Density‐dependent patterns of multivariate selection on sperm motility and morphology in a broadcast spawning mussel

Sperm cells exhibit extraordinary phenotypic variation, both among taxa and within individual species, yet our understanding of the adaptive value of sperm trait variation across multiple contexts is incomplete. For species without the opportunity to choose mating partners, such as sessile broadcast spawning invertebrates, fertilization depends on gamete interactions, which in turn can be strongly influenced by local environmental conditions that alter the concentration of sperm and eggs. However, the way in which such environmental factors impact phenotypic selection on functional gamete traits remains unclear in most systems. Here, we analyze patterns of linear and nonlinear multivariate selection under experimentally altered local sperm densities (densities within the capture zone of eggs) on a range of functionally important sperm traits in the broadcast spawning marine mussel, Mytilus galloprovincialis. Specifically, we assay components of sperm motility and morphology across two fertilization environments that simulate either sperm limitation (when there are too few sperm to fertilize all available eggs), or sperm saturation (when there are many more sperm than required for fertilization, and the risk of polyspermy and embryonic failure is heightened). Our findings reveal that the strength, form, and targets of selection on sperm depend on the prevailing fertilization environment. In particular, our analyses revealed multiple significant axes of nonlinear selection on sperm motility traits under sperm limitation, but only significant negative directional selection on flagellum length under sperm saturation. These findings highlight the importance of local sperm densities in driving the adaptation of sperm phenotypes, particularly those related to sperm motility, in broadcast spawning invertebrates.     

Mouse and Fly Sperm Motility Changes Differently under Modelling Microgravity

Sperm motility is essential for the natural fertilization process in most animal species. Despite the fact that evolution took place under conditions of constant gravity, the motility of spermatozoa of insects and mammals under microgravity conditions changes in different ways. In this work, an attempt was made to explain this effect. The sperm motility of the fruit fly Drosophila melanogaster and the mouse was evaluated after exposure to a random positioning machine for 6 h. Sodium fluoride was used to inhibit serine/threonine phosphatases, sodium orthovanadate was used to inhibit tyrosine phosphatases, and 6-(dimethylamino)purine was used to inhibit protein kinases. The results obtained indicate that simulated microgravity leads to an increase in the speed of movement of fly spermatozoa by 30% (p < 0.05), and this effect is blocked by sodium fluoride. In contrast, a 29% (p < 0.05) decrease in the speed of movement of mouse spermatozoa under simulated microgravity is prevented by 6-(dimethylamino)purine. Moreover, after 6 h of exposure, the content of tubulin cytoskeleton and actin proteins remains at the control level in the spermatozoa of flies and mice. However, the content of the actin-binding protein alpha-actinin in fly sperm decreases by 29% (p < 0.05), while in mouse sperm, the relative content of alpha-actinin1 increases by 94% (p < 0.05) and alpha-actinin4 by 121% (p < 0.05) relative to the control, as determined by 6 simulated microgravity tests. It can be assumed that the effect of simulated microgravity on the motility of mammalian spermatozoa is mediated through the regulation of phosphorylation and that of insects through the regulation of dephosphorylation of motor proteins; moreover, the development of a response to changes in external mechanical conditions has a different time scale.     

Can relaxed time constraints on sperm production eliminate protandry in an ectotherm?

