Distinct regulation of two flagella by calcium during chemotaxis of male gametes in the brown alga Mutimo cylindricus (Cutleriaceae, Tilopteridales).

Brown algal male gametes show chemotaxis to the sex pheromone that is released from female gametes. The chemotactic behavior of the male gametes is controlled by the changes in the beating of two flagella known as the anterior and posterior flagellum. Our previous study using Mutimo cylindricus showed that the sex pheromone induced an increment in both the deflection angle of the anterior flagellum and sustained unilateral bend of the posterior flagellum, but the mechanisms regulating these two flagellar waveforms were not fully revealed. In this study, we analyzed the changes in swimming path and flagellar waveforms with a high-speed recording system under different calcium conditions. The extracellular Ca2+ concentration at 10-3 M caused an increment in the deflection angle of the anterior flagellum only when ionomycin was absent. No sustained unilateral bend of the posterior flagellum was induced either in the absence or presence of ionomycin in extracellular Ca2+ concentrations below 10-2 M. Real-time Ca2+ imaging revealed that there is a spot near the basal part of anterior flagellum showing higher Ca2+ than in the other parts of the cell. The intensity of the spot slightly decreased when male gametes were treated with the sex pheromone. These results suggest that Ca2+-dependent changes in the anterior and posterior flagellum are regulated by distinct mechanisms and that the increase in the anterior flagellar deflection angle and sustained unilateral bend of the posterior flagellum may not be primarily induced by the Ca2+ concentration.

The Role of Soluble Adenylyl Cyclase in the Regulation of Flagellar Motility in Ascidian Sperm.

Flagellar motility in sperm is activated and regulated by factors related to the eggs at fertilization. In the ascidian Ciona intestinalis, a sulfated steroid called the SAAF (sperm activating and attracting factor) induces both sperm motility activation and chemotaxis. Cyclic AMP (cAMP) is one of the most important intracellular factors in the sperm signaling pathway. Adenylyl cyclase (AC) is the key enzyme that synthesizes cAMP at the onset of the signaling pathway in all cellular functions. We previously reported that both transmembrane AC (tmAC) and soluble AC (sAC) play important roles in sperm motility in Ciona. The tmAC plays a major role in the SAAF-induced activation of sperm motility. On the other hand, sAC is involved in the regulation of flagellar beat frequency and the Ca2+-dependent chemotactic movement of sperm. In this study, we focused on the role of sAC in the regulation of flagellar motility in Ciona sperm chemotaxis. The immunochemical analysis revealed that several isoforms of sAC protein were expressed in Ciona sperm, as reported in mammals and sea urchins. We demonstrated that sAC inhibition caused strong and transient asymmetrization during the chemotactic turn, and then sperm failed to turn toward the SAAF. In addition, real-time Ca2+ imaging in sperm flagella revealed that sAC inhibition induced an excessive and prolonged Ca2+ influx to flagella. These results indicate that sAC plays a key role in sperm chemotaxis by regulating the clearance of [Ca2+]i and by modulating Ca2+-dependent flagellar waveform conversion.

Positive selection on ADAM10 builds species recognition in the synchronous spawning coral Acropora.

The reef-building coral Acropora is a broadcast spawning hermaphrodite including more than 110 species in the Indo-Pacific. In addition, many sympatric species show synchronous spawning. The released gametes need to mate with conspecifics in the mixture of the gametes of many species for their species boundaries. However, the mechanism underlying the species recognition of conspecifics at fertilization remains unknown. We hypothesized that rapid molecular evolution (positive selection) in genes encoding gamete-composing proteins generates polymorphic regions that recognize conspecifics in the mixture of gametes from many species. We identified gamete proteins of Acropora digitifera using mass spectrometry and screened the genes that support branch site models that set the "foreground" branches showing strict fertilization specificity. ADAM10, ADAM17, Integrin α9, and Tetraspanin4 supported branch-site model and had positively selected site(s) that produced polymorphic regions. Therefore, we prepared antibodies against the proteins of A. digitifera that contained positively selected site(s) to analyze their functions in fertilization. The ADAM10 antibody reacted only with egg proteins of A. digitifera, and immunohistochemistry showed ADAM10 localized around the egg surface. Moreover, the ADAM10 antibody inhibited only A. digitifera fertilization but not the relative synchronous spawning species A. papillare. This study indicates that ADAM10 has evolved to gain fertilization specificity during speciation and contributes to species boundaries in this multi-species, synchronous-spawning, and species-rich genus.

