LIFE CYCLE OF A STEPHANODISCUS SP. (BACILLARIOPHYTA)1

Studies of the life cycle of a centric diatom, tentatively identified as Stephanodiscus neoastraea Håkansson & Hickel, showed that sexual reproduction occurred every year in a freshwater lake (Lough Neagh, Northern Ireland). Male and female gametes were produced in cells below 55% of the maximum diameter during a 3–4-week period in late summer, following the return of nitrate concentrations above 10 μM NO_3-N. The frequency of sexual reproduction was linked to the cycle of diameter size reduction and regeneration. The times of largest decreases in cell diameter were during nutrient stress in summer and low light conditions in late autumn, rather than during the main spring growth period. So, environmental conditions (combined with the limited life-spans of individual cells) affected the rate of diameter reduction and, therefore, the length of the life cycle (3–4 years).     

A NEW CELL STAGE IN THE HAPLOID‐DIPLOID LIFE CYCLE OF THE COLONY‐FORMING HAPTOPHYTE PHAEOCYSTIS ANTARCTICA AND ITS ECOLOGICAL IMPLICATIONS1

Few members of the well‐studied marine phytoplankton taxa have such a complex and polymorphic life cycle as the genus Phaeocystis. However, despite the ecological and biogeochemical importance of Phaeocystis blooms, the life cycle of the major bloom‐forming species of this genus remains illusive and poorly resolved. At least six different life stages and up to 15 different functional components of the life cycle have been proposed. Our culture and field observations indicate that there is a previously unrecognized stage in the life cycle of P. antarctica G. Karst. This stage comprises nonmotile cells that range in size from ～4.2 to 9.8 μm in diameter and form aggregates in which interstitial spaces between cells are small or absent. The aggregates (hereafter called attached aggregates, AAs) adhere to available surfaces. In field samples, small AAs, surrounded by a colony skin, adopt an epiphytic lifestyle and adhere in most cases to setae or spines of diatoms. These AAs, either directly or via other life stages, produce the colonial life stage. Culture studies indicate that bloom‐forming, colonial stages release flagellates (microzoospores) that fuse and form AAs, which can proliferate on the bottom of culture vessels and can eventually reform free‐floating colonies. We propose that these AAs are a new stage in the life cycle of P. antarctica, which we believe to be the zygote, thus documenting sexual reproduction in this species for the first time.     

LIFE HISTORIES OF BLIDINGIA MINIMA (CHLOROPHYCEAE), ESPECIALLY SEXUAL REPRODUCTION1

Blidingia minima (Näg. ex Kütz.) Kylin from Muroran, Hokkaido, Japan, has been shown to exhibit four patterns of life history in culture. Sexual reproduction, reported fully for the first time in the genus, occurs in two of them. I. An isomorphic-heteromorphic complex in which erect, tubular sporophytes alternate with dioecious gametophytes of the same forms, with the irregular production by both phases of discoid or pulvinate (pincushion-like) microthalli capable of both sexual and asexual reproduction. II. An alternation of heteromorphic phases in which an erect, tubular sporophyte alternated irregularly with a gametophytic microthallus. III. An asexual alternation of heteromorphic phases in which an erect, tubular frond alternates irregularly with a microthallus by means of quadriflagellate zoospores. IV. An asexual, monophasic life cycle in which erect, tubular fronds are perpetuated by quadriflagellate zoospores. The first pattern occurred in spring populations from one of three sites. The second occurred in populations from two sites. The third and fourth occurred in all populations tested. The life history of Blidingia minima from Muroran is similar to that of Kornmannia zostericola from Muroran.     

THE SEXUAL AND ASEXUAL LIFE CYCLE OF GLENODINIOPSIS STEINII (DINOPHYCEAE)

The sexual and asexual portions of the life cycle of Glenodiniopsis steinii were examined at both the light and scanning electron microscopic levels. Asexual reproduction by cell division occurs every 2 to 3 d under optimal conditions. Gamete formation and sexual reproduction can be induced by either nitrogen deprivation or aging of cultures. Fusion of gametes peaks about 10 hr into the light cycle and requires about 10 hr for completion. Cells remain motile during fusion but the zygote loses motility. The cell then undergoes changes that give it a thick-walled, warty appearance.     

