Photographs of Fertilization in the Smaller Species of Trichonympha

SYNOPSIS. The photographs illustrate male and female gametes before fertilization, several progressive stages in the entrance of the male gamete into the cytoplasm of the female, cytoplasmic fusion of gametes, loss of extranuclear organelles of male gamete, retention of extranuclear organelles of female gamete, movement of pronucleus of male gamete to that of female, progressive stages in fusion of pronuclei, and the formation of the zygote which possesses the extranuclear organelles of the female gamete. Some abortive attempts at fertilization, resulting from failure of gametes to differentiate, are shown.     

Gamete membrane dynamics during double fertilization in Arabidopsis

In the double fertilization of angiosperms, one sperm cell fertilizes an egg cell to produce a zygote, whereas the other sperm cell fertilizes a central cell to give rise to an endosperm. There is little information on gamete membrane dynamics during double fertilization even though the cell surface structure is critical for male and female gamete interactions. In a recent study, we analyzed gamete membrane behavior during double fertilization by live-cell imaging with Arabidopsis gamete membrane marker lines. We observed that the sperm membrane signals occasionally remained at the boundary of the female gametes after gamete fusion. In addition, sperm membrane signals entering the fertilized female gametes were detected. These findings suggested that plasma membrane fusion between male and female gametes occurred with the sperm internal membrane components entering the female gametes, and this was followed by plasmogamy.     

Mechanism of male gamete motility in araphid pennate diatoms from the genus Tabularia (Bacillariophyta)

During sexual reproduction, araphid pennate diatoms of the genus Tabularia (Kützing) D. M. Williams and Round released male gametes directly into the medium, sometimes at a considerable distance from the female gametes. This raised the question of how male gametes, suspended in water, manage to reach female ones, given that no locomotive organelles have been described in gametes of pennate diatoms. Optical microscopic investigation revealed cytoplasmic projections produced by male gametes of Tabularia tabulata (C. A. Agardh) Snoeijs and T. fasciculata (C. A. Agardh) D. M. Williams and Round. Morphology and behavior of these projections is consistent with pseudopodia, however, which specific type of pseudopodia they may be, remains inconclusive. The growth and retraction of the pseudopodia coincided with gamete motility and so we postulate that it explains the otherwise apparent random movement of male gametes. Spinning, shuffling and chaotic patterns of motility were documented. In theory, gamete mobility increases the probability of gamete encounter thus enhancing the probability of syngamy. This is the first known case where cytoplasmic projections have been described in diatom gametes, and possibly in mature gametes in general.     

Adaptive Lévy Walks in Foraging Fallow Deer

Background

Lévy flights are random walks, the step lengths of which come from probability distributions with heavy power-law tails, such that clusters of short steps are connected by rare long steps. Lévy walks maximise search efficiency of mobile foragers. Recently, several studies raised some concerns about the reliability of the statistical analysis used in previous analyses. Further, it is unclear whether Lévy walks represent adaptive strategies or emergent properties determined by the interaction between foragers and resource distribution. Thus two fundamental questions still need to be addressed: the presence of Lévy walks in the wild and whether or not they represent a form of adaptive behaviour.

Methodology/Principal Findings

We studied 235 paths of solitary and clustered (i.e. foraging in group) fallow deer (Dama dama), exploiting the same pasture. We used maximum likelihood estimation for discriminating between a power-tailed distribution and the exponential alternative and rank/frequency plots to discriminate between Lévy walks and composite Brownian walks. We showed that solitary deer perform Lévy searches, while clustered animals did not adopt that strategy.

Conclusion/Significance

Our demonstration of the presence of Lévy walks is, at our knowledge, the first available which adopts up-to-date statistical methodologies in a terrestrial mammal. Comparing solitary and clustered deer, we concluded that the Lévy walks of solitary deer represent an adaptation maximising encounter rates with forage resources and not an epiphenomenon induced by a peculiar food distribution.     

A Non-Lévy Random Walk in Chacma Baboons: What Does It Mean?

The Lévy walk is found from amoebas to humans and has been described as the optimal strategy for food research. Recent results, however, have generated controversy about this conclusion since animals also display alternatives to the Lévy walk such as the Brownian walk or mental maps and because movement patterns found in some species only seem to depend on food patches distribution. Here I show that movement patterns of chacma baboons do not follow a Lévy walk but a Brownian process. Moreover this Brownian walk is not the main process responsible for movement patterns of baboons. Findings about their speed and trajectories show that baboons use metal maps and memory to find resources. Thus the Brownian process found in this species appears to be more dependent on the environment or might be an alternative when known food patches are depleted and when animals have to find new resources.     