In most temperate-zone reptiles, as in many other ectothermic taxa, males emerge from periods of inactivity (e.g. hibernation) before females, a pattern referred to as protandry. In the large body of theory depicting its evolution it is assumed that as selection pressures on females moderate female emergence time, net sexual selection on males shifts male emergence time accordingly. This is because early-emerging males might (i) benefit from advantages in behavioural interactions (e.g. obtain more matings, better territories, or a higher social status), or (ii) produce and mature more spermatozoa before the mating season. These putative advantages of early emergence occur simultaneously in most temperate-zone reptile species, because sperm production and copulations occur soon after emergence from winter inactivity. Thus, it is difficult to distinguish between the two hypotheses. To do so would require a system in which behavioural interactions occur at one time of year and sperm production at another. The southern snow skink ( Niveoscincus microlepidotum ) provides such an example; adult males fight with each other immediately after emerging from hibernation, but examination of the testes and epididymes of males throughout the year show that males do not produce spermatozoa until around 3 months later. Thus, the 'behavioural interactions' hypothesis predicts protandry in snow skinks, whereas the 'sperm production' hypothesis predicts synchronous spring emergence in males and females. Our field data, collected over 5 years, show that emergence dates in spring are the same for male and female skinks. Hence, when the selection pressure for rapid sperm production is relaxed, we no longer see the protandric emergence pattern characteristic of most temperate-zone reptiles.     

Proteins with tandemly arranged repeats of a highly charged 16-amino-acid motif encoded by the Dhmst101 gene family are structural components of the outer sheath of the extremely elongated sperm tails of Drosophila hydei

Fruit fly species of the genus Drosophila show a remarkable variation in sperm length. Some of them produce gigantic sperm several times the total male body length. Sperm of Drosophila hydei, for example, are more than 20 mm long. Little is known about the advantage of such elongated sperm or about the proteins that stabilize their thin flagellar tails. Recently, two members of a novel gene family Dhmst101(1) and Dhmst101(2), whose gene products are associated with the sperm tail, were isolated and characterized. Here a third member of this gene family, Dhmst101(3), is described. It was previously demonstrated that all three genes are located in a single small cluster on chromosome 5 of D. hydei. They are located within 15 kb of genomic DNA, oriented in the same direction and transcribed testis-specifically. The encoded sperm tail-specific proteins are mainly composed of tandemly arranged repeats of a highly charged, cysteine-containing motif of 16 amino acids with the consensus sequence KKKCA/EEAAKKEKEAAE. Experiments with synthetic repeat monomers and dimers have demonstrated a tendency for alpha-helical rod formation, which increased strongly with an increase in repeat number. Therefore, Dhmst101 proteins with 7-60 repeats with regularly spaced cystein-residues are thus expected to form long alpha-helical rods cross-linked by numerous Cys-Cys bridges. Here we apply immunoelectron microscopy and monospecific antibodies, alpha-mst101, raised against the KKKCAEAAKKEKEAAE-motif to investigate the distribution of Dhmst101 proteins within the sperm tail of D. hydei. We show that Dhmst101 proteins are part of the outer sheath of the sperm tail where they presumably help to provide a tight but elastic envelope for the extremely extended spermatozoa of D. hydei.     

AN ACOUSTIC LINK BETWEEN BLUE WHALES IN THE EASTERN TROPICAL PACIFIC AND THE NORTHEAST PACIFIC1

Blue whale calls in the eastern North Pacific Ocean consist of a two-part call often termed the A-B call. This call has been described for regions offshore of Oregon, Washington, and California, USA and the Sea of Cortez, Mexico (reviewed in Rivers 1997). Data collected from moored hydrophones in the eastern tropical Pacific (ETP) indicate that the A-B pattern is common in this region as well. There is consistency in this call type throughout the eastern North Pacific and throughout the year. This acoustic evidence indicates continuity between blue whales in the ETP and those found west of North America. The acoustic data suggest that the population of blue whales generally referred to as the “Californi/Mexico” stock might better be termed the “northeast Pacific” stock of blue whales.     

