MASS SEXUAL REPRODUCTION IN THE TOXIGENIC DIATOMS PSEUDO‐NITZSCHIA AUSTRALIS AND P. PUNGENS (BACILLARIOPHYCEAE) ON THE WASHINGTON COAST,USA1

Sexual reproduction is documented for the first time in field populations of the pennate diatoms Pseudo‐nitzschia australis Freng. and P. pungens (Grunow ex Cleve) Hasle (var. cingulata Villac and hybrids between var. cingulata and var. pungens). A bloom dominated by these species began on June 26, 2006, along Kalaloch Beach, Washington, USA, coincident with a drop in the Si(OH)₄:NO₃ ratio to below two. Multimodal size distributions were detected for both species, and synchronous auxosporulation occurred within the smallest size class during a 3‐week window. Auxospores and initial cells created a new class of large cells, and cells in the intermediate size classes increased in abundance during auxosporulation. Mating cells of both species were attached to colonies of surf‐zone diatoms. Paired gametangia, gametes, zygotes, auxospores, and large initial cells were found. Auxosporulation began first for P. pungens (June 30), apparently once a critical, high cell concentration was reached, followed by P. australis (July 5), when the total Pseudo‐nitzschia cell concentration reached 929,000 cells · L^?1. Low frequencies of auxosporulation occurred throughout the bloom but increased 4‐fold for P. australis and 3‐fold for P. pungens when macronutrients were reduced to low levels on July 11. A 2‐year life cycle was estimated for P. australis and 3 years for P. pungens, both with annual auxosporulation. Domoic acid (DA) in razor clams reached a maximum of 38 μg DA · g^?1 on July 18. A significant relationship existed between the percent of cells within the new size range and DA concentrations in razor clams on the same beach.     

Variation in the sexual behaviour of Achnanthes longipes (Bacillariophyta). III. Progeny of crosses between monoecious and unisexual clones

The allogamous raphid diatom Achnanthes longipes C. A. Agardh possesses a complex breeding system involving interactions between three types of clone: monoecious, unisexual and bisexual. Previous studies showed that these three types can be crossed with each other, with a tendency for sexual characteristics to be inherited: inbred monoecious lineages gave rise to monoecious or, very rarely, to bisexual clones, while inbred unisexual lineages yielded unisexual and bisexual clones. The current paper reports on the progeny of crosses between monoecious and unisexual clones and their inbred offspring. All three types of clone appeared in the F₁ and F₂, although unisexual clones of opposite sex to the parental clone were not found. Inbreeding depression was observed and also a tendency for ‘normal’ auxosporulation (producing two auxospores per pair of gametangia) to be replaced by ‘reduced’ or ‘intermediate’ auxosporulation (producing one auxospore per pair). In addition, patterns of incompatibility were observed that were not seen during earlier studies of clones isolated directly from nature. These included the inability of some F₁ clones to mate with each other, in spite of compatibility with all other clones examined (unisexual, bisexual and monoecious).     

THE ROLE OF PHYTOPLANKTON SIZE ON PHOTOCHEMICAL RECOVERY DURING THE SOUTHERN OCEAN IRON EXPERIMENT1

Phytoplankton primary productivity in the Southern Ocean is controlled by complex interactions among iron, light, and grazing. This project interfaced with the Southern Ocean iron experiment (SOFeX) that created two iron‐enriched patches north and south of the Polar Front each with distinct silicic acid concentrations. We used pulse amplitude modulated fluorometry and measured the recovery of the maximum quantum yield of photochemistry (F_v/F_m) for three size fractions (whole, <5, <20 μm) and light adapted quantum yield (ΔF/F′_m) for single phytoplankton cells. The rates of recovery from iron stress were found to be unrelated to average cell size for both size‐fractioned and single‐celled measurements. The smallest cells appeared to exhibit more severe iron stress at the onset of the experiment than the larger taxa. The largest response detected in regression parameters was that of the pennate diatoms, which took only ～3.4 days to reach the maximum quantum yield, whereas the centric diatom Asteromphalus sp. reached maximum ΔF/F′_m after ～10.4 days. The north patch measurements showed a different response; the smallest cells never reached maximum ΔF/F′_m, whereas the size fraction containing the largest cells did. Single‐celled measurements made nearly 30 days after the initial iron enrichment suggested that diatoms were experiencing either silicic acid or iron limitation, whereas measurements of Phaeocystis sp. did not. These data represent the first study of in situ recovery rates of PSII for groups of diatoms, and may help elucidate the mechanisms of species change in response to environmental perturbation.     

