THE LIKELIHOOD OF SPERM COMPETITION IN MANATEES-EXPLAINING AN APPARENT PARADOX

Florida manatees ( Trichechus manatus latirostris ) are promiscuous, with multiple males mating with individual females. This suggests manatees are sperm competitors. Surprisingly, manatee testes are not relatively large. For adult males in non-winter, testicular size is approximately twice what is expected, based on allometry, for "typical" ( i. e. , non-sperm competitor) male mammals of similar size; for these manatees, combined testicular weight represents 0.19% of the body weight ( n = 27 manatees). Testicular weight was generally largest in manatees older than 7 yr in non-winter. Testicular size of some sperm competitors is as much as an order of magnitude (or more) larger than expected in a "typical" species. Perhaps in compensation for the testes not being remarkably large, the seminal vesicles of mature manatees may be larger than the testes. Production of notably large volumes of seminal fluid characterizes sperm competitor primate species and may have positive energy consequences for species such as the manatee that have extremely low metabolic rates. Another possible explanation for the observed relationship between testicular mass and body mass in manatees is that selection for a greatly expanded hindgut and extremely dense, heavy integument could make the body mass of manatees "artificially" high, and the relative testicular mass "artificially" low.     

Relative testis size and sperm morphometry across mammals: no evidence for an association between sperm competition and sperm length

Understanding why there is extensive variation in sperm form and function across taxa has been a challenge because sperm are specialized cells operating at a microscopic level in a complex environment. This comparative study collates published data to determine whether the evolution of sperm morphometry (sperm total length and separate component dimensions) is associated with sperm competition (when different males' sperm mix and compete for a female's ova) across 83 mammalian species. We use relative testes mass as an indicator of the intensity of sperm competition across taxa: relative investment into testes is widely accepted to predict the level of sperm competition that a species or population endures. Although we found evidence for positive associations between relative testes mass (controlling for allometry) and sperm morphometry across 83 mammalian species, these relationships were phylogenetically dependent. When we appropriately controlled for phylogenetic association using multiple regression within a phylogenetic framework, there was no relationship between relative testes mass and sperm length across mammals. Furthermore, we found no evidence for associations between relative testes mass and sperm head, mid-piece or flagellar lengths, nor was there a relationship with mid-piece or mitochondrial volumes. Results, therefore, indicate that sperm competition does not select for longer or shorter sperm across mammals, and alternative forces selecting on sperm form and function are discussed.     

Evolutionary change in testes tissue composition among experimental populations of house mice

Theory assumes that postcopulatory sexual selection favors increased investment in testes size because greater numbers of sperm within the ejaculate increase the chance of success in sperm competition, and larger testes are able to produce more sperm. However, changes in the organization of the testes tissue may also affect sperm production rates. Indeed, recent comparative analyses suggest that sperm competition selects for greater proportions of sperm‐producing tissue within the testes. Here, we explicitly test this hypothesis using the powerful technique of experimental evolution. We allowed house mice (Mus domesticus) to evolve via monogamy or polygamy in six replicate populations across 24 generations. We then used histology and image analysis to quantify the proportion of sperm‐producing tissue (seminiferous tubules) within the testes of males. Our results show that males that had evolved with sperm competition had testes with a higher proportion of seminiferous tubules compared with males that had evolved under monogamy. Previously, it had been shown that males from the polygamous populations produced greater numbers of sperm in the absence of changes in testes size. We thus provide evidence that sperm competition selects for an increase in the density of sperm‐producing tissue, and consequently increased testicular efficiency.     

