Browsing by Author "Brawley, S. H."
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- A model for signal transduction during gamete release in the fucoid alga Pelvetia compressaPublication . Pearson, Gareth; Brawley, S. H.Fucoid algae release gametes into-seawater following an inductive light period (potentiation), and gamete expulsion from potentiated receptacles of Pelvetia compressa began about 2 min after a light-to-dark transition. Agitation of the medium reversed potentiation, with an exponential time course completed in about 3 h. Light regulated two signaling pathways during potentiation and gamete expulsion: a photosynthetic pathway and a photosynthesis-independent pathway in which red light was active but blue light was not. Uptake of K+ appears to have an important role in potentiation, because a 50% inhibition of potentiation occurred in the presence of the tetraethylammonium ion, a K+-channel blocker. A central role of anion channels in the maintenance of potentiation is suggested by the premature release of gametes in the light when receptacles were incubated with inhibitors of slow-type anion channels. An inhibitor of tyrosine kinases, tyrphostin A63, also inhibited potentiation. A model for gamete release from P. compressa is presented that proposes that illumination results in the accumulation of ions (e.g. K+) throughout the cells of the receptacle during potentiation, which then move into the extracellular matrix during gamete expulsion to generate osmomechanical force, resulting in gamete release.
- Control of gamete release in fucoid algae: sensing hydrodynamic conditions via carbon acquisitionPublication . Pearson, G. A.; Serrão, Ester; Brawley, S. H.We investigated the environmental factors providing signals for gamete release in fucoid algae, with a particular focus on the inhibitory effect of water motion. The release of gametes by Fucus distichus occurred in still water under high light and was associated with the depletion of dissolved inorganic carbon (DIC) in tide pools isolated from the ocean during daytime low tides. Diurnal patterns of gamete release that lasted for 2-3 d, as in natural populations, were found in receptacles cultured in a 12 h light: 12 h dark photoperiod under calm conditions or in a simulated tidal regime. Constant light disrupted the diurnal component of release, however, suggesting that an endogenous circadian periodicity was absent. The effects of increased [DIC] on cultured receptacles under calm conditions were similar to the inhibitory effects of high water motion; both conditions reduced release in several species from both tide pool and intertidal habitats. Bicarbonate concentration, rather than carbon dioxide, or carbonate, was shown to be the component of the DIC system most closely correlated (inversely) with gamete release in the intertidal fucoid Pelvetia compressa. The hypothesis that gamete release is triggered by DIC depletion in the unstirred boundary layer during periods of low water motion in the light was further supported by experiments with P. compressa. These showed that gamete release was not inhibited by high water motion when DIC was absent. In the same experiments, high water motion inhibited release at 2 and 20 mmol/L DIC, confirming that the sensitivity of gamete release to water motion is DIC dependent. The ability of fucoid algae to couple physiological processes (e.g., photosynthesis) with life history events (gamete release) allows external fertilization to occur under favorable hydrodynamic conditions, and may contribute greatly to reproductive success in organisms inhabiting periodically turbulent environments.
- Distributional success of the marine seaweed Fucus vesiculosus L. in the brackish Baltic Sea correlates with osmotic capabilities of Baltic gametesPublication . Serrão, Ester; Kautsky, L.; Brawley, S. H.To understand the unique success of the marine seaweed Fucus vesiculosus L. (Phaeophyceae) in the brackish Baltic Sea, the performance of gametes from Baltic [4.1-6.5‰S (Salinity)] and marine populations was studied. Sperm from Baltic F. vesiculosus swam with a path velocity of c. 30-110 μm/s and could fertilize eggs in waters of salinities from 4 to 33‰S. In their natural water, Baltic sperm were not negatively phototactic, unlike marine sperm in seawater; this should decrease the sperm:egg concentration at the seafloor and reduce the likelihood of polyspermy. Marine (Iceland, Sweden) sperm in seawater had a path velocity of c. 80-100 μm/s, but performed poorly and could not fertilize eggs in natural or artificial Baltic water ≤‰S; therefore, Baltic populations have adapted or acclimated to their brackish habitat. Baltic populations appear better adapted to their natural low salinities because, even after culturing Baltic and marine individuals in water from both the Baltic (6.5‰S) and the marine Skagerrak (21‰S), Baltic sperm were in both cases still able to swim and fertilize eggs at lower salinities (4‰S) than marine sperm; fertilization never occurred between marine gametes at 4-6‰S. However, F. vesiculosus acclimates to some salinities, since sperm from Baltic and marine males that had been cultured at 21‰S swam better (higher velocity, proportion that were motile and/or linearity) in marine salinities (21-33‰S) than when they were cultured at 6.5‰S. The effects of salinity on sperm motility and fertilization were osmolar rather than due to specific ionic requirements, over the tested range. The osmolalities (< c. 100 mmol/kg) at which fertilization success of Baltic gametes decreases nearly to zero correspond to the osmolality of Baltic water at the northernmost limit of distribution of F. vesiculosus in the Baltic Sea. Therefore, the present range of F. vesiculosus in the Baltic appears to correspond to the osmotic tolerance of the gametes. Very small natural or anthropogenic increases in ambient osmolality would be likely to cause a substantial expansion of this species into the inner Baltic.
