A model for signal transduction during gamete release in the fucoid alga pelvetia compressa

Temporal windows of reproductive opportunity reinforce species barriers in a marine broadcast spawning assemblage

Prezygotic isolating mechanisms act to limit hybridization and maintain the genetic identity of closely-related species. While synchronous intraspecific spawning is a common phenomenon amongst marine organisms and plays an important role in reproductive success, asynchronous spawning between potentially hybridizing lineages may also be important in maintaining species boundaries. We tested this hypothesis by comparing reproductive synchrony over daily to hourly timescales in a sympatric assemblage of intertidal fucoid algae containing selfing hermaphroditic (Fucus spiralis and Fucus guiryi) and dioecious (Fucus vesiculosus and Fucus serratus) species. Our results confirm that gametes are released on semi-lunar cycles in all species. However, sister species with different mating systems showed asynchronous spawning at finer circadian timescales, thus providing evidence for a partial reproductive barrier between hermaphroditic and dioecious species. Finally, our data also emphasize the ecological, developmental, and/or physiological constraints that operate to restrict reproduction to narrow temporal windows of opportunity in the intertidal zone and more generally the role of ecological factors in marine speciation.

The sporulation of the green alga Ulva prolifera is controlled by changes in photosynthetic electron transport chain

Sporulation and spore release are essential phases of the life cycle in algae and land plants. Ulva prolifera, which is an ideal organism for studying sporulation and spore release, was used as the experimental material in the present study. The determination of photosynthetic parameters, combined with microscopic observation, treatment with photosynthetic inhibitors, limitation of carbon acquisition, and protein mass spectrometry, was employed in this experiment. Cycle electron transport (CEF) was found enhanced at the onset of sporangia formation. The inhibition effect of dibromothymoquinone (DBMIB) towards sporulation was always strong during the sporulation process whereas the inhibition effect of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) was continuously declined accompanied with the progress of sporulation. The changes of photosynthesis resulted from the limitation of CO₂ acquisition could stimulate sporulation onset. Quantitative protein analysis showed that enzymes involved in carbon fixation, including RUBISCO and pyruvate orthophosphate dikinase, declined during sporogenesis, while proteins involved in sporulation, including tubulin and centrin, increased. These results suggest that enhanced cyclic electron flow (CEF) and oxidation of the plastoquinone pool are essential for sporangia formation onset, and changes in photosynthetic electron transport chain have significant impacts on sporulation of the green algae.

Sex-biased gene expression in the brown alga Fucus vesiculosus

BACKGROUND

The fucoid brown algae (Heterokontophyta, Phaeophyceae) are increasingly the focus of ecological genetics, biodiversity, biogeography and speciation research. The molecular genetics underlying mating system variation, where repeated dioecious - hermaphrodite switches during evolution are recognized, and the molecular evolution of sex-related genes are key questions currently hampered by a lack of genomic information. We therefore undertook a comparative analysis of male and female reproductive tissue transcriptomes against a vegetative background during natural reproductive cycles in Fucus vesiculosus.

RESULTS

Over 300 k reads were assembled and annotated against public protein databases including a brown alga. Compared with the vegetative tissue, photosynthetic and carbohydrate metabolism pathways were under-expressed, particularly in male tissue, while several pathways involved in genetic information processing and replication were over-expressed. Estimates of sex-biased gene (SBG) expression were higher for male (14% of annotated orthologues) than female tissue (9%) relative to the vegetative background. Mean expression levels and variance were also greater in male- than female-biased genes. Major female-biased genes were carbohydrate-modifying enzymes with likely roles in zygote cell wall biogenesis and/or modification. Male-biased genes reflected distinct sperm development and function, and orthologues for signal perception (a phototropin), transduction (several kinases), and putatively flagella-localized proteins (including candidate gamete-recognition proteins) were uniquely expressed in males. Overall, the results suggest constraint on female-biased genes (possible pleiotropy), and less constrained male-biased genes, mostly associated with sperm-specific functions.