Zur Ursache des Hodenabstiegs (Descensus testiculorum) bei Säugetieren1

On the cause of the mammalian descent of the testes (Descensus testiculorum) Two explanations have been offered for the descent of the testes in mammals, both of which are frequently cited in the literature. Moore and co-workers argued that the phylogenetic rise of body temperature caused the translocation of the testes. According to Portmann , the descent of the testes was due to the evolution of the scrotum as a signal. However, both the sensibility of the extra-abdominal testes to temperature elevations and the optical effects of the scrotum can be interpreted as a consequence of testicular descent rather than as its cause. The hypothesis presented in this paper suggests a new adaptive explanation for the descent of the testes in mammals and regards its development as an example of evolutionary compromise. Obvious disadvantages such as reduced protection of the extra-abdominal testes, “perforation” of the inguinal abdominal wall, and an increased loss of heat from the body core have to be outbalanced by a strong selective advantage. This advantage is seen in the development of a completely new, fast mode of locomotion - the gallop. The strong flexions and extensions of the vertebral column during gallop should cause intense fluctuations of intra-abdominal pressure. Fluctuations of intra-abdominal pressure severely impede continuous flow of blood in the abdominal veins. Periodically reduced venous drainage resulting in fluctuations of intra-testicular pressure would impair the process of spermiohistogenesis, which is dependent on an absolutely constant pressure within the testis. Thus, it is the displacement of the pressure sensitive testes out of the abdominal cavity that allows for the evolution of a fast mode of locomotion accompanied by strong fluctuations of intraabdominal pressure. In the course of the phylogenetic translocation of the testes increasing specializations of the testicular blood vessels occur. In mammals possessing a scrotum the return flow of venous blood from the extra-abdominal testes to the abdominal cavity is supported by utilizing the energy of the arterial pulse (‘peripheral arterial pump’). A model for the successive stages of the descent of the testes is illustrated in Figures 7–10. The morphological changes related to the phylogenetic descent of the testes, such as the specialization of the testicular blood vessels, the forming of a cremasteric sack in the inguinal region, and the differentiation of the inguinal or perineal integument into a scrotum can all be interpreted as serving one purpose: they aid in maintaining a constant intra-testicular pressure in spite of increasing fluctuations of intra-abdominal pressure and venous blood flow during the evolution of the gallop. Although the blood vessels of the spermatic chord basically serve the same functions, they show markedly different specializations in Marsupials and Eutherians. This indicates that the descent of the testes has occurred independently, at least, in these two groups. The explanation put forward here postulates a causal relationship between the mode of locomotion and the position of the testes. Mammals possessing testes wnich reside permanently within the abdominal cavity (‘Testiconda’) cannot gallop, whereas mammals with the ability to gallop must have (periodical or permanent) extraabdominal testes (‘Testiphaena’).     

The Evolution of Large Testes: Sperm Competition or Male Mating Rate?

A positive relationship across species between the extent to which females mate with more than one male and relative testes mass has been demonstrated in a wide range of vertebrate taxa and certain insects. At least two hypotheses, which are not necessarily mutually exclusive, could account for this pattern: (1) the numerical sperm competition hypothesis, which assumes that larger testes enable the male to transfer more sperm to each female, giving the male an advantage in sperm competition and (2) the male mating rate hypothesis, which proposes that larger testes allow the male to produce a greater number of (potentially smaller) ejaculates to engage in frequent copulations with different females. Of these hypotheses, the former has won broad acceptance, while the latter has tended to be dismissed. Here, we argue that the lines of evidence commonly used to support the numerical sperm competition hypothesis in favour of the male mating rate hypothesis are not as clear cut or generally applicable as they are purported to be and that, consequently, the male mating rate hypothesis cannot be excluded with confidence on the basis of the current evidence. Furthermore, some evidence, such as the finding that ejaculate mass and/or sperm number is negatively correlated with testes mass across species in some insects and that larger testes in Drosophila can evolve in response to an increase in the number of females available for mating in the laboratory, provides support for the male mating rate hypothesis. Further work is needed to disentangle the relative effects of these selective pressures on the evolution of testes size.     

Differentiation autoantigen of testicular cells and spermatozoa in the guinea pig.

