Life history of Cladophora surera sp. nov. (Cladophorales, Ulvophyceae)

The life history of Cladophora surera sp. Nov., described from the south of the province of Buenos Aires, Argentina, was found to be diplobiontic and isomorphic with haploid gametophytic (n= 24) and sporophytic (2n= 48) plants. Other freshwater species, namely C, suhriana Kutz, and C. callicoma Kütz. have also been reported to be diplobiontic and isomorphic but differing from C. surera by their ploidy level. Plants exhibit great morphological variation, as the number of branches/mm² tends to increase with higher water velocity, thus adopting morphotypes that resemble very different species, that is, C. vagabunda (L.) Hoek, C. glomerata (L.) Kutz., C, laetevirens (Dillw.) Kutz., C. brasiliana Martens, C. dalmatica Kütz., C. vadorum (Aresch.) Kütz. and C. rivularis (L.) Hoek., but clearly differing from them by other morphological parameters; apical cell diameters, The ploidy level 24/48 has not been established for a species of Cladophora. Autogamy is the normal conjugation method and can take place even inside the gametangia before the gametes are released. Isogametes conjugate in a slow behavioral anisogamy, in which the contents of one migrate to the other. Tetra-flagellate zoospores result from meiosis.     

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.     

Mediterranean Species of Caulerpa Are Polyploid with Smaller Genomes in the Invasive Ones

Caulerpa species are marine green algae, which often act as invasive species with rapid clonal proliferation when growing outside their native biogeographical borders. Despite many publications on the genetics and ecology of Caulerpa species, their life history and ploidy levels are still to be resolved and are the subject of large controversy. While some authors claimed that the thallus found in nature has a haplodiplobiontic life cycle with heteromorphic alternation of generations, other authors claimed a diploid or haploid life cycle with only one generation involved. DAPI-staining with image analysis and microspectrophotometry were used to estimate relative nuclear DNA contents in three species of Caulerpa from the Mediterranean, at individual, population and species levels. Results show that ploidy levels and genome size vary in these three Caulerpa species, with a reduction in genome size for the invasive ones. Caulerpa species in the Mediterranean are polyploids in different life history phases; all sampled C. taxifolia and C. racemosa var. cylindracea were in haplophasic phase, but in C. prolifera, the native species, individuals were found in both diplophasic and haplophasic phases. Different levels of endopolyploidy were found in both C. prolifera and C. racemosa var. cylindracea. Life history is elucidated for the Mediterranean C. prolifera and it is hypothesized that haplophasic dominance in C. racemosa var. cylindracea and C. taxifolia is a beneficial trait for their invasive strategies.     

Alternation of generations – unravelling the underlying molecular mechanism of a 165‐year‐old botanical observation

Characteristically, land plants exhibit a life cycle with an ‘alternation of generations’ and thus alternate between a haploid gametophyte and a diploid sporophyte. At meiosis and fertilisation the transitions between these two ontogenies take place in distinct single stem cells. The evolutionary invention of an embryo, and thus an upright multicellular sporophyte, in the ancestor of land plants formed the basis for the evolution of increasingly complex plant morphologies shaping Earth's ecosystems. Recent research employing the moss Physcomitrella patens revealed the homeotic gene BELL1 as a master regulator of the gametophyte‐to‐sporophyte transition. Here, we discuss these findings in the context of classical botanical observations.     

Distributional ecology and behaviour of the early life stages of the box-jellyfish Chironex fleckeri

Laboratory observations on reared life stages of Chironex fleckeri (Cubozoa: Chirodropidae) have been combined with field sampling and observations to outline the life cycle of the species and the spatial and temporal distribution of populations in Queensland, Australia. A seasonal alternation of polypoid and medusoid generations from winter to summer respectively is accompanied by a shift in preferred habitat from tidal estuaries to the open eulittoral zone. The system appears to be constrained by predictable fluctuations in the suitability of the alternate habitats to their resident generations. Observations on behaviour, feeding, physiological tolerances, and sexual and asexual reproduction provide insights into the adaptiveness of this pattern.     

