‘Kamikaze’ sperm in mammals?

Hominid sexual nature

The aim of the paper is to identify psychosomatic evolutionary adaptations of hominids, which direct them at maximizing their reproductive success, and on the basis of which their various social structures are built. Selected features of the hominid last common ancestor were extracted; by reducing the influence of the social structure, they were defined as the hominid "sexual nature"; these considerations were supported by the analysis of sexual jealousy as a function of socio-environmental conditions. The "sexuality core" of a hominid female was defined as "selective polyandry"-the female selects the best males among those available; and of a hominid male as "tolerant promiscuity"-the male strives for multi-male and multi-female copulations with sexually attractive females. The extracted "sexuality cores" condemn hominids to a patriarchal social structure and thus to sexual coercion and jealousy. The source of male sexual jealousy is limited access to females. Hominid female jealousy of the male results mainly from the need for protection and support. Hominids' social structures are determined by females' sexual selectivity or opportunism and by their continuous or periodic proceptivity and estrus signaling. Evolutionary functions developed by women: out-estrus sexuality, copulation calls, multiple orgasms, allow them to obtain the best possible spermatozoid. The institution of marriage blocks the influence of sexual selection in the species Homo sapiens.

Sperm dimorphism in the Triatoma brasiliensis species complex and its applications

Morphological and structural features of the sperm of the Triatoma brasiliensis Neiva, 1911 (Hemiptera: Reduviidae) species complex were examined in this first study investigating the sperm of Heteroptera and the genus Triatoma. Males were dissected and their seminal vesicles removed. For measurement, seminal vesicles were squashed on glass slides, spread, fixed and observed under a photomicroscope. The images were analysed and measures of sperm were made. Data were analysed using one-way analysis of variance and Tukey's test to detect differentiation among taxa. Furthermore, seminal vesicles were prepared for studies of transmission electron microscopy. All taxa studied showed polymorphic (short and long) sperm. The sperm of Triatoma brasiliensis macromelasoma was significantly longer (in total length) than that of the other four members of the complex, which supports the hypothesis of hybrid speciation of this member of the complex as an increase in the size of typical hybrids under heterosis was previously shown. The sperm cells of the five taxa have similar ultrastructural morphology. The ultrastructural features observed confirm the hypothesis, raised by previous studies, that they are synapomorphic to the suborder Heteroptera.

Social dominance explains within-ejaculate variation in sperm design in a passerine bird

Background

Comparative studies suggest that sperm competition exerts stabilizing selection towards an optimal sperm design – e.g., the relative size and covariation of different sperm sections or a quantitative measure of sperm shape - that maximizes male fertility, which results in reduced levels of within-male variation in sperm morphology. Yet, these studies also reveal substantial amounts of unexplained within-ejaculate variance, and the factors presiding to the maintenance of such within-male variation in sperm design at the population level still remain to be identified. Sperm competition models predict that males should progressively invest more resources in their germline as their mating costs increase, i.e., the soma/germline allocation trade-off hypothesis. When access to fertile females is determined by social dominance, the soma/germline allocation trade-off hypothesis predicts that dominant males should invest less in the control of spermatogenesis. Hence, dominance should positively correlate with within-male variance in sperm design.

Results

In support of this hypothesis, we found that dominant house sparrow males produce ejaculates with higher levels of within-ejaculate variation in sperm design compared to subordinate males. However, after experimentally manipulating male social status, this pattern was not maintained.

Conclusions

Our results suggest that males might control variation in sperm design according to their social status to some extent. Yet, it seems that such within-ejaculate variation in sperm design cannot be rapidly adjusted to a new status. While variation in sperm design could result from various non-exclusive sources, we discuss how strategic allocation of resources to the somatic vs. the germline functions could be an important process shaping the relationship between within-male variation in sperm design and social status.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0914-2) contains supplementary material, which is available to authorized users.

