Functional and morphological diversity of sperm in Drosophila

Unlike mammals, where the males produce huge quantities of tiny spermatozoa, insects, and Drosophila in particular, exhibit a wide range of reproductive strategies. Sperm gigantism in Drosophila deviates from the rules that normally govern anisogamy, i.e. differences in the size and quantity of male and female gametes. Sperm gigantism has driven anatomical, physiological and cytological adaptations that affect the correlated evolution of the male and female reproductive systems, and has led to the evolution of a new structure, the roller, located between the testis and the seminal vesicle, and to sperm coiling to form pellets. The diversification of sperm strategy is investigated in the light of sexual selection processes that occur in the female genital tract after copulation. These processes, which bias paternity, result from interactions either between spermatozoa from different males, or between the spermatozoa and the environment within the female reproductive tract. In Drosophila, increased sperm size does not confer any reproductive advantage on the male. The evolution of sperm gigantism does not seem to be attributable to competition between spermatozoa from different males, as has been shown to occur in some vertebrate species. Alternative mechanisms, such as interactions between spermatozoa and the female reproductive system, are therefore currently viewed as being more likely explanations. In particular, the impact of sperm size on female reproductive physiology is being investigated to find out whether having large spermatozoa increases the likelihood of male reproductive success. Correlated adaptations of the spermatozoa and female storage organs also seem to be a major factor in determining sperm success, and their role in male-female conflicts is discussed briefly.     

Comparative studies of testicular structure and sperm morphology among copulatory and non-copulatory sculpins (Cottidae: Scorpaeniformes: Teleostei)

Testicular structure of 9 species and sperm head morphology of 19 species of Cottidae were observed in order to clarify relationships between morphological characteristics of the male reproductive organ and reproductive mode (copulation or non-copulation). Morphological structure of the testis was divided into the following five types based on the sperm transfer and reservoir system: (1) a non-duct type in which the sperm duct is not a distinct exterior structure, but the tube for sperm transport traverses along the testis as an interior structure; (2) an anterior duct type with distinct anterior sperm ducts traversing along the testis; (3) a posterior duct type with distinct anterior sperm ducts traversing along the dorsal hilus of testis and posterior sperm ducts extending to the rear of the testis; (4) an anterior duct posterior vesicle type with distinct anterior sperm ducts traversing along the testis, and the right and left sperm ducts fusing in the rear of testis, forming the seminal vesicle; (5) a non-duct posterior vesicle type in which sperm ducts do not accompany the testis, and the testis and seminal vesicle are connected directly or through posterior sperm ducts. It is thought that in Cottidae the non-duct type of reproductive organ is primitive, and the anterior duct type is common to all non-copulating species. The testes and accompanying seminal vesicle were seen only in copulating species. Sperm head morphology was divided into three types according to the length/width ratio: oval type ≤2, intermediate type >2 and ≤3, and slender type >3. The type of sperm head corresponded closely to the reproductive mode; non-copulating species had oval sperm head, and copulating species had intermediate or slender ones. These results suggest that the structure of the testis and the morphology of the sperm head evolved from testes with anterior sperm ducts and oval sperm heads to testes with an associated seminal vesicle and slender sperm heads in association with the evolution from non-copulatory to copulatory reproduction in Cottidae.     

The sperm roller: a modified testicular duct linked to giant sperm transport within the male reproductive tract

The male reproductive tracts of Drosophila display considerable variation in the relative size of their components, notably of the testes, but there are few structural differences between species. Here we report a remarkable coiled structure separating the testes from the seminal vesicles in the giant sperm species Drosophila bifurca. This evolutionary novelty, known as the 'sperm roller', seems to be an exaggeration in the size of the testicular duct as revealed by light and electron microscopic observations. It consists of a tubular monocellular epithelium lying on the basal laminae and muscle and conjunctive cells. The lumen of the roller contains crypts. The apical membrane of the epithelial cells presents numerous long microvilli protruding into the lumen. The sperm roller structure is probably involved in managing sperm during their transit through the male genital tract, because sperm are seen in bundles at the base of the testis, whereas they are singly rolled up by the time they enter the seminal vesicles. The hypercoiling of the individual spermatozoon within the roller probably occurs as the result of an osmotic process produced by features of the epithelial wall and the dramatically increased exchange surface. This is the first report of a specialized device of this type in Drosophila or, more generally, in insects.     

