SEXUAL REPRODUCTION IN NAVICULA CRYPTOCEPHALA (BACILLARIOPHYCEAE)1

Homothallic sexual reproduction and auxosporulation were studied in monoclonal cultures and seminatural populations of the freshwater epipelic diatom Navicula cryptocephala Kütz. Gametangia paired via the girdle, one gamete was formed per gametangium (and hence one zygote per pair of gametangia), and gamete fusion took place without the formation of any copulation envelope or copulation canal. Superfluous nuclei from meiosis survived unusually long, so that gametes and young zygotes were probably functionally polyploid; later, all but two haploid nuclei degenerated. Expanded auxospores had a swollen center, but during formation of the initial valves, the auxospore contracted away from the perizonium to produce linear‐lanceolate valves. The pattern of reproductive behavior found in N. cryptocephala can be classified as type IIA2a auxosporulation in Geitler's system. The same type of zygote and auxospore formation seen in clonal cultures was observed in seminatural material from four lakes in Scotland and the Czech Republic. Variation in nuclear structure and auxosporulation in the N. cryptocephala species complex is discussed, as is the evolution of type II auxosporulation (one zygote per pair of gametangia) from type I auxosporulation (two zygotes per pair). The penalty of smaller numbers of zygote produced in type II may be outweighed by formation of larger auxospores (prolonging the vegetative phase) or more vigorous auxospores. The variation present among members of the N. cryptocephala complex indicates that biogeographical analyses based on use of the name N. cryptocephala, as performed recently to support the ubiquity hypothesis of protist distributions, are almost meaningless.     

MATING SYSTEM,SEXUAL REPRODUCTION,AND AUXOSPORULATION IN THE ANOMALOUS RAPHID DIATOM EUNOTIA (BACILLARIOPHYTA)1

The diatom genus Eunotia is unusual among raphid diatoms in having a raphe system consisting of two short slits that are not integrated into the primary pattern center. This and other characteristics, particularly the presence of rimoportulae, are consistent with the hypothesis that Eunotia is a basal lineage within the raphid group. We studied auxosporulation in E. bilunaris (Ehrenberg) Mills and E. tropica Hustedt for comparison with other raphid pennate diatoms and with araphid pennates; E. bilunaris was studied in parental and F1 generations. Like araphid pennates, E. bilunaris and E. tropica are heterothallic. Clones of the same mating type did not interact sexually, and intraclonal sexual reproduction was absent or very rare. Clones retained the same sex throughout the life cycle, as shown by experiments using abrupt size reduction to produce clones of similar age but different size and using subclones derived from a single initial cell within six mitotic generations. Unlike in araphid pennate diatoms, in the Eunotia species the gametes are not visibly or behaviorally differentiated. Gametogenesis is merogenous, because the gametangium formed a supernumerary cell as well as a single gametic cell, both undergoing meiosis II to form a surviving functional nucleus and a nucleus that quickly degenerated. Plasmogamy is via papillae that grew out toward each other from the ends of the gametangia to create a copulation canal. After plasmogamy, the gametes moves bodily into the copulation canal, producing an elongate zygote, which expands to form a curved sausage‐like auxospore.     

