Nonnative Gracilaria vermiculophylla tetrasporophytes are more difficult to debranch and are less nutritious than gametophytes

Theory predicts that the maintenance of haplodiplontic life cycles requires ecological differences between the haploid gametophytes and diploid sporophytes, yet evidence of such differences remain scarce. The haplodiplontic red seaweed Gracilaria vermiculophylla has invaded the temperate estuaries of the Northern Hemisphere, where it commonly modifies detrital and trophic pathways. In native populations, abundant hard substratum enables spore settlement, and gametophyte:tetrasporophyte ratios are ~40:60. In contrast, many non‐native populations persist in soft‐sediment habitats without abundant hard substratum, and can be 90%–100% tetrasporophytic. To test for ecologically relevant phenotypic differences, we measured thallus morphology, protein content, organic content, “debranching resistance” (i.e., tensile force required to remove a branch from its main axis node), and material properties between male gametophytes, female gametophytes, and tetrasporophytes from a single, nonnative site in Charleston Harbor, South Carolina, USA in 2015 and 2016. Thallus length and surface area to volume ratio differed between years, but were not significantly different between ploidies. Tetrasporophytes had lower protein content than gametophytes, suggesting the latter may be more attractive to consumers. More force was required to pull a branch from the main axis of tetrasporophytes relative to gametophytes. A difference in debranching resistance may help to maintain tetrasporophyte thallus durability relative to gametophytes, providing a potential advantage in free‐floating populations. These data may shed light on the invasion ecology of an important ecosystem engineer, and may advance our understanding of life cycle evolution and the maintenance of life cycle diversity.     

Evidence of reproductive cost in the triphasic life history of the red alga Gracilaria chilensis (Gracilariales,Rhodophyta)

The extent of changes in basic physiological and demographic traits associated with reproduction was investigated in the highly cultivated haploid–diploid red alga, Gracilaria chilensis. Sixty individuals bearing vegetative and reproductive fronds collected in the natural population of Niebla (39°52′?S, 73°23′?W), in Chile, were cultivated under controlled culture conditions. Our results demonstrated that vegetative fronds have a higher survival rate and a better growth rate than reproductive ones irrespective of the type of individual analyzed (male gametophyte, female gametophyte, and tetrasporophyte). Moreover, the reproductive fronds clearly showed a decrease in photosynthetic activity compared to non-reproductive ones. In males and tetrasporophytes, the photosynthetic reduction in reproductive individuals could be explained by a physical effect of reproductive structure development as well as spores release, disrupting the continuity of the photosynthetic cortical tissues. Translocation of photoassimilates from nearby vegetative tissue or the previous accumulation of photosynthetic products seems to be a prerequisite for reproductive structure development in this species. Altogether, these results document for the first time in G. chilensis that reproduction has a strong physiological effect on male, female, and tetrasporophyte fronds. This trade-off between reproduction, growth, and survival suggest the existence of reproductive costs in the life history of G. chilensis.     