RELATIONSHIPS BETWEEN GROWTH RATE,CELL SIZE,AND INDUCTION OF SPERMATOGENESIS IN THE CENTRIC DIATOM THALASSIOSIRA WEISSFLOGII (BACILLARIOPHYTA)1

Patterns of changes in cell size, growth rate, and the inducibility of spermatogenesis were followed in eight sub‐clones of two isolates of the centric diatom Thalassiosira weissflogii (Grunow) Fryxell & Hasle grown at saturating light. One isolate originated from Long Island Sound, New York, USA and the other originated from Jakarta Harbor, Indonesia. As expected from previous studies, oscillations between intervals of cell size reduction and cell size enlargement were observed for each sub‐clone. For both isolates, sperm were easily detected, but cells resembling eggs and auxospores were rarely observed and fertilization was not confirmed, suggesting that the observed cell size increases may have resulted from a combination of asexual cell enlargement and rare auxosporulation. The two isolates differed in their minimum and maximum sizes, and the threshold size for the induction of sperm formation. However, the two sets of isolated sub‐clones displayed comparable relationships between growth rate, sperm inducibility, and cell size relative to the minimum, maximum, and threshold sizes. Growth rate increased as cell size decreased during vegetative divisions until the threshold for sperm inducibility was crossed. Below the size threshold for sperm inducibility, growth rate declined as cell size continued to decrease. Smaller cells were susceptible to failure of normal cytokinesis and valve deposition, resulting in the formation of abnormally long and often multinucleate cells. Culture conditions may select against restoration of cell size via auxosporulation due to the relationship between growth rate and cell size.     

Premeiotic endomitosis produces diploid eggs in the natural clone loach, Misgurnus anguillicaudatus (Teleostei: Cobitidae)

The natural clone loach produces unreduced eggs genetically identical to somatic cells of the mother fish and such diploid eggs normally develop as a clone without genetic contribution of sperm. Following the identification of clonal nature and diploidy of eggs, we conducted cytological studies to determine the mechanisms responsible for this unusual oogenesis. Cytolological observation of full-grown oocytes cultured in vitro revealed that oocytes of both the clone and the control loach underwent two successive meiotic divisions: formation of a bipolar spindle and metaphase in meiosis I and equal segregation of chromosomes, extrusion of the first polar body and the appearance of metaphase of meiosis II. However, spindle size of the clone was larger than that of the control. Bivalent chromosome number of germinal vesicle of oocytes was 25 in the control diploid, whereas 50 in the clone. The results suggest that chromosomes are duplicated by mitosis without cytokinesis before meiosis, i.e. premeiotic endomitosis and then oocytes differentiated from tetraploid oogonia undergo a quasinormal meiosis followed by two successive divisions to produce diploid eggs.     

The meiosis‐specific APC activator FgAMA1 is dispensable for meiosis but important for ascosporogenesis in Fusarium graminearum

Ascospores are the primary inoculum in Fusarium graminearum. Interestingly, 70 of its genes have premature stop codons (PSC) and require A‐to‐I editing during sexual reproduction to encode full‐length proteins, including the ortholog of yeast Ama1, a meiosis‐specific activator of APC/C. In this study, we characterized the function of FgAMA1 and its PSC editing. FgAMA1 was specifically expressed during sexual reproduction. The Fgama1 mutant was normal in growth and perithecium formation but defective in ascospogenesis. Instead of forming four‐celled, uninucleate ascospores, Fgama1 mutant produced oval, single‐celled, binucleated ascospores by selfing. Some mutant ascospores began to bud and underwent additional mitosis inside asci. Expression of the wild‐type or edited FgAMA1 but not the uneditable allele complemented Fgama1. In the Fgama1 x mat‐1‐1 outcross, over 60% of the asci had eight Fgama1 or intermediate (elongated but single‐celled) ascospores, suggesting efficient meiotic silencing of unpaired FgAMA1. Deletion of FgPAL1, one of the genes upregulated in Fgama1 also resulted in defects in ascospore morphology and budding. Overall, our results showed that FgAMA1 is dispensable for meiosis but important for ascospore formation and discharge. In F. graminearum, whereas some of its targets are functional during meiosis, FgAma1 may target other proteins that function after spore delimitation.     