No effect of sperm interactions or egg homogenate on sperm velocity in the blue mussel, Mytilus edulis (Bivalvia: Mytilidae)

We investigated the possible effects of sperm interactions and homogenized eggs on sperm velocity in blue mussels ( Mytilus edulis L., 1758) using computer-assisted sperm analysis. To test whether sperm competition results in an increase in sperm velocity, using seven pairs of males, we compared the mean curvilinear and average path velocities of sperm from two males measured separately with the corresponding values from a mixture of sperm from the same two males. To test whether the presence of eggs results in an increase in sperm velocity, we compared curvilinear and average path velocities from 11 individual males with the corresponding measures from the same 11 sperm samples mixed with aliquots of homogenized eggs. Neither experimental treatment resulted in an increase in sperm velocity. We interpret these results as consistent with the hypothesis that mussel sperm have been selected to immediately begin swimming at an optimal initial velocity that is adaptive for the particular environment in which they are located. Critical factors affecting the evolution of sperm velocity for broadcast spawning, external fertilizers such as M. edulis likely include population density and intraspecific spawning synchronicity. As has been suggested by others, the importance of sperm limitation (i.e., having much less than 100% of eggs being fertilized in the wild) may be as important an evolutionary driving force in broadcast spawning invertebrates as sperm competition is in internally or directly fertilized species.

The evolutionary psychology of men's coercive sexuality

Psychological adaptation underlies all human behavior. Thus, sexual coercion by men could either arise from a rape-specific psychological adaptation or it could be a side-effect of a more general psychological adaptation not directly related to rape. Determining the specific environmental cues that men's brains have been designed by selection to process may help us decide which these rival explanations is correct. We examine six testable predictions against existing data: (1) Both coercive and noncoercive will be associated with high levels of sexual arousal and performance in men. (2) Achieving physical control of a sexually unwilling woman will be sexually arousing to men. (3) Young men will be more sexually coercive than older men. (4) Men of low socioeconomic status will likewise be more sexually coercive. (5) A man's motivation to use sexual coercion will be influenced by its effects on social image. (6) Even in long-term relationships men will be motivated to use coercion when their mates show a lack of interest in resistance to sex because these are interpreted as signs of sexual infidelity. Current data support all six predictions and are hence consistent with the rape-specific hypothesis, but this does not eliminate the side-effect hypothesis, which is likewise compatible with the findings, as well as with the further evidence that forced matings increased the fitness of ancestral males during human evolution. We suggest some research that may help decide between the two hypotheses.

Postcopulatory sexual selection: Darwin's omission and its consequences

In one of his few major oversights, Darwin failed to appreciate that male-male competition and sexual selection can continue even after copulation has begun. The postcopulatory equivalents of both direct male-male battles (sperm competition) and female choice (cryptic female choice) occur within the female's body. Recognition of this hidden, but intense, sexual competition provides new insights into a variety of fields. These include the hyperdiverse and paradoxically elaborate morphology of both sperm and male genitalia, the equally puzzling and elaborate morphology of nongenitalic male structures that are specialized to grasp and stimulate females, powerful manipulative effects of substances in male semen on female reproductive physiology, paradoxical male courtship behavior that occurs after copulation has already begun, variability in parental investments, and the puzzlingly complex and diverse interactions between sperm and female products that surround animal eggs and between male gametophytes and female tissues in flowering plants. Many bizarre traits are involved, including male genitalia that are designed to explode or fall apart during copulation leaving behind parts within the female, male genitalia that "sing" during copulation, potent seminal products that invade the female's body cavity and her nervous system to influence her behavior, and a virtual Kama Sutra of courtship behavior performed after rather than before genital coupling, including male-female dialogues during copulation.

Sperm competition and sperm cooperation: the potential role of diploid and haploid expression

Sperm competition is a powerful selective force driving the evolution of sperm shape and function. Recent findings suggest that sperm cooperation is a potential evolutionary response to sperm competition. Sperm cooperation may enhance the performance of the ejaculate increasing a male's chance to outcompete rival males in competition for fertilisation. Whether and how sperm cooperation may evolve is the focal point of this review. The relative importance of haploid and diploid gene expression for the evolution of sperm cooperation and the potential conflict of interest between (i) haploid sperm and diploid male and (ii) among sibling sperm, since sibling sperm only share an average of 50% of their genes in a diploid organism, are discussed. Furthermore, sperm cooperation is defined and the literature for empirical evidence of sperm cooperation is reviewed in light of the author's definitions.