PHOTOSYNTHETIC RESPONSES TO LIGHT AND TEMPERATURE OF THE HETEROMORPHIC MARINE ALGA MASTOCARPUS PAPILLATUS (RHODOPHYTA)1

Physiological differentiation of the heteromorphic life-history phases of the red alga Mastocarpus papillatus (C. Agardh) Kützing was assessed. Photosynthetic responses to light and temperature of the erect, foliose gametophyte were compared to those of the crustose tetrasporophyte. Plants of both phases were collected from four locations on the Pacific coast of Baja California, Mexico, and California, USA, between 32–4l° N latitude. Within each location, the chlorophyll-specific, light-saturated photosynthetic rates of gametophytes were generally five times greater than those of tetrasporophytes. Initial slopes of photosynthesis: irradiance curves were greater for gametophytes than for tetrasporophytes. The crust and the blade from each location were similar with respect to dark respiration rates. For tetrasporophytes from all locations, the photosynthetic temperature optima were between 12–15° C. The photosynthetic temperature optima for gametophytes ranged from 15–17° C for plants from Trinidad Head (41° N) to 22–25° C for plants from Punta Descanso (32° N). Both gametophytes and tetrasporophytes from the northernmost location had significantly higher photosynthetic rates than the same phase from the other three locations. The photosynthetic responses to light support models for the life history in which life history phases have different ecological roles. The gametophyte, thought to be specialized for rapid growth and competition, may allocate more resources to photosynthetic machinery, hence the higher maximum photosynthetic rates. The tetrasporophyte, thought to be specialized for resistance to herbivores, may allocate more resources to structural or chemical defenses in preference to photosynthetic machinery. Consequently, the tetrasporophyte has lower photosynthetic capacity.     

Attachment strength differs amongst life‐history stages of an intertidal,isomorphic red alga

Complex haploid‐diploid life cycles amongst marine organisms may be maintained by ecological differences in life‐history phases. For red algal species within the Gigartinaceae, such differences may be driven, in part, by different cell wall composition and resultant biomechanical strengths of haploid and diploid phases. A field experiment tested the attachment strengths of gametophytes and tetrasporophytes of the isomorphic red alga, Chondrus verrucosus (with comparisons of fertile and vegetative fronds, with and without natural tissue damage across three wave‐exposed sites). Seventy‐nine percent of all fronds broke at the stipe‐holdfast junction. There were significant differences in attachment strength (break force and break stress), but not gross morphology (frond length, number of branch axes, wet weight and cross‐sectional area of fronds that dislodged at the stipe‐holdfast junction) of life‐history phases, with tetrasporophytes exhibiting weaker tissue strength and attachment, and therefore greater susceptibility to dislodgement by waves. However, fertility and tissue damage did not consistently influence dislodgement in pull‐to‐break tests simulating the effects of single waves. The ecological and evolutionary consequences of greater susceptibility to dislodgement of tetrasporophytes (relative to gametophytes) warrant further investigation.     

Genetic analysis and marker assisted identification of life phases of red alga <Emphasis Type="Italic">Gracilaria corticata</Emphasis> (J. Agardh)

The present study firstly reports the cytological and molecular marker assisted differentiation of isomorphic population of Gracilaria corticata (J. Agardh) with inter and intra-phasic genetic diversity analysis using ISSR markers. The genetic diversity of inbreeding population of G. corticata as determined in terms of percentage of polymorphic loci (PPL), average heterozygosity (He) and Shannon’s Weaver index (I) were 59.80, 0.59 and 1.21, respectively. The inter-phasic pair-wise average polymorphism were found to be 31.6% between male and female, 24.0% in male and tetrasporophyte and 25.3% in female and tetrasporophyte. The intra-phasic average polymorphisms were calculated as a maximum of 5.5% between females, 4.2% between males and the lowest 2.4% between tetrasporophytes. The primer 10 generated a marker of 800 bp specific to male and 650 bp to female gametophyte, while the primer 17 generated a marker of 2,500 bp specific to tetrasporophyte. Both the UPGMA based dendrogram and PCA analysis clustered all the three life phases differentially as distinct identity. Cytological analysis by chromosome count revealed 24 chromosomes in both haploid male and female gametophytes (N) and 48 for diploid (2 N) tetrasporophyte further confirming their genetic distinctness. The life phase specific markers reported in this study could be of help in breeding programmes where differentiation of life phases at the early developmental stages is crucial.     

Differences in response between haploid and diploid isomorphic phases of Gracilaria verrucosa (Rhodophyta: Gigartinales) exposed to artificial environmental conditions

This study tests the responses of juvenile gametophytes and tetrasporophytes (holdfast stage) of the isomorphic alga Gracilaria verrucosa under different environmental conditions.Estimations of survival and growth of holdfasts of haploid and diploid juvenile individuals were performed in natural sea-water and artificial culture medium, and under stringent conditions using lead as a toxin and ultra violet radiation as a mutagen. Results indicate that (i) holdfasts of haploid juveniles grow better than diploids in non-optimal medium conditions; (ii) holdfasts of diploid juveniles have a better tolerance to lead than haploids; and (iii) slight advantage of holdfasts of diploid juveniles grow better than haploids under U.V. radiation.     