CONCLUSIONS

Our results support the growing contention that males possess a large array of genes regulating male fitness, broadly supporting findings in evolutionarily distant heterogametic animal models. This work identifies an annotated set of F. vesiculosus gene products that potentially regulate sexual reproduction and may contribute to prezygotic isolation, one essential step towards developing tools for a functional understanding of species isolation and differentiation.

Comparative study of polysaccharides from reproductive and sterile tissues of five brown seaweeds

Sterile and reproductive tissues of five brown algae (Sargassum pallidum, Silvetia babingtonii, Fucus evanescens, Saccharina japonica, and Alaria ochotensis) from Russian Far East seas were compared for the content and monosaccharide composition of fucoidans as well as the content of laminarans and alginic acids. It was proved that reproduction has an apparent effect on fucoidan content and its monosaccharide composition. Fucoidan content in fertile tissues was shown to be 1.3-1.5 times as high as in sterile ones. Based on the present and previously reported data, we believe that fucoidan accumulation during the development of reproductive structure is a general trend for brown seaweeds. Reproduction also caused changes in the monosaccharide composition. As a result, fertile plants synthesized fucoidan with less heterogeneous monosaccharide composition in comparison with sterile ones. Structural changes of this polysaccharide are species-specific and perhaps depend on the type of the synthesized polysaccharide. The fertile plants of S. babingtonii yielded the highest fucoidan content [25% dry weight (dw)] among the tested species, whereas the maximum alginic acid content was found for the sterile tissues of S. japonica-up to 43% dw. There was no general trend in alginic acid content variation during sporulation. The alginic acid content was much higher in the sterile tissues of S. japonica in comparison with the fertile ones. On the contrary, the fertile tissues of A. ochotensis contained more alginic acid than the sterile ones.

Prezygotic barriers to hybridization in marine broadcast spawners: reproductive timing and mating system variation

Sympatric assemblages of congeners with incomplete reproductive barriers offer the opportunity to study the roles that ecological and non-ecological factors play in reproductive isolation. While interspecific asynchrony in gamete release and gametic incompatibility are known prezygotic barriers to hybridization, the role of mating system variation has been emphasized in plants. Reproductive isolation between the sibling brown algal species Fucus spiralis, Fucus guiryi (selfing hermaphrodite) and Fucus vesiculosus (dioecious) was studied because they form hybrids in parapatry in the rocky intertidal zone, maintain species integrity over a broad geographic range, and have contrasting mating systems. We compared reproductive synchrony (spawning overlap) between the three species at several temporal scales (yearly/seasonal, semilunar/tidal, and hourly during single tides). Interspecific patterns of egg release were coincident at seasonal (single peak in spring to early summer) to semilunar timescales. Synthesis of available data indicated that spawning is controlled by semidiurnal tidal and daily light-dark cues, and not directly by semilunar cycles. Importantly, interspecific shifts in timing detected at the hourly scale during single tides were consistent with a partial ecological prezygotic hybridization barrier. The species displayed patterns of gamete release consistent with a power law distribution, indicating a high degree of reproductive synchrony, while the hypothesis of weaker selective constraints for synchrony in selfing versus outcrossing species was supported by observed spawning in hermaphrodites over a broader range of tidal phase than in outcrossers. Synchronous gamete release is critical to the success of external fertilization, while high-energy intertidal environments may offer only limited windows of reproductive opportunity. Within these windows, however, subtle variations in reproductive timing have evolved with the potential to form ecological barriers to hybridization.