SCALY INCUNABULA,AUXOSPORE DEVELOPMENT,AND GIRDLE POLYMORPHISM IN SELLAPHORA MARVANII SP. NOV. (BACILLARIOPHYCEAE)1

Uniparental auxosporulation was observed in a monoclonal culture of a Sellaphora clone isolated from the epipelon of a fishpond in the Czech Republic. The cox1 sequence for the clone confirmed that it belonged to the Sellaphora pupula–bacillum species complex but showed significant differences from all previously characterized Sellaphora species, and it is therefore described as S. marvanii sp. nov. Protoplast, valve, and girdle structure resembled those of other Sellaphora species, but a novel finding for all diatoms was a change in girdle structure during the life cycle: the most advalvar girdle band (valvocopula) bore a single line of pores in enlarged postauxospore cells but was entirely plain in small cells and gametangia. The young auxospores were covered by incunabula containing large, delicate, ± circular scales, resembling those of centric diatom auxospores; similar scales have been reported in a few other raphid diatoms (Pseudo‐nitzschia multiseries, Diploneis sp.) but contrast with the strip incunabula of some Nitzschia and Pinnularia and the helmet‐like caps of Neidium. The scales persisted during auxospore expansion, mostly as two caps over the auxospore poles. The transverse perizonium comprised a very wide, closed primary band, flanked by numerous secondary bands whose open ends were strongly incurved toward the center. Initial valves were differentiated from their immediate descendants by the very strong external demarcation of the raphe sternum, irregular shape, and curved transapical profile.     

CELL ENLARGEMENT IN SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)1

Skeletonema costatum (Grev.) Cl. exhibits an asexual means, of increasing cell size that is more common than auxosporulation. This phenomenon accounts for the genetic stability of S. costatum in culture and for the deficiency of heterozygotes in natural populations, and has important implications far the life history of this species.     

OOGAMOUS REPRODUCTION,WITH TWO‐STEP AUXOSPORULATION,IN THE CENTRIC DIATOM THALASSIOSIRA PUNCTIGERA (BACILLARIOPHYTA)1

Thalassiosira species are common components of marine planktonic communities worldwide and are used intensively as model experimental organisms. However, data on life cycles and sexuality within the genus are fragmentary. A clone of the cosmopolitan marine diatom Thalassiosira punctigera Cleve emend. Hasle was isolated from the North Sea and oogamous sexual reproduction was observed in culture. Cells approximately 45 μm and smaller became sexualized. Oogonia were produced preferentially and spermatogenesis was infrequent. Unfertilized oogonia always aborted and their development was apparently arrested at prophase of meiosis I. Further progression through meiosis and auxospore formation occurred only after a sperm had penetrated into the oocyte. Many cells of the new large‐celled generation (approximately 90–120 μm in size) immediately became sexualized again but only oogonia were produced. A few of the large oogonia became auxospores and produced initial cells 132–153 μm in diameter. The second step of auxosporulation probably involved fertilization of large‐celled oocytes by the sperm of the small‐celled spermatogonangia that were still present in the culture. An F₁ clone obtained after selfing within the small‐celled auxosporulation size range was investigated. Like the parent clone, the F₁ clone was homothallic but no auxosporulation was observed: spermatogonangia were unable to produce viable sperm, apparently because of inbreeding depression. Aggregation and interaction of oogonia were documented, and may be relevant for understanding the mechanisms of signaling and recognition between sexualized cells and the evolution of sexuality in pennate diatoms.     

SEXUALITY, INCOMPATIBILITY, SIZE VARIATION, AND PREFERENTIAL POLYANDRY IN NATURAL POPULATIONS AND CLONES OF SELLAPHORA PUPULA (BACILLARIOPHYCEAE)

The capitate and rectangular demes of the freshwater epipelic diatom Sellaphora pupula (Kütz.) Mereschk. are dioecious, the first such report for any freshwater diatom. Sexual differentiation, which is probably determined genetically, involves recognition at the cell surface as well as differences in gamete behavior (one gametangium produces an active "male" gamete, the other a passive "female" gamete). In culture, successful sexual reproduction occurs only when compatible clones are mixed. All cells of a clone behave identically in interclonal crosses, being either male or female, regardless of the stage of the life cycle, in contrast to the sequential hermaphroditism of centric diatoms. Males and females have identical frustule morphology. As in other diatoms, there is an upper size threshold for sexual reproduction, below which cells become progressively easier to sexualize. In culture, sexual interactions occur in cells much smaller than those ever seen in natural populations, so that in nature the sexual size range is effectively open. Natural populations almost always contain sexualizable cells; often, most of the cells are below the upper sexual size threshold. Male gametangia are, on average, slightly larger than females in the capitate deme, which may be produced by preferential polyandry, depleting the population of males and making them younger at mating. Rarely, selfing occurs producing zygotes, but these abort before producing initial cells. The sizes of the gametangia and initial cells are correlated but this does not invalidate the use of "cardinal points" of the life cycle in taxonomy. No interbreeding occurs between the rectangular and capitate demes. However, when males of one deme are mixed with females of the other, there is a stimulation of activity, as during the early stages of pairing in compatible intrademic crosses.     