BIOGEOGRAPHY OF ASEXUAL AND SEXUAL REPRODUCTION IN CALOGLOSSA (DELESSERIACEAE,RHODOPHYTA) FROM AUSTRALIA AND NEW ZEALAND

Caloglossa species are widely distributed in mangroves and salt marshes around the world and their life history patterns are being investigated in laboratory culture. In Australia all isolates of C. monosticha, C. postiae and C. ogasawaraensis have Polysiphonia‐type (P‐type) sexual life histories. Among the 70 C. leprieurii isolates from Australia and New Zealand P‐type sexual reproduction also is dominant. However, ten isolates of C. leprieurii from the Spencer Gulf and the Gulf of St. Vincent in South Australia give rise to successive tetrasporphyte generations without gametophytes. Moreover, one isolate from Queensland is asexual. Only one South Australia isolate, obtained from Lake Alexandrina at the mouth of the Murray River, is sexual. South Australia and Pacific Mexico are two regions in which asexual reproduction is dominant. In another mangrove dwelling red alga Bostrychia moritiziana (Rhodomelaceae) non‐sexual reproduction also is frequent in Australia, New Caledonia and Bali (Indonesia). This asexual reproductive pattern of tetrasporophytic recycling appears to have arisen independently among individual populations of various red algal species in different regions. Investigations are underway on the molecular phylogeny of the Caloglossa leprieurii isolates.     

A STUDY OF THE SEXUAL REPRODUCTION AND DETERMINATION OF MATING TYPE OF GYMNODINIUM NOLLERI (DINOPHYCEAE) IN CULTURE1

Sexual reproduction of Gymnodinium nolleri ( Ellegaard & Moestrup 1999 ) was studied by intercrossing experiments in all combinations of six clonal strains and backcrossing of five clonal F1 offspring. The results indicated that the conjugation of G. nolleri responded to the existence of more than two sexual types (complex heterothallism) and that compatibility between progeny of one cyst (inbreeding) was the rule. Sexual fusion, planozygote formation and development, cyst formation, and germination and planomeiocyte division were followed using time‐lapse photography. This study revealed many similarities between the sexual stages and life cycle pattern of G. nolleri and the related G. catenatum and the existence under culture conditions of an alternative cycle between vegetative cells and zygotes without a hypnozygote stage. The fate of zygotes, division or encystment, was influenced by the nutritional status of the external medium. The division of G. nolleri planozygotes was promoted by high levels of external nutrients, whereas the maximum percentage of encystment was recorded when phosphates were reduced in the isolation medium. The division of zygotes might be different from both vegetative and planomeiocyte division because it resulted in two‐cell chains with the cells not oriented in parallel.     

Population characteristics and life cycle of the Lake Baikal invader <Emphasis Type="Italic">Gmelinoides fasciatus</Emphasis> (Stebbing, 1899) (Crustacea: Amphipoda in Lake Ladoga

Dynamics of the size-age structure and sexual structure of the population and characteristics of reproduction and life cycle have been studied in the Baikal invader Gmelinoiudes fasciatus (Stebbing, 1899) in Lake Ladoga. The studied characteristics display both similar features with and differences from populations of this species in other waterbodies. It is found that the size and age structure of the G. fasciatus population changes in the circannian aspect under the influence of complex of environmental factors. The sexual structure of the population is dynamic, but a sex ratio of approximately 1: 1 is maintained in the circannian cycle. Six principal periods of hatching are recognized. The main factor determining the timing of the start and termination of the reproduction period in G. fasciatus is temperature. The weight of eggs oviposited by the female over the whole life cycle (about 21.5 cal in the energy equivalent) amounts to about 60% of the female body weight.     

Biogeography of sexual and asexual populations in Bostrychia moritziana (Rhodomelaceae, Rhodophyta)

Bostrychia moritziana (Sonder ex Kützing) J. Agardh is recorded from many regions around the world. Our laboratory culture investigations have verified a sexual life cycle in isolates from Australia, Venezuela, Colombia, South Africa, Fiji, New Zealand and Indonesia. By contrast, asexual isolates producing successive generations of tetrasporophytes in laboratory culture and, presumably, in the field, are known from Australia. New Caledonia and Japan. In Australia, asexual reproduction is absent only in Victoria. In Western Australia, Northern Territory and Queensland, 99% of the isolates have asexual reproduction. In New South Wales (NSW), asexual and sexual populations are often intermixed. Of the 176 worldwide field collections, 58% were vegetative, 39% were tetrasporic, 2% were female and 1% were male. After several years of observations on the asexual isolates in culture, at least 30 successive asexual tetrasporophytic generations have developed. Only two asexual isolates (3558 and 3575) from NSW have formed a single male and female gametophyte in culture. In a self-cross of 3568, the carpospores developed into tetrasporophytes that recycled asexually. All outcrosses done with normal sexual isolates produced normal carposporophytes and the carpospores developed into tetrasporophytes that also recycled asexually. Asexual populations may arise repeatedly by loss of meiosis in tetrasporangia of sexual populations. Asexual reproduction apparently does not diminish the overall dispersal and abundance in the field. Our present bio-geographic data show that sexually reproducing popuiations of B. moritziana occur worldwide, while asexuaily reproducing populations are confined to the western Pacific. Bostrychia bispora West et Zuccarello, initially described on the basis of its asexual reproduction to distinguish it from B, moritziana, is now reduced to synonymy with B. moritziana.     