Calaxin is required for asymmetric bend initiation and propagation in sperm flagella.

Regulation of waveform asymmetry in flagella is critical for changes in direction when sperm are swimming, as seen during the chemotaxis of sperm towards eggs. Ca2+ is an important regulator of asymmetry in flagellar waveforms. A calcium sensor protein, calaxin, is associated with the outer arm dynein and plays a key role in the regulation of flagellar motility in a Ca2+-dependent manner. However, the underlying mechanism of regulating asymmetric waves by means of Ca2+ and calaxin remains unclear. To clarify the calaxin-dependent mechanism for generating Ca2+-dependent asymmetric flagellar waveforms, we analyzed the initial step of flagellar bend formation and propagation in the sperm of the ascidian Ciona intestinalis. Our experiment used demembranated sperm cells, which were then reactivated by UV flash photolysis of caged ATP under both high and low Ca2+ concentrations. Here, we show that initial bends in the flagella are formed at the base of the sperm and propagate towards the tip during waveform generation. However, the direction of the initial bend differed between asymmetric and symmetric waves. When a calaxin inhibitor (repaglinide) was applied, it resulted in the failure of asymmetric wave formation and propagation. This was because repaglinide had no effect on initial bend formation, but it significantly inhibited the generation of the subsequent bend in the reverse direction. Switching of dynein sliding activity by mechanical feedback is crucial for flagellar oscillation. Our results suggest that the Ca2+/calaxin mechanism plays an important role in the switching of dynein activity from microtubule sliding in the principal bend into the suppressed sliding in the reverse bend, thereby allowing the sperm to successfully change direction.

Spawning-Induced pH Increase Activates Sperm Attraction and Fertilization Abilities in Eggs of the Ascidian, Phallusia philippinensis and Ciona intestinalis.

In Phlebobranchiata ascidians, oocytes and spermatozoa are stored in the oviduct and spermiduct, respectively, until spawning occurs. Gametes in the gonoducts are mature and fertilizable; however, it was found that the gametes of the ascidians Phallusia philippinensis and Ciona intestinalis could not undergo fertilization in the gonoductal fluids. The body fluids of the ascidians, especially in the gonoducts, were much more acidic (pH 5.5-6.8) than seawater (pH 8.2), and the fertilization rate was low under such acidic conditions. Hence, we examined the effect of pH on gametes. Pre-incubation of gonoductal eggs at pH 8.2 prior to insemination increased fertilization rates, even when insemination was performed under low pH conditions. Furthermore, an increase in ambient pH induced an increase in the intracellular pH of the eggs. It was also found that an increase in ambient pH triggered the release of sperm attractants from the egg and is therefore necessary for sperm chemotaxis. Hence, acidic conditions in the gonoductal fluids keep the gametes, especially eggs, infertile, and the release of eggs into seawater upon spawning induces an increase in ambient pH, which enables egg fertilization.

Fertilization modes and the evolution of sperm characteristics in marine fishes: Paired comparisons of externally and internally fertilizing species.

Fertilization mode may affect sperm characteristics, such as morphology, velocity, and motility. However, there is little information on how fertilization mode affects sperm evolution because several factors (e.g., sperm competition) are intricately intertwined when phylogenetically distant species are compared. Here, we investigated sperm characteristics by comparing seven externally and four internally fertilizing marine fishes from three different groups containing close relatives, considering sperm competition levels. The sperm head was significantly slenderer in internal fertilizers than in external fertilizers, suggesting that a slender head is advantageous for swimming in viscous ovarian fluid or in narrow spaces of the ovary. In addition, sperm motility differed between external and internal fertilizers; sperm of external fertilizers were only motile in seawater, whereas sperm of internal fertilizers were only motile in an isotonic solution. These results suggest that sperm motility was adapted according to fertilization mode. By contrast, total sperm length and sperm velocity were not associated with fertilization mode, perhaps because of the different levels of sperm competition. Relative testis mass (an index of sperm competition level) was positively correlated with sperm velocity and negatively correlated with the ratio of sperm head length to total sperm length. These findings suggest that species with higher levels of sperm competition have faster sperm with longer flagella relative to the head length. These results contradict the previous assumption that the evolution of internal fertilization increases the total sperm length. In addition, copulatory behavior with internal insemination may involve a large genital morphology, but this is not essential in fish, suggesting the existence of various sperm transfer methods. Although the power of our analyses is not strong because of the limited number of species, we propose a new scenario of sperm evolution in which internal fertilization would increase sperm head length, but not total sperm length, and change sperm motility.