A review of the life history,reproduction and phenology of Gracilaria

The basic life history of the red alga Gracilaria is of the three-phase Polysiphonia type but a number of species show deviations. Plants can bear both gametangia and tetrasporangia, either on separate parts of the thallus or on the same. Explanations include the in situ germination of tetraspores (allowing gametophytic thalli to be epiphytic on tetrasporophytes), the coalescence of spores or developing discs (resulting in chimaeras), mitotic recombination during cell division in the mature diploid thallus (resulting in patches of diploid male and female cells on the tetrasporophyte), a mutation eliminating the repression of female expression allowing haploid male plants to be bisexual and initial failure of cell walls to form during the development of tetraspores. Polyploids can be produced from plants with diploid gametangia. The sexes and phases are usually morphologically identical but gametophytes or their parts may be smaller. The growth rates of the sexes may differ and diploid juveniles may survive better than haploid. Neither polyploidy nor hybridization results in superior growth. The sex ratio is probably 1:1 but females may appear to be more abundant. Diploid and haploid phases are usually either about equal or diploids predominate, often depending on the type of substratum. At high latitudes reproduction peaks in late summer whereas in the tropics it may be high all year. In temperate regions growth rate is fastest and biomass highest in late summer; in the tropics peak biomass is mainly in the winter. Spermatia are effective for only a few h. Spores vary in size around 25 \m, diploid ones usually being larger. Cystocarps or tetrasporangia in the field may not currently be releasing spores. In the laboratory spore release shows a diurnal rhythm, peaking during the night or day according to the species. All the above attributes are potentially important in planning and executing Gracilaria cultivation.     

A simulation model for an Iridaea splendens (Gigartinales,Rhodophyta) population in Vancouver,Canada

Populations of Iridaea splendens at Brockton Point, Stanley Park, Vancouver, Canada were observed to alternate in dominance between the gametophytic phase in summer and tetrasporophytic phase in winter. The mechanism regulating this alternation is not clear. Using a matrix projection model to simulate population growth, we show that this alternation is possible if there are differential survival and recruitment rates of the two phases in summer and winter. Sensitivity and elasticity analyses indicate the relative importance of perennation vs. recruitment. Recruitment from tetrasporophytes and from gametophytes both contribute about 25% to the population growth. Perennation among gametophytes is more important than among tetrasporophytes. The implication of this is that if this population is to be harvested, more tetrasporophytes can be harvested than gametophytes without resulting in the depletion of the resource. This is simulated in the matrix model by comparing the relative effects on population growth of increasing the mortality rate of the perennation phase of tetrasporophyte and gametophyte by 50 to 75%, and increasing recruitment rate in either phase, from summer to winter or from winter to summer.Dedicated to Dr T. Bisalputra of the University of British Columbia on the occasion of his retirement.     

ADDITIONAL EVIDENCE FOR ECOLOGICAL DIFFERENCES AMONG ISOMORPHIC REPRODUCTIVE PHASES OF IRIDAEA LAMINARIOIDES (RHODOPHYTA: GIGARTINALES)1

