Sporophyte and gametophyte generations differ in their thermotolerance response in the moss Microbryum

BACKGROUND AND AIMS: Actively growing post-embryonic sporophytes of desert mosses are restricted to the cooler, wetter months. However, most desert mosses have perennial gametophytes. It is hypothesized that these life history patterns are due in part to a reduced thermotolerance for sporophytes relative to gametophytes. METHODS: Gametophytes with attached embryonic sporophytes of Microbryum starckeanum were exposed whilst desiccated to thermal episodes of 35 degrees C (1 hr), 55 degrees C (1 hr), 75 degrees C (1 hr) and 75 degrees C (3 hr), then moistened and allowed to recover for 35 d in a growth chamber. KEY RESULTS: All of the gametophytes survived the thermal exposures and produced protonemata, with the majority also producing shoot buds. Symptoms of gametophytic stress (leaf burning and discoloration of entire shoots) were present in lower frequencies in the 55 degrees C exposure. Sporophyte resumption of growth and maturation to meiosis were significantly negatively affected by thermal treatment. Not a single sporophyte exposed to the two higher thermal treatments (75 degrees C for 1 h and 75 degrees C for 3 h) survived to meiosis, and those sporophytes exposed to 75 degrees C that survived to the post-embryonic phenophase took significantly longer to reach this phase. Furthermore, among the thermal treatments where some capsules reached maturity (35 degrees C and 55 degrees C), maternal shoots that produced a meiotic capsule took longer to regenerate through protonemata than maternal shoots aborting their sporophyte, suggestive of a resource trade-off between generations. CONCLUSIONS: Either (1) the inherent sporophyte thermotolerance is quite low even in this desert moss, and/or (2) a gametophytic thermal stress response controls sporophyte viability.     

Developing sporophytes transition from an inducible to a constitutive ecological strategy of desiccation tolerance in the moss Aloina ambigua: effects of desiccation on fitness

Background and Aims Two ecological strategies of desiccation tolerance exist in plants, constitutive and inducible. Because of difficulties in culturing sporophytes, very little is known about desiccation tolerance in this generation and how desiccation affects sexual fitness.Methods Cultured sporophytes and vegetative shoots from a single genotype of the moss Aloina ambigua raised in the laboratory were tested for their strategy of desiccation tolerance by desiccating the shoot–sporophyte complex and vegetative shoots at different intensities, and comparing outcomes with those of undried shoot–sporophyte complexes and vegetative shoots. By using a dehardened clonal line, the effects of field, age and genetic variance among plants were removed.Key Results The gametophyte and embryonic sporophyte were found to employ a predominantly inducible strategy of desiccation tolerance, while the post-embryonic sporophyte was found to employ a moderately constitutive strategy of desiccation tolerance. Further, desiccation reduced sporophyte fitness, as measured by sporophyte mass, seta length and capsule size. However, the effects of desiccation on sporophyte fitness were reduced if the stress occurred during embryonic development as opposed to postembryonic desiccation.Conclusions The effects of desiccation on dehardened sporophytes of a bryophyte are shown for the first time. The transition from one desiccation tolerance strategy to the other in a single structure or generation is shown for only the second time in plants and for the first time in bryophytes. Finding degrees of inducible strategies of desiccation tolerance in different life phases prompts the formulation of a continuum hypothesis of ecological desiccation tolerance in mosses, where desiccation tolerance is not an either/or phenomenon, but varies in degree along a gradient of ecological inducibility.     

