal photoreceptors located in the pineal gland Okano et al.

Among the 12 opsin superfamily genes in the chicken genome, OPN5 is localized to the cerebrospinal fluid CSF -contacting neurons in the hypothalamic paraventricular organ PVO Nakane et al. Cell bodies of these neurons project dendrites to the third ventricle, and their axons terminate in the vicinity of the PT Nakane et al. In Xenopus oocytes with forced OPN5 expression, exposure to light ranging from ultraviolet UV -B to blue activates a membrane current under voltage clamp conditions, demonstrating that OPN5 is a short-wavelength-sensitive photopigment Nakane et al.

Testicular growth is observed in eye-patched, pinealectomized quail upon exposure to light in the UV-to-blue region of spectrum, supporting the hypothesis that OPN5 is a deep brain photoreceptor that regulates photoperiodicity in quail Nakane et al. The current understanding of the signal transduction pathway for photoperiodicity is summarized in Fig.

Figure 2 Download Figure Download figure as PowerPoint slide A schematic diagram of the signal transduction cascade for seasonal reproduction in birds. Light information is received by deep brain photoreceptors, such as OPN5, and then transmitted to the pars tuberalis PT of the pituitary gland, where TSH is induced. T 3 induces morphological changes in GnRH nerve terminals and glial processes, thereby facilitating GnRH secretion, resulting in gonadal development. The retina is the only photoreceptive organ in mammals.

Light information from the retina is transmitted to the pineal gland via the suprachiasmatic nucleus SCN , which is the location of the master circadian clock Morin et al. Melatonin is a serotonin-derived hormone produced in the pineal gland Reiter Upon synthesis, melatonin is immediately released into circulation. Therefore, information regarding environmental light—darkness cycles is physiologically represented by the duration of melatonin synthesis and secretion by the pineal gland.

Melatonin receptors are highly expressed in the PT Dardente et al. Intraperitoneal injection with melatonin inhibits DIO2 expression and regression of the testis even under LD conditions Watanabe et al. It is important to note that the phototransduction mechanism is different in mammals and birds, but the downstream signaling pathway regulating seasonal reproduction is conserved among avian and mammalian species Fig.

TSHB functions as a master control gene of seasonal reproduction in both mammals and birds. Figure 3 Download Figure Download figure as PowerPoint slide Universality and diversity of photoperiodic signaling mechanisms in vertebrates. In mammals, light information received in the retina is transmitted to the pineal gland via the SCN, the location of the master circadian clock.

Melatonin synthesis and secretion in the pineal gland is a reflection of photoperiod, because light and the circadian clock regulate its production, which is higher in the night than in the day. In fish, the entire photoperiodic signaling machinery is integrated in the coronet cells located in the fish-specific SV C. TH is involved in seasonal reproduction also in SD breeding mammals.

The photoperiodic TSH—TH signaling pathway is widely conserved; however, the downstream events may be different among species. In LD breeding rodents, melatonin inhibits secretion of kisspeptin from the hypothalamic arcuate nucleus Revel et al. In mammals, the effects of RFRPs on seasonal reproduction are different among species. Bales and Caroline M. Hostetler Bhatnagar Tardif, and Julienne N.

Rutherford Pelch, Joseph M. Beeman, Bridget A. Niebruegge, Stacey R. Winkeler, and Susan C. Nagel show more. About David O. Norris Dr. David Norris has done research in environmental endocrinology and neuroendocrinology for more than 50 years. He received his bachelor's degree from Baldwin-Wallace College and his Ph. Norris has worked in the area of forensic botany with Dr. Jane H. Bock, since , primarily on developing the use of plant cells in the gastrointestinal tract to aid in homicide investigations. Norris and Dr. Bock have been involved in investigations in numerous states as well as throughout the State of Colorado.

Norris has been certified as an expert witness in this area for the State of Colorado.