Gametophytes are more abundant thou sporophytes in wave exposed rocky intertidal populations of Iridaea laminarioides Bory in Central Chile. In this study we experimental tested the differential effects of selected ecological factors on karyologically different life history phases. In the field, gametophytes dominated at higher elevation and during summer; tetrasporophytes were most abundant low in the intertidal and during the fall. Laboratory responses correlated with these patterns. Gametophytes exhibited greater desiccation tolerance than tetrasporophytes. Optimum growth of gametophytes occurred at higher temperatures (20°C) and longer photoperiods (16:8 h LD) than sporophytes (15°C and 12:12 h LD). Grazing preferences changed with the developmental stage of the alga, but all herbivores tested had increased preference for diploid tissues as compared to haploid. Number of spores produced with respect to total plant surface, or total rocky surface, or settlement of spores and their germination rate did not show significant differences between phases but showed great variability in space and time. Spontaneous spore release, however, was always higher in cystocarpic than in tetrasporangial thalli. Such a combination of results suggests that some real ecological differences exist between the two life history phases of I. laminarioides. Such ecological differences permit a prediction of vertical and temporal patterns of distribution for both phases. Horizontal patterns of distribution cannot be explained because the several selection factors probably interact differently in various habitats.     

Ploidy determination of three Kappaphycus alvarezii strains (Rhodophyta, Gigartinales) by confocal fluorescence microscopy

Kappaphycus alvarezii is a red alga that is commercially important as a source of carrageenan. Since K. alvarezii presents large phenotypic plasticity and rarely develops reproductive structures in culture, identification of gametophytic and tetrasporophytic phases in cultivation systems are difficult. The aim of this study was to determine the ploidy of three K. alvarezii strains previously identified as brown “tetrasporophyte”, brown “gametophyte” and “Edison de Paula” (EP). Nuclei from these strains were stained with DAPI, and analyzed using confocal fluorescence microscopy and ImageJ software. The brown “tetrasporophyte” had the highest nuclear fluorescence intensity, consistent with a diploid tetrasporophyte (2N). The brown “gametophyte” and “EP” strains had nuclear fluorescence intensities of 55.78% and 57.10% in relation to the tetrasporophyte, respectively, consistent with haploid gametophytes (N). The present study demonstrated that this technique can be used as a rapid and effective tool to distinguish between haploid (gametophytic) and diploid (tetrasporophytic) plants of K. alvarezii, in addition to help identify new strains developed through alterations of ploidy level.     

Ploidy Distribution of the Harmful Bloom Forming Macroalgae Ulva spp. in Narragansett Bay,Rhode Island,USA, Using Flow Cytometry Methods

Macroalgal blooms occur worldwide and have the potential to cause severe ecological and economic damage. Narragansett Bay, RI is a eutrophic system that experiences summer macroalgal blooms composed mostly of Ulva compressa and Ulva rigida, which have biphasic life cycles with separate haploid and diploid phases. In this study, we used flow cytometry to assess ploidy levels of U. compressa and U. rigida populations from five sites in Narragansett Bay, RI, USA, to assess the relative contribution of both phases to bloom formation. Both haploid gametophytes and diploid sporophytes were present for both species. Sites ranged from a relative overabundance of gametophytes to a relative overabundance of sporophytes, compared to the null model prediction of √2 gametophytes: 1 sporophyte. We found significant differences in cell area between ploidy levels for each species, with sporophyte cells significantly larger than gametophyte cells in U. compressa and U. rigida. We found no differences in relative growth rate between ploidy levels for each species. Our results indicate the presence of both phases of each of the two dominant bloom forming species throughout the bloom season, and represent one of the first studies of in situ Ulva life cycle dynamics.     

Demographic models to simulate the stable ratio between ecologically similar gametophytes and tetrasporophytes in populations of the Gigartinaceae (Rhodophyta)

