Fertile Sporophytes Research Articles

Field survey of the phase and sex ratios of the brown alga Dictyota dichotoma (Dictyotales, Phaeophyceae) using sex‐specific molecular markers

SUMMARYDominance of one phase (i.e. diploid or haploid) has been reported in various isomorphic macroalgae; however, studies of the relative frequencies of the two phases are limited to a few taxa owing to the difficulty in identifying phases of sterile thalli. In Dictyota dichotoma, we have previously documented the dominance of fertile sporophytes. In the present study, the phase and sex ratios of sterile thalli of D. dichotoma were determined using newly developed sex‐specific markers. Differentially expressed genes were detected by transcriptome analyses of female and male gametophytic strains and a sporophytic strain aiming to identify candidate sex‐specific genes. These candidates were aligned with genome sequences on the sex chromosomes of Ectocarpus siliculosus and were identified as putative sex‐specific genes of D. dichotoma. Two primer sets were designed to discriminate phase and sex by the presence or absence of PCR products, and these markers were applied to examine the phase and sex ratios of D. dichotoma in various habitats and seasons. The proportion of sporophytes against 33–50 thalli examined in each of the five habitats at Ebisu Island, Shizuoka Prefecture was 88–100%, and the proportion of fertile sporophytes against total sporophytes (fertility proportion) varied among the habitats (0–54%). The fertile sporophytes were remarkably larger than the sterile sporophytes. At Tateyama, Chiba Prefecture, the dominance of sporophytes occurred in all seasons. The fertility proportion of sporophyte was relatively high from February (62.4%) to May (91.9%) and lower than 23% in other seasons. When artificial substrates were anchored in the middle of the D. dichotoma population, newly recruited thalli were all sporophytes, suggesting infrequent reproduction via tetraspores in nature. Based on these findings, we discuss potential fitness differences between the phases of D. dichotoma.

Exposure time modulates the effects of climate change-related stressors on fertile sporophytes and early-life stage performance of a habitat-forming kelp species

Understanding the impact of increases in pCO2 (OA) and extreme changes in temperature on marine organisms is critical to predicting how they will cope with climate change. We evaluated the effects of OA as well as warming and cooling trend temperature on early reproductive traits of Lessonia trabeculata, a bio-engineer kelp species. Sori discs were maintained for an exposure time (ET) of 3 (T3) and 7 (T7) days to one of two contrasting pCO2 levels (450 and 1100 μatm). In addition, at each pCO2 level, they were subjected to three temperature treatments: 15 °C (control), 10 °C (cool) and 19 °C (warm). Subsequently, we compared sorus photosynthetic performance (Fv/Fm), the number of meiospores released (MR) and their germination rate (GR) after 48 h of settlement, with values obtained from sori discs not exposed (DNE) to the treatments. The Fv/Fm measured for DNE was lower than at T3 and T7 at 10 and 15 °C but not at 19 °C. Regardless of temperature, we found no significant differences between MR measured at T0 and T3. MR at T7 was significantly lower at 19 °C than at 10 and 15 °C. We found only a significant reduction in MR in response to elevated pCO2 at T3. The GR of meiospores released by DNE and then maintained for 48 h to 19 °C decreased significantly by ~33% when compared with those maintained for the same time at 10 and 15 °C. A similar, but more drastic reduction (~54%) in the GR was found in meiospores released by sori discs exposed for T3 and maintained for 48 h to 19 °C. We suggest that OA and warming trend will threaten the early establishment of this species with further consequences for the functioning of the associated ecosystem.

Obtaining spores for the production of Saccharina latissima: seasonal limitations in nature, and induction of sporogenesis in darkness

When cultivating the kelp Saccharina latissima, knowledge on the availability of seeding material for the production is essential. Applying a spore seeding approach requires spores from the reproductive organs of the fertile sporophytes (sori). As sori are generally not present during the time of seeding in late summer, the production of spores (sporogenesis) can be artificially induced by removing the meristematic part of the sporophyte and keeping the sporophyte under short day, temperate, and nutrient-replete conditions. Only limited information is available on the effect of light intensities including darkness on the sporogenesis of S. latissima. This study examined the natural pattern of sporogenesis in S. latissima from Middelfart, Denmark, and the effect of four different light regimes (0, 20, 60, or 120 μmol photons m−2 s−1) on the artificial induction of sporogenesis in S. latissima. Natural reproductivity and availability of spores in Denmark peaked in early winter, with 86% of the population being reproductive in November. Reproductive material was available from October until late spring, but with a variable spore release from 11 × 103 to 1.2 × 106 spores cm−2 sori. The artificial induction of sporogenesis was optimal in darkness with > 90% of sporophytes developing sori after 49 days, with an average spore release density of 1.15 ± 0.38 × 106 spores cm−2 sori. The results confirmed that S. latissima in Denmark follows the general pattern of reproduction of S. latissima in North Atlantic regions and demonstrated for the first time that sporogenesis in S. latissima can be efficiently induced in darkness.