PERIZONIUM AND INITIAL VALVE FORMATION IN THE DIATOM NAVICULA CUSPIDATA (BACILLARIOPHYCEAE)1

This paper describes the perizonium and initial valve formation in Navicula cuspidata Kütz., based on light microscope (LM) and scanning electron microscope (SEM) observations. The perizonium consists of concentric over-lapping bands, laid down sequentially at the tips of the expanding biconical auxospore during its elongation. The central perizonial band has fimbriate edges and is considerably more rigid than the more distal bands. During auxospore elongation and the band secretion, the chloroplasts continuously oscillate between the two ends of the cell; this oscillation ceases once the elongation is complete. The initial valves, formed within the perizonium, are molded into the basically biconical shape of the perizonium except for a central flattening of each valve face. In contrast to the raphes in gametangial and vegetative valves which are surrounded by a smooth axial area, the raphes in initial valves lie within a raised ridge running along the apical axis of the valve. The regular pattern of apically oriented ridges on the outer surface of vegetative valves is also lacking on initial valves. Comparison of pore–pore spacing within striae of gametangial valves, initial values and post-initial valves (first division and vegetative cells) reveals that the pore–pore distance within striae is conserved at all sexual stages. However, the distance between striae is considerably larger in initial valves than in gametangial and post-initial valves. Vegetative interstriae spacing as well as the planar morphology of the valve face is regained at the first division of the initial cell. This suggests that the spacing between striae is dependent on the sexual stage of the cell during valve formation (i.e. not directly dependent on the cell size) and can be altered independently of the pore–pore spacing.     

SEXUAL REPRODUCTION,MATING SYSTEM,AND PROTOPLAST DYNAMICS OF SEMINAVIS (BACILLARIOPHYCEAE)1

Cell division, the mating system, and auxosporulation were studied in the marine epipelic diatom Seminavis cf. robusta Danielidis & D. G. Mann. The interphase protoplast contains two girdle‐appressed chloroplasts, each with an elongate bar‐like pyrenoid, and also a central nucleus, located in a bridge between two vacuoles. Before cell division, the chloroplasts divide transversely and translocate onto the valves. The nucleus relocates to the ventral side for mitosis. After cytokinesis and valve formation, the chloroplasts move back to the girdle, showing a constant clockwise movement relative to the epitheca of the daughter cell. Seminavis cf. robusta is dioecious, and sexual reproduction is possible once cells are less than 50 μm. In crosses of compatible clones, gametangia pair laterally, without the formation of a copulation envelope, and produce two gametes apiece. The intensity of sexualization increases as cells reduce further in size below the 50‐μm threshold. At plasmogamy, the gametangia dehisce fully and the gametes, which were morphologically and behaviorally isogamous, fuse in the space between the gametangial thecae. The auxospore forms a transverse and longitudinal perizonium. After expansion is complete, there is an unequal contraction of the protoplast within the perizonium, creating the asymmetrical shape of the vegetative cell. Apart from this last feature, almost all characteristics exhibited by the live cell and auxospores of Seminavis agree with what is found in Navicula sensu stricto, supporting the classification of both in the Naviculaceae. Haploid parthenogenesis and polyploid auxospores were found, lending support to the view that change in ploidy may be a significant mechanism in diatom evolution.     