Two distinct compartments of a ctenophore comb plate provide structural and functional integrity for the motility of giant multicilia.

Comb plates are large ciliary structures uniquely seen in comb jellies (ctenophores).1,2,3 A comb plate is constructed from tens of thousands of cilia that are bundled together by structures called compartmenting lamellae (CLs).4,5,6 We previously reported the first component of the CL, CTENO64, and found that it was specifically found in ctenophores and was essential for the determination of ciliary orientation.3 However, CTENO64 is localized only in the proximal region of the CL; therefore, the molecular architecture of the CL over the entire length of a comb plate has not been elucidated. Here, we identified a second CL component, CTENO189. This ctenophore-specific protein was present in the distal region of comb plates, with a localization clearly segregated from CTENO64. Knockdown of the CTENO189 gene using morpholino antisense oligonucleotides resulted in complete loss of CLs in the distal region of comb plates but did not affect the formation of comb plates or the orientation of each cilium. However, the hexagonal distribution of cilia was disarranged, and the metachronal coordination of comb plates along a comb row was lost in the CTENO189 morphants. The morphant comb plate showed asymmetric ciliary-type movement in normal seawater, and in a high-viscosity solution, it could not maintain the normal waveforms but showed a symmetric flagellar-type movement. Our findings demonstrated two distinct compartments of a comb plate: the proximal CL as the building foundation that rigidly fixes the ciliary orientation, and the distal CL that reinforces the elastic connection among cilia to overcome the hydrodynamic drag of giant multiciliary plates.

Mass spectrometry of short peptides reveals common features of metazoan peptidergic neurons.

The evolutionary origins of neurons remain unknown. Although recent genome data of extant early-branching animals have shown that neural genes existed in the common ancestor of animals, the physiological and genetic properties of neurons in the early evolutionary phase are still unclear. Here, we performed a mass spectrometry-based comprehensive survey of short peptides from early-branching lineages Cnidaria, Porifera and Ctenophora. We identified a number of mature ctenophore neuropeptides that are expressed in neurons associated with sensory, muscular and digestive systems. The ctenophore peptides are stored in vesicles in cell bodies and neurites, suggesting volume transmission similar to that of cnidarian and bilaterian peptidergic systems. A comparison of genetic characteristics revealed that the peptide-expressing cells of Cnidaria and Ctenophora express the vast majority of genes that have pivotal roles in maturation, secretion and degradation of neuropeptides in Bilateria. Functional analysis of neuropeptides and prediction of receptors with machine learning demonstrated peptide regulation of a wide range of target effector cells, including cells of muscular systems. The striking parallels between the peptidergic neuronal properties of Cnidaria and Bilateria and those of Ctenophora, the most basal neuron-bearing animals, suggest a common evolutionary origin of metazoan peptidergic nervous systems.

Axonemal Growth and Alignment During Paraspermatogenesis in the Marine Gastropod Strombus luhuanus.

Parasperm are non-fertilizing sperm that are produced simultaneously with fertile eusperm. They occur in several animal species and show considerable morphological diversity. We investigated the dynamics of axonemes during paraspermatogenesis in the marine snail S. luhuanus. Mature parasperm were characterized by two lateral undulating membranes for motility and many globular vesicles. Axonemes were first observed as brush-like structures that extruded from the anterior region. Multiple axonemes longer than the brush then started to extend inside the cytoplasm towards the posterior region. The mass of the axonemes separated into two lateral rows and formed an undulating membrane that drives bidirectional swimming in the mature parasperm. The central pair of axonemes was aligned in the undulating membrane, resulting in cooperative bend propagation. During paraspermatogenesis, centrioles were largely diminished and localized to the anterior region. CEP290, a major component of the transition zone, showed a broad distribution in the anterior area. Axonemes in the posterior region showed a 9 + 0 structure with both outer and inner arm dyneins. These observations provide a structural basis for understanding the physiological functions of parasperm in marine reproductive strategies.