In populations of the Gigartinaceae (Rhodophyta), gametophytes often predominate numerically over tetrasporophytes. Several hypotheses have been proposed to explain this dominance, based on the usually implicit assumption that the stable ratio between gametophytes and tetrasporophytes (G:T ratio) should be 1 if both reproductive phases are ecologically similar. We developed demographic models to test this assumption, for which we considered that both phases are ecologically similar. Defining ecologic similarity for most demographic rates is relatively straightforward, except for rates of spore output. The first set of models considered the same spore output per thallus of both phases as representing ecologic similarity. Model iterations led to stable G:T ratios of 1 for triennial and for perennial thalli, regardless of the initial G:T ratio, but not for annual thalli with initial G:T ratios different from 1. However, equal spore output may not represent ecologic similarity, due to size differences between carpospores and tetraspores. The second set of models considered the lowest possible spore output for each phase, according to the life history of this family: only one carposporangium, with one carpospore, is produced from every two gametophytes and only one tetrasporangium, with four tetraspores, is produced by every tetrasporophyte. Model iterations led to stable G:T ratios of 2.8 for most cases, a ratio of 1 being obtained only every 2 years for annual thalli with an initial G:T ratio of 1. Increasing absolute spore output, without altering the relative output between phases and incorporating density-independent mortality through a matrix model, given the same mortality rate for both phases, did not modify results. We suggest that the combination of both modeling and field research may uncover more rapidly than otherwise the most relevant ecologic differences between phases, if any, that underlie the G:T ratio observed for a given population.     

Gametophyte-sporophyte coalescence in populations of the intertidal carrageenophyte <Emphasis Type="Italic">Mazzaella laminarioides</Emphasis> (Rhodophyta)

Mazzaella laminarioides has consistently been reported as a typical coalescent/clump species with a triphasic life history of the Polysiphonia-type in which the haploid gametophyte is the predominant phase with respect to the diploid sporophyte. Preliminary observations of intertidal populations revealed that, in some instances, cystocarpic and tetrasporic fronds emerged from the same clump (G-T clumps), implying a coalescent process of haploid and diploid thalli by fusion of their corresponding adjacent basal holdfasts. Population surveys at three sites in Coliumo Bay, central Chile, were carried out to characterize frond demography as well as to asses the frequency of gametophyte-tetrasporophyte (G-T) coalescence. Visual and resorcinol methods were employed to determine the phases of the fronds collected over central transects of 15 randomly sampled clumps. Coalescence of G-T clumps was infrequent, with gametophytes dominating over tetrasporophyte thalli.     

Cultivation of the red alga Chondracanthus chamissoi: sexual reproduction and seedling production in culture under controlled conditions

The red alga Chondracanthus chamissoi (Gigartinales) is endemic to the southern-central region of South America. In the Pacific Ocean, it is distributed from north-central Peru to Chiloe Island. This species is of economic importance because it is edible and used for carrageenan production. The tetrasporophyte phase was grown in the laboratory, obtaining male and female gametophytes that were incubated under different photoperiod, pH, salinity and temperature conditions. These gametophytes developed and generated reproductive structures that led to in vitro maturation. Subsequently, fertilisation occurred and formation of cystocarps was observed. Finally, carpospores were released and the formation of sporophytes completed the life history of this species under laboratory conditions. Reproductive phase growth rates were recorded for each of the different culture conditions used. Sporophytes reached the highest daily growth rate (22%), while gametophyte’s daily growth rate was slower (9%). This research confirms, in vitro, the assumption that C. chamissoi has a sexual triphasic life history Polysiphonia type with isomorphic gametophytes and tetrasporophytes. The development of the complete life history took 20 months in the laboratory.     

Seasonal variation in the mode of reproduction of Ulva intestinalis in a brackish water environment