Sperm ultrastructure in the inseminating Macropsobrycon uruguayanae (Teleostei: Characidae: Cheirodontinae)

Macropsobrycon uruguayanae is a small, inseminating characid (tetra) of the tribe Compsurini. Although spermatozoa can be found within the ovarian cavity close to oocytes, the exact moment of fertilization has not yet been determined. Spermatozoa have moderately elongate nuclei with electron-dense chromatin. During spermiogenesis, nuclear rotation takes place. Elongate mitochondria with lamellar cristae are found posterior to the nucleus. Centrioles are parallel to one another with the proximal centriole slightly anterior to the longer distal one. The anterior tip of the proximal centriole is located within a shallow nuclear fossa. Electron-dense spurs are associated within the anterior and posterior ends of the distal centriole. Striated centriolar rootlets radiate both anteriorly and posteriorly from the distal centriole. Nine longitudinal accessory microtubules surround the axoneme in the proximal flagellum. The flagellum has a typical 9 + 2 axoneme with no intratubular differentiation. Atypical spermatozoa are also found in the testicular lumen. These cells resemble spermatozoa in most aspects, except that their nuclei are variable in shape, with the granular chromatin less electron-dense than that seen in spermatozoa. The origin and function of these cells could not be determined. The specializations seen in the spermatozoa are discussed as possible adaptations related to the habit of insemination.

Testis structure, spermatogenesis, spermatocytogenesis, and sperm structure in cardinal fish (Apogonidae, Perciformes)

The testes in all 16 of the studied cardinal fish species are shown to be bilobed, with spermatogonia dispersed throughout the gametogenic epithelium of the seminiferous tubules. Each testicular lobe is covered luminally by an epithelium consisting of primary germ cells and Sertoli cells. At maturation the seminiferous tubules reach around 0.6-2.3 mm in length. They number from 60 in the smallest species to over 300 in the largest one, increasing both in dimension and number with increase in length of the male, and are species-specific. The highest number of spermatogonia is found at the apical ends of the tubules. During maturation extensions of Sertoli cells surround single or small groups of B-spermatogonia, forming the spermatocysts, the final dimensions of which reflect the final number of contained spermatozoids. Back-calculations of serial sections reveal that within the spermatocysts the spermatogonia undergo eight generations of mitotic divisions before the first and second meiotic divisions and formation of spermatids. The largest mature spermatocysts in large species attain around 180 microm in diameter, a volume of 25 mm(3), and contain around 8,200 spermatids. The total volume of sperm in the mature spermatocysts leaves enough space for the discarded cytoplasm and developing flagella. The bursting cysts liberate the ripe sperm and maturing spermatids, into the tubule lumen and spermduct, with the spermatids often still connected by cytoplasm bridges. The sperm, with one or two flagella, features round or oval heads and a cytoplasmic collar bearing a few mitochondria. The percentage of biflagellate or monoflagellate sperm differs in proportion in males of different lengths and in different species. Differences in spermatogenesis of small and larger species of cardinal fish are discussed.

Psychological and Physiological Adaptations to Sperm Competition in Humans

Postcopulatory competition between males, in the form of sperm competition, is a widespread phenomenon in many animal species. The extent to which sperm competition has been an important selective pressure during human evolution remains controversial, however. The authors review critically the evidence that human males and females have psychological, behavioral, and physiological adaptations that evolved in response to selection pressures associated with sperm competition. The authors consider, using evidence from contemporary societies, whether sperm competition is likely to have been a significant adaptive problem for ancestral humans and examine the evidence suggesting that human males have physiological and psychological mechanisms that allow for “prudent” sperm allocation in response to variations in the risk of sperm competition.

Sperm competition in mammals

Although Darwin identified the evolutionary significance of competition between males in the context of reproduction, it is only in the past few decades that we have begun to appreciate the importance of competition at the gametic level. Sperm competition, defined as competition between the sperm of two or more males for fertilization of the same set of ova, is now recognised as a key selective force shaping male reproductive anatomy, physiology and behaviour across diverse animal groups, including mammals. The aim of this article is to provide a brief review of the selective consequences of sperm competition in mammals, with emphasis on recent theoretical advances and empirical controversies. Evidence of female influences on sperm competition outcomes in mammals is also discussed, and it is concluded that understanding the selective pressures driving coevolution between male and female reproductive traits remains a major challenge for researchers in this field.