Seascape genetics of the stalked kelp Pterygophora californica and comparative population genetics in the Santa Barbara Channel.

We conducted a population genetic analysis of the stalked kelp, Pterygophora californica, in the Santa Barbara Channel, California, USA. The results were compared with previous work on the genetic differentiation of giant kelp, Macrocystis pyrifera, in the same region. These two sympatric kelps not only share many life history and dispersal characteristics but also differ in that dislodged P. californica does not produce floating rafts with buoyant fertile sporophytes, commonly observed for M. pyrifera. We used a comparative population genetic approach with these two species to test the hypothesis that theability to produce floating rafts increases the genetic connectivity among kelp patches in the Santa Barbara Channel. We quantified the association of habitat continuity and oceanographic distance with the genetic differentiation observed instalked kelp, like previously conducted for giant kelp. We compared both overall (across all patches) and pairwise (between patches) genetic differentiation. We found that oceanographic transit time, habitat continuity, and geographic distance were all associated with genetic connectivity in P.californica, supporting similar previous findings for M. pyrifera. Controlling for differences in heterozygosity between kelp species using Jost's DEST , we showed that global differentiation and pairwise differentiation were similar among patches between the two kelp species, indicating that theyhave similar dispersal capabilities despite theirdifferences in rafting ability. These results suggest that rafting sporophytes do not play asignificant role in effective dispersal of M. pyrifera at ecologically relevant spatial and temporal scales.

Stress physiology and reproductive phenology of Arctic endemic kelp Laminaria solidungula J. Agardh

The reproductive phenology and the responses of the early life history stages of the Arctic endemic kelp Laminaria solidungula to global climate change stressors were investigated. This species inhabits low-light areas at depths up to 18 m and under the canopy of other large kelps in shallower depths. Around Svalbard, fertile sporophytes had been collected in late spring to early autumn. This observation suggests that sporogenesis starts in late autumn, continuing throughout the winter and ending in early spring, or that reproduction may occur throughout the year, especially among deepwater population where diel fluctuating low light may be perceived as short days to induce sporogenesis. Fertile sporophytes were induced to release spores that were subsequently exposed to different temperatures (2, 7, 12, 17 °C) in isolation and in interaction with ultraviolet radiation (UVR). Results showed a high temperature affinity up to 12 °C for meiospore germination and growth, but its photosynthesis was low-light-adapted and susceptible to UVR. After 16 days of cultivation, germlings died under 17 °C and gametophyte size was largest at 12 °C. After 48-day cultivation under continuous daylight, gametogenesis was not observed. The projected ocean warming will unlikely negatively affect photosynthesis and growth of micro- and macrostages of L. solidungula. Conversely, the susceptibility of meiospores and gametophytes to high PAR and UVR can disrupt the completion of the life cycle. However, sporogenesis and gametogenesis, which are the key processes for the survival of this Arctic endemic species that requires low temperature and short days, can still occur in deepwater habitats.

Prevailing sea surface temperatures inhibit summer reproduction of the kelp Laminaria digitata at Helgoland (North Sea).

The impact of abiotic factors on kelp sporophyte reproduction has rarely been investigated. Laminaria digitata (Hudson) J.V. Lamouroux is one of the few summer fertile Laminaria species worldwide and reproduction is subjected to relatively high water temperatures. We investigated the impact of prevailing summer temperatures (~18°C in August) on the induction of sporangia, meiospore release, and germination at the island of Helgoland (North Sea). At Helgoland, fertile sporophytes are found between April and December with a maximum in late summer. While released meiospore numbers were constant between June and October, germination rates decreased significantly in summer. Short-term exposure of mature sori to 17°C-22°C induced a significantly higher meiospore release indicating enhancement of sporulation by elevated temperatures. Induction of sporangia on vegetative blade disks was not possible at 20°C, and fertility was only 20% at 18°C-19°C, but it was 100% in cool temperatures of 1°C-10°C. It was shown for the first time in a kelp species that "sporogenesis" is the life-cycle process with the narrowest temperature window compared to growth or survival of the sporophyte or reproduction, growth, and survival of the gametophyte. We incorporated several parameters (induction time, fertile area, and relative fertility) into a "Reproductive efficiency index." This indicates that sporogenesis of L.digitata is a cold-adapted process with an optimum at (5)-10°C. The results show that the population at Helgoland is at its reproduction limit despite the existence of other geographically more southerly located populations.