We explored the reproductive modes of Ulva intestinalis in the inner part of the Baltic Sea during three consecutive years by using five microsatellite loci to estimate the relative abundance of diploid sporophytes and haploid gametophytes. Our results suggest that both diploid sporophytes and haploid gametophytes occur regularly in the Baltic Sea. The ratio of haploid to diploid individuals changes with seasons. Sporophytes are more abundant than gametophytes throughout the year, but the proportion of haploids increases from 10% in early summer to 35% in September. The over-wintering takes primarily place as diploid spores released by sporophytes. The sporophytes appear to reproduce both sexually and asexually in the Baltic Sea, since clones were found for this life phase. The fraction of individuals which belonged to an apparent diploid clone was higher in spring (62%) than in autumn (33%). We also found evidence for asexual clones in haploid gametophytes. The presence of both diploid and haploid individuals and the pattern of genetic and genotypic diversity provide evidence of sexual reproduction in the Baltic Sea. Thus the sporophytes and gametophytes do not function as two reproductively separate units. Compared with many other algal species with a reduced reproductive cycle in low salinity, U. intestinalis differs by having a multitude of reproductive modes also in the brackish water Baltic Sea, which can in part explain the dynamic propagation and high adaptability of the species.     

Seasonal variation in the mode of reproduction of Ulva intestinalis in a brackish water environment

We explored the reproductive modes of Ulva intestinalis in the inner part of the Baltic Sea during three consecutive years by using five microsatellite loci to estimate the relative abundance of diploid sporophytes and haploid gametophytes. Our results suggest that both diploid sporophytes and haploid gametophytes occur regularly in the Baltic Sea. The ratio of haploid to diploid individuals changes with seasons. Sporophytes are more abundant than gametophytes throughout the year, but the proportion of haploids increases from 10% in early summer to 35% in September. The over-wintering takes primarily place as diploid spores released by sporophytes. The sporophytes appear to reproduce both sexually and asexually in the Baltic Sea, since clones were found for this life phase. The fraction of individuals which belonged to an apparent diploid clone was higher in spring (62%) than in autumn (33%). We also found evidence for asexual clones in haploid gametophytes. The presence of both diploid and haploid individuals and the pattern of genetic and genotypic diversity provide evidence of sexual reproduction in the Baltic Sea. Thus the sporophytes and gametophytes do not function as two reproductively separate units. Compared with many other algal species with a reduced reproductive cycle in low salinity, U. intestinalis differs by having a multitude of reproductive modes also in the brackish water Baltic Sea, which can in part explain the dynamic propagation and high adaptability of the species.     

Evolution and maintenance of haploid–diploid life cycles in natural populations: The case of the marine brown alga Ectocarpus

The evolutionary stability of haploid–diploid life cycles is still controversial. Mathematical models indicate that niche differences between ploidy phases may be a necessary condition for the evolution and maintenance of these life cycles. Nevertheless, experimental support for this prediction remains elusive. In the present work, we explored this hypothesis in natural populations of the brown alga Ectocarpus. Consistent with the life cycle described in culture, Ectocarpus crouaniorum in NW France and E. siliculosus in SW Italy exhibited an alternation between haploid gametophytes and diploid sporophytes. Our field data invalidated, however, the long‐standing view of an isomorphic alternation of generations. Gametophytes and sporophytes displayed marked differences in size and, conforming to theoretical predictions, occupied different spatiotemporal niches. Gametophytes were found almost exclusively on the alga Scytosiphon lomentaria during spring whereas sporophytes were present year‐round on abiotic substrata. Paradoxically, E. siliculosus in NW France exhibited similar habitat usage despite the absence of alternation of ploidy phases. Diploid sporophytes grew both epilithically and epiphytically, and this mainly asexual population gained the same ecological advantage postulated for haploid–diploid populations. Consequently, an ecological interpretation of the niche differences between haploid and diploid individuals does not seem to satisfactorily explain the evolution of the Ectocarpus life cycle.     

