Is the hook of muroid rodent's sperm related to sperm train formation?

Competition between spermatozoa of rival males to gain fertilizations has led to a wide array of modifications in sperm structure and function. Sperm cells of most muroid rodents have hook‐shaped extensions in the apical–ventral tip of the head, but the function of this structure is largely unknown. These ‘hooks’ may facilitate aggregation of spermatozoa in so‐called ‘trains’, as an adaptation to sperm competition, because sperm in trains may swim faster than free‐swimming cells. However, there is controversy regarding the role of the hook in train formation, and in relation to whether it is selected by sperm competition. We examined spermatozoa from muroid rodents with varying levels of sperm competition to assess whether (i) sperm aggregates are common in these taxa, (ii) presence of a hook relates to the formation of sperm aggregations, and (iii) formation of sperm aggregations is explained by sperm competition. Our analyses in 25 muroid species revealed that > 92% of spermatozoa swim individually in all species, with the exception of the wood mouse, Apodemus sylvaticus, which has ~50% spermatozoa swimming freely. Species with hooked spermatozoa had higher sperm competition levels and longer sperm than species whose sperm lack a hook. Neither the presence of hook nor sperm competition levels were related to the percentage of sperm in aggregations. Thus, (i) sperm aggregates in muroid rodents are an exceptional trait found only in a few species, (ii) evolution of the sperm hook is associated to sperm competition levels, but (iii) the hook is unlikely to be related to the formation of sperm aggregates. The evolutionary significance of the sperm head hook thus remains elusive, and future studies should examine potential roles of this pervasive structure in sperm's hydrodynamic efficiency and sperm–female tract interactions.     

Porcine oviduct sperm binding glycoprotein and its deleterious effect on sperm: a mechanism for negative selection of sperm?

In their journey through the oviduct some subpopulations of sperm are preserved in a reservoir, while others are negatively selected. Sperm binding glycoprotein (SBG) is a pig oviductal epithelial cell glycoprotein that produces, under capacitating conditions, acrosome alteration, p97 tyrosine-phosphorylation and reduction of the motility of sperm. In this paper, we show that SBG is accessible at the extracellular surface of the oviductal epithelial cells, supporting a sperm interaction biological role in situ. We analyze the possible dependence of the tyrosine-phosphorylation of p97 on the PKA mechanism, finding that apparently it is not PKA dependent. Also, after SBG treatment the phosphorylated proteins locate mainly at the detached periacrosomal region and at the tail of sperm; the latter may be related to SBG's motility reduction effect. The study of the time course effect of SBG on sperm as detected by chlortetracycline (CTC) staining and of its binding to sperm by immunodetection in conjunction with CTC, shows results in agreement with the hypothesis that this glycoprotein is involved in the alteration of acrosomes in a specific sperm subpopulation. The results suggest that SBG may be part of a mechanism for negative selection of sperm.     

Effects of cytoplasmic genes on sperm viability and sperm morphology in a seed beetle: implications for sperm competition theory?

Sperm competition theory predicts that sperm traits influencing male fertilizing ability will evolve adaptively. However, it has been suggested that some sperm traits may be at least partly encoded by mitochondrial genes. If true, this may constrain the adaptive evolution of such traits because mitochondrial DNA (mtDNA) is maternally inherited and there is thus no selection on mtDNA in males. Phenotypic variation in such traits may nevertheless be high because mutations in mtDNA that have deleterious effects on male traits, but neutral or beneficial effects in females, may be maintained by random processes or selection in females. We used backcrossing to create introgression lines of seed beetles (Callosobruchus maculatus), carrying orthogonal combinations of distinct lineages of cytoplasmic and nuclear genes, and then assayed sperm viability and sperm length in all lines. We found sizeable cytoplasmic effects on both sperm traits and our analyses also suggested that the cytoplasmic effects varied across nuclear genetic backgrounds. We discuss some potential implications of these findings for sperm competition theory.     