Stress ecology in fucus: abiotic, biotic and genetic interactions

Stress regimes defined as the synchronous or sequential action of abiotic and biotic stresses determine the performance and distribution of species. The natural patterns of stress to which species are more or less well adapted have recently started to shift and alter under the influence of global change. This was the motivation to review our knowledge on the stress ecology of a benthic key player, the macroalgal genus Fucus. We first provide a comprehensive review of the genus as an ecological model including what is currently known about the major lineages of Fucus species with respect to hybridization, ecotypic differentiation and speciation; as well as life history, population structure and geographic distribution. We then review our current understanding of both extrinsic (abiotic/biotic) and intrinsic (genetic) stress(es) on Fucus species and how they interact with each other. It is concluded that (i) interactive stress effects appear to be equally distributed over additive, antagonistic and synergistic categories at the level of single experiments, but are predominantly additive when averaged over all studies in a meta-analysis of 41 experiments; (ii) juvenile and adult responses to stress frequently differ and (iii) several species or particular populations of Fucus may be relatively unaffected by climate change as a consequence of pre-adapted ecotypes that collectively express wide physiological tolerences. Future research on Fucus should (i) include additional species, (ii) include marginal populations as models for responses to environmental stress; (iii) assess a wider range of stress combinations, including their temporal fluctuations; (iv) better differentiate between stress sensitivity of juvenile versus adult stages; (v) include a functional genomic component in order to better integrate Fucus' ecological and evolutionary responses to stress regimes and (vi) utilize a multivariate modelling approach in order to develop and understand interaction networks.

CONTROL OF REPRODUCTION RHYTHMICITY BY ENVIRONMENTAL AND ENDOGENOUS SIGNALS IN ULVA PSEUDOCURVATA (CHLOROPHYTA)(1)

A field population of Ulva pseudocurvata Koeman et C. Hoek (hereafter termed Ulva) at Sylt Island (North Sea, Germany) exhibited biweekly peaks of gametophytic reproduction during the colder seasons and approximately weekly peaks during summer. The reproductive events lasted 1-5 d and were separated from each other by purely vegetative phases. Under constant conditions in the laboratory, a free-running rhythm was observed with reproductive peaks occurring approximately every 7 d. When artificial moonlight was provided every 4 weeks, fewer reproductive events occurred, and the reproductive rhythm became synchronized to the environmental artificial moonlight rhythm. In the laboratory, apical disks were entirely converted into reproductive tissue after 8 d cultivation, while almost all basal disks stayed vegetative, which prevented the entire loss of the vegetative thallus during reproductive events. Seasonal size reduction of the thallus occurred from late autumn onward and was determined to be controlled by a genuine photoperiodic response, since size reduction could be induced from May onward by experimental short-day (SD) treatment but was prevented in a long-day (LD) or night-break regime (NB). A daily fine-tuning occurred with gamete release early in the morning at the first sign of daylight, following an obligatory dark ("night") period of at least 1 h duration. No release took place if the overnight dark phase was replaced by continuous light. Blue, green, or red light all triggered gamete release after a dark phase at an irradiance of 0.1 μmol photons · m(-2)  ·s(-1) , while 0.001 μmol photons · m(-2)  · s(-1) was equivalent to a dark control.

Involvement of Reactive Oxygen Species and Reactive Nitrogen Species in the Wound Response of Dasycladus vermicularis

We investigated the signaling events involved in the wound response of the marine macroalga Dasycladus vermicularis, finding nitric oxide (NO) production in relation to injury. The addition of exogenous H2O2 to aliquots of injured algae accelerated the kinetics of NO production in the wounded region. Similarly, the addition of an NO donor caused an increase in detectable H2O2 around the site of injury. By wounding or incubating uninjured algae with an NO donor, peroxidase activity was enhanced. Based on the use of selected pharmacological probes, our results indicate that H2O2 production involves the upstream activation of signaling events similar to those observed in the physiology of higher plants.

Regulation of gamete release in the economic brown seaweed Hizikia fusiforme (Phaeophyta)

Gamete release is an essential event in artificial seeding of the economic brown seaweed, Hizikia fusiforme. Mass egg release occurred in the dark, with few eggs being discharged in the light. Release of eggs was elicited with eight practical salinity units (one PSU identical with 1 g sea salts l(-1)) and was inhibited by salinity levels >32 PSU. Egg release was optimal at 23 degrees C, and was decreased by 72% in agitated seawater compared to unstirred seawater. Inhibitors of photosynthesis and ions channels suppressed egg release, indicating that this process was physiologically associated with photosynthetic activity and ion transport.