No association between sperm competition and sperm length variation across dung flies (Scathophagidae)

Sperm length is extremely variable across species, but a general explanation for this variation is lacking. However, when the risk of sperm competition is high, sperm length is predicted to be less variable within species, and there is some evidence for this in birds and social insects. Here, we examined intraspecific variation in sperm length, both within and between males, and its potential associations with sperm competition risk and variation in female reproductive tract morphology across dung flies. We used two measures of variation in sperm size, and testis size was employed as our index of sperm competition risk. We found no evidence of associations between sperm length variation and sperm competition or female reproductive tract variation. These results suggest that variation in sperm competition risk may not always be associated with variation in sperm morphology, and the cause(s) of sperm length variation in dung flies remains unclear.     

Testis asymmetry and sperm length in Rhacophorus omeimontis

Theory predicts that the degree of testes asymmetry should be positively correlated with male body condition in species with directional testis asymmetry. We tested this prediction in Rhacophorus omeimontis, a species in which females mate with more than one male. Our results showed that the treefrogs did not exhibit the absence of directional asymmetry in testis size, but rather the occurrence of fluctuating asymmetry. Moreover, we also tested differences in body size, body mass, testis mass, testis asymmetry, and sperm size among initially paired, jointly paired, and unpaired males. We found that body size and mass, testis mass, testis asymmetry and sperm length did not differ among the three male types. Testis mass showed a positive relationship with soma mass, but the correlations between the extent of fluctuating testis asymmetry and sperm length, and between testis mass and sperm length were not significant. Our data suggest that testes size and sperm length do not play an important role in determining male mating success in the presence of sperm competition.     

The influence of mating system on seminal vesicle variability among gobies (Teleostei, Gobiidae)

A variety of sexual selection mechanisms have been implicated to drive the variability of the male reproductive tract in internal fertilizers, while studies on external fertilizers have been largely limited to exploring the influence of sperm competition on testis size and sperm number. Males in the Gobiidae, a speciose teleost family of demersal spawners with external fertilization, are known to be characterized by accessory structures to the sperm duct called seminal vesicles. These seminal vesicles secrete a mucus-enriched seminal fluid. Seminal vesicle size and function have been demonstrated to be influenced by sperm competition at the intraspecific level. With the aim to test the factors influencing the development of these male organs at the interspecific level, an independent contrast analysis was performed on 12 species, differing in mating system type, sperm competition risk, and duration of egg deposition. The type of mating system appears to be the main factor significantly affecting development of seminal vesicles, with males of monogamous species completely lacking or having extremely reduced organs.     

Co-evolution of male and female reproductive characters across the Scathophagidae (Diptera)

Sperm morphometry is extremely variable across species, but a general adaptive explanation for this diversity is lacking. As sperm must function within the female, variation in sperm form may be associated with variation in female reproductive tract morphology. We investigated this and other potential evolutionary associations between male and female reproductive characters across the Scathophagidae. Sperm length was positively associated with the length of the spermathecal (sperm store) ducts, indicating correlated evolution between the two. No association was found between sperm length and spermathecal size. However, the size of the spermathecae was positively associated with testis size indicating co-evolution between male investment in sperm production and female sperm storage capacity. Furthermore, species with a higher degree of polyandry (larger testes) had longer spermathecal ducts. However, no associations between sperm length or length variation and testis size were found which suggests greater sperm competition sensu stricto does not select for longer sperm.     

Effect of castration on epididymal sperm storage in male musk shrews (Suncus murinus) and mice (Mus musculus)

Reproductively mature male musk shrews and mice were bilaterally castrated. Epididymal sperm numbers and motility were assessed 0, 2, 4 and 6 weeks after surgery. Seminal vesicle weights and plasma concentrations of total androgens were also measured. In male musk shrews, 30% of the original epididymal sperm numbers were still present 2 weeks after castration and motile spermatozoa were present in 2 of 7 individuals. By 4 and 6 weeks after castration the numbers of spermatozoa remaining declined to about 10% and no sperm motility was noted. Seminal vesicle weights were maintained at about 30% of their original size even up to 6 weeks after castration. In male mice, epididymal sperm numbers, seminal vesicle weights, and androgen levels declined more dramatically after castration. Although androgen concentrations in gonadally intact male musk shrews were approximately 50% of the values in male mice, after castration the concentrations in musk shrews were approximately 2-fold higher than in mice at all times. The results suggest that post-castration retention of epididymal sperm and seminal vesicle weights in the male musk shrew as compared with male mice, is facilitated either by a relatively greater adrenal contribution to circulating androgen levels and/or greater target tissue sensitivity.     