TAXONOMY,LIFE CYCLE,AND AUXOSPORULATION OF NITZSCHIA FONTICOLA (BACILLARIOPHYTA)1

Nitzschia fonticola (Grunow) Grunow is a member of Nitzschia sect. Lanceolatae, a group of taxonomically intractable but ecologically important and widespread diatoms. We investigated the morphology and life cycle in three clones of N. fonticola and all exhibited reduced sexuality, with pedogamous production of auxospores in unpaired gametangia. The auxospores of all clones contained tangles of striplike elements that lay outside the perizonium and were distinct from it in structure and ontogeny. We introduce a new term, incunabula, to refer to such components of the auxospore wall. Semicryptic variation was detected: one clone differed from the other two in valve size and shape, stria density, and fibula density, as well as its nuclear large subunit ribosomal DNA (LSU rDNA) sequence. The implications of reduced sexuality for the taxonomy of sect. Lanceolatae are discussed. A lectotype is designated for N. fonticola from among original material of Grunow, and the application of the name is clarified further by designating illustrations and the LSU sequence AM182191 from one of our clones as epitypes.     

AUXOSPORE FORMATION BY THE SILICA‐SINKING,OCEANIC DIATOM FRAGILARIOPSIS KERGUELENSIS (BACILLARIOPHYCEAE)1

Size restoration by the auxospore that develops from the zygote is a crucial stage in diatom life cycles. However, information on sexual events in pelagic diatom species is very limited. We report for the first time auxospore formation by the pennate diatom Fragilariopsis kerguelensis (O'Hara) Hustedt during an iron‐induced bloom in the Southern Ocean (EIFEX, European Iron Fertilization EXperiment). Auxospores of F. kerguelensis resembled those described for Pseudo‐nitzschia species. The auxospore was characterized by an outer coating, the perizonium; two caps, one at each distal end; and four chloroplasts, one at each end and two in the central part. Different stages of auxospore elongation were recorded, with a length of 24–91 μm, but only the largest auxospores contained the initial cell, whose apical axis ranged between 76 and 90 μm. Gametangial cell walls were often attached to the auxospores and ranged from 10 to 31 μm in length. Auxospore abundances were consistently higher in the fertilized patch, where an increase in the F. kerguelensis population was observed, as compared with surrounding waters.     

SOME ASPECTS OF MORPHOLOGICAL VARIATION IN STEPHANODISCUS NIAGARAE (BACILLARIOPHYCEAE)1

Valves of S. niagarae var. niagarae Ehr. and S. niagarae var. magnifica Fricke from geographically dispersed sediment and plankton collections were observed by light microscopy and scanning electron microscopy (LM and SEM). Measurements made by LM can be arranged so that means and ranges of diameter, areolar density, or strial density intergrade from one population into the next. Mean diameter is negatively related to increasing areolar and strial densities. No unique features observable by SEM distinguish the two described varieties. Thus, S. niagarae var. magnifica, having large diameter valves with low areolar and strial densities, may represent one end of a trend in overall variation in S. niagarae. At the opposite extreme are the populations with small valves and high areolar densities which are often erroneously referred to as S. astraea. Type material of S. niagarae lies intermediate to these forms. Three populations considered in this study have distinct morphological characteristics. Valves from Yellowstone Lake sediments have spine placements distinctly different from valves of all other populations. Specimens from Lake Superior have nearly flat central areas. Ribs of valves from Grand Traverse Bay (Lake Michigan) are covered with granules.     