Taxonomic features of the red alga Meristotheca coacta (Solieriaceae, Gigartinales)

Vegetative and reproductive structures of the little‐known red alga, Meristotheca coacta Okamura (Solieriaceae, Gigartinales) were described on the basis of specimens collected from three localities in southern Japan, including an island close to the lectotype locality. Meristotheca coacta was shown to be a strongly supported monophyletic clade in our rbcL analyses. The taxonomic features of this species were reassessed. This species is characterized as follows: (i) the thalli consist of relatively thick (300–1400 μm from the uppermost to basal regions), elastic and rose‐red blades; (ii) the blades are irregularly lobed and branched into variously shaped segments with undulate and crispate margins; (iii) tetrasporangial initials are laterally attached to their parental cells and the lateral pit‐connections remain in mature tetrasporangia; (iv) gametophytes are dioecious; (v) carpogonial branches are three‐celled (occasionally two‐ or four‐celled); (vi) a darkly staining auxiliary cell complex is present and recognizable prior to diploidization; and (vii) cystocarps are produced along (or near) the margins of the blades and on the marginal proliferations and lack spinous outgrowths.     

Cytoplasmic streaming of calcium oxalate crystals in Callipsygma wilsonis (Bryopsidales, Chlorophyta)

Light microscopic study of the giant‐celled, marine green alga Callipsygma wilsonis J. Agardh (Udoteaceae, Bryopsidales) revealed numerous birefringent crystalline inclusions in the terminal segments of the assimilatory axes. The inclusions were thin plates with a triangular shape in face view, a base up to 75 μm in length, and a height that was one‐seventh the length of the base. Crystals of various sizes commonly were stacked face‐to‐face with one or more edges coinciding, but removal of organic material by treatment in sodium hypochlorite resulted in disaggregation. The crystals were soluble in dilute hydrochloric acid without effervescence but were insoluble in acetic acid. These diagnostic chemical solubility tests and a positive reaction to the Yasue staining reaction indicated that the crystals were composed of calcium oxalate. Scanning electron microscopy showed that most crystals had smoothly curving edges, but some had truncate or beveled margins. Calcium oxalate crystals have been reported to occur in the large central vacuoles of several bryopsidalean species, but the crystals in C. wilsonis were present in the parietal cytoplasm, which was evident from the presence of crystals in streaming cytoplasm. Calcium oxalate crystals, amyloplasts, chloroplasts, and other cytoplasmic constituents moved along cytoskeletal cables at rates of approximately 2.8 μm s⁻¹. These findings add to a growing body of evidence that calcium oxalate crystals in diverse algae may be present in cellular compartments other than the central vacuole.     

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.     

Analysis of gemini pollen 3 mutant suggests a broad function of AUGMIN in microtubule organization during sexual reproduction in Arabidopsis

In flowering plants, male gametes arise via meiosis of diploid pollen mother cells followed by two rounds of mitotic division. Haploid microspores undergo polar nuclear migration and asymmetric division at pollen mitosis I to segregate the male germline, followed by division of the germ cell to generate a pair of sperm cells. We previously reported two gemini pollen (gem) mutants that produced twin‐celled pollen arising from polarity and cytokinesis defects at pollen mitosis I in Arabidopsis. Here, we report an independent mutant, gem3, with a similar division phenotype and severe genetic transmission defects through pollen. Cytological analyses revealed that gem3 disrupts cell division during male meiosis, at pollen mitosis I and during female gametophyte development. We show that gem3 is a hypomorphic allele (aug6‐1) of AUGMIN subunit 6, encoding a conserved component in the augmin complex, which mediates microtubule (MT)‐dependent MT nucleation in acentrosomal cells. We show that MT arrays are disturbed in gem3/aug6‐1 during male meiosis and pollen mitosis I using fluorescent MT‐markers. Our results demonstrate a broad role for the augmin complex in MT organization during sexual reproduction, and highlight gem3/aug6‐1 mutants as a valuable tool for the investigation of augmin‐dependent MT nucleation and dynamics in plant cells.     

Recombination suppression in heterozygotes for a pericentric inversion induces the interchromosomal effect on crossovers in Arabidopsis

During meiosis, recombination ensures allelic exchanges through crossovers (COs) between the homologous chromosomes. Advances in our understanding of the rules of COs have come from studies of mutations including structural chromosomal rearrangements that, when heterozygous, are known to impair COs in various organisms. In this work, we investigate the effect of a large heterozygous pericentric inversion on male and female recombination in Arabidopsis. The inversion was discovered in the Atmcc1 mutant background and was characterized through genetic and next‐generation sequencing analysis. Reciprocal backcross populations, each consisting of over 400 individuals, obtained from the mutant and the wild type, both crossed with Landsberg erecta, were analyzed genome‐wide by 143 single‐nucleotide polymorphisms. The negative impact of inversion became evident in terms of CO loss in the rearranged chromosome in both male and female meiosis. No single‐CO event was detected within the inversion, consistent with a post‐meiotic selection operating against unbalanced gametes. Cytological analysis of chiasmata in F₁ plants confirmed that COs were reduced in male meiosis in the chromosome with inversion. Crossover suppression on the rearranged chromosome is associated with a significant increase of COs in the other chromosomes, thereby maintaining unchanged the number of COs per cell. The CO pattern observed in our study is consistent with the interchromosomal (IC) effect as first described in Drosophila. In contrast to male meiosis, in female meiosis no IC effect is visible. This may be related to the greater strength of interference that constrains the CO number in excess of the minimum value imposed by CO assurance in Arabidopsis female meiosis.     