Genome size and the nucleolar number as estimators of ploidy level in Dactylis glomeratain the Slovenian Alps

 We studied five natural populations of Dactylis glomerata L. (Poaceae) growing at different altitudes in the south-eastern fringe of the Alps in northern Slovenia to determine the subspecies. The stomatal length, the pollen diameter and chromosome counts were consistent with the tetraploid taxon D. glomerata subsp. glomerata (2n=4x =28). Genome size was measured in 55 individuals. The mean 2C value was 8.6 pg DNA. The mean 2C values of populations growing at different altitudes showed only 2.1% variation, and no correlation was observed between altitude and genome size. In D. nk;glomerata subsp. glomerata eight nucleoli were observed in late telophase, indicating that the nucleolus-organising regions inherited from both diploid parent species are functional. We demonstrate that both genome size and the number of nucleoli may be used to determine the ploidy level as an alternative to chromosome counting. Received May 31, 2001; accepted March 5, 2002 Published online: November 14, 2002 Addresses of the authors: Dr. Barbara Vilhar (e-mail: barbara.vilhar@uni-lj.si), Tatjana Vidic, Nejc Jogan, Prof. Marina Dermastia, University of Ljubljana, Biotechnical Faculty, Department of Biology, Večna pot 111, 1001 Ljubljana, Slovenia.     

Meiotic analysis of four cross-pollinated generations in a synthetic autotetraploid population of husk tomato (<i>Physalis ixocarpa</i>)

The cultivated husk tomato (Physalis ixocarpa) (2n = 2x = 24) is native from Mexico and Central America and shows a wide genetic variation. Presently, it is the fourth horticultural crop in cultivation surface in Mexico. The working team of this research previously developed an autotetraploid population by using colchicine. The objectives of the present work were to analyze the ploidy level and meiotic behavior of the subsequent generations (C3, C4, C5, C6) from the original (C2) composed only by plants with the duplicated genome from the Rendidora cultivar, and to determine pollen viability. As a diploid control the cultivar Rendidora of P. ixocarpa was used. Ploidy level was determined by flow citometry and meiotic analysis. For the meiotic study, the microsporocytes were prepared by the squash method, stained with carmin and analyzed in diakinesis. Pollen viability was evaluated through 0.01% Buffalo Black staining. The tetraploid condition prevailed through four cross-pollinating generations, maintaining a constant chromosome number 2n = 4x = 48. In diakinesis, the chromosomes of the diploid cultivar were associated into bivalents, whereas in tetraploid plants the chromosomes associated into univalents, bivalents and trivalents. Highly significant differences in bivalent pairing were detected between autotetraploid plants and between generations. Pollen viability did not show significant differences between generations and allowed reproduction. These results indicate that it is possible to develop an autotetraploid cultivar, because the polyploid state is naturally maintained and the plants are fertile. Furthermore, given the differences in bivalent pairing between plants and generations, a response to selection toward meiotic stability is expected.     

Phylogenetic relationships and gene flow between sympatric diploid and tetraploid plants ofDactylis glomerata (Gramineae)

Phylogenetic relationships between sympatric, morphologically indistinguishable diploid and tetraploid plants ofDactylis glomerata L. (Gramineae) in Galicia (Spain) were assessed using allozyme markers for 6 distinct systems. The study exploited recent introduction in Galicia and subsequent hybridization of an alien 4xDactylis subspecies possessing distinct allozymes from those of all the native plants. Opportunities for gene exchanges between the ploidies were estimated from in situ observations of flowering, examination of progenies in 2x/4x natural and experimental crosses, and enzyme analyses. Results show a high genetic similarity between the Galician diploids and tetraploids, which possess peculiar alleles in common. Although the ploidy levels usually have distinct flowering periods, interploidal crosses do occasionally occur. Gene flow is likely much more important from the diploid to the tetraploid level. A good genetic intermixing occurs between the Galician and the alien tetraploid entities which have simultaneous flowering. Autopolyploidization of the diploids followed by various rates of hybridization is proposed as one very probable origin of natural tetraploids inDactylis.     

Parasexuality and Ploidy Change in Candida tropicalis

Candida species exhibit a variety of ploidy states and modes of sexual reproduction. Most species possess the requisite genes for sexual reproduction, recombination, and meiosis, yet only a few have been reported to undergo a complete sexual cycle including mating and sporulation. Candida albicans, the most studied Candida species and a prevalent human fungal pathogen, completes its sexual cycle via a parasexual process of concerted chromosome loss rather than a conventional meiosis. In this study, we examine ploidy changes in Candida tropicalis, a closely related species to C. albicans that was recently revealed to undergo sexual mating. C. tropicalis diploid cells mate to form tetraploid cells, and we show that these can be induced to undergo chromosome loss to regenerate diploid forms by growth on sorbose medium. The diploid products are themselves mating competent, thereby establishing a parasexual cycle in this species for the first time. Extended incubation (>120 generations) of C. tropicalis tetraploid cells under rich culture conditions also resulted in instability of the tetraploid form and a gradual reduction in ploidy back to the diploid state. The fitness levels of C. tropicalis diploid and tetraploid cells were compared, and diploid cells exhibited increased fitness relative to tetraploid cells in vitro, despite diploid and tetraploid cells having similar doubling times. Collectively, these experiments demonstrate distinct pathways by which a parasexual cycle can occur in C. tropicalis and indicate that nonmeiotic mechanisms drive ploidy changes in this prevalent human pathogen.     