Trypanosomes and mammalian sperm: one of a kind?

Flagellar-mediated motility is an indispensable function for cell types as evolutionarily distant as mammalian sperm and kinetoplastid parasites, a large group of flagellated protozoa that includes several important human pathogens. Despite the obvious importance of flagellar motility, little is known about the signalling processes that direct the frequency and wave shape of the flagellar beat, or those that provide the motile cell with the necessary environmental cues that enable it to aim its movement. Similarly, the energetics of the flagellar beat and the problem of a sufficient ATP supply along the entire length of the beating flagellum remain to be explored. Recent proteome projects studying the flagella of mammalian sperm and kinetoplastid parasites have provided important information and have indicated a surprising degree of similarities between the flagella of these two cell types.     

Species-specific interaction of seminal plasma on sperm–neutrophil binding

Bovine semen is naturally deposited in the vagina and spermatozoa migrate through the cervix into the uterus leaving the bulk of seminal plasma (SP) behind. In equine, both spermatozoa and SP are deposited directly in the uterus and SP reduces sperm binding to neutrophils and prevents the formation of DNA-based neutrophil extracellular traps (NETs). We investigated the role of bovine SP on sperm–neutrophil binding using the four most common bovine semen extenders. Contrary to equine, bovine spermatozoa removed from SP had low binding to neutrophils for up to 3 h, but as little as 10% SP increased sperm–neutrophil binding and NETs formation over time. Similar results were obtained with neutrophils isolated from peripheral blood or from the uterus. Scanning electron microscopy showed that the binding can be mediated by NETs or by direct attachment of the cell membranes for both species. The increased binding with SP reduced the number of free spermatozoa indicating that sperm transport to the site of fertilization (and thus fertility) may be hindered. Surprisingly, egg yolk negated the role of bovine SP on sperm–neutrophil binding compared to all the other semen extenders, but did not alter equine sperm binding to neutrophils. Current artificial insemination in bovine relies heavily on egg yolk extender and introduces variable amounts of SP into the uterus, which naturally remains in the vagina. Our results indicate a need to re-evaluate the composition of semen extenders and the semen processing procedures in relation to sperm transport, longevity and fertilizing ability.     

What women want in their sperm donor: A study of more than 1000 women’s sperm donor selections

Reproductive medicine and commercial sperm banking have facilitated an evolutionary shift in how women are able to choose who fathers their offspring, by notionally expanding women’s opportunity set beyond former constraints. This study analyses 1546 individual reservations of semen by women from a private Australian assisted reproductive health facility across a ten year period from 2006 to 2015. Using the time that each sample was available at the facility until reservation, we explore women’s preference for particular male characteristics. We find that younger donors, and those who hold a higher formal education compared to those with no academic qualifications are more quickly selected for reservation by women. Both age and education as proxies for resources are at the centre of Parental Investment theory, and our findings further build on this standard evolutionary construct in relation to female mate preferences. Reproductive medicine not only provides women the opportunity to become a parent, where previously they would not have been able to, it also reveals that female preference for resources of their potential mate (sperm donor) remain, even when the notion of paternal investment becomes redundant. These findings build on behavioural science’s understanding of large-scale decisions and human behaviour in reproductive medical settings.     

The physiological acquisition of amoeboid motility in nematode sperm: Is the tail the only thing the sperm lost?

Nematode spermatozoa are highly specialized amoeboid cells that must acquire motility through the extension of a single pseudopod. Despite morphological and molecular differences with flagellated spermatozoa (including a non‐actin‐based cytoskeleton), nematode sperm must also respond to cues present in the female reproductive tract that render them motile, thereby allowing them to locate and fertilize the egg. The factors that trigger pseudopod extension in vivo are unknown, although current models suggest the activation through proteases acting on the sperm surface resulting in a myriad of biochemical, physiological, and morphological changes. Compelling evidence shows that pseudopod extension is under the regulation of physiological events also observed in other eukaryotic cells (including flagellated sperm) that involve membrane rearrangements in response to extracellular cues that initiate various signal transduction pathways. An integrative approach to the study of nonflagellated spermatozoa will shed light on the identification of unique and conserved processes during fertilization among different taxa. Mol. Reprod. Dev. 77: 739–750, 2010. © 2010 Wiley‐Liss, Inc.     