HOST IMMUNE STATUS DETERMINES SEXUALITY IN A PARASITIC NEMATODE

We examine the hypothesis that sexual reproduction by parasites is an adaptation to counter the somatic evolution of vertebrate immune responses. This is analogous to the idea that antagonistic coevolution between hosts and their parasites maintains sexual reproduction in host populations. Strongyloides ratti is a parasitic nematode of rats. It can have a direct life cycle, with clonal larvae of the wholly parthenogenetic parasites becoming infective, or an indirect life cycle, with clonal larvae developing into free-living dioecious adults. These free-living adults produce infective larvae by conventional meiosis and syngamy. The occurrence of the sexual cycle is determined by both environmental and genetic factors. By experimentally manipulating host immune status using hypothymic mutants, corticosteroids, whole-body γ-irradiation and previous exposure to S. ratti, we show that larvae from hosts that have acquired immune protection are more likely to develop into sexual adults. This effect is independent of the method of manipulation, larval density, and the number of days postinfection. This immune-determined sexuality is consistent with the idea that sexual reproduction by parasites is adaptive in the face of specific immunity, an idea which, if true, has clinical and epidemiological consequences.     

TWO NEW SPECIES OF CHLAMYDOMONAS1,2

Two new species of Chlamydomonas, C. surtseyiensis and C. archibaldii, are described as is their sexual reproduction which occurs in clonal culture.     

MORPHOLOGY AND DEVELOPMENT OF AHNFELTIA PLICATA (RHODOPHYTA): PROPOSAL OF AHNFELTIALES ORD. NOV.1

Ahnfeltia plicata (Hudson) Fries, the type species of Ahnfeltia Fries, is currently assigned to the Phyllophoraceae (Gigartinales). Several morphological and biochemical characters distance A. plicata from the Phyllophoraceae but, because sexual reproduction has never been demonstrated, an alternative placement has not been possible. A. plicata now is shown to have a heteromorphic sexual life history. Erect branched gametophytes are dioecious. In male sori, spermatangia are cut off transversely from spermatangial mother cells. Female sori form numerous terminal sessile carpogonia. Following fertilization, several zygotes in each sorus fuse facultatively with undifferentiated intercalary cells of the female sorus and cut off gonimoblast initials obliquely outwards. These initials give rise to branching gonimoblast filaments that fuse with apical and intercalary female sorus cells and with each other, then grow radially outward in the compound external carposporophyte and terminate in carposporangia. Carpospores develop in culture into crustose tetrasporophytes identical to Porphyrodiscus simulans Batters. Field-collected P. simulans tetraspores grew into erect A. plicata axes. Tetrasporangia are formed by division and enlargement of crust apical cells followed by sequential enlargement and maturation of tetrasporocytes in an erosive process. Monosporangia are formed in sori on male gametophytes. Pit plugs of both gametophyte and tetrasporophyte phases consist of naked plug cores without cap layers of membranes. Gametophytes exhibit both cell fusions and secondary pit connections whereas tetrasporophytes form cell fusions but lack secondary pit connections. On the basis of the unique female and postfertilization reproductive development and in conjunction with the pit plug structure which is unique among florideophytes, the order Ahnfeltiales, containing the family Ahnfeltiaceae, is proposed.     

Life cycle of Chattonella marina (Raphidophyceae) inferred from analysis of microsatellite marker genotypes

Red tides of Chattonella spp. have caused continuous damage to Japanese aquaculture, however, the life cycle of this organism remains incompletely understood. To further investigate this matter, we assessed genotypes at 14 microsatellite markers in three varieties of Chattonella marina, viz., C. marina var. antiqua, C. marina var. marina, and C. marina var. ovata, to establish whether Chattonella undergoes asexual diploidization or sexual reproduction. After genotyping 287 strains of C. marina, all but one of these strains was shown to be heterozygous for at least some loci, and thus, in the diploid state, suggesting that Chattonella strains undergo sexual reproduction. In addition, we performed single‐cell amplification on ‘small cells’ that are derived from vegetative cells under dark and low‐nutrient conditions. The results indicated the existence of two types of small cells. The ‘Small cell Type 1’ was found to be heterozygous, genotypically equivalent to the vegetative cells, and is therefore diploid. These small cells may change to resting cells (cysts) directly. The ‘Small cell Type 2’ was homozygous at all analyzed loci, suggesting that these small cells are haploid and may be derived by meiosis. As fusion between small cells has previously been observed, the ‘Small cell Type 2’ may be the gamete of Chattonella. We present a construct of the full life cycle of Chattonella marina based on our own and previous results.     