远缘杂交中不育基因的位置和效应的最大似然估计

提出了一种统计方法,利用标记位点的异常分离,来估计远缘杂交中不育基因位点的位置和效应,在回交群体中,用最大似然法对不育基因与标记位点之间的生组值和配子存活率进行估计。将表现连续分布的育性指标转化为百连续变异的遗传标记的分离,可以避免对育性直接观测所带来的重组值估计结果的不稳定,还可以同时估计雌雄配子的存活率。     

Turn designation, sampling rate and the misidentification of power laws in movement path data using maximum likelihood estimates

Many authors have claimed to observe animal movement paths that appear to be Lévy walks, i.e. a random walk where the distribution of move lengths follows an inverse power law. A Lévy walk is known to be the optimal search strategy of a particular class of random walks in certain environments; hence, it is important to distinguish correctly between Lévy walks and other types of random walks in observed animal movement paths. Evidence of a power law distribution in the step length distribution of observed animal movement paths is often used to classify a particular movement path as a Lévy walk. However, there is some doubt about the accuracy of early studies that apparently found Lévy walk behaviour. A recently accepted method to determine whether a movement path truly exhibits Lévy walk behaviour is based on an analysis of move lengths with a maximum likelihood estimate using Akaike weights. Here, we show that simulated (non-Lévy) random walks representing different types of animal movement behaviour (a composite correlated random walk; pooled data from a set of random walks with different levels of correlation and three-dimensional correlated random walks projected into one dimension) can all show apparent power law behaviour typical of Lévy walks when using the maximum likelihood estimation method. The probability of the movement path being identified as having a power law step distribution is related to both the sampling rate used by the observer and the way that ‘turns’ or ‘reorientations’ in the movement path are designated. However, identification is also dependent on the nature and properties of the simulated path, and there is currently no standard method of observation and analysis that is robust for all cases. Our results indicate that even apparently robust maximum likelihood methods can lead to a mismatch between pattern and process, as paths arising from non-Lévy walks exhibit Lévy-like patterns.     

Analysis of gamete and zygote motility in Allomyces

To study the mechanisms of chemotaxis in eukaryotes, the motility patterns of the gametes and zygotes and the chemotactic responses of the male gametes of the lower eukaryote Allomyces macrogynus were examined. Dark-field microscopy of the male gametes showed a smooth swimming pattern interrupted by very brief ‘jerks’ of the cell body that caused a change in swimming direction. Female gametes had a slower swimming velocity than the males and underwent more jerks or turns which accounted for their sluggish motility. The zygotes swam with the fastest velocity and were observed to have a helical swimming pattern involving a continuous turning of the cell body, a behavior absent from the gametes. Introduction of female gametes that produce the chemoattractant sirenin brought about an immediate change in the behavior of the male gametes. They moved in spirals (or helices) towards the source of the chemoattractant (the female gametes), undergoing only a few jerks to reorient the male cells. When very near the female cells, or in high concentrations of added sirenin, many very short motility tracks were observed that finally resulted in contact between the two gamete types. The results indicate that the poor swimming ability of the female gametes facilitates gamete contact, resulting in as many as 30–40 male gametes clustered on a single female cell. Further, male gamete orientation to the sirenin gradient is caused by the chemoattractant suppressing the jerk motion.     

Gametic behavior in a marine green alga, Monostroma angicava: an effect of phototaxis on mating efficiency

The role of phototactic behavior of gametes was tested experimentally in the slightly anisogamous marine green alga Monostroma angicava Kjellman, and the effect of phototaxis on mating efficiency was discovered. Both male and female gametes showed positive phototaxis in response to a white light source. In contrast, they did not respond to a red light source. Their swimming velocity did not differ between these two illuminating light sources. It was, therefore, suggested that the search ability of the gamete itself might not vary between phototactic and non-phototactic conditions. The number of zygotes formed during the mating process may be expressed as the product of the number of encounters between male and female gametes and the fraction of encounters that result in sexual fusion. In this study, with high densities of male and female gametes mixed in test tubes, almost all minor (fewer in number) gametes fused sexually within 10 min. After dilution of the gamete suspensions by half, mating efficiency in test tubes illuminated by white light from above was higher than that in dark controls. This suggests that male and female gametes gathered at the water surface through their positive phototaxis, thus increasing the rate of encounters. Mating efficiency also decreased if the test tubes were illuminated from above by white light and also shaken. Since negative phototaxis is clearly shown in planozygotes, we suggest that positive phototaxis of male and female gametes in M. angicava is an adaptive trait for increasing the rate of gametic encounters rather than for the dispersal of zygotes as previously reported for zoospores of some marine algae. Received: 12 February 1999 / Revision accepted: 24 May 1999     