The Roles of Two CNG Channels in the Regulation of Ascidian Sperm Chemotaxis.

Spermatozoa sense and respond to their environmental signals to ensure fertilization success. Reception and transduction of signals are reflected rapidly in sperm flagellar waveforms and swimming behavior. In the ascidian Ciona intestinalis (type A; also called C. robusta), an egg-derived sulfated steroid called SAAF (sperm activating and attracting factor), induces both sperm motility activation and chemotaxis. Two types of CNG (cyclic nucleotide-gated) channels, Ci-tetra KCNG (tetrameric, cyclic nucleotide-gated, K+-selective) and Ci-HCN (hyperpolarization-activated and cyclic nucleotide-gated), are highly expressed in Ciona testis from the comprehensive gene expression analysis. To elucidate the sperm signaling pathway to regulate flagellar motility, we focus on the role of CNG channels. In this study, the immunochemical analysis revealed that both CNG channels are expressed in Ciona sperm and localized to sperm flagella. Sperm motility analysis and Ca2+ imaging during chemotaxis showed that CNG channel inhibition affected the changes in flagellar waveforms and Ca2+ efflux needed for the chemotactic turn. These results suggest that CNG channels in Ciona sperm play a vital role in regulating sperm motility and intracellular Ca2+ regulation during chemotaxis.

Morphological differences in tardigrade spermatozoa induce variation in gamete motility.

BACKGROUND: Fertilization is an event at the beginning of ontogeny. Successful fertilization depends on strategies for uniting female and male gametes that developed throughout evolutionary history. In some species of tardigrades, investigations of reproduction have revealed that released spermatozoa swim in the water to reach a female, after which the gametes are stored in her body. The morphology of the spermatozoa includes a coiled nucleus and a species-specific-length acrosome. Although the mating behaviour and morphology of tardigrades have been reported, the motility of male gametes remains unknown. Here, using a high-speed camera, we recorded the spermatozoon motilities of two tardigrades, Paramacrobiotus sp. and Macrobiotus shonaicus, which have longer and shorter spermatozoa, respectively. RESULTS: The movement of spermatozoa was faster in Paramacrobiotus sp. than in M. shonaicus, but the beat frequencies of the tails were equal, suggesting that the long tail improved acceleration. In both species, the head part consisting of a coiled nucleus and an acrosome did not swing, in contrast to the tail. The head part of Paramacrobiotus sp. spermatozoa swung harder during turning; in contrast, the tail of M. shonaicus moved more widely than the head. Finally, after mating, the spermatozoa that reached the female aggregated around the cloaca while waiting to enter her body in both tested species. CONCLUSIONS: This study provides results for the first observations and analyses of individual spermatozoon motility in tardigrades. A comparison of the spermatozoon movements of the two tardigrades suggested that the motilities of the male gametes were affected by morphological differences, where the longer spermatozoa swam faster and the shorter ones showed more stable swimming. Swimming was mainly induced by tail movement, but the long head of Paramacrobiotus sp. spermatozoa might be especially important for turning. In addition, observations of mated female cloacae suggested that the head parts of the spermatozoa were required for aggregation around the cloaca of a mated female.

A dynein-associated photoreceptor protein prevents ciliary acclimation to blue light.

Light-responsive regulation of ciliary motility is known to be conducted through modulation of dyneins, but the mechanism is not fully understood. Here, we report a novel subunit of the two-headed f/I1 inner arm dynein, named DYBLUP, in animal spermatozoa and a unicellular green alga. This subunit contains a BLUF (sensors of blue light using FAD) domain that appears to directly modulate dynein activity in response to light. DYBLUP (dynein-associated BLUF protein) mediates the connection between the f/I1 motor domain and the tether complex that links the motor to the doublet microtubule. Chlamydomonas lacking the DYBLUP ortholog shows both positive and negative phototaxis but becomes acclimated and attracted to high-intensity blue light. These results suggest a mechanism to avoid toxic strong light via direct photoregulation of dyneins.