The photosynthetic performance of sterile and fertile sporophytes in a natural population of the fern Dryopteris affinis

We studied the photosynthetic performance of sterile and fertile sporophytes in a natural population of the fern Dryopteris affinis growing within a riparian forest (Central Italy) using chlorophyll (Chl) a fluorescence transients, the OJIP phase, where O is for the minimum fluorescence, P is for the peak (the maximum), and J and I are inflections. The "vitality" of the samples was assessed by the maximum quantum yield of primary photochemistry obtained indirectly from the fluorescence data (Fv/Fm); in the same way, the so-called performance index (PIABS) was obtained from fluorescence data. The photosynthetic performance (inferred from PIABS) of D. affinis changed significantly with the seasonal development of the fronds. The highest photosynthetic performance was recorded in the summer, corresponding to the period of spore release. The photosynthetic performance decreased in the winter, down to the minimal values of senescent fronds reached at the end of the seasonal cycle (May-June). On the whole, during the seasonal development, sterile and fertile fronds had a similar photosynthetic behaviour, as inferred from fluorescence data. At the end of spore maturation and dispersal (September-October), the fertile fronds showed somewhat lower photosynthetic performance than the sterile fronds, as revealed by PIABS. Being a long-lived fern, confined to humid and undisturbed sites in the Mediterranean, D. affinis deserves to be further investigated as a potential indicator of ecological continuity in Mediterranean riparian forests.

Thermal ecophysiology of gametophytes cultured from invasive Undaria pinnatifida (Harvey) Suringar in coastal California harbors

Given the ecological and economic impact of invasive species, knowledge of traits that allow the invasive kelp Undaria pinnatifida (Harvey) Suringar to expand its range is important for developing prevention and eradication efforts. Physiological mechanisms of thermotolerance may be important as they can influence both local and regional patterns of distribution and abundance. An established measure of thermotolerance is the heat shock response where heat shock proteins (Hsps) are produced in response to high temperature and protect cellular proteins from misfolding and degradation. It is hypothesized that Hsp production varies with developmental stage in many species, and U. pinnatifida, gametophytes have been shown to have a broader temperature range for growth and survival than sporophytes. Thus, we examined up-regulation of the hsp70 gene in gametophytes of invasive U. pinnatifida and compared it to values previously measured for sporophytes. Fertile sporophytes were collected from harbors in Monterey, Santa Barbara, Los Angeles, and San Diego, California, and gametophytes were cultured from them in the laboratory. After being grown in controlled light and temperature conditions, gametophyte tissue was exposed to a temperature gradient and quantitative real-time PCR was used to determine relative amounts of hsp70 transcript. Expression of hsp70 was extremely low in gametophytes. An investigation of gametophyte thermal ecophysiology using oxygen evolution measures and pulse-amplitude-modulated (PAM) fluorometry at a subset of temperatures revealed that U. pinnatifida gametophytes decrease dark-adapted yield with increasing temperature from 17 to 31 °C. Photosynthetic rates declined but remained positive at all incubation temperatures, while respiration increased with increasing temperatures up to 26 °C and then declined. Thus, U. pinnatifida gametophytes remained metabolically active at temperatures well above normal environmental conditions, indicating they are broadly thermotolerant. This tolerance, however, likely is conferred by mechanisms other than up-regulation of the hsp70 gene as no significant increases in expression related to temperature were observed in this study.

MORPHOLOGY AND MOLECULAR PHYLOGENY OF AUREOPHYCUS ALEUTICUS GEN. ET SP. NOV. (LAMINARIALES, PHAEOPHYCEAE) FROM THE ALEUTIAN ISLANDS(1).