Pollen and sperm heteromorphism: convergence across kingdoms?

Sperm competition theory predicts that males should produce many, similar sperm. However, in some species of animals and plants, males exhibit a heteromorphism that results in the production of at least two different types of sperm or pollen grains. In animals, sperm heteromorphism typically corresponds to the production of one fertile morph and one (or more) sterile morph(s), whereas in plants two or more pollen morphs (one of which can be either sterile or fertile) are produced in all flowers but sometimes in different anthers. Heteromorphism has arisen independently several times across phyla and at different phylogenetic levels. Here, we compare and contrast sperm and pollen heteromorphism and discuss the evolutionary hypotheses suggested to explain heteromorphism in these taxa. These hypotheses include facilitation, nutritive contribution, blocking, cheap filler, sperm flushing or killing for animals; outcrossing and precise cross-pollen transfer or bet-hedging strategy for plants; cryptic female choice for both. We conclude that heteromorphism in the two phyla is most likely linked to a general evolutionary response to sexual selection, either to increase one male's sperm or pollen success in competition with other males, or mediate male/female interactions. Therefore, although sperm and pollen are not homologous, we suggest that heteromorphism represents an example of convergence across kingdoms.

Exceptional sperm cooperation in the wood mouse

Spermatozoa from a single male will compete for fertilization of ova with spermatozoa from another male when present in the female reproductive tract at the same time. Close genetic relatedness predisposes individuals towards altruism, and as haploid germ cells of an ejaculate will have genotypic similarity of 50%, it is predicted that spermatozoa may display cooperation and altruism to gain an advantage when inter-male sperm competition is intense. We report here the probable altruistic behaviour of spermatozoa in an eutherian mammal. Spermatozoa of the common wood mouse, Apodemus sylvaticus, displayed a unique morphological transformation resulting in cooperation in distinctive aggregations or 'trains' of hundreds or thousands of cells, which significantly increased sperm progressive motility. Eventual dispersal of sperm trains was associated with most of the spermatozoa undergoing a premature acrosome reaction. Cells undergoing an acrosome reaction in aggregations remote from the egg are altruistic in that they help sperm transport to the egg but compromise their own fertilizing ability.

Sperm viability and sperm competition in insects

Sperm quality plays an important role in vertebrates in determining which male has the advantage when two or more males compete to fertilize a female's ova. In insects, however, the importance of sperm quality has never been considered, despite sperm competition being widespread and well studied in this group. We tested the hypothesis that sperm viability, measured as the proportion of live sperm, covaried with the intensity of sperm competition in insects. In a pairwise comparison of seven closely related species pairs, each comprising a monandrous and a polyandrous species (i.e., with and without sperm competition, respectively), we found that in all cases the polyandrous species had a higher proportion of live sperm in their sperm stores. The distribution of the percentage of live sperm showed considerable inter- and intraspecific variation, suggesting that, all else being equal, males will vary in their ability to fertilize ova on the basis of sperm viability alone. Our results suggest that sperm viability is one of a suite of male adaptations to sperm competition in insects.

Conditions for the evolution of soldier sperm classes

There has been wide disagreement as to whether sperm competition among animals can produce a soldier class of sperm to fight against other males. Utilizing mathematical models, we analyze the appropriate conditions for the evolution and maintenance of a soldier sperm class. We conclude that: (1) soldier sperm evolve even if one soldier sperm can kill or block less than one competing sperm; (2) soldier sperm evolve faster when there is a large variance in the number of competing sperm; (3) soldier ratio increases until reproductive sperm are too scarce to fertilize all ova or a sperm intensely refuses to become a soldier; and (4) soldier sperm are more likely to be smaller than reproductive sperm. Our models suggest that the conditions for the evolution of a soldier sperm class are not stringent.