Nexin-Dynein regulatory complex component DRC7 but not FBXL13 is required for sperm flagellum formation and male fertility in mice.

Flagella and cilia are evolutionarily conserved cellular organelles. Abnormal formation or motility of these organelles in humans causes several syndromic diseases termed ciliopathies. The central component of flagella and cilia is the axoneme that is composed of the '9+2' microtubule arrangement, dynein arms, radial spokes, and the Nexin-Dynein Regulatory Complex (N-DRC). The N-DRC is localized between doublet microtubules and has been extensively studied in the unicellular flagellate Chlamydomonas. Recently, it has been reported that TCTE1 (DRC5), a component of the N-DRC, is essential for proper sperm motility and male fertility in mice. Further, TCTE1 has been shown to interact with FBXL13 (DRC6) and DRC7; however, functional roles of FBXL13 and DRC7 in mammals have not been elucidated. Here we show that Fbxl13 and Drc7 expression are testes-enriched in mice. Although Fbxl13 knockout (KO) mice did not show any obvious phenotypes, Drc7 KO male mice were infertile due to their short immotile spermatozoa. In Drc7 KO spermatids, the axoneme is disorganized and the '9+2' microtubule arrangement was difficult to detect. Further, other N-DRC components fail to incorporate into the flagellum without DRC7. These results indicate that Drc7, but not Fbxl13, is essential for the correct assembly of the N-DRC and flagella.

CTENO64 Is Required for Coordinated Paddling of Ciliary Comb Plate in Ctenophores.

Ctenophores, or comb jellies, are one of the earliest branching basal metazoan groups, whose phylogenetic position continues to be controversial. They have eight rows of iridescent structures, called comb plates, which are huge multiciliated paddle-like structures used for locomotion and uniquely found in this group of animals [1]. Despite a number of morphological and physiological studies over the past 50 years, the molecular nature of comb plates remains completely unknown. Here, we identified a protein CTENO64 that is specifically localized in the comb plates. This protein is only found in ctenophores and not in other animals or eukaryotic species that possess multiciliary cells or tissues. It is localized to regions, called compartmenting lamella (CL), which are uniquely seen in ctenophore multicilia, connecting adjacent cilia in the comb plates. Knockdown of the CTENO64 gene did not affect the formation of comb plates but caused the loss or misformation of CLs and the disruption of ciliary orientation, resulting in aberrant and non-planar waveforms in the mid-distal region of the comb plates. We propose that CLs have been convergently acquired in ctenophores to overcome the hydrodynamic constraints of possessing extremely long multicilia. Our findings provide the initial step in unveiling the molecular structure and evolutionary significance of ciliary comb plates and shed light not only on the hidden biology of ctenophores but also on the unique evolutionary pathway of these animals. VIDEO ABSTRACT.

Erratum: Publisher Correction: Calaxin is required for cilia-driven determination of vertebrate laterality.

[This corrects the article DOI: 10.1038/s42003-019-0462-y.].

High potassium seawater inhibits ascidian sperm chemotaxis, but does not affect the male gamete chemotaxis of a brown alga.

Male gamete chemotaxis towards the female gamete is a general strategy to facilitate the sexual reproduction in many marine eukaryotes. Biochemical studies of chemoattractants for male gametes of brown algae have advanced in the 1970s and 1980s, but the molecular mechanism of male gamete responses to the attractants remains elusive. In sea urchin, a K+ channel called the tetraKCNG channel plays a fundamental role in sperm chemotaxis and inhibition of K+ efflux through this channel by high K+ seawater blocks almost all cell responses to the chemoattractant. This signalling mechanism could be conserved in marine invertebrates as tetraKCNG channels are conserved in the marine invertebrates that exhibit sperm chemotaxis. We confirmed that high K+ seawater also inhibited sperm chemotaxis in ascidian, Ciona intestinalis (robusta), in this study. Conversely, the male gamete chemotaxis towards the female gamete of a brown alga, Mutimo cylindricus, was preserved even in high K+ seawater. This result indicates that none of the K+ channels is essential for male gamete chemotaxis in the brown alga, suggesting that the signalling mechanism for chemotaxis in this brown alga is quite different from that of marine invertebrates. Correlated to this result, we revealed that the channels previously proposed as homologues of tetraKCNG in brown algae have a distinct domain composition from that of the tetraKCNG. Namely, one of them possesses two repeats of the six transmembrane segments (diKCNG) instead of four. The structural analysis suggests that diKCNG is a cyclic nucleotide-modulated and/or voltage-gated K+ channel.