PHOTOSYNTHETIC PIGMENT ORGANIZATION IN DIATOMS (BACILLARIOPHYCEAE)1

The isolation and characterization of six pigment-protein complexes from five diatom species (Phaeodactylum tricornutum Bohlin, Chaetoceros gracilis Schutt, Nitzschia sp. Mono Lake, Nitzschia laevis Hust. and Thalassiosira pseudonana (Hust.) Hasle and Heimdal) was accomplished by membrane dissociation with digitonin followed by gel electrophoresis. Six analogous complexes obtained from all species were correlated in spectral characteristics and relative mass with complexes from higher plants obtained by the same procedure. The largest of these complexes, comprising about 15% of the total Chl a, contained reaction centers of Photosystem I (PSI) and antenna pigments (LHC₁).Some PSI complexes also separated from LHC₁ in the gel. For the first time in diatoms, a Photosystem II complex was isolated and identified from its position in the gels, absorption and fluorescence spectra, lack of P700, and enrichment carotene. Three antenna pigment-protein complexes in addition to LHC₁ occurred in varying proportion under different experimental conditions but in sum, they accounted for 70% of the total Chl a. All three bands were highly enriched in Chl c and fucoxanthin, although the ratio of Chl c/ xanthophyll decreased between the slowest migrating LHC₂ and fastest moving LHC₄ LHC₃ contained the highest proportio of pigment-protein and was composed primarily of polypeptides of about 18,000 D. Essentially all α- and β-carotene was bound to the reaction center complexes. The Nitzschia from Mono Lake differed from the other species in that PSI complexes could not be readily dissociated from its membrane by digitonin treatment, a characteristic which may reflect a different chloroplast membrane structure in this alga.     

AUXOSPORULATION IN CYCLOTELLA OCELLATA (BACILLARIOPHYCEAE) UNDER NATURAL AND EXPERIMENTAL CONDITIONS1

Growth and sexual reproduction in a population of Cyclotella ocellata Pantocseck were studied during one annual cycle in a reservoir and in short-term enclosure experiments performed in situ involving different nutrient conditions and concentrations of zooplankton species. Three phases of auxosporulation in this diatom were distinguishable morphologically: 1) preauxospore, from the beginning of zygote formation until the valves were longitudinally separated, 2) primary auxospore, when the zygote grew too large to fit inside the valves and before it reached its full size, and 3) mature auxospore, characterized by a well-developed, markedly scalloped edge. Under experimental and natural conditions, sexual reproduction was associated with changes in cell size. In the natural system, the auxospore appeared to act as a resting structure during conditions adverse for population growth. A threshold population of small cells appeared to be necessary for sexual reproduction in the natural system, whereas auxosporulation was associated with phosphorus fertilization in the enclosures. In both environments only cells smaller than 9.5 μm in diameter were capable of auxospore formation. Our results suggest that, once having reached the critical cell size, the factors that trigger sexual reproduction may depend on ambient environmental conditions.     

MICROSATELLITE MARKER DEVELOPMENT AND GENETIC VARIATION IN THE TOXIC MARINE DIATOM PSEUDO‐NITZSCHIA MULTISERIES (BACILLARIOPHYCEAE)1

The genetic structure of phytoplankton populations is largely unknown. In this study we developed nine polymorphic microsatellite markers for the domoic acid–producing marine diatom Pseudo‐nitzschia multiseries (Hasle) Hasle. We then used them in the genotyping of 25 physiologically diverse field isolates and six of their descendants: 22 field isolates originated from eastern Canadian waters, two from European waters, and one from Russian waters. The numbers of alleles per locus ranged from three to seven and the observed heterozygosities from 0.39 to 0.70. A substantial degree of genetic variation was observed within the field isolates, with 23 different genotypes detected. The Russian isolate was the most genetically distinct, although there was also evidence of genetic differentiation at a more local scale. Mating experiments demonstrated that alleles were inherited in a Mendelian manner. Pseudo‐nitzschia multiseries primer pairs were tested on DNA from four congeners: P. calliantha Lundholm, Moestrup et Hasle; P. fraudulenta (P. T. Cleve) Hasle; P. pungens (Grunow ex P. T. Cleve) Hasle; and P. seriata (P. T. Cleve) H. Peragallo. Cross‐reactivity was only observed in P. pungens. Our results are a first step in understanding the genetic variation present at the Pseudo‐nitzschia“species” level and in determining the true biogeographic extent of Pseudo‐nitzschia species.     