Phenology and wintering capacity of sporophytes and gametophytes of ferns native to northern Japan

Summary To investigate life history adaptations to cold climates, the leaf development, sporulation period, growing stage of gametophytes, and the frost and drought resistance of sporophytes and gametophytes of 67 fern species native to Kokkaido were studied. Most ferns common in Hokkaido are summer-green with leaves developing during late May to June and decaying during October. Most of the ferns in Hokkaido sporulate during August to early September. Spores dispersed from June to September germinate before winter begins, forming vegetative prothallia. Gametophytes mature only in the following summer. Thus in Hokkaido the gametophytes as well as perennial sporophytes are exposed to severe winter conditions. In order to correlate the life cycles of temperate ferns with winter cold stress, frost resistance of gametophytes, rhizomes, and leaves of sporophytes were determined. Maximal frost resistance of rhizomes reflects the stress conditions of their habitats: rhizomes of forest understory ferns are damaged at-5°to -17.5°C, epiphytic ferns and ferns of habitats exposed to severe frost sustained temperatures of -20° to-40°C. The leaves of winter-green and evergreen ferns resist frost ranging from -25° to -40°C. The leaves of summer-green ferns are killed by late frost below -5°C. With some exceptions, gametophytes of ferns growing on the forest floor resist frost to -40°C and are much hardier than sporophytes. These results suggest the possible restrictive effects of cold climate on the life span of leaves as well as on the sporulation period. If winter cold is one of the decisive factors for seasonality expression and habitat distribution of ferns, the sensitive generation must be the sporophyte rather than the gametophyte. The hardier gametophyte is therefore able to colonize habitats in which the sporophyte is excluded by frost if mechanisms of vegetative propagation are evolved.Contribution No. 2451 from The Institute of Low Temperature Science     

Biological and nutritional aspects involved in fern multiplication

Gametophytes of several species of ferns were mechanically triturated and the resulting homogenates cultured in vitro for propagation purposes. Differences in the time period from spore culture to sporophyte development were perceivable between species. For those species with a fast life cycle and high sporophyte production such as Woodwardia virginica and Dryopteris affinis sp. affinis, homogenization of gametophytes can be considered to be excellent method for propagation, yielding hundreds of sporophytes in a short period of time. Sporophyte formation was inhibited in O. regalis by the succesive application of homogenization to gametophytes regenerated by this technique. The effect of the culture medium composition on fern production was also studied in O. regalis and P. ensiformis gametophytes. In these species, sporophyte formation increased when the gametophytes were cultured in a medium containing water+0.7% agar. Addition of sucrose inhibited gametophyte development and induced their necrosis. The 1/2 dilution of Murashige and Skoog basal medium, without sucrose, favoured leaf expansion in P. ensiformis sporophytes. This revised version was published online in June 2006 with corrections to the Cover Date.     

CONTROL OF SEX RATIOS IN HAPLOID POPULATIONS OF THE MOSS,CERATODON PURPUREUS

Female biased sex ratios occur in a number of unrelated mosses. Such ratios refer to the relative numbers of male and female gametophytes in moss populations and are therefore more comparable to the numbers of pollen grains and ovules in populations of seed plants than to the numbers of male (microsporangiate) and female (megasporangiate) sporophytes. A survey of 11 populations of the moss, Ceratodon purpureus, showed that sex ratios are heterogeneous, but that female biases occur in more than half the populations. One hundred and sixty single spore isolates representing 40 sporophytes from one population demonstrated that female gametophytes outnumbered males by a ratio of 3:2 at the time of germination. Female gametophytic clones formed significantly more biomass than male clones, and individual female shoots were more robust. Male clones, however, produced more numerous stems. These sexually dimorphic traits may be related to life history differences between male and female gametophytes since females must provide nutritional support to the “parasitic” sporophyte generation, a burden that males do not share.     

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.     