SEASONAL CHANGE IN THE DISTRIBUTION OF CELL SIZE OF COCCONEIS SCUTELLUM VAR. ORNATA (BACILLARIOPHYCEAE) IN RELATION TO GROWTH AND SEXUAL REPRODUCTION1

Growth and sexual reproduction of the marine littoral diatom Cocconeis scutellum Ehrenb. var. ornata Grun. were investigated at 30 different combinations of temperature (5, 10, 14, 18, 22° C), irradiance (20, 60, 100 μE·m^?2·s^?1) and daylength (14:10 and 10:14 h LD cycle). Growth occurred at all combinations. The optimal growth was observed at 14–18° C, long daylength and highest-to-moderate irradiance, and at 18° C, short daylength and highest irradiance. Sexual reproduction on the other hand occurred between 5 and 18° C, and the optimal condition was 10–14° C and short daylength. Annual cyclic, and sesonal changes in the distribution of cell size (valve length) were observed in a field population. These changes were characterized by an annual minimum in mean cell size in autumn, an annual maximum in winter, a slight decrease from the mean in spring–middle summer, a rapid decrease from the mean in late summer–early autumn, and appearance of bimodal distribution of cell size in winter. These changes were caused by sexual reproduction in autumn, rapid growth in late summer–early autumn and slow growth in other seasons, and poor viability of small cells near the lower end of the size range.     

CURRENT KNOWLEDGE OF THE LIFE CYCLES OF PHAEOCYSTIS GLOBOSA AND PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE)1

Despite continuous efforts since the 1950s and more recent advances in culturing flagellates and nonflagellate cells of the prymnesiophyte Phaeocystis, a number of different life‐cycle models exist today that appear to apply for P. globosa Scherff. and P. antarctica G. Karst., both spherical colony formers. In one such model, this life cycle consists of three different flagellates and one nonmotile cell stage that is embedded in carbohydrate matrix‐forming colonies of different sizes and forms. Recently, noncolonial aggregates of diploid nonmotile cells attached to surfaces of diatoms were put forward as a new stage in the sexual life cycle of P. antarctica. However, it can be discussed that these “attached aggregates” (AAs) are an intermediate between motile diploid flagellates, with their well‐known tendency to adhere to surfaces, and the young spherical colony with its diploid nonmotile cells, which in nature is commonly found attached to diatoms. A life‐cycle model pertaining to both P. globosa and P. antarctica is presented.     

Light is a crucial signal for zoosporogenesis and gametogenesis in some green microalgae

Patterns of reproduction were investigated in some microalgal species of Chlorophyceae (Botryosphaerella sudetica, Neochloris aquatica, Neochloris vigensis, Bracteacoccus minor). Under continuous light, the microalgae reproduced asexually producing autospores. However, appropriate manipulation of external conditions led to a change in the reproduction pattern towards production of zoospores or gametes. Production of zoospores and gametes was inhibited by light; motile cells emerged when microalgae were cultivated in darkness. The period of dark treatment necessary for zoosporogenesis or gametogenesis differed substantially among species that were tested. Sexual reproduction was observed in Neochloris vigensis and Bracteacoccus minor, whose generative life cycle had not been previously reported. The morphology of motile cells, the mode of sexual reproduction, and the efficiency of both the production of motile stages and mating events, were investigated. In order to gain detailed insights into patterns of reproduction, Botryosphaerella sudetica was selected for investigation under different light treatments. Non-actinic red light applied in the early phase of dark cultivation (up to 2 h) suppressed both zoosporogenesis and gametogenesis. However, after a 3-h dark pre-treatment, red light treatment had no effect on zoosporogenesis or gametogenesis. In contrast, non-actinic blue light did not block zoosporogenesis or gametogenesis, regardless of the time of treatment. The possible role of a red-light photoreceptor in zoosporogenesis and gametogenesis is discussed.     