Guinea pigs, immunized with autologous testes, produce antibody that reacts with surface antigen(s) of epididymal sperm and testicular cells. Both IgG and F(ab')₂ fragments of the antibody molecules bind to testicular cells. Only 50% of the testicular cells had detectable surface antigens and they are identified by rosette formation with Staphylococcus aureus, which bind only to testicular cells and sperm coated with guinea pig IgG. Rosette-positive testicular cells (percentage of cells positive) are: early round spermatids (8%), oval late spermatids (98%), cytoplasmic droplets (93%), and heads (and midpieces) of testicular and epididymal sperm (100%). As determined by a quantitative cellular radioimmunobinding assay, antibody to testicular cells is absorbed by testis or sperm but not by other male or female guinea pig tissues. The antigen, which could be multiple, is therefore sperm and late spermatid specific, and is designated testicular cell-sperm differentiation autoantigen. The surface distribution of this antigen is modulated by antibody to exhibit temperature-dependent translational movement along the plasma membrane.     

Effects of dietary cobalt on testicular structure

Adult male rats were maintained on a diet containing 265 ppm cobalt for up to 98 days. Three rats were sacrificed weekly and assayed for testicular damage by light and electron microscopy. Testicular damage was first apparent after 70 days of treatment, followed by a progressive deterioration of cell architecture and decrease in testicular volume. The degenerative changes were of a very general nature; e.g., thickening of basal lamina and basement membranes, increased packing of red blood cells in veins and arteries, formation of "giant" cells, loss of sperm tail filaments, and degeneration of sperm mitochondria. No cobalt residues could be detected by energy dispersive x-ray microanalysis. These data indicate that testicular degeneration was not a primary response to cobalt and suggest that the testes become hypoxic due both to blockage of veins and arteries by red blood cells and to changes in permeability caused by thickening of basal lamina and basement membranes.     

PCNA indexing as a preclinical immunohistochemical biomarker for testicular toxicity

It is well known that toxicants such as cyclophosphamide and ethanol can have deleterious effects on normal spermatogenesis. End points such as testis weight and sperm counts have been used widely to assess gross structural and functional changes in testes resulting from toxicant exposure. Histopathological assessments are more sensitive measures of testicular health, but generally they are neither quantitative nor sensitive enough to detect early toxicity. Recently, immunolabeling cells with proliferating cell nuclear antigen (PCNA) has been used to identify proliferating spermatogonia; however, there have been no systematic attempts to quantify these changes. We have developed a sensitive, reliable and quantitative assay using immunohistochemistry on formalin fixed, paraffin embedded rat testes to assess the degree of proliferation-related toxicity. An indexing scheme was derived based on the determination of radially positioned PCNA-positive cells within similarly staged seminiferous tubules presenting a single layer of PCNA-positive cells along the basement membrane of the basal tubular compartment. An average of 60 tubules in the testes were counted per animal. Our results show significant decreases in the PCNA index in rats treated with an experimental compound that has been shown to produce testicular histopathology. The analysis provides a quick, reliable, sensitive, and quantitative means for assessing early testicular toxicity. The assay has potential utility as an in vivo biomarker for detecting early testicular toxicity of experimental compounds in preclinical development as well as for refining follow-up compounds with reduced testicular toxicity.     

Effects of molybdenum on fertility of male rats

Sodium molybdate was administered orally to adult male rat at dose level of 10, 30, and 50 mg kg body weight (5 days per week) for 60 days. At higher dose levels significant decrease in absolute and organ-to-body weight ratios of testes, epididymides, seminal vesicles and ventral prostate was observed. The sperm abnormality, associated with decrease in sperm motility and sperm count was also observed. Significant alterations in the activities of marker testicular enzymes, viz. sorbitol dehydrogenase (decreases), lactate dehydrogenase (increases) and -glutamyl transpeptidase (increases) associated with histopathological changes in testes was also observed. Accumulation of molybdenum in testes, epididymides and seminal vesicles was also observed. The study reveals that the oral ingestion of molybdenum may affect the histoarchitecture of testes and sperm morphology. The testicular and spermatotoxic changes may be responsible for observed male mediated developmental toxic effects.     

