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In haplodiplontic lineages, sexual reproduction occurs in haploid parents without meiosis. Although widespread in multicellular lineages such as brown algae (Phaeophyceae), haplodiplontic gametogenesis has been little studied at the molecular level. We addressed this by generating an annotated reference transcriptome for the gametophytic phase of the sugar kelp, Saccharina latissima. Transcriptional profiles of microscopic male and female gametophytes were analysed at four time points during the transition from vegetative growth to gametogenesis. Gametogenic signals resulting from a switch in culture irradiance from red to white light activated a core set of genes in a sex-independent manner, involving rapid activation of ribosome biogenesis, transcription and translation related pathways, with several acting at the post-transcriptional or post-translational level. Additional genes regulating nutrient acquisition and key carbohydrate-energy pathways were also identified. Candidate sex-biased genes under gametogenic conditions had potentially key roles in controlling female- and male-specific gametogenesis. Among these were several sex-biased or -specific E3 ubiquitin-protein ligases that may have important regulatory roles. Females specifically expressed several genes that coordinate gene expression and/or protein degradation, and the synthesis of inositol-containing compounds. Other female-biased genes supported parallels with oogenesis in divergent multicellular lineages, in particular reactive oxygen signalling via an NADPH-oxidase. Males specifically expressed the hypothesised brown algal sex-determining factor. Male-biased expression mainly involved upregulation of genes that control mitotic cell proliferation and spermatogenesis in other systems, as well as multiple flagella-related genes. Our data and results enhance genome-level understanding of gametogenesis in this ecologically and economically important multicellular lineage., Competing Interests: The authors have declared that no competing interests exist.

Two morphologically similar digitate kelp species, Laminaria digitata and Hedophyllum nigripes, co-occur along a shallow sublittoral depth gradient in the Arctic but, in contrast to L. digitata, very few ecophysiological data exist for H. nigripes. We investigated growth, survival, photosynthetic characteristics and carbon:nitrogen ratios of juvenile sporophytes, and recruitment and survival of gametophytes in genetically verified Arctic isolates of both species along temperature gradients (0–25 °C) over 14 days. Laminaria digitata gametophytes survived 23–24 °C, while sporophytes survived 21–22 °C. Hedophyllum nigripes had lower temperature affinities. Gametophytes survived 19–21 °C, while sporophytes survived 18 °C. Male gametophytes were more heat-tolerant than female gametophytes in both species. The pronounced cold adaption of H. nigripes compared to L. digitata also became apparent in different sporophyte growth optima (L. digitata: 15 °C; H. nigripes: 10 °C) and gametogenesis optima (L. digitata: 5–15 °C; H. nigripes: 0–10 °C). Higher carbon:nitrogen ratios in H. nigripes suggest an adaptation to nutrient poor Arctic conditions. The overall temperature performance of H. nigripes possibly restricts the species to Arctic–Sub-Arctic regions, while Arctic L. digitata behaved similarly to cold-temperate populations. Our data suggest that a future increase in seawater temperatures may hamper the success of H. nigripes and favour L. digitata in Arctic environments. [ABSTRACT FROM AUTHOR]

Kelps are globally important bioengineering species with high ecological and economic value, but are increasingly threatened by climate-driven geographic range shifts. The inheritance of economically important traits from parents to offspring is poorly understood in kelps but it is of utmost interest to seaweed farmers wishing to select strains with superior performance and resilience to environmental change. For two allopatric kelp species (N-Atlantic Laminaria digitata and S-Atlantic L. pallida), we compared the speed of gametogenesis and reproductive success in parental gametophytes, and produced intraspecific and reciprocal interspecific crosses of female × male gametophyte parents isolated from the two species. We then compared the upper thermal resilience of microscopic and macroscopic sibling sporophytes in an exposure experiment over two weeks. The upper thermal limit of the sporophytes resulting from intraspecific crosses of the two species deviated by 1°C. In contrast, sporophytes from both interspecific hybrid crosses had a 2–3°C higher upper thermal tolerance than single species sporophytes, indicating heterosis for thermal tolerance. This phenotypic response appears partially sex-dependent in our study, with female parents being more important in determining the thermal-response phenotype than male parents. The presence of male gametophytes generally enhanced female reproductive success. Both gametogenesis rate and reproductive success differed among the types of reciprocal crosses. Although the interspecific crosses were artificial in an ecological sense, they may provide a tool for understanding the molecular basis of heterosis and thermal tolerance in kelps (e.g. by investigating species-specific gene expression), or for aquaculture breeding programmes against a background of rapid environmental change. [ABSTRACT FROM AUTHOR]

Kelp beds worldwide are under pressure from ongoing climate and environmental change. Along European coastlines increases in seawater temperature and changes in nutrient conditions occur where upwelling events are disrupted and also along eutrophicated coasts. In addition, seaweed responses to change may interact with seasonal daylength cycles. We performed a factorial experiment to examine the combined effects of seawater temperatures, nutrient regimes and photoperiod (long and short days) in order to better understand how latitudinal or seasonal differences in daylengths affect the sensitivity of transient microscopic kelp stages of Laminaria digitata from the North Sea to warming and eutrophication. While the optimal temperature range for vegetative gametophyte growth was 10°C-18°C under long summer photoperiod conditions, gametogenesis was induced at lower temperatures between 5°C and 15°C, with maximum sporophyte development under long photoperiods and enriched nutrient regimes, which represent local late spring conditions. Although gametogenesis was fastest at 10°C-15°C, sporophyte recruitment was highest at 5°C. As these particular early life cycle processes in L. digitata have different temperature optima, this may drive the seasonal cycle of recruitment in the field. Increasing summer temperatures due to global warming will increase gametophyte size due to enhanced vegetative growth and inhibition of gametogenesis. This will probably lead to delayed but enhanced recruitment of new sporophytes under cooler autumn to spring conditions over a wide geographical scale, preventing the formation of juvenile sporophytes under stressful summer conditions and possibly changing annual recruitment patterns. [ABSTRACT FROM AUTHOR]

Our objective for this study was to evaluate the influence of preindustrial and expected future atmospheric CO2 concentrations (280 μatm and 700 μatm pCO2, respectively) on different life-cycle stages of the kelp Laminaria hyperborea from Helgoland (Germany, North Sea). Zoospore germination, gametogenesis, vegetative growth, sorus formation and photosynthetic performance of vegetative and fertile tissue were examined. The contribution of external carbonic anhydrase (exCA) to C-supply for net-photosynthesis (net-PS) and the Chl a- and phlorotannin content were investigated. Female gametogenesis and vegetative growth of sporophytes were significantly enhanced under the expected future pCO2. rETR(max) and net-PS of young vegetative sporophytes tended to increase performance at higher pCO2. The trend towards elevated net-PS vanished after inhibition of exCA. In vegetative sporophytes, phlorotannin content and Chl a content were not significantly affected by pCO2. [ABSTRACT FROM AUTHOR]