Calaxin is required for cilia-driven determination of vertebrate laterality.

Calaxin is a Ca2+-binding dynein-associated protein that regulates flagellar and ciliary movement. In ascidians, calaxin plays essential roles in chemotaxis of sperm. However, nothing has been known for the function of calaxin in vertebrates. Here we show that the mice with a null mutation in Efcab1, which encodes calaxin, display typical phenotypes of primary ciliary dyskinesia, including hydrocephalus, situs inversus, and abnormal motility of trachea cilia and sperm flagella. Strikingly, both males and females are viable and fertile, indicating that calaxin is not essential for fertilization in mice. The 9 + 2 axonemal structures of epithelial multicilia and sperm flagella are normal, but the formation of 9 + 0 nodal cilia is significantly disrupted. Knockout of calaxin in zebrafish also causes situs inversus due to the irregular ciliary beating of Kupffer's vesicle cilia, although the 9 + 2 axonemal structure appears to remain normal.

Single-cell genomics unveiled a cryptic cyanobacterial lineage with a worldwide distribution hidden by a dinoflagellate host.

Cyanobacteria are one of the most important contributors to oceanic primary production and survive in a wide range of marine habitats. Much effort has been made to understand their ecological features, diversity, and evolution, based mainly on data from free-living cyanobacterial species. In addition, symbiosis has emerged as an important lifestyle of oceanic microbes and increasing knowledge of cyanobacteria in symbiotic relationships with unicellular eukaryotes suggests their significance in understanding the global oceanic ecosystem. However, detailed characteristics of these cyanobacteria remain poorly described. To gain better insight into marine cyanobacteria in symbiosis, we sequenced the genome of cyanobacteria collected from a cell of a pelagic dinoflagellate that is known to host cyanobacterial symbionts within a specialized chamber. Phylogenetic analyses using the genome sequence revealed that the cyanobacterium represents an underdescribed lineage within an extensively studied, ecologically important group of marine cyanobacteria. Metagenomic analyses demonstrated that this cyanobacterial lineage is globally distributed and strictly coexists with its host dinoflagellates, suggesting that the intimate symbiotic association allowed the cyanobacteria to escape from previous metagenomic studies. Furthermore, a comparative analysis of the protein repertoire with related species indicated that the lineage has independently undergone reductive genome evolution to a similar extent as Prochlorococcus, which has the most reduced genomes among free-living cyanobacteria. Discovery of this cyanobacterial lineage, hidden by its symbiotic lifestyle, provides crucial insights into the diversity, ecology, and evolution of marine cyanobacteria and suggests the existence of other undiscovered cryptic cyanobacterial lineages.

Microtubule stabilizer reveals requirement of Ca2+-dependent conformational changes of microtubules for rapid coiling of haptonema in haptophyte algae.

A haptonema is an elongated microtubule-based motile organelle uniquely present in haptophytes. The most notable and rapid movement of a haptonema is 'coiling', which occurs within a few milliseconds following mechanical stimulation in an unknown motor-independent mechanism. Here, we analyzed the coiling process in detail by high-speed filming and showed that haptonema coiling was initiated by left-handed twisting of the haptonema, followed by writhing to form a helix from the distal tip. On recovery from a mechanical stimulus, the helix slowly uncoiled from the proximal region. Electron microscopy showed that the seven microtubules in a haptonema were arranged mostly in parallel but that one of the microtubules often wound around the others in the extended state. A microtubule stabilizer, paclitaxel, inhibited coiling and induced right-handed twisting of the haptonema in the absence of Ca2+, suggesting changes in the mechanical properties of microtubules. Addition of Ca2+ resulted in the conversion of haptonematal twist into the planar bends near the proximal region. These results indicate that switching microtubule conformation, possibly with the aid of Ca2+-binding microtubule-associated proteins is responsible for rapid haptonematal coiling.