Loci affecting male fertility in hybrids between <Emphasis Type="Italic">Mus macedonicus</Emphasis> and C57BL/6

Male F₁ hybrids between inbred strains and Mus macedonicus have very small testes and are sterile. Cytological analysis of testes shows very few meioses. To determine the genetic basis for this sterility, (C57BL/6J × Mus macedonics) F₁ females were mated to males from C57BL/10J. In about half the male progeny no meiosis I was observed. About half of the animals that progressed through meiosis I showed other indications of low fertility and the balance appeared fertile. QTL analysis of the progeny suggested that loci on proximal Chrs 17 and X were involved in the sterility and a locus on Chr X in variation of body weight. There is also evidence that X//Y dissociation of the pseudo-autosomal region occurs. The QTLs on Chrs X and 17 together account for about 37% of the variance for testis weight. Congenic lines B.MAC-X(1-38), and B.MAC-17(1-23) have been constructed using a modified speed congenic approach. Testis tubules from B.MAC-X(1-38) are narrow and vacuolated. They contain only Sertoli cells and mitotically dividing spermatogonia. Very occasionally a meiotic metaphase can be observed, but no sperm are produced. Homozygous males from B.MAC-17(1-23) are sterile, producing sperm heads but no complete sperm.     

THE OOGONIA OF MACROALGA UNDARIA PINNATIFIDA ARE ALKALINE‐PHOSPHATASE POSITIVE AND CONTAIN GERMINAL BODY‐LIKE STRUCTURES1

It was observed that in the female gametophyte of Undaria pinnatifida (Harv.) Suringar (Phaeophyta, Laminariales) gametangial initials and maturing oogonia demonstrated different levels of alkaline‐phosphatase activity (APA). The oogonia exhibited a higher level of APA than in its initials. Electron‐dense granular ovoid structures ～0.5–0.6 μm were present in the cytoplasm of oogonia. These inclusions were not membrane bound and do not appear to be associated with any particular organelles. The number of the inclusions was 1 to 2 in a single section of the cell. In essential details, the specific APA and subcellular germinal body‐like structure of the developing female gamete in U. pinnatifida were very similar to those in metazoan oocytes.     

Activity of the ovarian germinal epithelium in the freshwater catfish,Pimelodus maculatus (Teleostei: Ostariophysi: Siluriformes): Germline cysts,follicle formation and oocyte development

Distinct types of oogonia are found in the germinal epithelium that borders the ovarian lamellae of Pimelodus maculatus: A‐undifferentiated, A‐differentiated and B‐oogonia. This is similar to the situation observed for spermatogonia in the vertebrate testis. The single A‐undifferentiated oogonia divide by mitosis giving rise to A‐groups of single differentiated oogonia, each enclosed by epithelial cells that are prefollicle cells. Subsequently, the single A‐differentiated oogonia proliferate to generate B‐oogonia that are interconnected by cytoplasmic bridges, hence, forming germline cysts. The prefollicle cells associated with them also divide. Within the germline cysts, B‐oogonia enter meiosis becoming oocytes. Meiotic prophase and early folliculogenesis occur within the germline cysts. During folliculogenesis, prefollicle cells grow between the oocytes, encompassing and individualizing each of them. The intercellular bridges disappear, and the germline cysts are broken down. Next, a basement membrane begins to form around the nascent follicle, separating an oocyte and its associated prefollicle cells from the cell nest. Folliculogenesis is completed when the oocyte and the now follicle cells are totally encompassed by a basement membrane. Cells derived from the ovarian stroma encompass the newly‐formed ovarian follicle, and become the theca, thereby completing the formation of the follicle complex. Follicle complexes remain attached to the germinal epithelium as they share a portion of basement membrane. This attachment site is where the oocyte is released during ovulation. The postovulatory follicle complex is continuous with the germinal epithelium as both are supported by a continuous basement membrane. The findings in P. maculatus reinforce the hypothesis that ovarian follicle formation represents a conserved process throughout vertebrate evolution. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Sexual auxosporulation of the marine diatom Navicula directa var. directa

Sexual auxosporulation was observed in a mixed culture of two strains of Navicula directa var. directa. Two gametes did not re‐arrange in their gametangium and each adhered to the inner surfaces of the gametangial theca. Each of the two gametes of one gametangium fused with a gamete of the other gametangium iso‐gamously. As a result, two zygotes and hence two auxo‐spores were produced per paired gametangia. As the gametangial thecae kept close to the gametes during fusion, the zygote became associated with two different thecae. The presence of type IB2a of Geitler's (1973) system was confirmed by the present observations.     