Homologous Versus Antithetic Alternation of Generations and the Origin of Sporophytes

The late-nineteenth/early-twentieth century debate over homologous versus antithetic alternation of generations is reviewed. Supporters of both theories, at first, used Coleochaete as a model for the origin of land-plant life cycles. The early debate focused on the morphological interpretation of the sporophyte and on whether vascular cryptogams had bryophyte-like ancestors. The terms of the debate shifted after the discovery that the alternation of morphological generations was accompanied by an alternation of chromosome number. Supporters of homologous alternation now promoted a model in which land plants had been derived from an algal ancestor with an isomorphic alternation of haploid and diploid generations whereas supporters of antithetic alternation favored a model in which land plants were derived from a haploid algal ancestor with zygotic meiosis. Modern evidence that embryophytes are derived from charophycean green algae is more compatible with an updated version of the antithetic theory.     

Strategies providing success in a variable habitat: I. Relationships of environmental factors and dominance of Cladophora glomerata

Dominance of macrophytes and their response to environmental factors were studied in the river Ilm, Thuringia/ Germany with special reference to Cladophora glomerata (L.) Kütz. Macroalgae showed a growth peak in spring with C. glomerata, Ulothrix zonata Kütz., Lemanea fluviatilis (L.) C. Agardh and Audouinella sp. being the dominant species. Shortly after this peak, a rapid decline of macrophyte substrate coverage was observed. Only C. glomerata revealed a second growth peak in late summer/early autumn. Frequent disturbances of the macrophyte assemblage by floods resulted repeatedly in an almost complete wash out of benthic organisms. After summer floods C. glomerata was the species that recolonized the substrate. At high‐light sites, faster recovery of C. glomerata was observed as compared to low‐light sites. This is discussed in relation to the life cycle of C. glomerata. Among the physical and chemical parameters that were analysed, irradiance, current velocity, pH, soluble reactive phosphorus and ammonia‐nitrogen accounted for most of the observed patterns of dominance of C. glomerata.     

Powdery mildew suppresses herbivore‐induced plant volatiles and interferes with parasitoid attraction in Brassica rapa

The co‐occurrence of different antagonists on a plant can greatly affect infochemicals with ecological consequences for higher trophic levels. Here we investigated how the presence of a plant pathogen, the powdery mildew Erysiphe cruciferarum, on Brassica rapa affects (1) plant volatiles emitted in response to damage by a specialist herbivore, Pieris brassicae; (2) the attraction of the parasitic wasp Cotesia glomerata and (3) the performance of P. brassicae and C. glomerata. Plant volatiles were significantly induced by herbivory in both healthy and mildew‐infected plants, but were quantitatively 41% lower for mildew‐infected plants compared to healthy plants. Parasitoids strongly preferred Pieris‐infested plants to dually‐infested (Pieris + mildew) plants, and preferred dually infested plants over only mildew‐infected plants. The performance of P. brassicae was unaffected by powdery mildew, but C. glomerata cocoon mass was reduced when parasitized caterpillars developed on mildew‐infected plants. Thus, avoidance of mildew‐infested plants may be adaptive for C. glomerata parasitoids, whereas P. brassicae caterpillars may suffer less parasitism on mildew‐infected plants in nature. From a pest management standpoint, the concurrent presence of multiple plant antagonists can affect the efficiency of specific natural enemies, which may in turn have a negative impact on the regulation of pest populations.     