A previously unknown species of kelp was collected on Kagamil Island, Aleutian Islands. The species can be easily distinguished from any known laminarialean alga: the erect sporophytic thallus is composed of a thin lanceolate blade attaining ∼2 m in height and ∼0.50 m in width, without midrib, and the edge of the blade at the transition zone is thickened to form a V-shape; the stipe is solid and flattened, slightly translucent, attaining ∼1 m in length; the holdfast is semidiscoidal and up to 0.15 m in diameter. Anatomically, the blade has the typical trumpet-shaped hyphae characteristic of the Chordaceae and derived foliose laminarialean species (i.e., Alariaceae/Laminariaceae/Lessoniaceae). No hair pits or mucilaginous structures were observed on the blade or stipe. No fertile sporophytes were collected, but abundant juvenile sporophytes were observed in the field. In the molecular phylogenetic analyses using chloroplast rbcL gene, nuclear ITS1-5.8S-ITS2 rDNA, and mitochondria nad6 DNA sequences, the new species (Aureophycus aleuticus gen. et sp. nov.) showed a closer relationship with Alariaceae of conventional taxonomy, or the "Group 1" clade of Lane etal. (2006) including Alaria and related taxa than with other groups, although the species was not clearly included in the group. Aureophycus may be a key species in elucidating the evolution of the Alariaceae within the Laminariales. Because of the lack of information on reproductive organs and insufficient resolution of the molecular analyses, we refrain from assigning the new species to a family, but we place the new species in a new genus in the Laminariales.

New records of benthic marine algae from the Canary Islands (eastern Atlantic Ocean): morphology, taxonomy and distribution

Abstract Four species of marine algae are reported from the Canary Islands for the first time. Our report of the western Atlantic Gelidiella setacea (Gelidiales, Rhodophyta) is the first from the eastern Atlantic Ocean. Pseudotetraspora marina (Tetrasporales, Chlorophyta) previously known on the eastern side of the Atlantic Ocean from temperate saltmarshes only, is now reported growing in the shallow sublittoral zone, the habitat in which tropical western Atlantic populations also occur. The presence of Lomentaria chylocladiella (Rhodymeniales, Rhodophyta) represents the first report in the Atlantic Ocean of a species previously thought to be endemic to the Mediterranean Sea. The record of the widely distributed Acrochaetium hallandicum (Acrochaetiales, Rhodophyta) was not unexpected. Specimens of A. hallandicum have vegetative cells with a single lobate parietal chloroplast with a single pyrenoid, a feature that among acrochaetioid algae occurs exclusively in the genus Colaconema (Colaconematales), and consequently the species is transferred to this genus. Fertile sporophytes are described for Gelidiella setacea, a species previously known only in its vegetative condition. Cruciately to irregularly divided sporangia are regularly arranged in transverse rows in stichidia laterally formed on the axes. The species is transferred to the genus Parviphycus on the basis of the morphology of the stichidia and the distichous pattern of apical division exhibited by the axes, both exclusive features of this genus.

Sexual compatibility and hybrid formation between the giant kelp species Macrocystis pyrifera and M. integrifolia (Laminariales, Phaeophyceae) in Chile

Two species of giant kelp inhabit the coast of Chile: Macrocystis integrifolia and M. pyrifera, representing important economic resources. As part of our efforts to domesticate these kelps for mariculture, and to obtain superior cultivars, we studied their biological relationship. Hybridization experiments with clonal gametophyte cultures showed reciprocal cross-fertility and produced fertile hybrid sporophytes with intermediate morphological characters. This hybridization potential in the laboratory contrasts with the persistence of two morphologically well-defined sister taxa in natural habitats on the Pacific coast of South America. We conclude that M. integrifolia and M. pyrifera are conspecific and speculate that unknown mechanisms support the co-existence of two morphologically distinct taxa on the subspecific level.

Morphological, ecological and genetic evidence for distinguishing Anastrophyllum joergensenii Schiffn. and A. alpinum Steph. (Jungermanniopsida: Lophoziaceae)

The liverwort Anastrophyllum joergensenii Schiffn., reported from Norway, Scotland, Alaska and the Sino-Himalaya is shown to consist of two distinct species, A. alpinum Steph. (treated before as a synonym of A. joergensenii) in the Himalaya, western China, Alaska and Scotland, and A. joergensenii Schiffn. s. str. in Norway, Scotland and western China. The two species are distinguished on genetic characters, size, leaf and perianth characters, and appear to have different ecological preferences. Anastrophyllum alpinum, although the more widespread of the two in Scotland, is there known only as non-fertile plants, whereas in the Sino-Himalaya fertile populations and sporophytes are not infrequent; in contrast, the rarer A. joergensenii can produce perianths in Scotland, Norway and Yunnan but androecia and sporophytes are unknown. The differences between the two are detailed and the ecology and distribution outlined.