SPERM COMPETITION SELECTS BEYOND RELATIVE TESTES SIZE IN BIRDS

Sperm morphology varies considerably across taxa, and postcopulatory sexual selection is thought to be one of the main forces responsible for this diversity. Several studies have investigated the effects of the variation in sperm design on sperm function, but the consequences of variation in sperm design on testis morphology have been overlooked. Testes size or architecture may determine the size of the sperm they produce, and selection for longer sperm may require concomitant adaptations in the testes. Relative testes size differs greatly between species and is often used as an index of sperm competition, but little is known about whether larger testes have more sperm-producing tissue or produce sperm at a faster rate. Using a comparative approach in New World Blackbirds (Icteridae), we found (1) a strong link between testis histology and sperm length, suggesting selection on testis architecture through selection on sperm size, and (2) that species under intense sperm competition had a greater proportion of sperm-producing tissue within their testes. These results support the prediction that sperm competition fosters adaptations in reproductive organs that extend beyond testes size, and raise questions about the trade-offs influencing reproductive investment.     

The evolutionary history of testicular externalization and the origin of the scrotum

This paper re-examines the evolution of the scrotum and testicular descent in the context of the recent phylogeny of mammals. The adaptive significance of testicular descent and scrotality is briefly discussed. We mapped four character states reflecting the position of testes and presence of scrotum onto recent mammalian phylogeny. Our results are interpreted as follows: as to the presence of testicondy in Monotremata and most of Atlantogenata, which represent the basal group of all eutherians, we argue that primary testicondy represents a plesiomorphic condition for Eutheria as well as for all mammals. This is in opposition to the previous hypothesis of Werdelin and Nilsonne that the scrotum may have evolved before the origin of mammals and then repeatedly disappeared in many groups including monotremes. We suggest that the scrotum evolved at least twice during the evolutionary history of mammals, within Marsupialia and Boreoeutheria, and has subsequently been lost by many groups; this trend is especially strong in Laurasiatheria. We suggest that the recent diversity in testicular position within mammals is the result of multiple selection pressures stemming from the need to provide conditions suitable for sperm development and storage, or to protect the male gonads from excessive physical and physiological disturbance.     

Anomalies de position des testicules dans l’enfance: conséquences à l’âge adulte

Non descended testes in the low scrotum is a common anomaly at birth, with about 4% of the newborn males affected. Only one quarter of these newborn babies will still have non descended testes when one year old. However, the testes that will descend within the first year of life seem no more to be considered as normally descended testes. Moreover, the retractile testis, which represents a secondary anomaly of testicular position occuring after the babies are older than one year, is no more to be regarded as a physiological variant of the normally descended testis, since several reports indicate histological and clinical modifications in such cases. The testicular non descent can be associated with two consequences in adult life. Firstly, an history of non descended testis is the only known risk factor for the testicular cancer. Secondly, such an history is a risk factor for the male fertility because of spermatogenesis alterations, as indicated by qualitative and quantitative histological analyses of the testicular tissue, and by depressed spermatozoa output and quality (motility, normal forms); moreover, testicular volumes are reduced, and the time to pregnancy as well as the rate of infertility are increased. Time is arrived for a reappraisal of the consequences in adult life of the abnormal testicular location (either congenital or acquired) during childhood.     

Development to blastocyst is impaired when intracytoplasmic sperm injection is performed with abnormal sperm from infertile mice harboring a mutation in the protein phosphatase 1cgamma gene

Idiopathic azoospermia, characterized by abnormal spermatogenesis, is commonly treated by performing intracytoplasmic sperm injection (ICSI) with sperm retrieved from testicular biopsies. However, no controlled experiments have been performed using an animal model to assess the efficacy or safety of the procedure. We have performed ICSI with testicular sperm obtained in a similar manner from testes of male mice homozygous for a null mutation in the protein phosphatase 1cgamma gene (PP1cgamma) or those of their wild-type littermates. PP1cgamma mutant testicular sperm are less resistant to sonication than are wild-type sperm and display a range of morphological abnormalities, similar to those reported for testicular sperm from idiopathic azoospermic men. PP1cgamma mutant sperm are unable to support development to the blastocyst stage, resulting in arrested development either before or just after compaction. A comparison of testicular and epididymal sperm from wild-type males revealed that the epididymal sperm caused embryos to fragment at an elevated rate. These results suggest that ICSI with any kind of testicular sperm carries an increased risk of embryo fragmentation and that abnormal testicular sperm has an added risk of embryo wastage at later preimplantation stages.     