Identification of sperm morphometric subpopulations in the canine ejaculate: do they reflect different subpopulations in sperm chromatin integrity?

A statistical approach using sequentially principal component analysis (PCA) clustering and discriminant analysis was developed to disclose morphometric sperm subpopulations. In addition, we used a similar approach to disclose subpopulations of spermatozoa with different degrees of DNA fragmentation. It is widely accepted that sperm morphology is a strong indicator of semen quality and since the sperm head mainly comprises the sperm DNA, it has been proposed that subtle changes in sperm head morphology may be related to abnormal DNA content. Semen from four mongrel dogs (five replicates per dog) were used to investigate DNA quality by means of the sperm chromatin structure assay (SCSA), and for computerized sperm morphometry (ASMA). Each sperm head was measured for nine primary parameters: head area (A), head perimeter (P), head length (L), head width (W), acrosome area (%), midpiece width (w), midpiece area (a), distance (d) between the major axes of the head and midpiece, angle (theta) of divergence of the midpiece from the head axis; and four parameters of head shape: FUN1 (L/W), FUN2 (4pi A/P2), FUN3 ((L - W)/(L + W)) and FUN 4 (pi LW/4A). The data matrix consisted of 2361 observations, (morphometric analysis on individual spermatozoa) and 63,815 observations for the DNA integrity. The PCA analysis revealed five variables with Eigen values over 1, representing more than 79% of the cumulative variance. The morphometric data revealed five sperm subpopulations, while the DNA data gave six subpopulations of spermatozoa with different DNA integrity. Significant differences were found in the percentage of spermatozoa falling in each cluster among dogs (p < 0.05). Linear regression models including sperm head shape factors 2, 3 and 4 predicted the amount of denatured DNA within each individual spermatozoon (p < 0.001). We conclude that the ASMA analysis can be considered a powerful tool to improve the spermiogram.     

Current status of sperm cryopreservation: why isn't it better?

Cryopreservation extends the availability of sperm for fertilization; however, the fertilizing potential of the frozen-thawed sperm is compromised because of alterations in the structure and physiology of the sperm cell. These alterations, characteristics of sperm capacitation, are present in the motile population and decrease sperm life-span, ability to interact with female tract, and fertilizing ability. The etiology of such alterations may represent a combination of factors, such as inherited fragility of the sperm cell to withstand the cryopreservation process and the semen dilution. Although the former is difficult to address, approaches that make-up for the dilution of seminal fluid may be sought. The aim of this work is to review aspects of sperm cryopreservation paralleled by events of capacitation and evaluate the possible roles of sperm membrane cholesterol, reactive oxygen species, and seminal plasma as mediators of cryopreservation effects on sperm function.     

Is there a relationship between sperm chromosome abnormalities and sperm morphology?

This review explores the relationship between sperm chromosomal constitution and morphology. With the advent of techniques for obtaining information on the chromosome complements of spermatozoa, this relationship has been studied in fertile men and in men with a high frequency of chromosomal abnormalities. Using human sperm karyotype analysis, no relationship between sperm chromosome abnormalities and morphology was found in fertile men, translocation carriers or post-radiotherapy cancer patients. Fluorescence in situ hybridization (FISH) analysis has not generally revealed a specific association between morphologically abnormal sperm and sperm chromosome abnormalities, but has indicated that teratozoospermia, like other forms of abnormal semen profiles (aesthenozoospermia, oligozoospermia) is associated with a modest increase in the frequency of sperm chromosome abnormalities. However, FISH studies on some infertile men and mouse strains have suggested that certain types of morphologically abnormal spermatozoa, such as macrocephalic multitailed spermatozoa, are associated with a very significantly increased frequency of aneuploidy. Thus, there may be an association between sperm morphology and aneuploidy in infertile men with specific abnormalities.     

