W. G. Breed

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.

By hook or by crook? Morphometry, competition and cooperation in rodent sperm.

Background Sperm design varies enormously across species and sperm competition is thought to be a major factor influencing this variation. However, the functional significance of many sperm traits is still poorly understood. The sperm of most murid rodents are characterised by an apical hook of the sperm head that varies markedly in extent across species. In the European woodmouse Apodemus sylvaticus (Muridae), the highly reflected apical hook of sperm is used to form sperm groups, or “trains,” which exhibited increased swimming velocity and thrusting force compared to individual sperm. Methodology/Principal Findings Here we use a comparative study of murine rodent sperm and demonstrate that the apical hook and sperm cooperation are likely to be general adaptations to sperm competition in rodents. We found that species with relatively larger testes, and therefore more intense sperm competition, have a longer, more reflected apical sperm hook. In addition, we show that sperm groups also occur in rodents other than the European woodmouse. Conclusions Our results suggest that in rodents sperm cooperation is more widespread than assumed so far and highlight the importance of diploid versus haploid selection in the evolution of sperm design and function.

BODY MASS, TESTES MASS, AND SPERM SIZE IN MURINE RODENTS

Abstract The relationships among testes mass, sperm size (as indicated by sperm tail length), and body mass of Old World rats and mice were investigated. A near isometric relationship between testes mass and body mass was found but not between body mass and sperm size. In a few lineages, testes mass deviated considerably from the regression line with relatively very small testes occurring in Bunomys fratrorum, Bandicota indica, and B. savilei of Asia, Aethomys ineptus of Africa, some Pseudomys, and all Notomys of Australia. By contrast, relatively large testes mass occurred in Apodemus, Berylmys, and Maxomys bartelsii of Asia, Pogonomys of New Guinea, and some Australasian Rattus, Melomys, and Mastacomys. Considerable variability in sperm size was also evident; some species that had relatively small testes also had relatively small spermatozoa. The reason(s) for interspecific variation in relative testes mass and sperm size is unknown, but the data provide an opportunity to test the hypothesis that differences in relative testes mass, and perhaps sperm size, relate to interspecific differences in the amount of intermale sperm competition and in breeding systems.

Why so many mammalian spermatozoa - a clue from marsupials ?

Mammals generally ejaculate many more spermatozoa than seem to be needed for fertilization. This apparent profligacy has not been explained, but observations made in marsupials may shed light on it. The Virginia opossum, Didelphis virginiana, inseminates only about three million spermatozoa, a very low number. As a corollary, relatively few (ca. 13 x 106) are stored in each cauda epididymidis. However, some 5% of the spermatozoa that the opossum ejaculates populate the oviduct about 12 h later when ovulation can be anticipated - a success rate in the female orders of magnitude greater than in eutherian mammals. I t is not certain what determines the unusually efficient transport to and the high survival rate of spermatozoa in the oviduct of Didelphis, but two unusual features suggest themselves as possible contributors. Didelphis (and all other American marsupial) spermatozoa undergo a head-to-head pairing in the epididymis by the acrosomal face; this serves to isolate the acrosome of ejaculated spermatozoa from the female milieu until the pairs separate in the oviduct. Secondly, spermatozoa are housed in special crypts in the isthmus of the oviduct. Australian marsupials, which usually lack such features, store spermatozoa in the epididymis in numbers more close to those in comparably sized eutheriam mammals. Exceptions which store very low sperm numbers there can be seen in one Australian Family, the Dasyuridae. The spermatozoa of dasyurids are not paired, but the species examined possess distinctive sperm storage crypts in the oviducal isthmus similar to those in the opossum. The present findings suggest that where mechanisms exist that could protect the acrosome and, or, the whole spermatozoon in the female tract, a much lower level of sperm production can be maintained without compromising fertility. While the number ejaculated typically by any one species is probably determined ultimately by several interacting factors, it therefore seems likely that a most important one in this respect relates to conditions spermatozoa face in the female tract.

Changes in distribution of labile zinc in mouse spermatozoa during maturation in the epididymis assessed by the fluorophore Zinquin

The Zn(II)-specific fluorophore Zinquin was used to determine the regional distribution of free or loosely-bound Zn(II) in mouse spermatozoa. Spermatozoa from the testes exhibited bright fluorescence over the entire head; those from the caput epididymides generally fluoresced more brightly in the post-acrosomal region; and spermatozoa from the caudae epididymides fluoresced less brightly, with foci of fluorescence over the sperm head which were lost after extraction with Triton X-100 and hence appeared to be membrane-associated. Treatment of cauda sperm with sodium dodecyl sulfate resulted in a bright uniform Zinquin fluorescence in the heads, similar to that observed in caput sperm, indicating that the two types of sperm have similar amounts of head Zn(II) but that the availability of Zn(II) for binding Zinquin is different. By contrast, the intensity of tail fluorescence was similar in spermatozoa from different regions of the male reproductive tract and was largely unaffected by Triton X-100 extraction, consistent with an intracellular location. Similar differences were observed between caput sperm and cauda sperm in the rat. It is concluded that visualization and measurement of free or loosely-bound Zn(II) in subcellular compartments of spermatozoa should facilitate investigation of the role of this metal in the development and function of spermatozoa and abnormalities that might accompany infertility and Zn(II) deficiency.