The rotifers of Lake Peipus

In the northern part of Lake Peipus, 140 taxa of rotifers were identified, with species of Anuraeopsis, Conochilus, Keratella, Polyarthra and Synchaeta dominating. Two main periods of sexual reproduction occur, in the spring and autumn. Different life cycle patterns are represented. Rotifer number and biomass have two maxima between spring and early autumn. The contribution of rotifers to total zooplankton production varies from 13.6% (Oct.) to 89.8% (May). The average production of grazing rotifers is 485.1 kJ m^–2, while that of predatory rotifers (Asplanchna) is 10.0 kJ m^–2.     

Tracking bacterial growth in liquid media and a new bacterial life model

By increasing viscosity of liquid media above 8.4 centipoise (cp) i.e. 0.084 g· cm^-1 · s^-1, individual growth and family formation ofEscherichia coli was continuously observed in real-time for up to 6 h. The observations showed primarily unidirectional growth and reproduction ofE. coli and suggested more than one reproduction in the observed portion ofE. coli life span. A new bacterial life model is proposed: each bacterium has a stable cell polarity that ultimately transforms into two bacteria of different generations; the life cycle of a bacterium can contain more than one reproduction cycle; and the age of a bacterium should be defined by its experienced chronological time. This new bacterial life model differs from the dominant concepts of bacterial life but complies with all basic life principles based on direct observation of macroorganisms.     

Sex at the origin: an Asian population of the rice blast fungus Magnaporthe oryzae reproduces sexually

Sexual reproduction may be cryptic or facultative in fungi and therefore difficult to detect. Magnaporthe oryzae, which causes blast, the most damaging fungal disease of rice, is thought to originate from southeast Asia. It reproduces asexually in all rice‐growing regions. Sexual reproduction has been suspected in limited areas of southeast Asia, but has never been demonstrated in contemporary populations. We characterized several M. oryzae populations worldwide both biologically and genetically, to identify candidate populations for sexual reproduction. The sexual cycle of M. oryzae requires two strains of opposite mating types, at least one of which is female‐fertile, to come into contact. In one Chinese population, the two mating types were found to be present at similar frequencies and almost all strains were female‐fertile. Compatible strains from this population completed the sexual cycle in vitro and produced viable progenies. Genotypic richness and linkage disequilibrium data also supported the existence of sexual reproduction in this population. We resampled this population the following year, and the data obtained confirmed the presence of all the biological and genetic characteristics of sexual reproduction. In particular, a considerable genetic reshuffling of alleles was observed between the 2 years. Computer simulations confirmed that the observed genetic characteristics were unlikely to have arisen in the absence of recombination. We therefore concluded that a contemporary population of M. oryzae, pathogenic on rice, reproduces sexually in natura in southeast Asia. Our findings provide evidence for the loss of sexual reproduction by a fungal plant pathogen outside its centre of origin.     

Genes involved in Dictyostelium discoideum sexual reproduction

Macrocyst formation in the cellular slime moulds is a sexual process induced under dark and humid conditions. Normal development life cycle in these organisms involves proliferation by cell division and, upon starvation, formation of multicellular aggregates and fruiting bodies, consisting of spores and stalk cells. Macrocyst formation, cell division by binary fission and spore formation are thus three alternative modes of reproduction, for which it is of interest to understand how a choice is made. The genetic basis of asexual development and fruiting body formation is well known, by contrast information on the genetic control of sexual reproduction during macrocyst formation is scarce. In Dictyostelium discoideum, the most widely used species, several cell-surface proteins relevant to sexual cell fusion have been identified using cell fusion-blocking antibodies, but isolation of the relevant genes has been unsuccessful. Analysis of sexually deficient mutants, some of which are normal for asexual development, has shown that sexual reproduction is regulated by both specific genes and genes that are also involved in asexual development. Reverse genetic analysis of 24 genes highly enriched in a gamete-specific subtraction library has revealed four genes involved in the regulation of sexual cell interactions. One of them was found to be a novel regulator of the cAMP signalling pathway specific to sexual development. Studies on the molecular genetic control of the sexual cycle will be reviewed and their contribution to our understanding of the organization and function of the D. discoideum genome as a whole discussed.     

THE LIFE HISTORY OF POLYSIPHONIA DENUDATA (DILLWYN) KÜTZING IN CULTURE1

The sequence of somatic phases in the life history of the red alga Polysiphonia denudata has been determined in unialgal culture, using supplemented seawater media. Tetraspores gave rise to sexual plants bearing either antheridia or carpogonia in 50 days. Fertilization occurred within 1 week and the fertilized carpogonium developed to produce the mature carposporophytic phase in approximately another week. Carpospores formed by the carposporophyte germinated to produce the tetrasporangium-bearing phase, which released tetraspores in 44 days, thus completing the life history in 3^1/2% months. No deviations from this pattern have so far been observed.