- Ecological genetics in the North Atlantic: environmental gradients and adaptation at specific lociPublication . Schmidt, P. S.; Serrão, Ester; Pearson, G. A.; Riginos, C.; Rawson, P. D.; Hilbish, Thomas J.; Brawley, S. H.; Trussell, G. C.; Carrington, E.; Wethey, D. S.; Grahame, J. W.; Bonhomme, F.; Rand, D. M.The North Atlantic intertidal community provides a rich set of organismal and environmental material for the study of ecological genetics. Clearly defined environmental gradients exist at multiple spatial scales: there are broad latitudinal trends in temperature, meso-scale changes in salinity along estuaries, and smaller scale gradients in desiccation and temperature spanning the intertidal range. The geology and geography of the American and European coasts provide natural replication of these gradients, allowing for population genetic analyses of parallel adaptation to environmental stress and heterogeneity. Statistical methods have been developed that provide genomic neutrality tests of population differentiation and aid in the process of candidate gene identification. In this paper, we review studies of marine organisms that illustrate associations between an environmental gradient and specific genetic markers. Such highly differentiated markers become candidate genes for adaptation to the environmental factors in question, but the functional significance of genetic variants must be comprehensively evaluated. We present a set of predictions about locus-specific selection across latitudinal, estuarine, and intertidal gradients that are likely to exist in the North Atlantic. We further present new data and analyses that support and contradict these simple selection models. Some taxa show pronounced clinal variation at certain loci against a background of mild clinal variation at many loci. These cases illustrate the procedures necessary for distinguishing selection driven by internal genomic vs. external environmental factors. We suggest that the North Atlantic intertidal community provides a model system for identifying genes that matter in ecology due to the clarity of the environmental stresses and an extensive experimental literature on ecological function. While these organisms are typically poor genetic and genomic models, advances in comparative genomics have provided access to molecular tools that can now be applied to taxa with well-defined ecologies. As many of the organisms we discuss have tight physiological limits driven by climatic factors, this synthesis of molecular population genetics with marine ecology could provide a sensitive means of assessing evolutionary responses to climate change.
- Evolution of the Fucaceae (Phaeophyceae) inferred from nrDNA-ITSPublication . Serrão, Ester; Alice, L. A.; Brawley, S. H.Sequences of the internal transcribed spacer region (ITS-1, 5.8S, and ITS-2) of nuclear ribosomal DNA were obtained from 16 species representing all six genera of Fucaceae (Ascophyllum, Fucus, Hesperophycus, Pelvetia, Pelvetiopsis, and Xiphophora) plus one outgroup (Hormosira). Parsimony analysis indicated that the family Fucaceae is monophyletic and that the northern hemisphere taxa are highly divergent from the only southern hemisphere genus, Xiphophora. The genus Pelvetia is not monophyletic because the European P. canaliculata is more closely related to Fucus, Hesperophycus, and Pelvetiopsis than to other Pelvetia species. We establish Silvetia, gen. nov. and transfer the 3 Pacific species of Pelvetia to the new genus. Fucus is monophyletic and not ancestral in the Fucaceae. The ITS sequences identified two strongly supported lineages within Fucus, one with F. serratus sister to the clade containing F. gardneri, F. distichus, and F. evanescens and a second including F. vesiculosus, F. spiralis, F. ceranoides, and F. virsoides. The ITS was not useful for resolving relationships within each of these clusters and between populations of F. vesiculosus. Within-individual variation in ITS sequences is high in Fucus, a derived genus, compared to Ascophyllum, a more ancestral genus. Mapping of the two characters that form the basis of Powell’s model for speciation in the Fucaceae showed that 1) number of eggs per oogonium has not followed a gradual reduction and that 2) monoecy/dioecy has changed several times during evolution of this family.