Decline in sperm counts: an artefact of changed reference range of "normal"?

OBJECTIVE--To investigate a reported fall in sperm counts during 1940-90 in relation to the reduced lower reference value of "normal" during the same period by assuming the null hypothesis that no change had occurred in the probability distribution of the sperm concentration. DESIGN--Analysis by using various mathematical models of the probability distribution of sperm concentration together with experimental data which supported a model employing a logarithmic distribution. SUBJECTS--235 men presenting for stimulated in vitro fertilisation at Midland Fertility Services, Aldridge, in 1992 together with samples of 20 ejaculates from each of five men attending the same centre during 1992-3. RESULTS--The effect of the change in lower reference value for the "normal" sperm concentration (from 60 x 10(9) to 20 x 10(9)/l) depended on the probability distribution of the concentration in the population. If that distribution was normal or uniform, then very little of the reported decline was a consequence of the change in lower reference value. If it was heavily skewed, then most or all of the reported decline may have been a consequence of that change. The limited experimental data available indicate that the distribution was heavily skewed. CONCLUSIONS--Depending on the actual distribution of sperm concentration in the population, the reported decline in concentration may have been accounted for entirely or in part by the change in lower reference value. The original evidence does not support the hypothesis that the sperm count declined significantly between 1940 and 1990.     

Is the sperm type of the Nemertodermatida close to that of the ancestral Platyhelminthes?

Results from a transmission electron microscope study of the spermiogenesis and spermatozoon of Meara stichopi (Nemertodermatida, Platyhelminthes) indicate that the sperm type of the Nemertodermatida has evolved from the primitive metazoan sperm type rather than from an aberrant biflagellar sperm type as found in many other flatworms. The spirally coiled mitochondrial derivative in the mature spermatozoon develops from two large oval mitochondria in the early spermatid stages. A single flagellum grows out from a peripheral basal body adjacent to a perpendicularly placed accessory centriole. The basal body moves to a distal depression of the nucleus, and becomes equipped with an anchoring fibre apparatus. Most of the flagellum becomes axially incorporated into the developing spermatid. No trace of a second flagellum was found in any stage of the spermiogenesis. Rounded vesicles appear around the proximal, tapering end of the elongating nucleus. Most probably these vesicles form a thin acrosomal structure in the mature spermatozoon. No dense bodies, characteristic of many other ‘turbellarian’ flatworm sperm types, were found. This revised version was published online in July 2006 with corrections to the Cover Date.     

How severe is sperm limitation in natural populations of marine free-spawners?

Successful fertilization in marine organisms that release sperm into seawater is potentially limited by the rapid dilution of gametes; cases of severe sperm limitation have been demonstrated in nature. However, recent surveys of naturally spawning populations indicate fairly high fertilization levels in many taxa. The extreme selection exerted by sperm limitation has resulted in numerous adaptations to reduce sperm limitation and enhance fertilization. Thus, most taxa show indications of the evolutionary consequences of sperm limitation even when population level, ecological effects are minimal.     

Defining the roles of Ca2+ — permeable channels in sperm

Ion channels exert a vital role in the dialogue between male and female gametes and thus in the generation of new individuals in many species. Intracellular Ca²⁺ is possibly the key messenger between gametes. Different Ca²⁺-permeable channels have been detected in the plasma membrane and in the organelle-like acrosome membrane of sperm, which play vital roles in determining sperm fertilizing ability. Recent reports from several laboratories have adequately documented that the Ca²⁺-permeable channels of a sperm control a variety of functions ranging from motility to the acrosome reaction. In this article, we have reviewed the data from our and other laboratories, and have documented the mechanisms of different Ca²⁺-permeable channels involved in the fertilization event.     