Male reproductive effect of nickel sulphate in mice

Nickel sulphate was administered orally to adult male mice at dose level of 5 and 10 mg/kg body weight (5 days per week) for 35 days. There was no change in body weight. However a significant decrease in absolute and organ-to-body weight ratios of testes, epididymides, seminal vesicles and prostate gland was observed. The sperm abnormality, associated with decrease in sperm motility and sperm count was also observed. Significant alterations in the activities of marker testicular enzymes, viz. sorbitol dehydrogenase (decreases), lactate dehydrogenase (increases) and -glutamyl transpeptidase (increases) associated with histopathological changes in testes, epididymides and seminal vesicles, were also observed. Accumulation of nickel in testes, epididymides and seminal vesicles was also observed. The study reveals that the oral exposure to nickel may affect the histology of testes, epididymides, seminal vesicles and sperms morphology. These testicular and spermatotoxic changes may be responsible for observed male mediated developmental toxic effects.     

Sperm competition and the evolution of testes size in birds

Comparative analyses suggest that a variety of ecological and behavioural factors contribute to the tremendous variability in extrapair mating among birds. In an analysis of 1010 species of birds, we examined several ecological and behavioural factors in relation to testes size; an index of sperm competition and the extent of extrapair mating. In univariate and multivariate analyses, testes size was significantly larger in species that breed colonially than in species that breed solitarily, suggesting that higher breeding density is associated with greater sperm competition. After controlling for phylogenetic effects and other ecological variables, testes size was also larger in taxa that did not participate in feeding their offspring. In analyses of both the raw species data and phylogenetically independent contrasts, monogamous taxa had smaller testes than taxa with multiple social mates, and testes size tended to increase with clutch size, which suggests that sperm depletion may play a role in the evolution of testes size. Our results suggest that traditional ecological and behavioural variables, such as social mating system, breeding density and male parental care can account for a significant portion of the variation in sperm competition in birds.     

Testicular migration, spermatogenesis, temperature regulation and environment of the sheath-tail bat, Taphozous georgianus

The testes of the common sheath-tail bat of tropical Australia undergo a seasonal migration between the abdomen and the scrotal pouches, while each cauda epididymidis is permanently maintained in the scrotal pouch. Straps of smooth muscle attach to both the cranial and caudal poles of the testes, and these extend cranially to the diaphragm and caudally to the cauda epididymidis. The testicular arteries are not coiled. Among the environmental factors investigated, maximum temperature correlated most significantly with testicular descent, and the number of spermatogonia per bat also correlated most significantly with maximum temperature. Body temperature of a captive bat ranged from 25 to 38 degrees C and this was closely related to body weight and ambient temperature. It seems likely that the scrotal pouch provides a temperature slightly below that of the body and so facilitates sperm storage in the permanently scrotal cauda epididymidis. Migration of the testes probably serves to ameliorate the seasonal temperature fluctuations to which they are exposed while the relatively high correlation between maximum environment temperature and spermatogonial numbers suggests that temperature may be a proximate influence on reproduction in the sheath-tail bat.     