Blue- and green-light signals for gamete release in the brown alga, Silvetia compressa

The intertidal brown alga Silvetia compressa releases gametes from receptacles (the reproductive tissue) rapidly upon a dark transfer (following a photosynthesis-dependent period in the light, termed potentiation). In this study, the wavelength-dependence of this process was investigated. During the potentiation period in white light (WL), gametes are not released. However, gametes were released during potentiation in blue light (BL), or in low red light/blue light (RL/BL) ratios, but not in RL alone, high RL/BL ratios, or in broadband blue-green light (B-GL) (presence of BL, but absence of RL). RL was as effective as WL for potentiation, i.e., both lead to gamete release following transfer to darkness. Rates of linear photosynthetic electron transport were similar in RL and BL. Gamete release in BL was inhibited by equal amounts of additional narrow-waveband light between the green and red regions of the spectrum, with light-induced gamete release restricted between <491 nm and 509 nm. Very little light-induced gamete release occurred between 530 nm and 650 nm. It is proposed that a BL-responsive photoreceptor is responsible for light-induced gamete release. Transfer of WL-potentiated receptacles to GL near 530 nm resulted in significant de-potentiation and reduced gamete release during a subsequent dark transfer. This effect was not seen at 509 nm or 560 nm and revealed the presence of a second photoreceptor system repressing or counteracting potentiation in the light. We propose that the restriction of gamete release to periods when irradiance is blue-shifted may constitute a depth-sensing mechanism for this intertidal alga, allowing controlled release of gametes at high tide and/or less turbid periods, thus minimizing gamete dilution, and promoting fertilization success.

WATER MOTION, MARINE MACROALGAL PHYSIOLOGY, AND PRODUCTION

Water motion is a key determinant of marine macroalgal production, influencing directly or indirectly physiological rates and community structure. Our understanding of how marine macroalgae interact with their hydrodynamic environment has increased substantially over the past 20 years, due to the application of tools such as flow visualization to aquatic vegetation, and in situ measurements of seawater velocity and turbulence. This review considers how the hydrodynamic environment in which macroalgae grow influences their ability to acquire essential resources and how macroalgae might respond physiologically to fluctuations in their hydrodynamic regime with a focus on: (1) the biochemical processes occurring within the diffusion boundary layer (DBL) that might reduce rates of macroalgal production; (2) time scales over which measurements of velocity and DBL processes should be made, discussing the likelihood of in situ mass transfer limitation; (3) if and how macroalgal morphology influences resource acquisition in slow flows; and (4) ecobiomechanics and how hydrodynamic drag might influence resource acquisition and allocation. Finally, the concept that macroalgal production is enhanced in wave-exposed versus sheltered habitats is discussed.

Inhibition of the establishment of zygotic polarity by protein tyrosine kinase inhibitors leads to an alteration of embryo pattern in Fucus

Fucoid algae, including the genus Fucus and Pelvetia, are recognized as model systems to study early embryogenesis in plants. In particular the zygotes of these fucoid algae are highly suitable experimental systems for investigating the establishment of polarity and its requirement for later embryogenesis. However, the transduction pathways involved in the initiation of polarization are still poorly understood, and the link between the early polarization processes and embryo long-term patterning has never been experimentally demonstrated. We, therefore, have investigated the putative role of protein phosphorylation in the regulation of early embryogenesis, using a combined pharmacological and biochemical approach. Among the various protein kinase inhibitors tested, a subset of well-known PTK inhibitors, including genistein, prevented germination but had no effect on growth of germinated zygotes and embryos. Inhibition of germination appeared to be a direct consequence of prevention of polarization since genistein and other PTK inhibitors specifically inhibited axis formation in a light-independent manner. Genistein inhibited cellular events associated with polarization such as polarized secretion of cell wall sulfated compounds. Anchorage of F-actin at the rhizoid pole was also inhibited and F-actin redistributed in response to a new light vector. Zygotes inhibited in the polarization process over the period of axis formation recovered from the treatment and displayed differentiated cellular structures after a few days. However, they exhibited a deeply disorganized pattern, suggesting that the early polarization process is essential for normal patterning of the embryo. Western blot analysis of protein phosphorylation showed that the patterns of protein phosphorylation changed during development and were disturbed by treatments with genistein. This drug also inhibited in vitro autophosphorylation. The nature of the genistein-sensitive kinases required for polarization and long-term patterning is discussed in light of these data.