Cytogeography of Solidago gigantea (Asteraceae) and its invasive ploidy level

Aim Newly formed polyploids experience problems of establishment and spread similar to those faced by newly introduced alien species. To understand the significance of polyploidy in biological invasions, we mapped the distribution of ploidy levels in Solidago gigantea Aiton in its native range in North America, and in Europe and East Asia where it is invasive. Location North America, Europe and East Asia. Methods Flow cytometry was used to measure ploidy levels in a total of 834 plants from 149 locations. Together with data from contributors and a literature review, ploidy‐level data were assembled for 336 locations. Cytogeographical maps from North America and Europe were prepared, incorporating new and previously published ploidy‐level data. Results In the native range, diploid, tetraploid and hexaploid plants were found, and also one triploid and one pentaploid plant (2n = 3x and 2n = 5x, respectively, each being new reports for this species). There was a high degree of geographical separation among the ploidy levels, and populations with mixed ploidy were rare. However, four zones were identified where plants of different ploidy could come into contact. In Europe and East Asia, only tetraploid plants were found. Main conclusions The geographical pattern in North America suggests that the ploidy levels are ecologically differentiated, although further investigations are needed to identify the nature of these differences. Alien populations appear to be exclusively tetraploid, but it is not clear whether this is because tetraploids were selectively introduced or because diploids were unsuccessful. In any case, comparisons between native and introduced populations need to account for ploidy level.     

LIFE HISTORIES IN THE PHYLLOPHORACEAE (RHODOPHYTA: GIGARTINALES) FROM THE PACIFIC COAST OF NORTH AMERICA. I. GYMNOGONGRUS LINEARIS AND G. LEPTOPHYLLUS1

Carpospores of Gymnogongrus linearis (C. Ag.) J. Ag. collected from Sonoma Co., California were cultured and gave rise to crustose plants. Tetrasporogenesis could not be induced. However, tetraspores from field-collected crustose tetrasporophytes found near G. linearis from San Mateo Co., California were cultured. These field crusts superficially resemble Petrocelis middendorffii (Ruprecht) Kjellman, but differ in size, color, number of tetrasporangia per filament, and distal dichotomous branching of the perithallial filaments. Tetraspores gave rise to upright plants identical to G. linearis. Gymnogongrus leptophyllus J. Ag. collected from California and Baja California, Mexico were found as narrow and wide forms. Narrow form isolates recycled directly without producing a crustose tetrasporophyte. These are interpreted as apogamous. Carpospores of the wide form grew into crusts resembling Petrocelis (=Erythrodermis) haematis Hollenberg. Tetrasporogenesis was induced in culture by abrasion or dehydration. Tetraspores from field-collected crusts and laboratory cultured tetrasporophytes grew into plants identical to G. leptophyllus, completing a sexual life history with an alternation of heteromorphic generations.     

A COMPREHENSIVE STUDY OF THE LIFE CYCLE OF A SOUTH AMERICAN POPULATION OF STIGEOCLONIUM TENUE (CHAETOPHORALES,CHLOROPHYTA)1

The diplobiontic–haplodiplontic life cycle with alternating isomorphic generations in Stigeoclonium tenue (C. Agardh) Kütz. is described for the first time. Sporophytes (2n = 10) arise from tetraflagellate zoospores that are produced by meiosis. Sporic meiosis might be inferred from the cruciform divisions formed during zoosporogenesis and is confirmed through observations of prophase I substages. Zoospores do not germinate directly but produce a haploid cyst that germinates to give rise to a gametophyte (n = 5). Gametophytes produce biflagellate isogametes, which fuse to produce zygotes that germinate by mitosis into the sporophytic stage. Gametophytes and sporophytes reproduce asexually both via mitotic tetraflagellate zoospores and by thallus fragmentation. Results from this study indicate that both the cosmopolitan distribution and dominance of S. tenue in many periphytic communities might be due to its multiple reproductive strategies.     

Ploidy doubling by in vitro culture of excised protocorms or protocorm-like bodies in <Emphasis Type="Italic">Phalaenopsis</Emphasis> species

Endopolyploidy was observed in the protocorms of diploid Phalaenopsis aphrodite subsp. formosana with ploidy doubling achieved by in vitro regeneration of excised protocorms, or protocorm-like bodies (PLBs). Thirty-four per cent of the PLBs regenerated from the first cycle of sectioned protocorms were found to be polyploids with ploidy doubled once or twice as determined by flow-cytometry. The frequency of ploidy doubling increased as the sectioning cycles increased and was highest in diploid followed by the triploid and tetraploid. Regeneration of the endopolyploid cells in the tissue of the protocorms or PLBs is proposed as the source of the development of ploidy doubled plantlets. The frequency of ploidy doubling was similar in seven other Phalaenopsis species, although the rate of increase within cycles was genotype specific. In two species, a comparison of five parameters between 5-month-old diploid and tetraploid potted plants showed only the stomata density differed significantly. The flowers of the tetraploid plant were larger and heavier than those of the diploids. This ploidy doubling method is a simple and effective means to produce large number of polyploid Phalaenopsis species plants as well as their hybrids. The method will be beneficial to orchid breeding programs especially for the interspecific hybridization between varieties having different chromosome sizes and ploidy levels.     