Relationship of intratesticular testosterone content of stallions to age, spermatogenesis, Sertoli cell distribution and germ cell-Sertoli cell ratios

Testes were obtained from 47 1-20-year-old stallions during the natural breeding season. Total testicular testosterone and testosterone/g testis increased with age (P less than 0.005), and total testicular testosterone was associated with larger testis size (P less than 0.05). Neither testosterone per gram nor per paired testes were related to total Sertoli cell number (P greater than 0.05), but greater testosterone per paired testes was associated with fewer Sertoli cells per unit of seminiferous tubule length (P less than 0.005) or basement membrane area (P less than 0.02) and with a higher number of germ cells supported per Sertoli cell (P less than 0.05). Although values for testosterone per gram and per paired testes were unrelated (P greater than 0.10) to sperm production/g testis or to the yield of spermatids/spermatogonium, testosterone per paired testes was positively related to sperm production per paired testes (P less than 0.05). It is concluded that intratesticular testosterone increases with age, is related in a positive manner to quantitative rates of sperm production, and can account for some of the differences in sperm production among individual stallions within a single breeding season.     

Inflammatory changes in the epididymis after vasectomy in the Lewis rat

The epididymides of Lewis rats were studied at intervals up to 7 months after vasectomy, vasectomy followed 3 months later by vasovasostomy, or sham operations. Epididymal histology was related to testicular alterations and to serum antisperm antibodies as determined with an enzyme-linked immunosorbent assay. In vasectomy and vasovasostomy groups. 13 of 33 rats had testicular alterations, which consisted mainly of pronounced depletion of germ cells. Over half of the rats with testicular alterations also had severe epididymal lesions that included interstitial changes characteristic of an inflammatory response. These consisted of aggregates of mononuclear cells, including lymphocytes, plasma cells, and macrophages. The lumina of epididymides with interstitial changes contained polymorphonuclear leukocytes and/or macrophages. All animals with altered testes had greatly decreased numbers of epididymal sperm. In many instances, the lumen of the proximal cauda epididymidis was collapsed, and columnar cells of the epididymal epithelium contained many very large lysosomes. The distal cauda epididymidis was distended with sperm and debris. None of the rats that lacked testicular alterations showed epididymal changes. Mean serum antisperm antibody levels were significantly higher for rats with epididymal interstitial changes than for animals without such epididymal alterations. Infiltrations of inflammatory cells into the epididymal interstitium and lumen are part of the constellation of changes that occurs after immunization with testicular homogenates to produce experimental allergic orchitis. The observations reported here support the hypothesis that reproductive tract alterations after vasectomy in this model have an immune basis.     

The effects of postnatal FSH substitution on Sertoli cell number and the sperm production capacity of the adult boar

The role of FSH for Sertoli cell establishment and sperm production in the boar is not definitely known. In order to elucidate its function FSH was substituted postnatally in male pigs and the resulting effects on testicular histological traits and sperm production capacity were investigated when the boars had reached maturity. Six male piglets received pFSH from 18 to 48 days postnatally. Another six piglets instead received saline and served as controls. Blood samples were drawn to measure FSH, LH, testosterone and estradiol. After 28 weeks, the boars were trained to mount a dummy so that the spermatogenic capacity was tested by increasing the frequency of semen collection at the age of 30 weeks. Libido (latency time) and ejaculate criteria (volume, motility, morphological abnormalities) were determined. Thereafter the boars were killed and their testes analyzed for morphology, number of Sertoli cells, germ cells and Leydig cells as well as the ratio between mitosis and apoptosis in the tubules.FSH concentrations were twofold due to FSH application when compared to the controls. LH was low during the first 2 weeks of FSH treatment. Thereafter concentrations increased in three of the six treated animals but not in controls. Testosterone increased slightly over the application period both in the controls and the treated piglets. Estradiol levels were similar in both groups. Increased ejaculation frequency reduced sperm concentrations and sperm motility in all boars and the percentage of morphologically abnormal sperm increased. Ejaculate volumes and the time of latency were not significantly altered. No differences were observed between the controls and the FSH treated boars. The testicular parameters of both FSH- and control boars were identical for morphology, number of spermatogenic and somatic cells as well as mitosis–apoptosis equilibrium. The data demonstrated that a prolonged postnatal period of FSH concentrations does not influence the sperm production of the adult boar.