INTERGENERIC HYBRIDIZATION AMONG FIVE GENERA OF THE FAMILY LESSONIACEAE (PHAEOPHYCEAE) AND EVIDENCE FOR POLYPLOIDY IN A FERTILE PELAGOPHYCUS×MACROCYSTIS HYBRID

Hybridization was attempted by combining gametophytes between intergeneric pairs among the following taxa in the Lessoniaceae: Macrocystis pyrifera ( L.) C. Agardh , M. integrifolia Bory, M. angustifolia Bory , Pelagophycus porra ( Leman) Setch ., Nereocystis luetkeana ( Mert.) Post & Rupr ., Dictyoneurum californicum Rupr ., and Dictyoneuropsis reticulata ( Saud.) Smith. Hybrid sporophytes were produced in some combinations involving Macrocystis × Pelagophycus and Macrocystis × Dictyoneurum, and in all combinations of Dictyoneuropsis × Dictyoneurum. This is the first report of intergeneric hybrids involving Dictyoneurum. Gametophytes of P. porra had 16–24 chromosomes. Gametophytes from a fertile Macrocystis-Pelagophycus hybrid were crossed with Macrocystis and Pelagophycus gametophytes. Hybrid male gametophytes and Pelagophycus female gametophytes produced sporophyte progeny, but hybrid males with Macrocystis females did not. A single hybrid female gametophyte did not produce gametophytes in combination with hybrid males , Pelagophycus males or Macrocystis males. The hybrid gametophytes had approximately 30 chromosomes. It is hypothesized that the hybrid is an alloploid, containing a complete set of Macrocystis and Pelagophycus chromosomes, which may have allowed meiosis and sporogenesis to proceed normally in the hybrid sporophyte found in the sea. Thus, reproductive isolating mechanisms appear to operate at both pre- and postzygotic stages, and both can be overcome in intergeneric hybrids .     

Costs of sporophyte production in the moss,Dicranum polysetum

We investigated the cost of sporophyte production in the moss Dicranum polysetum both by examining patterns of growth and reproduction in unmanipulated shoots and by experimentally manipulating sexual reproduction.The estimated proportion of total carbon investment allocated to sexual reproduction in sporophyte-producing shoots over the study period was 74.8%. Unmanipulated shoots that aborted all sporophytes had a significantly higher growth in the top shoots than shoots that produced sporophytes. In sporophyte-producing shoots, total apical growth decreased proportionately with the number of sporophytes.Experimental prevention of sporophyte development resulted in significantly higher total apical growth of the gametophytes. Shoots where current perichaetia were lacking when marked had a mass increase in the top shoots similar to manipulated sporophytic shoots whereas sporophytic control shoots grew significantly less than these two categories.The difference between control shoots and manipulated shoots in the mass of vegetative apical growth was mainly because of different length increments whereas mass per unit length was similar between groups. The probability to reproduce sexually in the year after the manipulation, and the biomass allocated to this reproduction, were not affected by the experimental treatment.     

Phylogenetic analyses of morphological evolution in the gametophyte and sporophyte generations of the moss order Hookeriales (Bryopsida)

Morphological characters from the gametophyte and sporophyte generations have been used in land plants to infer relationships and construct classifications, but sporophytes provide the vast majority of data for the systematics of vascular plants. In bryophytes both generations are well developed and characters from both are commonly used to classify these organisms. However, because morphological traits of gametophytes and sporophytes can have different genetic bases and experience different selective pressures, taxonomic emphasis on one generation or the other may yield incongruent classifications. The moss order Hookeriales has a controversial taxonomic history because previous classifications have focused almost exclusively on either gametophytes or sporophytes. The Hookeriales provide a model for comparing morphological evolution in gametophytes and sporophytes, and its impact on alternative classification systems. In this study we reconstruct relationships among mosses that are or have been included in the Hookeriales based on sequences from five gene regions, and reconstruct morphological evolution of six sporophyte and gametophyte traits that have been used to differentiate families and genera. We found that the Hookeriales, as currently circumscribed, are monophyletic and that both sporophyte and gametophyte characters are labile. We documented parallel changes and reversals in traits from both generations. This study addresses the general issue of morphological reversals to ancestral states, and resolves novel relationships in the Hookeriales.     