Critical Contents of Nitrogen and Phosphorus for Sorus Formation in Four Laminaria Species

The critical contents of nitrogen and phosphorus for sorus formation in Laminaria angustata, L. japonica, L. ochotensis and L. religiosa were measured using cultured discs cut from the sporophytes. In L. japonica, fertile sporophytes were also used. Discs were cultured in Provasoli's enriched seawater or nutrient-poor seawater at 10 °C under 100 μmol photons m−2 s−1 of irradiance (12L:1̅2̅D̅). Regardless of the nutrient conditions in the culturemedia, the contents of nitrogen and phosphorus in the sorus part of discs and sporophytes of L. japonica were above 1.78 mgN cm−3 and 0.19 mgP cm−3 respectively, showing these values are critical for sorus formation in this species. Discs of L. angustata, L. ochotensis and L. religiosa in PES had higher contents of nitrogen and phosphorus in sorus parts than in L. japonica. The sorus area increased with accumulation of nitrogen or phosphorus in all species, but in L. angustata it was small despite the high contents of nitrogen and phosphorus, showing that this alga has a high nutritional cost in reproduction that might be a physiological adaptation to cold and nutrient-rich habitats.

Mariculture of Laminaria japonica (Laminariales, Phaeophyceae) using protoplast regeneration

SUMMARY The conditions for culture of viable protoplasts from Laminaria japonica were investigated and the regenerative processes were observed in detail. As a result of culturing at four water temperatures (5, 10, 15, and 18°C), we found that low water temperature was better for survival, division and rhizoidal formation of protoplast-derived cells. Only epidermis-derived protoplasts developed into normal sporophytes through a direct developmental process. Protoplast-derived cells divided after 5 days and 2–10 celled germlings formed the first rhizoids after 15 days. Only initial sporophytes with the first rhizoids grew to normal sporophytes with multilayered blades, stipes and holdfasts. When these young sporophytes were transplanted into the sea, they grew to normal fertile sporophytes.

The life history and phenology of Colpomenia peregrina (Scytosiphonales, Phaeophyceae) from Japan

Life-history studies on Colpomenia peregrina (Sauvageau) Hamel from Hokkaido, Japan, were carried out in field and culture. In the field, minute gametophytes appeared in late autumn, were fertile from late winter until early summer, and disappeared from mid-summer onward. Sporophytes were found from spring to autumn, bearing ectocarpoid plurilocular sporangia during spring and summer and unilocular sporangia in autumn. Fertile sporophytes were minute and discoid, 1–3 mm in diameter, epilithic or epiphytic, consisting of prostrate and erect filaments, and bearing ascocysts. In culture, zygotes and unfused gametes developed into prostrate sporophytes that were similar in morphology to field-collected sporophytes. These sporophytes produced plurilocular sporangia at 5–20°C, mainly in long-day photoperiods, and unilocular sporangia mainly at 15 and 20°C, both in short days. Spores released from plurilocular sporangia on the sporophytes developed again into sporophytes; however, unispores developed into saccate gametophytes. In cultures from field sporophytes, saccate thalli identical with C. peregrina were obtained. Seasonality of gametophytes of C. peregrina can be explained by photoperiodic and temperature responses in sporangial production of sporophytes.

CROSSING EXPERIMENTS, LIPID COMPOSITION, AND THE SPECIES CONCEPT IN ECTOCARPUS SILICULOSUS AND E. FASCICULATUS (PHAEOPHYCEAE, ECTOCARPALES)1

ABSTRACT Sporophytes of Ectocarpus siliculosus (Dillwyn) Lyngbye and E. fasciculatus Harvey were collected in the vicinity of Roscoff Brittany, France. Gametophytes derived from meiospores were used for intra‐ and interspecific crosses. Intraspecific gamete combinations gave viable zygotes, which developed into fertile sporophytes. Interspecific crosses were unsuccessful. Gamete fusions did not occur between female gametes of E. fasciculatus and male gametes of E. siliculosus. Hybrid zygotes were formed in the reciprocal combination but died soon after germination. We conclude that the two species of Ectocarpus at Roscoff represent distinct taxonomic entities, which are separated by pre‐ and postzygotic compatibility barriers. These biological findings are confirmed by the differential occurrence of the chemotaxonomic marker betaine‐lipid diacetylglycerylhydroxymethyltrimethyl‐β‐alanine, which is present in our cultures of E. fasciculatus but absent in E. siliculosus.