Sperm competition experiments between lines of crickets producing different sperm lengths.

Sperm numbers can be important determinants of fertilization success in sperm competition. However, the importance of variation in sperm size is less well understood. Sperm size varies significantly both between and within species and comparative studies have suggested that some of this variance can be explained by sperm competition. In this study we examine whether variation in sperm length has consequences for fertilization precedence using controlled sperm competition experiments in the field cricket Gryllus bimaculatus. This species is an ideal model for such investigations because the mechanism of sperm competition generates complete mixing of different males' spermatozoa in the female (thereby allowing individual sperm to express their own competitive abilities). We successfully bred lines of crickets, the males of which produced short, medium and long sperm types with narrow and non-overlapping distributions. Males of different lines were then sequentially mated with control females in order to create two-male sperm competitions. The paternity outcomes of these competitions were measured after matings using an irradiated male technique (with a full reciprocal design that controls for natural fertility and any irradiation effects on gamete competitiveness) over a 12 day oviposition period. However, having successfully bred diverging sperm length lines and competing males that differed in sperm length, we found no evidence that a male's sperm size explained any of the variation in their relative fertilization success. Males from lines producing longer sperm showed no fertilization advantage over males producing shorter sperm across 97 double matings. There was also no advantage for males producing a sperm length close to the population mean over those competitors whose sperm length had been selectively diverged across 63 matings.     

Coevolution of sperm and female reproductive tract morphology in stalk-eyed flies

Sperm and female reproductive tract morphology are among the most rapidly evolving characters known in insects. To investigate whether interspecific variation in these traits results from divergent coevolution we examined testis size, sperm length and female reproductive tract morphology for evidence of correlated evolution using 13 species of diopsid stalk-eyed flies. We found that sperm dimorphism (the simultaneous production of two size classes of sperm by individual males) is ancestral and occurs in four genera while sperm monomorphism evolved once and persists in one genus. The length of ''long-sperm'' types, though unrelated to male body or testis size, exhibits correlated evolution with two regions of the female reproductive tract, the spermathecae and ventral receptacle, where sperm are typically stored and used for fertilization, respectively. Two lines of evidence indicate that ''short sperm'', which are probably incapable of fertilization, coevolve with spermathecae. First, loss of sperm dimorphism coincides phylogenetically with reduction or loss of spermathecae. Second, evolutionary change in short-sperm length correlates with change in spermathecal size but not spermathecal duct length or ventral receptacle length. Morphological coevolution between sperm and female reproductive tracts is consistent with a history of female-mediated selection on sperm length.     

The evolution of sperm and non-sperm producing organs in male Drosophila

The male ejaculate is made up of two components: sperm and non-sperm. There has been little consideration of how these two basic compartments evolve. If they are subject to trade-offs, theory predicts that when the sperm competition raffle is unfair, when seminal fluid proteins stimulate fecundity and/or when ejaculate components alter fertilization success, there will be differential selection on sperm versus non-sperm ejaculate characteristics. However, the fundamental assumption that there are trade-offs between sperm and non-sperm ejaculate compartments in Drosophila has not yet been tested. To address this, we examined testis (sperm producing) and accessory gland (non-sperm producing) size across 22 species of Drosophila . We also examined how these characters varied with copulation duration, which may represent an additional target for sperm competition. The results showed no evidence of a trade-off between testis length and accessory gland length. Copulation duration correlated negatively with accessory gland length and there was a positive correlation with testis length, but only after correcting for body size. Overall, the results suggest no evidence for gross trade-offs in sperm versus non-sperm compartments across these Drosophila species, and motivate more detailed examination of ejaculate investment patterns.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 505–512.     