Effect of cooling and cryopreservation on sperm motility and morphology of several species of marsupial

The effects of long-term cooling and freezing on sperm motility are described for six marsupial species: the fat-tailed dunnart, koala, brushtail possum, long-footed potoroo, northern brown bandicoot and ring-tailed possum. The effects of up to eight days of cooling at 4 degrees C on the motility of dunnart spermatozoa and the effect of cryopreservation on spermatozoa of the other species were determined. The cryoprotectant used was a Tris-citrate-fructose-egg yolk-glycerol diluent. The percentage and rating of sperm motility, and sperm structure, as determined by light microscopy, were investigated. Sperm motility in the fat-tailed dunnart was retained for up to six days when cooled to 4 degrees C, suggesting that sperm from this species have some degree of tolerance to cold shock. After this time, however, the percentage of motile spermatozoa and their motility rating declined. In all species except the fat-tailed dunnart, reinitiation of motility following cryopreservation occurred across a range of glycerol concentrations (4-17%). Cryoprotectant containing 6% and/or 8% glycerol resulted in little change of motility rating or of the percentage of live sperm after thawing, although there was some decline in the percentage of motile sperm. The unusual structural and motility characteristics of dunnart spermatozoa may account for the lack of success of sperm cryopreservation in this species.

Variation in sperm morphology in the Australian rodent genus, Pseudomys (Muridae)

SummarySperm morphology among the various members of the genus Pseudomys showed great interspecific variation. In P. apodemoides, P. desertor, P. fumeus, P. gracilicaudatus, P. hermannsburgensis, P. higginsi and P. nanus the sperm head was bilaterally flattened and possessed three hooks, the length of which varied somewhat between species. The top hook contained nuclear, subacrosomal and acrosomal material, whereas the two lower ventral hooks displayed largely only an extension of the subacrosomal material. Four other species of Pseudomys had no ventral hooks in their sperm head. In P. novaehollandiae there was a single long top hook, whereas in P. pilligaensis it was almost nonexistent. Neither the sperm head of P. delicatulus, nor that of P. shortridgei possessed any hooks; the former became narrower towards the tip, whereas the latter was spatulate in shape. The acrosome covered about two thirds of the nucleus and varied considerably in shape between species. Insertion of the connecting piece of the tail occurred on the lower ventral side of the sperm head in all species with a long top hook but was nearly basal in P. delicatulus and midbasal in P. shortridgei. The ultrastructure of mid and principal pieces was similar among species, but the number of gyres of mitochondria ranged from about 64 to 100.

15 – Reproduction, Mating Strategies and Sperm Competition in Marsupials and Monotremes

The chapter presents both phylogenetic and nonphylogenetic comparative analysis of how testes mass, sperm numbers, and sperm length scale with body mass in Marsupials and Monotremes. In eutherian mammals, a negative correlation between body mass and sperm size, as indicated by the length of the sperm tail, occurs. When data for individual species are examined, it became clear that for species within the same genus and often family, the data shows similar trends suggesting a phylogenetic influence, and that, for some groups there is a marked deviation from the average body mass. However, data on social organization and information on breeding systems, copulatory behavior and paternity guards are relatively sparse. But, in some cases, the occurrence of a multimale breeding system is evident, with the potential for intermale sperm competition. These species tend to have large relative testis size and high sperm numbers, thus the presence or absence of intermale sperm competition is likely to be one of the factors responsible for the variation in relative mass of the testis in marsupial species. Sperm competition is one of the driving forces that influence testis size and sperm numbers in this group of animals.

Evolution of the Spermatozoon in Australasian Rodents

The head of the spermatozoon in eutherian mammals contains a nucleus, acrosomal cap and cytoskeleton. It is generally spatulate, paddle-shaped or pear-shaped, but in most murid rodents it is hook-shaped with the anterior region of the nucleus surrounded by an elaborate acrosome and an extension of the subacrosomal cytoskeleton as a perforatorium. This type of spermatozoon is present in Australasian Rattus, together with several other New Guinean genera. However, in most Australasian hydromyine rodents a far greater complexity of structural organisation of the sperm head has evolved in which two further elaborate processes extend from its upper concave surface. These processes contain a huge extension of the cytoskeleton within which filamentous actin is present. By contrast, the form of the sperm head in a few species of Pseudomys, Notomys and Solomys is highly divergent and is either truncated, spatulate or pear- shaped. The evolutionary trends of change in sperm head shape are discussed and it is suggested that the falciform sperm head with the two extra processes in most of the hydromyine rodents is one of the most morphologically complex sperm head types to have evolved in eutherian mammals; it contains a far more extensive development of the cytoskeleton than that of any other mammalian spermatozoon.

Protein Composition of the Ventral Processes on the Sperm Head of Australian Hydromyine Rodents

Abstract The sperm head of the plains rat, an Australian hydromyine rodent, is highly complex in structure and contains, in addition to an apical hook, two large ventral processes (VPs) that extend from its upper concave surface and that are largely composed of a huge extension of the sperm head cytoskeleton surrounded by postacrosomal dense lamina. In this study we have attempted to determine their protein composition. For this, the VPs were isolated, the proteins within them separated by SDS-PAGE, and the resultant polypeptide bands Western blotted and probed with antibodies against laboratory rat perforatorial and bull perinuclear theca sperm proteins. Antibodies were also used to determine the perforatorial and perinuclear theca proteins by immunogold labeling of transmission electron microscopic sections. The results indicate that the material within the VPs is largely composed of perforatorial cross-reacting proteins together with F-actin with the dominant protein being PERF 15. The perinuclear theca proteins are, by contrast, restricted to a narrow region adjacent to the acrosomal and nuclear membranes. In conclusion, this study has shown that the VPs of the spermatozoa of Australian rodents are perforatorial-like appendages that contain similar proteins to the perforatorium of the apical hook together with F-actin; their functional significance remains unknown.