DIEL VARIATION IN MIGRATION RATES OF EASTERN PACIFIC GRAY WHALES MEASURED WITH THERMAL IMAGING SENSORS

We recorded the blows of gray whales during their southbound migration past central California in January 1994, 1995, and 1996, using thermal imaging sensors. For our sampling purposes, we defined day (0730–1630) and night (1630–0730) to coincide with the on/off effort periods of the visual counts being conducted concurrently. We pooled data across the three years of sampling and tested for diel variation in surfacing interval, pod size, offshore distance, migration rate, and swimming speed by comparing paired day/night means for samples collected within the respective 24-h period. We performed these tests using data from the entire migration period and then repeated the tests for samples collected prior to and after the approximate median migration date (15 January). Over the entire migration period we observed larger diurnal pod sizes (x?_day= 1.75 ± 0.280, x?_night= 1.63 ± 0.232) and greater diurnal offshore distances (x?_day= 2.30 ± 0.328 km, x?_night= 2.03 ± 0.356 km) but found no diel variation in surfacing interval. For the entire migration period, the nocturnal migration rate (average number of whales passing per hour) was higher than the diurnal rate. During the first half of the migration we detected no diel variation in pod size or surfacing interval, but diurnal offshore distances were larger than at night (x?_day= 2.28 ± 0.273 km, x?_night= 1.96 ± 0.318 km). Diurnal and nocturnal migration rates prior to 15 January were not different. During the second half of the migration, there was no diel variation in surfacing interval, pod size, or distance offshore, but the nocturnal migration rate was higher (28%, SE = 11.6%) than the diurnal rate. We found no diel variation in swimming speed in any comparison. We propose that later migrants socialize more during the day, which effectively slows their diurnal rate of migration relative to nocturnal rates.     

Conformational properties of native sperm whale apomyoglobin in solution.

Apomyoglobin from sperm whale is often used for studies of ligand binding, protein folding, and protein stability. In an effort to describe its conformational properties in solution, homonuclear and heteronuclear (13C and 15N) NMR methods were applied to the protein in its native state. Assignments were confirmed for nuclear Overhauser effects (NOEs) involving side chain and backbone protons in the folded regions of the structure. These NOEs were used to derive distance restraints. The shifts induced by the hydrophobic dye 8-anilino-1-naphthalenesulfonic acid (ANS) were inspected in the regions remote from its binding site and served as an indicator of conformational flexibility. 3JalphaH-NH values were obtained to assess dihedral angle averaging and to provide additional restraints. A family of structures was calculated with X-PLOR and an ab initio simulated annealing protocol using holomyoglobin as a template. Where the structure appeared well defined by chemical shift, line width, ANS perturbation, and density of NOEs, the low resolution model of apomyoglobin provides a valid approximation for the structure. The new model offers an improved representation of the folded regions of the protein, which encompass the A, B, E, helices as well as parts of the G and H helices. Regions that are less well defined at this stage of calculations include the CD corner and the end of the H-helix. The EF-F-FG segment remains uncharacterized.     

The reactivation of demembranated human spermatozoa lacking outer dynein arms is independent of pH

The effect of pH, Mg-ATP, and free calcium on activity of the inner dynein arm was investigated using demembranated human spermatozoa lacking the outer dynein arms (LODA). The results were compared with those obtained for demembranated-reactivated normal spermatozoa to evaluate the functional properties of the inner and outer dynein arms in axonemal motility. The reactivation of Triton X-100–demembranated LODA spermatozoa was analysed at various pHs and concentrations of Mg-ATP and calcium using video recordings. The percentage of reactivated LODA spermatozoa as a function of Mg-ATP concentration was not dependent on pH, whereas reactivation of normal human spermatozoa is pH dependent. This suggests that there may be a pH-dependent regulatory mechanism associated with the outer dynein arms. A delay in the principal bend propagation of normal and LODA reactivated cells was found at pH 7.1. This disappeared at pH 7.8 in normal but not in LODA populations. This suggests a role for outer dynein arms in the initiation of the propagation of flagellar bends at alkaline pH. The level of LODA and normal sperm reactivation both depended on the calcium concentration in the medium. At lower free calcium concentrations, the reactivation level and beat frequency of reactivated cells were higher. Our results suggest a functional difference between outer and inner dynein arms of human spermatozoa based on a differential pH sensitivity. Moreover, calcium seems to exert its regulatory action elsewhere than on the outer dynein arms. Mol. Reprod. Dev. 49:416–425, 1998. © 1998 Wiley-Liss, Inc.