Gametophyte contribution to sporophyte growth on the basis of carbon gain in the fern <Emphasis Type="Italic">Thelypteris palustris</Emphasis>: effect of gametophyte organic-matter production on sporophytes

At an early stage of growth gametophytes support the sporophytes of ferns. Young sporophytes become independent of gametophytes when the first leaves develop. Although large fern gametophytes produce multiple archegonia simultaneously, only one sporophyte is typically established on one gametophyte. The number of sporophytes is believed to be controlled in two possible directions, from gametophyte to sporophyte or from preceding sporophyte to another sporophyte. To investigate the effects of gametophytes on their sporophytes, we studied the relationship between organic matter production by gametophytes and the growth of young sporophytes of Thelypteris palustris. We cut gametophytes in half (CGs) to reduce the gametophytes’ production of matter. There was no significant difference between the growth of sporophytes on intact gametophytes (IGs) and that on CGs. According to our estimates, based on the rate of organic matter production, the large gametophyte was able to produce two or more sporophytes. The resources required for CGs to make similar-sized sporophytes was twice that for IGs. In polyembryony each of the multiple sporophytes was similar in size to the single sporophytes. Resource limitation does not seem to explain why fern gametophytes establish single sporophytes.     

EFFECTS OF LIGHT AND TEMPERATURE ON NET PHOTOSYNTHESIS AND DARK RESPIRATION OF GAMETOPHYTES AND EMBRYONIC SPOROPHYTES OF MACROCYSTIS PYRIFERA1

The rates of net photosynthesis as a function of irradiance and temperature were determined for gametophytes and embryonic sporophytes of the kelp, Macrocystis pyrifera (L.) C. Ag. Gametophytes exhibited higher net photosynthetic rates based on oxygen and pH measurements than their derived embryonic sporophytes, but reached light saturation at comparable irradiance levels. The net photosynthesis of gametophytes reached a maximum of 66.4 mg O₂ g dry wt^?1 h^?1 (86.5 mg CO₂ g dry wt^?1 h^?1), a value approximately seven times the rate reported previously for the adult sporophyte blades. Gametophytes were light saturated at 70 μE m^?2 s^?1 and exhibited a significant decline in photosynthetic performance at irradiances 140 μE m^?1 s^?1. Embryonic sporophytes revealed a maximum photosynthetic capacity of 20.6 mg O₂ g dry wt^?1 h^?1 (25.3 mg CO₂ g dry wt^?1 h^?1), a rate about twice that reported for adult sporophyte blades. Embryonic sporophytes also became light saturated at 70 μE m^?2 s^?1, but unlike their parental gametophytes, failed to exhibit lesser photosynthetic rates at the highest irradiance levels studied; light compensation occurred at 2.8 μE m^?2 s^?1. Light-saturated net photosynthetic rates of gametophytes and embryonic sporophytes varied significantly with temperature. Gametophytes exhibited maximal photosynthesis at 15° to 20° C, whereas embryonic sporophytes maintained comparable rates between 10° and 20° C. Both gametophytes and embryonic sporophytes declined in photosynthetic capacity at 30° C. Dark respiration of gametophytes was uniform from 10° to 25° C, but increased six-fold at 30° C; the rates for embryonic sporophytes were comparable over the entire range of temperatures examined. The broader light and temperature tolerances of the embryonic sporophytes suggest that this stage in the life history of M. pyrifera is well suited for the subtidal benthic environment and for the conditions in the upper levels of the water column.     

苔藓植物耐旱机制研究进展

耐旱藓类快速脱水并存活的能力可由快速建立起来的对环境变化的耐受机制来反映,保护细胞完整性的组成型机制与修复细胞损伤的诱导机制协同作用使苔藓植物渡过干旱胁迫.再水化时光合系统原初恢复非常迅速;ABA处理可显著改变PSⅡ的生理特征;基因表达的变化主要由翻译调控引起;脱水组织中贮存mRNPs既保护了mRNAs,又加快了再水化修复速度.山墙藓(Tortula ruralis)是耐旱研究较多的一个种,已建立了表达序列文库(EST),将会成为耐旱研究的重要模式植物.     