Intraspecific variation in movement patterns: modeling individual behaviour in a large marine predator

In large marine predators, foraging entails movement. Quantitative models reveal how behaviours can mediate individual movement, such that deviations from a random pattern may reveal specific search tactics or behaviour. Using locations for 52 grey seals fitted with satellite-linked recorders on Sable Island; we modeled movement as a correlated random walk (CRW) for individual animals, at two temporal scales. Mean move length, turning angle, and net squared displacement (R²_n: the rate of change in area over time) at successive moves over 3 to 10 months were calculated. The distribution of move lengths of individual animals was compared to a Lévy distribution to determine if grey seals use a Lévy flight search tactic. Grey seals exhibited three types of movement as determined by CRW model fit: directed movers – animals displaying directed long distance travel that were significantly underpredicted by the CRW (23% of animals); residents – animals remaining in the area surrounding Sable Island that were overpredicted by the model (29% of animals); and correlated random walkers – those (48% of animals) in which movement was predicted by the CRW model. Kernel home range size differed significantly among all three movement types, as did travel speed, mean move length, mean R²_n and total distance traveled. Sex and season of deployment were significant predictors of movement type, with directed movers more likely to be male and residents more likely to be female. Only 30% of grey seals fit a Lévy distribution, which suggests that food patches used by the majority of seals are not randomly distributed. Intraspecific variation in movement behaviour is an important characteristic in grey seal foraging ecology, underscoring the need to account for such variability in developing models of habitat use and predation.     

Auxosporulation of Licmophora communis (Bacillariophyta) and a review of mating systems and sexual reproduction in araphid pennate diatoms

The auxosporulation of Licmophora communis is allogamous and dioecious. Pairing between sessile, shortstalked cells of compatible clones is followed by meiosis and gametogenesis, to form two gametes in each gametangium. The behavior of the gametes differs between the gametangia. In the male gametangium, the gametes detach from the frustule, round up, and migrate out of the gametangium after its dehiscence at the broader, unattached pole. In the female gametangium, both gametes remain attached to the adjacent theca over almost their whole length and do not move. Plasmogamy therefore occurs within the female gametangium and this is where the zygotes are formed and remain. After fertilization, the zygotes detach from the thecae of the female gametangia, contract, and become ellipsoidal, before expanding parallel to the apical axis of the gametangium. We review the types of auxosporulation in other pennate diatoms and the systems used for classifying these. Dioecy and cis‐type anisogamy (in which one gametangium produces active gametes and the other produces passive gametes), as in L. communis, are probably primitive within the pennate group (although there is no information on the AsterionellopsisRhaphoneis clade). However, size can also be restored in various araphid pennates by allogamous sexual reproduction involving the formation of only one gamete per gametangium, or in rare cases by automixis or (apparently) vegetative enlargement.     

Performance of non-motile male gametes in the sea: analysis of paternity and fertilization success in a natural population of a red seaweed,Gracilaria gracilis

In haploid–diploid red seaweeds, the dispersal of male gametes is presumed limited due to their lack of flagella. It has been suggested that this group suffers from sperm limitation and, consequently, that fertilization is relatively inefficient. Fertilization in most floridean rhodophytes results in the formation a cystocarp, a swelling on the haploid female thallus housing the diploid zygote and its thousands of diploid daughter spores. To study the performance of non-motile male gametes in the sea, we evaluated both female and male fertilization success in a natural population of the red marine alga Gracilaria gracilis. Female fertilization success, estimated by cystocarp yield per unit female thallus, was evaluated with respect to the availability of male gametes. Male fertilization success, estimated by the individual contribution of different males to zygotes, was assessed by paternity analyses on 350 cystocarps produced in one reproductive season using two microsatellite loci. The results show that cystocarp yield is not sperm limited and that the large variation in male fertilization success cannot be solely explained by the distance travelled by the male gamete to find a mate. Taken together, the results suggest that, not only is fertilization efficient, but that male–male competition and/or female choice may play a role in shaping population mating patterns.     