Correlation between nuclear DNA values and differing optimal ploidy levels inNarcissus,Hyacinthus andTulipa cultivars

4C nuclear DNA amounts were determined in 16 large decorative cultivars ofNarcissus (Amaryllidaceae), 13 ofHyacinthus (Hyacinthaceae) and 12 ofTulipa (Liliaceae) at different levels of ploidy. Within each genus, nuclear DNA amounts and ploidy levels are positively correlated, with no DNA loss in polyploids.Based on wide surveys of chromosome numbers, the maximum numbers of cultivars, interpreted as the optimum levels of selective success or horticultural fitness, were found to be at the tetraploid level inNarcissus (2n=4x=28), the triploid inHyacinthus (2n=3x=24) and the diploid inTulipa (2n=2x=24). All these ploidy optima were shown to correspond to a small range of nuclear DNA amounts (4C=96-139 pg), which could suggest the existence of a single DNA value optimal for the three biologically similar but unrelated genera. In each case the optimum is at an equilibrium reached between enhanced size and other morphological characteristics on one hand and reduced growth rate on the other, both resulting from increase in ploidy and nuclear DNA amounts.     

Spatial and temporal diversity in hyperparasitoid communities of Cotesia glomerata on garlic mustard,Alliaria petiolata

1. Interactions between two trophic levels can be very intimate, often making species dependent on each other, something that increases with specialisation. Some specialised multivoltine herbivores may depend on multiple plant species for their survival over the course of a growing season, especially if their food plants are short‐lived and grow at different times. Later generations may exploit different plant species from those exploited by previous generations. 2. Multivoltine parasitoids as well as their natural enemies must also find their hosts on different food plants in different habitats across the season. Secondary hyperparasitoid communities have been studied on cocoons of the primary parasitoid, Cotesia glomerata (Hymenoptera: Braconidae), on black mustard (Brassica nigra) – a major food plant of its host, the large cabbage white (Pieris brassicae) – which grows in mid‐summer. 3. Here, hyperparasitoid communities on C. glomerata pupal clusters were studied on an early‐season host, garlic mustard, Alliaria petiolata, over ‘time’ (one season, April–July) in six closely located ‘populations’ (c. 2 km apart), and within two different ‘areas’ at greater separation (c. 100 km apart). At the plant level, spatial effects of pupal ‘location’ (canopy or bottom) on the plant were tested. 4. Although large‐scale separation (area) did not influence hyperparasitism, sampling time and small‐scale separation (population) affected hyperparasitism levels and composition of hyperparasitoid communities. Location on the plant strongly increased proportions of winged species in the canopy and proportions of wingless species in bottom‐located pupae. 5. These results show that hyperparasitism varies considerably at the local level, but that differences in hyperparasitoid communities do not increase with spatial distance.     

Differential susceptibility to grazers in two phases of an intertidal alga: Advantages of heteromorphic generations

The intertidal red alga Gigartina papillata (C. Ag.) J. Ag. alternates between two major forms in its life cycle, an upright blade and a fleshy crust. The evolution and functional significance of such alternation of heteromorphic generations is little understood. This study, done on the outer coast of Washington state (U.S.A.), considers the rôle of physical factors, measured by intertidal position, and a biological factor, interactions with herbivores, as selective pressures potentially responsible for this pattern. Observations along transects and a herbivore exclusion experiment showed that the two phases of this alga simultaneously occupy similar intertidal positions but differ significantly and oppositely in their response to grazing. The number of crust plants declined in the absence of grazers, probably due to being overgrown and shaded by a thick algal film. It is, therefore, suggested that the crust form is grazer-dependent. In contrast, the number of blade phase plants increased in the absence of grazers. “Bet-hedging” is suggested in explanation of the maintenance of two phases in one cycle; the crust is more persistent but susceptible to overgrowth, while the more productive blade is more susceptible to grazing.     

The life cycle of Laodicea indica (Laodiceidae,Leptomedusae, Cnidaria)

The life cycle of Laodicea indica is described. In the Bismarck Sea this species shows a normal alternation of generations in the wet season; but in the dry season the cycle is contracted, and planulae give rise to gonothecae without formation of a hydroid colony. Medusae are liberated about 3 d after planula settlement. This life cycle pattern is previously unreported in hydromedusae. The possible adaptive value of such a life cycle and the evolution of polyp reduction in hydromedusan life cycles are discussed.