Mycothallic/mycorrhizal symbiosis of chlorophyllous gametophytes and sporophytes of a fern, Pellaea viridis (Forsk.) Prantl (Pellaeaceae, Pteridales)

Gametophytes of Pellaea viridis that appeared spontaneously on the surface of substratum originating from an ultramafic area were found to form mycothallic symbiosis with arbuscular mycorrhizal fungi (AMF) under laboratory conditions. In gametophytes and sporophytes grown with Glomus tenue, abundant arbuscule formation was observed at both stages. In gametophytes, the fungus was found in the region where the rhizoids are initiated. If G. intraradices was added to the soil, the gametophytes were colonised mostly by G. tenue, and roots of sporophytes were colonised by G. intraradices. The presence of AM fungi in both gametophytes and sporophytes of P. viridis resulted in the development of larger leaf area and root length of the sporophyte. The analysis of gametophytes from the Botanical Garden in Krakow (Poland) showed that cordate gametophytes of Pteridales, namely Pellaea viridis (Pellaeaceae), Adiantum raddianum and A. formosum (Adiantaceae), were also mycothallic.     

苔藓植物耐旱机制研究进展

耐旱藓类快速脱水并存活的能力可由快速建立起来的对环境变化的耐受机制来反映,保护细胞完整性的组成型机制与修复细胞损伤的诱导机制协同作用使苔藓植物渡过干旱胁迫。再水化时光合系统原初恢复非常迅速;ABA处理可显著改变PSⅡ的生理特征;基因表达的变化主要由翻译调控引起;脱水组织中贮存mRNPs既保护了mRNAs, 又加快了再水化修复速度。山墙藓(Tortula ruralis)是耐旱研究较多的一个种,已建立了表达序列文库(EST),将会成为耐旱研究的重要模式植物。     

Gametogenesis and chromosome number in Postelsia palmaeformis (Laminariales, Phaeophyceae)

Gametophytes of the ‘sea palm’, the kelp Postelsia palmaeformis Ruprecht, produced gametes whether or not chelated iron was supplied in the culture medium, in contrast to the inhibition of gametogenesis seen with the absence of iron in many other kelps. As gametogenesis proceeded, every cell of the gametophytes was converted into a gamete so that the gametophytes did not continue to grow vegetatively. The portion of the life history from spore release through germination, gametophyte growth, gametogenesis, fertilization and growth of the young sporophyte was completed in 9–10 days under laboratory conditions. Chromosome counts showed that sporophytes had a diploid number of 26–34 chromosomes while sporangia and gametophytes had a haploid number of 14–17 chromosomes, indicating a typical haplodiplophasic life history as seen in other Laminariales.     

PROBLEMS IN THE IDENTITY AND ORIGIN OF THE APPALACHIAN VITTARIA GAMETOPHYTE,A SPOROPHYTELESS FERN OF THE EASTERN UNITED STATES

Populations of a fern gametophyte presumed to be of the genus Vittaria occur commonly in the uplands of the southeastern United States. The gametophytes occur on non-calcareous rock outcrops of various composition in areas which provide continuous moisture and protection from temperature extremes. Gametophytes in these habitats are robust and long lived, frequently forming the dominant vegetation in areas covering several square feet. Reproduction is exclusively vegetative by production of gemmae. Although sex organs are present in most populations, viable sporophytes are never produced. The gametophytes have previously been considered most likely to be V. lineata, which occurs in Florida, but morphological and physiological comparisons do not support that conclusion. Significant differences between Appalachian and Florida gametophytes occur in growth form, growth rate, cold hardiness, sporophyte production, and patterns of gemma production. The distribution of the Appalachian gametophytes correlates with old, unglaciated land masses suggesting antiquity rather than recent introduction. Present evidence favors the interpretation that the Appalachian Vittaria gametophytes either belong to a tropical American species from which they have long been separated, or that they represent a distinct species of which the sporophyte no longer exists.     