- Gamete release at low tide in fucoid algae: Maladaptive or advantageous?Publication . Brawley, S. H.; Johnson, L. E.; Pearson, G. A.; Speransky, V.; Li, R.; Serrão, EsterThis review discusses three questions pertaining to gamete release by fucoid algae at low tide: 1) Are gametes viable and does fertilization occur at low tide?, 2) How many gametes are released at low tide versus at high tide? and 3) Is gamete release at low tide maladaptive or is it selectively advantageous? Gamete release at low tide (LT) in fucoid algae is observed commonly in monoecious species from the lower and mid-intertidal zones (e.g., Fucus distichus, F. evanescens, F. gardneri, Pelvetia compressa); in dioecious species, intact antheridia (containing sperm) are commonly released at low tide (e.g., in Ascophyllum nodosum, F. vesiculosus). Fertilization at low tide can be determined with a calcofluor white assay and occurs in at least three species (F. distichus, F. gardneri, P. compressa). In general, fucoid algae have high levels of fertilization success, but substantial mortality occurs during early embryogenesis in some intertidal zones due to physical stresses. The agarose bead assay is useful to assess desiccation on an egg-sized scale. Constitutive dehydrin-like proteins are present in sperm, eggs, and embryos. The release of gametes at low tide appears to be a consequence of a mechanism selected to permit gamete release under calm conditions in seawater; this mechanism is described. The timing of adhesion by zygotes at low and high tide is unknown. Such information and additional data on the relative proportions of zygotes produced by monecious species at low tide versus high tide are required in order to assess effects of gamete release at low tide upon dispersal and population structure.
- Isolation and cross-species amplification of microsatellite loci from the fucoid seaweeds fucus vesiculosus, f. serratus and Ascophyllum nodosum (heterokontophyta, fucaceae)Publication . Engel, C. R.; Brawley, S. H.; Edwards, K. J.; Serrão, EsterThe Fucaceae is a family of brown seaweeds that dominate and frequently co-occur on North Atlantic rocky shores. We developed nine polymorphic microsatellite markers for the fucoid seaweeds Fucus vesiculosus, F. serratus and Ascophyllum nodosum using a combined, enriched library. Six of these loci were polymorphic in at least two species, showing from two to eight alleles with heterozygosities ranging from 0.41 to 0.85. Loci were also tested on F. spiralis, revealing five polymorphic microsatellite loci in this species.
- Reproductive success of Fucus vesiculosus (Phaeophyceae) in the Baltic SeaPublication . Serrão, Ester; Brawley, S. H.; Hedman, J.; Kautsky, L.; Samuelsson, G.Marine organisms colonizing brackish habitats such as the Baltic Sea must cope with the negative effects of low salinities on reproductive success because these may reduce gamete viability and/or increase polyspermy. Reproductive characteristics of the marine seaweed Fucus vesiculosus L. were studied in several brackish habitats, particularly in the northern Baltic Sea, to understand its ability to reproduce where few other marine species survive. Polyspermy and fertilization success were variable at the boundary of the continuous distribution of F. vesiculosus in the Baltic Sea, and polyspermy was high (10%–30%) when fertilization was successful. A strong female bias (80%–86%, ca. 5.5:1) was found at the northernmost limit of Baltic F. vesiculosus. Electrophysiological studies showed that many eggs have a high input resistance (519 ± 150 MΩ[mean ± SE, n = 14] at Drivan, 1995), which may be helpful in preventing polyspermy in this brackish habitat. The polyspermy block remains sodium-dependent in the northern Baltic. Sperm bound quickly to northern Baltic eggs in natural water, but fertilization was delayed compared to marine F. vesiculosus. A subset of northern Baltic eggs studied during an optimal reproductive period (7–11 July 1995) had a membrane potential (Em) of ca. −100 mV and an effective fertilization potential (FP) of ca. 2 min with a plateau of −25 mV, but repolarized too rapidly for the FP to be protective. Pronuclear migration and cell wall secretion occurred more slowly in Baltic than in marine zygotes. The reproductive success of theseboundary populations may be dependent upon windows of opportunity when there are favorable combinations of the levels of salinity, water motion, population density, and sex ratio. These factors and the short duration of the reproductive season in the northern Baltic Sea may result in reproductive failure in some years.
- Successful external fertilization in turbulent environmentsPublication . Serrão, Ester; Pearson, G. A.; Kautsky, L.; Brawley, S. H.Mathematical and experimental simulations predict that external fertilization is unsuccessful in habitats characterized by high water motion. A key assumption of such predictions is that gametes are released in hydrodynamic regimes that quickly dilute gametes. We used fucoid seaweeds to examine whether marine organisms in intertidal and subtidal habitats might achieve high levels of fertilization by restricting their release of gametes to calm intervals. Fucus vesiculosus L. (Baltic Sea) released high numbers of gametes only when maximal water velocities were below ca. 0.2 m/s immediately prior to natural periods of release, which occur in early evening in association with lunar cues. Natural fertilization success measured at two sites was always close to 100%. Laboratory experiments confirmed that (i) high water motion inhibits gamete release by F. vesiculosus and by the intertidal fucoids Fucus distichus L. (Maine) and Pelvetia fastigiata (J. Ag.) DeToni (California), and (ii) showed that photosynthesis is required for high gamete release. These data suggest that chemical changes in the boundary layer surrounding adults during photosynthesis and/or mechanosensitive channels may modulate gamete release in response to changing hydrodynamic conditions. Therefore, sensitivity to environmental factors can lead to successful external fertilization, even for species living in turbulent habitats.