The evolution of sperm length in moths

Sperm form and function remain poorly understood despite being of fundamental biological importance. An instructive approach has been to examine evolutionary associations across comparable taxa between sperm characters and other, potentially selective reproductive traits. We adopt this approach here in a comparative study examining how sperm lengths are associated with male and female reproductive characters across moths. Primary data have revealed Lepidoptera to be an ideal order for examination: there is profound variation in the dimensions (but not organization) of the reproductive traits between closely related species which all share a monophyletic ancestry, for example, eupyrene sperm length varies from 110 to 12,675 microm. Eupyrene (normal fertilizing) and apyrene (anucleate and non-fertile) sperm lengths are positively correlated across taxa and both sperm types show positive associations with mating pattern (as measured by the residual testis size). At fertilization, eupyrene sperm must migrate down the often elongated female spermathecal duct from storage to unite with the ovum. Across taxa, the elongation of this duct is associated with increased eupyrene sperm length, suggesting a positive female influence on sperm size since longer, more powerful sperm may be selected to migrate and/or compete successfully down greater ductal lengths. Apyrene sperm length is not associated with female reproductive tract dimensions. However, we found a positive relationship between the residual testis volume and spermathecal volume, suggesting coevolution between male investment in spermatogenesis and the extent of the female sperm storage capacity. Within males, there is a positive association between the two organs which form the ejaculate-containing spermatophore: the testes and the accessory gland. The 'trade-up' in investment to these components is discussed in relation to paternal investment and mating patterns.     

Geographical variation in sperm morphology in the red-winged blackbird (<Emphasis Type="Italic">Agelaius phoeniceus</Emphasis>)

Many species differ genetically, physiologically, and morphologically between geographically distinct populations, typically in response to variation in ecological and climatic variables. Little is known, however, about geographical variation in sperm morphology. Sperm morphology is under strong sexual selection, has been shown to evolve rapidly, and often co-varies with other reproductive traits (e.g., testis size or mating system) that differ between populations in some species. The aim of this study was to establish whether sperm morphology varies between populations of the red-winged blackbird (Agelaius phoeniceus), a species with an enormous breeding range and marked inter-population variation in both body size and mating system. We found (1) highly significant variation in sperm morphology among study sites, (2) a gradual increase in sperm length from the southwest to the northeast of the breeding range, and (3) a strong negative association between sperm length and body size. However, the relationship with the mating system remains unclear. Several hypotheses to explain these patterns are proposed.     

角马蜂贮精囊及精子的形态学观察

昆虫贮精囊和精子的形态多样性是重要的分类和系统发育分析特征之一, 然而在马蜂亚科乃至整个胡蜂科中却鲜有涉及。本文首次解剖了角马蜂Polistes chinensis antennalis Pérez的雄性生殖系统, 着重对其贮精囊的超微结构进行描述, 并简要报道了精子的外部形态。角马蜂的贮精囊由输精管亚前端膨大而成, 有一层发达的柱状上皮细胞贴在基底膜内壁： 细胞核位于柱状细胞基部, 上皮细胞端半部线粒体密集, 顶膜特化成微绒毛。角马蜂精子头长21.4 μm, 体长94 μm, 是已报道胡蜂科精子中长度最短、 相对头长最长的种类。研究结果为胡蜂科昆虫系统发育以及繁殖生理提供理论依据。     

Convergent evolution of sperm gigantism and the developmental origins of sperm size variability in Caenorhabditis nematodes

Sperm cells provide essential, if usually diminutive, ingredients to successful sexual reproduction. Despite this conserved function, sperm competition and coevolution with female traits can drive spectacular morphological change in these cells. Here, we characterize four repeated instances of convergent evolution of sperm gigantism in Caenorhabditis nematodes using phylogenetic comparative methods on 26 species. Species at the extreme end of the 50‐fold range of sperm‐cell volumes across the genus have sperm capable of comprising up to 5% of egg‐cell volume, representing severe attenuation of the magnitude of anisogamy. Furthermore, we uncover significant differences in mean and variance of sperm size among genotypes, between sexes, and within and between individuals of identical genotypes. We demonstrate that the developmental basis of sperm size variation, both within and between species, becomes established during an early stage of sperm development at the formation of primary spermatocytes, while subsequent meiotic divisions contribute little further sperm size variability. These findings provide first insights into the developmental determinants of inter‐ and intraspecific sperm size differences in Caenorhabditis. We hypothesize that life history and ecological differences among species favored the evolution of alternative sperm competition strategies toward either many smaller sperm or fewer larger sperm.     