Phenology and photosynthetic activity in sterile and fertile sporophytes of Dryopteris filix-mas (L.) Schott.

This study describes the phenology of sporophytes of the fern Dryopteris filix-mas in relation to whole plant development. Sterile and fertile potted sporophytes were set out at an exposed site and the seasonal development of the fronds was measured from the commencement of unfolding, through the phase of increasing length, up to discoloration. The physiological activity of the fronds was determined by measuring photosynthetic gas exchange. The fronds of sterile sporophytes unfolded in April, about a week earlier than those of fertile plants, but the colour had already begun to turn in September and their life span was 1-2 months shorter. However, between mid-June and the end of August the sterile sporophytes put out several sets of new fronds: these overwintered without changing color and were still photosynthetically active in the following spring. All types of fronds were fully expanded 1-2 months from the beginning of unfolding and, with a natural supply of CO2, had similar maximum net photosynthetic rates of 8-9 μmol/m2 · s. The decline in photosynthetic performance began before symptoms of senescence were visible and was due to decreased efficiency of the mesophyll. It is concluded that the phenology of D. filix-mas changes with transition from the sterile to the fertile phase. Whereas fertile sporophytes are genuinely summergreen, the sterile sporophytes with their summer fronds remain green throughout the winter and should therefore be termed semi-evergreen. The formation of overwintering summer shoots clearly extends the period of photosynthetic productivity of sterile sporophytes.

SPOROCARPS OF AZOLLA MEXICAN A PRESL I. ISOLATION AND PURIFICATION OF MEGASPOROCARPS AND MICROSPORANGIA*

SUMMARY Azolla mexicana Presl, a heterosporous aquatic fern, consistently sporulates under routine culture conditions, producing both megasporocarps and microsporocarps in abundance. As a prelude to studies on germ plasm preservation, gametogenesis, embryogenesis, and efforts to establish interspecific hybrids, methods were developed to isolate and separate the reproductive structures. Fertile sporophytes were agitated to release the sporocarps. During this process mature microsporocarps ruptured to release microsporangia. A series of stainless steel sieves, ranging from 1130 to 200 μm in pore size, was used to separate megasporocarps and microsporangia from sporophyte debris and immature microsporocarps. Purified preparations of megasporocarps and microsporangia were obtained using sorbitol step gradients.

Copper effects in early stages of the Kelp, Laminaria saccharina

The effects of copper on the Laminaria saccharina were examined by culturing plants in 5, 10, 50, 100 and 500 μg Cu l −1 filtered and filtered plus uv treated seawater media. Fertile sporophytes were used to determine effects of copper on meiospore release. Untreated meiospores also were obtained and cultured in seawater media, followed by copper additions at various stages of development through the early sporophyte stage. The release of meiospores from copper-treated sorus materials was reduced by concentrations ≧ 50 μg l −1 . Settlement and germination of meiospores were not affected by concentrations up to 500 μg l −1. Development of gametophytes and gametogenesis were delayed in concentrations ≧ 50 μg l −1 . Growth of sporophytes, in terms of length increase or relative blade area increase, was inhibited with increasing concentrations > 10 μg l −1.

Aspects of the reproductive phenology ofSaccorhiza dermatodea(Phaeophyta, Laminariales) in Newfoundland

The phenology of Saccorhiza dermatodea is described from the island of Newfoundland, Canada, and the possible roles of daylength and temperature in controlling the annual cycle are examined. New sporophytes first appear in late March or early April and continue to appear until late May or early June; there is, however, considerable year-to-year and local variation in the timing of the first appearance of the new sporelings. The reproductive maturation of the sporophytes is rapid, commencing at the end of September. Within 2 weeks of the first appearance of sorus tissue, 100% of the population is fertile. Fertile sporophytes may persist until July of the following year. Viable zoospores were obtained from the beginning of the fertile period until April of the following year. The annual cycle is correlated with seasonal temperature and daylength fluctuations. The new generation of sporophytes appears when water temperature is at its winter low, nutrients are maximal and daylength and light intensity are inc...