THE NATURAL LIFE CYCLE IN WILD POPULATIONS OF DIATOMA MONILIFORMIS (BACILLARIOPHYCEAE) AND ITS DISRUPTION IN AN ABERRANT ENVIRONMENT1

We studied how size variation in populations of Diatoma moniliformis Kütz. was influenced by environmental effects on the diatom life cycle. One of the two populations sampled monthly in the northern Baltic Sea grew under natural conditions; the other population was in a cooling water discharge channel of a nuclear power plant, where the temperature and flow rate of the water were artificially higher. The life cycle was synchronous at the natural site, with sexual reproduction occurring in the winter; most of the initial cells were found in March-April. After this, a reduction in cell size occurred, and the vegetative life cycle consisted of two parts. During the first part, cell volume decreased, whereas the surface area to volume ratio increased, and during the second part of the cycle, both of these parameters decreased. No direct evidence was found for the existence of a supra-annual life cycle in D. moniliformis, as convincing modes fm large cells were lacking in the size-frequency distributions. It was concluded from extrapolations of the data that the natural life cycle of D. moniliformis probably lasts 2 or 3 years. The changes in cell proportions during the life cycle fit well with annual growth cycles of D. moniliformis at the natural site (i.e. the cells had high surface area to volume ratios during the period of optimal growth in late spring [May-June]). At the site affected by cooling water discharge, the synchronization of the natural life cycle was disrupted, but some seasonal size variation did occur. Under natural conditions, auxosporulation is probably triggered by a combination of small cell size, low water temperature (0–3° C), and rapidly increasing light intensity or daylength in late winter to early spring. When these conditions were not met (e.g. at the heated site, the required low temperature was absent), auxosporulation did not occur simultaneously. This paper also presents scanning electron photomicrographs showing the typical shape and fine structure of the initial cell of D. moniliformis. These cells are semispherical in cross section, possess a pronounced curvature along the longitudinal axis, and are bent in the perualvar plane.     

Auxosporulation of Licmophora communis (Bacillariophyta) and a review of mating systems and sexual reproduction in araphid pennate diatoms

The auxosporulation of Licmophora communis is allogamous and dioecious. Pairing between sessile, shortstalked cells of compatible clones is followed by meiosis and gametogenesis, to form two gametes in each gametangium. The behavior of the gametes differs between the gametangia. In the male gametangium, the gametes detach from the frustule, round up, and migrate out of the gametangium after its dehiscence at the broader, unattached pole. In the female gametangium, both gametes remain attached to the adjacent theca over almost their whole length and do not move. Plasmogamy therefore occurs within the female gametangium and this is where the zygotes are formed and remain. After fertilization, the zygotes detach from the thecae of the female gametangia, contract, and become ellipsoidal, before expanding parallel to the apical axis of the gametangium. We review the types of auxosporulation in other pennate diatoms and the systems used for classifying these. Dioecy and cis‐type anisogamy (in which one gametangium produces active gametes and the other produces passive gametes), as in L. communis, are probably primitive within the pennate group (although there is no information on the AsterionellopsisRhaphoneis clade). However, size can also be restored in various araphid pennates by allogamous sexual reproduction involving the formation of only one gamete per gametangium, or in rare cases by automixis or (apparently) vegetative enlargement.     

THE COMPLEX POLYSACCHARIDES OF THE RAPHID DIATOM PINNULARIA VIRIDIS (BACILLARIOPHYCEAE)1

A combination of carbohydrate analysis and atomic force microscopy (AFM) was used to characterize the polysaccharides of the pennate diatom, Pinnularia viridis (Nitzsch) Ehrenberg. Polymeric substances were fractionated into those in the spent culture medium (SCM) and those sequentially extracted from the cells with water at 45° C (WW), NaHCO₃ containing EDTA at 95° C (HB), and 1 M NaOH containing NaBH₄ at 95° C. Carbohydrate, protein, and sulfate were detected in all the fractions, but their relative proportions differed significantly. Nineteen sugars were identified, including pentoses, hexoses, 6‐deoxyhexoses, O‐methylated sugars, aminohexoses, and traces of uronic acids. To some extent, the same constituent monosaccharides and a proportion of the linkage patterns occurred in all four fractions, indicating the fractions contained a spectrum of highly heterogeneous but structurally related polysaccharides. Several carbohydrates were enriched in specific fractions. A soluble, partially substituted, 3‐linked galactan was slightly enriched in the SCM. The WW fraction was highly enriched in 3‐linked glucan, presumably derived from chrysolaminaran. Chemical and AFM data for the WW and HB fractions indicated that compositional differences were associated with substantial changes in the morphology and properties of the cell surface mucilage. Soluble polymers relatively enriched in fucose conferred a degree of softness and compressibility to the mucilage, whereas most of the mucilage comprised firmer more gelatinous polymers comparatively enriched in rhamnose. The frustule residue dissolved during extraction with NaOH, and a partially substituted 3‐linked mannan, together with relatively large amounts of protein, was obtained.     