MAP kinase, a universal suppressor of sperm centrosomes during meiosis?

We reported previously that inhibition of MAP kinase during meiosis in Urechis caupo eggs caused premature sperm aster formation and we reviewed indirect evidence that the suppression of sperm asters by MAPK during meiosis might be a universal mechanism (M. C. Gould and J. L. Stephano, 1999, Dev. Biol. 216, 348-358). We tested this proposition with oyster (Crassostrea gigas) and starfish (Asterina miniata) eggs, utilizing the MEK inhibitors U0126 and PD98059. Centrosomes, asters, and meiotic spindles were visualized by normal epifluorescence and confocal microscopy following indirect immunocytochemical staining for anti-beta-tubulin. When MAPK activation was inhibited, sperm asters in both species developed prematurely and tended to move toward the egg centrosomes, sometimes even fusing with the egg spindle or centrosomes. Meiotic spindles and polar body formation were also abnormal when MAPK was inhibited.     

The dependence of the duration of sperm motility in Ob–Irtysh basin coregonidae on temperature

The duration of sperm motility in four whitefish species in the Ob–Irtysh basin, namely, Coregonus tugun, the river and lake forms of C. peled, C. lavaretus pidschian, and C. nasus, has been studied. It is shown that the duration of sperm motility in these species has a statistically significant inversely proportional dependence on the temperature of the water that is used for activation. The total duration of sperm motility at the spawning temperature in the range from 0.1 to 5.0°C amounts to 331 ± 107 s, on average, while the duration of forward movement amounts to 149 ± 44 s, on average. At temperatures that exceed the spawning values (above 7.1°C), these parameters constitute 190 ± 47 and 86 ± 15 s, respectively. The highest interspecific variability in the duration of sperm motility was observed within the temperature range from 0.1 to 2.5 and above 7.1°C. C. tugun shows less dependence of the duration of sperm motility on temperature than the other species under study. At a temperature of 0.7°C, the total duration and duration of the forward movement of C. tugun sperm are 201 and 108 s, respectively. When the temperature was increased to 13.4°C, these parameters decreased by 1.2 and 1.1 times only, i.e., to 172 and 100 s, respectively.     

Distribution, crypticity, stability, and localization of α-L-fucosidase of mouse cauda epididymal sperm

Sperm‐associated and semen‐specific isoforms of α‐L ‐fucosidase are thought to function in fertilization in numerous organisms. Here, we report the localization, distribution, crypticity, and stability of this enzyme in mouse cauda epididymal sperm and cauda fluid. Western analysis revealed that the sperm‐associated α‐L ‐fucosidase is present as two isoforms (Mr ～49 and 56 kDa), whereas the cauda fluid α‐L ‐fucosidase shows a single band at 50 kDa. α‐L ‐Fucosidase activity was detected using the fluorogenic substrate 4‐MU‐FUC. Of the total α‐L ‐fucosidase activity recovered in the cauda epididymal contents, 74% was found in the cell‐free cauda fluid and about 7% was found in sperm cells. During capacitation or permeabilization, cryptic intracellular stores of soluble enzyme were released to the supernatant, while leaving bound enzyme concentrated within the small volume of sperm. Moreover, membrane‐associated enzyme activity was still detectable in acrosome‐reacted cells. Immunofluorescence studies support the presence of α‐L ‐fucosidase (originally localizing at the acrosomal area) at the equatorial segment after the acrosome reaction. α‐L ‐Fucosidase activity of both cauda fluid and sperm at 37°C, 5% CO₂ was relatively stable and detectable up to 72 hr. The stability and appearance of mouse sperm‐associated α‐L ‐fucosidase in the equatorial segment after the acrosome reaction suggest that α‐L ‐fucosidase may be involved in sperm–egg interaction. Mol. Reprod. Dev. 79: 208–217, 2012. © 2011 Wiley Periodicals, Inc.