Female control of male gamete delivery during fertilization in Arabidopsis thaliana

Fertilization in both animals and plants relies on the correct targeting of the male gametes to the female gametes. In flowering plants, the pollen tube carries two male gametes through the maternal reproductive tissues to the embryo sac, which contains two female gametes. The pollen tube then releases its two male gametes into a specialized receptor cell of the embryo sac, the synergid cell. The mechanisms controlling this critical step of gamete delivery are unknown. Here, data based on the new sirène (srn) mutant of Arabidopsis thaliana provide the first evidence for female control over male gamete delivery. Live imaging of fertilization shows that wild-type pollen tubes do not stop their growth and do not deliver their contents in srn embryo sacs.     

Optimising the success of random destructive searches: Lévy walks can outperform ballistic motions

We re-evaluate the long standing and widely held belief that ballistic movements (i.e. straight-lines movements) outperform Lévy walks when searching for targets that once located are not revisited. The belief stems from the results of analyses of one-dimensional searches, analyses which have not accounted for the fact that target numbers can be continually depleted during the search process. This is a crucial oversight because continual depletion promotes the searching efficiencies of some Lévy walks above that of ballistic motion. The continual depletion effect is not so important for two- and three-dimensional searches. Nevertheless, we show that Lévy walks and ballistic movements can be equally or almost equally effective when searching within two- and three-dimensional environments for randomly and sparsely distributed targets or when searching for targets that are occasionally concealed. We also show that Lévy walks are advantageous when searching for targets that can occasionally evade capture. These situations represent common predator–prey interactions in which predators are involved in ‘imperfect destructive’ searches. Our model suggests that accounting for coevolutionary arms races at the predator–prey detection/reaction scales can explain to some extent Lévy walk searching patterns of predators at larger scales. This result provides new insights into the Lévy walk movement patterns of some destructive foragers.     

Production of anisogametes and gamete motility dimorphism in Monostroma angicava

 The reproductive strategy of a marine alga with a heteromorphic biphasic life cycle was studied by analyzing various sexual reproductive characters in light of the evolution of anisogamy. Gametophytes of Monostroma angicava were dioecious and their gametes were slightly anisogamous. Volume of gametangium, density of gametangia and area of mature gametangial parts on each gametophyte did not differ from male to female. Therefore, the reproductive biomass investment for gamete production was considered to be the same for each sex. Anisogamy in this alga appeared to be derived from the difference in the number of cell divisions during gametogenesis, because the majority of male gametangia each produced 64 (2⁶) gametes and the female produced 32 (2⁵) gametes. This corresponded with measurements of cell size in male and female gametes. Further, the sex ratio was 1:1 for sexually mature plants sampled at Charatsunai. Therefore, it was suggested that in the field twice as many male gametes are released as female gametes. Liberated gametes of both sexes showed positive phototaxis. The swimming velocity of freshly liberated male gametes was a little higher than that of female gametes. Male gametes had the potential to swim for ca. 72 h and female gametes for ca. 84 h. The difference in gamete motility between the two sexes seemed to be related to cell size. Planozygotes were negatively phototactic and swam more rapidly than gametes of either sex. Received: 5 March 1997 / Revision accepted: 18 July 1997     

Scavengers on the Move: Behavioural Changes in Foraging Search Patterns during the Annual Cycle

Background

Optimal foraging theory predicts that animals will tend to maximize foraging success by optimizing search strategies. However, how organisms detect sparsely distributed food resources remains an open question. When targets are sparse and unpredictably distributed, a Lévy strategy should maximize foraging success. By contrast, when resources are abundant and regularly distributed, simple Brownian random movement should be sufficient. Although very different groups of organisms exhibit Lévy motion, the shift from a Lévy to a Brownian search strategy has been suggested to depend on internal and external factors such as sex, prey density, or environmental context. However, animal response at the individual level has received little attention.

Methodology/Principal Findings

We used GPS satellite-telemetry data of Egyptian vultures Neophron percnopterus to examine movement patterns at the individual level during consecutive years, with particular interest in the variations in foraging search patterns during the different periods of the annual cycle (i.e. breeding vs. non-breeding). Our results show that vultures followed a Brownian search strategy in their wintering sojourn in Africa, whereas they exhibited a more complex foraging search pattern at breeding grounds in Europe, including Lévy motion. Interestingly, our results showed that individuals shifted between search strategies within the same period of the annual cycle in successive years.