The circadian rhythm of sperm release in the codling moth, Cydia pomonella

The release of sperm bundles from testes to the vas deferens is controlled by a circadian clock in several moth species. We investigated the pattern of sperm release in the codling moth, Cydia pomonnella L. (Lepidoptera: Tortricidae). Sperm release in the codling moth follows a two-step rhythm in light-dark cycles: sperm is released from the testis before lights-off, remains in the vas deferens during the dark phase and is transferred to the seminal vesicles after lights-on. This rhythm continues in constant darkness indicating that it has a circadian nature. The release of sperm is asynchronous in moths held in constant light. In contrast to previously investigated moths, constant light has no adverse effects on the male reproductive capacity in the codling moth.     

多肠目薄背涡虫生殖系统结构及一氧化氮合酶组化研究

运用常规组织学方法和NADPH-d组织化学方法,研究了薄背涡虫 Notoplana humilis 生殖系统的组织结构和一氧化氮合酶的分布.其雄性生殖系统包括精巢、储精囊、阴茎、雄性生殖孔,精巢壁由一薄层薄膜组成,每个精巢内都含有不同发育时期的雄性生殖细胞,且精子发育无明显同步性;储精囊呈螺旋状排列在雄性生殖孔附近,囊壁由单层扁平上皮组成;阴茎为粗大的球形,外壁由柱状上皮细胞和数层肌细胞组成.雌性生殖系统包括输卵管、生殖腔、雌性生殖孔和受精囊,但不形成集中的卵巢和卵黄腺.雌雄生殖孔、生殖腔、受精囊、阴茎等部位呈NADPH-d强阳性反应.     

In vitro studies on the functional maturation and locomotion of sperm in a free-living nematode, Panagrellus redivivus

In the seminal vesicle of male Panagrellus redivivus the sperm are normally rounded, non-motile and have cytoplasmic organelles randomly scattered throughout the whole cell body. Sperm become amoeboid in the uterus of the female with a clear anterior region capable of producing pseudopodia and an arch-shaped rigid posterior region containing numerous organelles. The sperm arrange themselves in the form of a chain in the uterus attaching themselves anterio-posteriorly, however sperm entering the post-vulvar uterine sac do not form a chain and remain scattered. Approximately eight hours after insemination the sperm in the uterus stop producing pseudopodia. Pseudopodial formation recommences in the seven to eight anteriormost sperm in the chain as they reach seminal receptacle.     

Implications of diversity in sperm size and function for sperm competition and fertility

Sperm competition is now recognised as a potent selective force shaping many male reproductive traits. While the influence of sperm competition on sperm number is widely accepted, its effects upon sperm size remain controversial. It had been traditionally assumed that there is a trade-off between sperm number and sperm size, so that an increase in sperm number would result in a decrease in sperm size, under conditions of sperm competition. Contrary to this prediction, we proposed some time ago that sperm competition favours an increase in sperm size, because longer sperm swim faster and are more likely to win the race to fertilize ova. Comparative studies between species show that in many taxa such a relationship exists, but the consequences of an increase in sperm size may vary between taxa depending on the environment in which spermatozoa have to compete. We present new evidence showing that in mammals longer sperm swim at higher speeds. We also show that mean swimming speed is highly correlated with maximum swimming speed, so even if the fastest swimming sperm are more likely to fertilize, both measures are informative. When individuals of the same species are compared, ratios between the dimensions of different sperm components, as well as the shape of the head, seem better at explaining sperm swimming velocity. Finally, we show that mean and maximum sperm swimming speed determine male fertility. Other studies have shown that in competitive contexts, males with faster swimming sperm have higher fertilization success. We conclude that the available evidence supports our original hypothesis.     

Variability in sperm form and function in the context of sperm competition risk in two Tupinambis lizards

In polyandrous species, sperm morphometry and sperm velocity are under strong sexual selection. Although several hypotheses have been proposed to explain the role of sperm competition in sperm trait variation, this aspect is still poorly understood. It has been suggested that an increase in sperm competition pressure could reduce sperm size variation or produce a diversity of sperm to maximize male fertilization success. We aim at elucidating the variability of sperm morphometric traits and velocity in two Tupinambis lizards in the context of sperm competition risk. Sperm traits showed substantial variation at all levels examined: between species, among males within species, and within the ejaculate of individual males. Sperm velocity was found to be positively correlated with flagellum: midpiece ratio, with relatively longer flagella associated with faster sperm. Our results document high variability in sperm form and function in lizards.