ENDOBACTERIA IN THE DIATOM PINNULARIA (BACILLARIOPHYCEAE). II. HOST CELL CYCLE‐DEPENDENT TRANSLOCATION AND TRANSIENT CHLOROPLAST SCARS1

Rod‐shaped bacteria were found together with the “secondary chloroplast” in the endoplasmic reticulum (ER) in the pennate diatom Pinnularia. During the host interphase, bacteria bore cavities into the chloroplast and interacted with thylakoids, forming a functional unit with the plastid of the engulfed and reduced eukaryotic endocytobiont, without perforating the relict cell membrane and the chloroplast envelope. Fluorescently labeled antibodies directed against the large subunit of RUBISCO elicited the same pattern as DAPI–DNA complexes. In pre‐prophase, bacteria started to dissociate from the plastids and left scars at their previous locations. In profile, scars appeared as granular amorphous regions interrupting several stacks of thylakoids, whereas images of glancing sections showed foci of circumscribed spiral domains. The vortex pattern was interpreted as the result of a screwing movement of the bacteria during their withdrawal, initiated while structurally still connected to the thylakoids. Scars became effaced, and thylakoids reoriented during prophasic translocation of the complex plastid from the girdle to the valve. In the transition to prophase, bacteria were found as clusters inside ER cisternae near the nucleus, where the microtubule‐organizing center was accompanied by the spindle precursor. Microtubules were seen to be aligned with ER profiles (containing bacteria). TEM images indicated that intracellular long‐distance transport is controlled by the host cytoskeleton acting upon the ER. Restricted to this stage, bacteria appeared to divide and occasionally bore tentacle‐like appendages. Their intraluminal localization at the nucleus in prophase, near the new plasmalemma after cleavage, and their return to the chloroplasts shortly afterward revealed a cell cycle‐dependent translocation with the ER as circulatory system and, together with the obvious retardation of their division cycle, a domestication by the host.     

Repeated evolution of uniparental reproduction in Sellaphora (Bacillariophyceae)

Diatoms possess a remarkable life cycle in which cell size decreases slowly during vegetative cell division and then increases rapidly via special expanding cells called ‘auxospores’, which are usually formed as a result of biparental sexual reproduction. However, auxospores are sometimes produced by single unpaired cells, i.e. uniparentally. We examined the nature of uniparental auxosporulation in Sellaphora and used a two-gene dataset to study phylogenetic relationships between uniparental and biparental Sellaphora demes and species; we tested whether uniparental reproduction has evolved once or repeatedly in the genus. In at least two of the uniparental demes auxosporulation occurred through autogamy (i.e. intra-tetrad mating within an undivided cell). Maximum likelihood phylogenies indicated four lineages of uniparental Sellaphora and significance tests of alternative topologies, in which combinations of uniparental Sellaphora were constrained to be monophyletic, coupled with likelihood reconstruction of ancestral character states, led to rejection of the hypothesis that uniparental auxosporulation evolved only once in the genus. Uniparentally reproducing lineages appear to arise not infrequently in diatoms but do not persist. Two small extranuclear bodies, apparently containing DNA and lying outside the chloroplast (one close to each pole of the cell), were revealed by DAPI staining.