Conclusions/Significance

Results could be primarily explained by the different environmental conditions in which foraging activities occur. However, the high degree of behavioural flexibility exhibited during the breeding period in contrast to the non-breeding period is challenging, suggesting that not only environmental conditions explain individuals'' behaviour but also individuals'' cognitive abilities (e.g., memory effects) could play an important role. Our results support the growing awareness about the role of behavioural flexibility at the individual level, adding new empirical evidence about how animals in general, and particularly scavengers, solve the problem of efficiently finding food resources.     

Eyespot behavior during the fertilization of gametes in Ulva arasakii Chihara (Ulvophyceae, Chlorophyta)

Behavior of the eyespots during the fertilization of Ulva arasakii Chihara was studied using field emission scanning electron microscopy (FE‐SEM). FE‐SEM enabled the visualization of the eyespot of biflagellate male and female gametes. The smaller male gamete has one protruded smaller (1.3 ± 0.15 μm× 1.0 ± 0.29 μm) eyespot and the larger female gamete has a larger (1.6 ± 0.2 μm× 1.1 ± 0.13 μm) one on a posterior position of the cell. The cell membrane over the eyespot region is relatively smooth compared to other parts of the cell body and exhibits hexagonal arranged lipid globules. Because the size of the cell and the morphology of the eyespot are different between male and female gametes, we could follow the fate of the eyespots during the fertilization. The initial cytoplasmic contact and fusion of the gametes takes place at their anterior end, slightly posterior to the flagellar base. The morphology of the fusing gametes followed two clearly distinguishable patterns. About half the gamete pairs lie side‐by‐side with their longitudinal axes nearly parallel, while the rest are oriented anti‐parallel to each other. In all cases, the larger female gamete fused along the same side as the eyespot, while the smaller male gamete fused along the side away from its eyespot. As fusion proceeds, the gamete pair is transformed into the quadriflagellate planozygote, in which the eyespots are positioned side‐by‐side on the region of cell fusion. These observations indicated that the opposite positioning of the eyespot relative to the cell fusion site in male and female gametes is important for the proper arrangement of the eyespots in the planozygote. The significance of this feature in advanced green algae is briefly discussed.     

Female‐induced remote regulation of sperm physiology may provide opportunities for gamete‐level mate choice

In sedentary externally fertilizing species, direct interactions between mating partners are limited and prefertilization communication between sexes occurs largely at the gamete level. Certain combinations of eggs and sperm often have higher fertilization success than others, which may be contingent on egg‐derived chemical factors that preferentially attract sperm from compatible males. Here, we examine the mechanisms underlying such effects in the marine mussel Mytilus galloprovincialis, where differential sperm attraction has recently been shown to be associated with variation in offspring viability. Specifically, we focus on the sperm surface glycans, an individually unique layer of carbohydrates that moderate self‐recognition and other cellular‐level interactions. In many species egg‐derived factors trigger remarkable changes in the sperm's glycan layer, physiology, and swimming behavior, and thus potentially moderate mate choice at the gamete level. Here, we show that sperm glycan modifications and the strength of acrosome reaction are both dependent on specific male–female interactions (male–female combination). We also find associations between female‐induced sperm glycan changes and the Ca²⁺ influx into sperm–‐a key regulator of fertilization processes from sperm capacitation to gamete fusion. Together, our results suggest that female‐induced remote regulation of sperm physiology may constitute a novel mechanism of gamete‐level mate choice.     

Sexual reproduction inEudorina elegans (chlorophyta,volvocales)

Sexual reproduction inEudorina elegans Ehr. was studied in detail in laboratory cultures, with particular regard to conjugation between gametes and gone colony formation. Male and female gametes fused after being induced by changing the medium. The anterior end, including the flagellar base, of the male gamete entered the anterior region of the female gamete. Fusion of the two protoplasts proceeded laterally and posteriorly. The male gamete bore a slender cytoplasmic protrusion at the base of the flagella. This structure has not been previously described in the male gamete ofEudorina, and may participate in plasmogamy. A biflagellate gone cell swam from the germinating zygote and secreted a gelatinous envelope. It then divided to form a gone colony within the gelatious envelope, which moved during colony formation by means of the two flagella which were retained intact from the original gone cell.