No evidence for killer sperm or other selective interactions between human spermatozoa in ejaculates of different males in vitro

This study examines one of the possible mechanisms of sperm competition, i.e. the kamikaze sperm hypothesis. This hypothesis states that sperm from different males interact to incapacitate each other in a variety of ways. We used ejaculates from human donors to compare mixes of semen in vitro from the same or different males. We measured the following parameters: (i) the degree of sperm aggregation, velocity and proportion of morphologically normal sperm after 1 and 3 h incubation in undiluted semen samples, (ii) the proportion of viable sperm plus the same parameters as in (i) in 'swim-up' sperm suspensions after 1 and 3 h incubation, (iii) the degree of self and non-self sperm aggregation using fluorescent dyes to distinguish the sperm of different males, and (iv) the extent of sperm capacitation and acrosome-reacted sperm in mixtures of sperm from the same and different males. We observed very few significant changes in sperm aggregation or performance in mixtures of sperm from different males compared with mixtures from the same male and none that were consistent with previously reported findings. The incapacitation of rival sperm therefore seems an unlikely mechanism of sperm competition in humans.

Wise, winsome, or weird? Mechanisms of sperm storage in female animals

Female sperm storage is an integral part of the reproductive pattern of many species. In the female, sperm become sequestered in specialized storage organs or reservoirs, where they may remain for several days, weeks, months, or years before being used to fertilize eggs. Several different but interrelated mechanisms are used by animals to target the sperm to the portion of the female genital tract adapted for sperm storage. Both males and females influence this process. This review describes themes among the mechanisms and molecules necessary for sperm to become efficiently stored in females and the roles that the female storage organs play in the nourishment, protection, and release of stored sperm.

Sib competition and sperm competitiveness: an answer to 'why so many sperms?' and the recombination/sperm number correlation

Sperm, or inter-ejaculate, competition leads to the evolution of many sperm per ejaculate. However, sperm competition theory does not predict the correlation between sperm number and chiasma, or that between sperm number and haploid chromosome number. Firstly, we show that phylogenetic inertia cannot account for at least the latter relation, and secondly, a model is presented which incorporates sib competition (intra-ejaculate competition) and sperm competitiveness to explain the relation between sperm numbers and recombination and the question 'Why so many sperms?'. It is argued that if there are deleterious mutations which affect sperm competitiveness this will lead to sib or intra-ejaculate competition. If inter-ejaculate competition also exists, then there will be selection for increased recombination. A chiasmate male heterozygous for n mutations which reduce sperm competitiveness can produce gametes with 0...n mutations. The proportion of gametes with 0 mutations per ejaculate is s = 0.5n, which is a small fraction. This means that to ensure on average one sperm with 0 mutations per ejaculate, a chiasmate male must produce 1/s sperm. We may therefore expect that 1/s will be positively correlated with sperm numbers. If inter-ejaculate competition leads to an optimum sperm number of x, then the optimum number of sperm per ejaculate is x/s. Sperm numbers will be increased by: (i) the number of loci which affect sperm competitiveness in the haploid state; (ii) the mutation rate; and (iii) the recombination rate. A correlation between recombination rates and sperm numbers is therefore to be expected.

Sperm competition games: sperm size and number under gametic control

This paper examines sperm competition games in which both size and number of gametes in an ejaculate can be varied strategically, but under the control of the haploid sperm. It is a companion to the previous paper (Parker (Proc. R. Soc. Lond. B 253, 245 (1993))) for the case where the strategic control is by the diploid parent. Under haploid control, the optimal balance between size and number of sperm in an ejaculate becomes quite different from that predicted for diploid control; there is a conflict between parental and gametic interests over sperm size and number. This occurs because both intra-ejaculate and inter-ejaculate sperm competition are present with haploid control, whereas intra-ejaculate competition is absent under diploid control. The magnitude of this conflict is affected by the risk of inter-ejaculate sperm competition, and by the fact that gametic interests depend on the nature of available mutations. If ejaculate expenditure trades off against numbers of matings achieved, with haploid control there is escalation towards maximal expenditure on the ejaculate. If the ejaculate expenditure is fixed by the diploid parent, but there is a size-number trade-off under haploid control, solutions depend on the mode of action of the mutation affecting sperm size or number. If mutant-bearing sperm deviate entirely at the expense of non-mutant sperm, any increase in size or number will spread. If mutant sperm deviate entirely at their own expense, the evolutionarily stable strategy (ESS) is the same as for diploid control; there is no conflict.(ABSTRACT TRUNCATED AT 250 WORDS)