Biomass variation and reproductive phenology of Gracilaria gracilis in a Patagonian natural bed (Chubut, Argentina)

The biomass variation and the reproduction of the natural Gracilaria gracilis bed in Bahía Bustamante (Patagonia, Argentina) were analyzed for 2 years, with the aim of determining the present situation of the population for an updated status overview; establishing the relevant features of the temporal variation in both biomass and reproductive states in relation to environmental factors, epiphytes and associated algae; and assessing carpospore availability for future spore-culture development. Field measurements and sampling were performed monthly between March 2006 and February 2008. In both years, G. gracilis biomass presented marked seasonal variations, with a minimum in winter and a maximum in late spring and in summer. During both years, coexistence of the three life-cycle phases was found, with dominance of tetrasporophytes. Two data sets from individuals originated from sexual reproduction (tetraspores and carpospores) and from asexual reproduction by thallus fragmentation were analyzed separately. In the fragmentation fraction, tetrasporophyte frequencies remained higher than those for gametophytes. However, in the spore-originated fraction, a generation ratio close to 0.5 was observed. Female gametophytes bearing cystocarps were always present, with a maximum in summer and autumn. Biological data were related to environmental factors by means of canonical correspondence analysis (CCA). The first year was characterized by higher biomass values of G. gracilis and Undaria pinnatifida, lower epiphytism, larger Gracilaria thalli and greater proportion of mature tetrasporophytes and gametophytes. The second year was characterized by a high proportion of Gracilaria vegetative thalli and high epiphyte density. The best time to obtain spores from cystocarpic thalli would be in summer and early autumn.     

Spatial and temporal variations of Chondrus crispus (Gigartinaceae, Rhodophyta) carrageenan content in natural populations from Galicia (NW Spain)

Qualitative and quantitative differences in carrageenan composition of gametophytes and tetrasporophytes of Chondrus crispus were observed in this study. Carrageenans in gametophytes belong to the kappa family (κ-, ι-, ν- and μ-carrageenan). The dominant fractions were κ- and ι-carrageenan (more than 50% of the total carrageenans). In tetrasporophytes, the presence of λ-carrageenan was confirmed. Carrageenan content in gametophytes (37.4?±?1.68% DW) was higher than in tetrasporophytes (29.13?±?0.76% DW). Spatial and temporal variation in carrageenan content in both life cycle phases appears to be related mainly to seawater and air temperatures, insolation, water movement and desiccation. The highest values of carrageenan content were recorded in those localities where higher values of precipitation, wind speed or water movement occurred. A bimodal temporal pattern on carrageenan content was observed. Fronds showed a high carrageenan content in spring and autumn. During these seasons, the content was over 40% in gametophytes and 30% in the tetrasporophytes. In summer and winter, these values down in both life cycle phases below 30%. In general the highest carrageenan contents were related to highest seawater temperatures. On the contrary, high air temperature and high insolation appeared to be unfavourable for carrageenan production. GLM models were obtained to predict carrageenan production from natural C. crispus populations, along Galician coast.     

Testing the effects of heterozygosity on growth rate plasticity in the seaweed Gracilaria chilensis (Rhodophyta)

Heterozygosity has been positively associated with fitness and population survival. However, the relationship between heterozygosity and adaptive phenotypic plasticity (i.e., plasticity which results in fitness homeostasis or improvement in changing environments) is unclear and has been poorly explored in seaweeds. In this study, we explored this relationship in the clonal red seaweed, Gracilaria chilensis by conducting three growth rate plasticity experiments under contrasting salinity conditions and by measuring heterozygosity with five microsatellite DNA markers. Firstly, we compared growth rate plasticity between the haploid and diploid phases. Secondly, we compared growth rate plasticity between diploids with different numbers of heterozygous loci. Finally, we compared growth rate plasticity between diploid plants from two populations that are expected to exhibit significant differences in heterozygosity. We found that, (i) diploids displayed a higher growth rate and lower growth rate plasticity than haploids, (ii) diploids with a higher number of heterozygous loci displayed lower growth rate plasticity than those exhibiting less heterozygosity, and (iii) diploid sporophytes from the population with higher heterozygosity displayed lower growth rate plasticity than those with lower heterozygosity. Accordingly, this study suggests that heterozygosity is inversely related to growth rate plasticity in G. chilensis. However, better genetic tools in seaweeds are required for a more definitive conclusion on the relationship between heterozygosity and phenotypic plasticity.