Contribution of fern gametophytes to the growth of produced sporophytes on the basis of carbon gain

Sporophytes appeared on most gametophytes of Thelypteris palustris (Salisb.) Schott that reached a certain size, which is interpreted to be a critical size of gametophytes for the production of sporophytes. After sporophytes were produced, attached gametophytes ceased dry weight growth, but the gametophytes which did not produce sporophytes grew successively. It was hypothesized that matter produced by gametophytes was being supplied to young sporophytes. Photosynthesis and respiration of gametophytes with attached sporophytes were not significantly different from that of gametophytes without sporophytes. Photosynthetic activity of gametophytes dropped from 0.18 to 0.03 mol CO₂ g^–1 s^–1 during the growth period. The higher photosynthetic rates of gametophytes in the early growth stage were important for reaching the critical size for sporophyte production in a short time. Sporophytes in the one leaf stage averaged 0.14 mol CO₂ g^–1 s^–1 of photosynthetic activity. The results show that sporophytes that had expanded the first leaf grow by their own photosynthetic production. Gametophytes allocated the photosynthate for sporophytes and it was an important aid before the one-leaf stage. The supportive role of gametophytes ended at that stage.     

A MULTISPECIES LABORATORY ASSESSMENT OF RAPID SPOROPHYTE RECRUITMENT FROM DELAYED KELP GAMETOPHYTES1

Recent work suggests that the ability to delay reproduction as resistant haploid gametophytes may be important for seaweeds that experience unpredictable disturbances or seasonal periods of poor conditions that result in adult sporophyte absence. Further, delayed gametophytes of some kelp species (order Laminariales) may produce sporophytes more rapidly than if they had never experienced a delay, conferring a competitive advantage when conditions improve or after disturbance events. Here, it was determined that the gametophytes of the canopy‐forming kelp Macrocystis pyrifera (L.) C. Agardh could delay reproduction in a one‐ to two‐cell state (<50 μm) for at least 7 months when grown under nutrient‐limiting conditions. These stages retained reproductive viability and produced sporophytes within 5 d once nutrients were increased. This finding suggests that gametophytes could potentially promote recovery of M. pyrifera populations after extended periods of sporophyte absence. In addition, the time required for sporophyte production between gametophytes of the four most conspicuous kelp species in Southern California that had delayed reproduction and gametophytes that had not was compared. For these four kelp species, a delay of at least 30 d conferred a 40%–76% reduction in the time required for sporophyte production once nutrients were received. Fecundity did not decrease with delay duration, suggesting there is no apparent cost of delayed development for kelps as has been observed in other organisms. Thus, delayed development may be a viable strategy for surviving and initially dominating in environments with variable quality.     

Comparative study of the sympatric ferns Culcita macrocarpa and Woodwardia radicans: Sexual phenotype

The sexual phenotypes of 1152 gametophytes from four populations of Culcita macrocarpa and Woodwardia radicans were monitored over a 1-year period. Gametophytes were maintained under three experimental conditions: (1) isolated, (2) pairs from the same sporophyte, or (3) pairs from different sporophytes. The frequencies of the sexual phenotypes did not vary significantly among these three conditions, and although there were some quantitative differences between populations, the sexual-phenotype sequences observed were species-specific. Gametophytes of C. macrocarpa were first male and then hermaphrodite: this sequence, together with the absence of antheridiogens, favours intragametophytic selfing. Natural populations of C. macrocarpa are presumably androdioecious. Gametophytes of W. radicans were first female and then hermaphrodite: this sequence and antheridiogen activity favour intergametophytic and even xenogamous mating. Despite these laboratory findings, populations of W. radicans are probably trioecious (because of the effects of antheridiogen). Few sporophytes of W. radicans were obtained in the present study, and none of C. macrocarpa: this is attributable to limiting illumination or substrate.