Historical influences dominate the population genetic structure of a sedentary marine fish,Atlantic wolffish (Anarhichas lupus), across the North Atlantic Ocean

Genetic variation was assessed in Atlantic wolffish, Anarhichas lupus, across the North Atlantic Ocean using microsatellite and amplified fragment length polymorphism (AFLP) markers. Despite unusual life history attributes such as large benthic eggs, large larvae, a limited pelagic stage and relatively sedentary adults, which suggest potential for strong population structure, range‐wide F_ST values were comparable to other marine fishes (≤0.035). Nevertheless, both significant genetic differentiation among regions and isolation by distance were observed, suggesting limited dispersal in this species. AFLP loci, evaluated on a subset of samples, revealed slightly higher F_ST values, but similar patterns of differentiation and isolation‐by‐distance estimates, compared to microsatellites. The genetic structure of Atlantic wolffish has likely been shaped by its post‐glacial history of recolonization, North Atlantic current patterns and continuity of habitat on continental shelves.     

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.     

PATTERNS OF INTERSPECIFIC INTERACTIONS IN THE ULVA‐DOMINATED INTERTIDAL COMMUNITY IN A SOUTHERN COAST OF KOREA

The reproductive success of marine species with external fertilization depends on environmental conditions during gamete release. There is special interest presently in whether water motion causes sperm limitation under natural conditions. We investigated gamete release of Fucus vesiculosus from an exposed shore to ascertain: 1) when gametes are released during the tidal cycle, 2) when fertilization occurs, and 3) what the natural sperm:egg ratios are. Water samples were collected and concentrated over five minutes every half hour off Pemaquid Point, ME from three replicate sites within each of two locations using a pump‐filter device. Immunofluorescence microscopy revealed that gamete release occurred only on the two calmest spring tides. Sperm became present in the water column at the same time as oogonia (30 min^?1 h prior to high tide [HT]) and reached peak concentration at exactly HT. The sperm:egg ratio was 76:1 on 8 Oct 1999 and 21:1 on 8 Nov 1999 at exactly 30 min prior to HT and dropped sharply after HT. Gametes continued to be collected for several hours after HT but analysis of pronuclear position in aceto‐iron‐hematoxylin stained eggs revealed that all fertilization occurred at approximately HT. We modelled the total number of days when reproduction was possible using these results and wind and wave data from the National Data Buoy Center. Our research provides evidence that gamete release by F. vesiculosus occurs at slack HT on calm days and that sperm are not a limiting factor in fertilization for this species.     

B12 SPECIFICITY OF MARINE CENTRIC DIATOMS1, 2

Twenty-one species (23 isolates) of marine diatoms were examined for their capacity to utilize analogs of cyanocobalamin for growth at the ecologically significant concentration of 4 ng 1-¹. Yields due to the analogs were compared to those produced by B₁₂. Responses of the various clones to the analogs were not all-or-none, but varied continuously; thus, assigning the clones to the conventional B₁₂ specificity types is a convenient but arbitrary classification. The use of 10 and 1% levels of response is suggested for such classification. At the 10% level of response, 11 clones had coliform, 4 lactobacillus, and 8 mammalian specificity patterns. At the 1% response level, 14 had coliform, 5 lactobacillus, and 4 mammalian specificities. All clones exceed the 10% response level on all benzimi-dazole-containine analogs tested. Few clones showed definite enough patterns of response to make them potentially useful for differential bioassay. The clones suggested are clone 675-D (Bidclulphia sp.?), clone F^;-3 (Fragilaria sp.?), and the estuarine clone of Cyclotella nana (3H).     

AUTOGAMOUS AUXOSPORULATION IN PINNULARIA NODOSA (BACILLARIOPHYCEAE)1

Auxosporulation of the freshwater epipelic diatom Pinnularia nodosa (Ehrenb.) W. Sm. was studied in a clonal culture. Interphase cells possessed two chloroplasts with invaginated pyrenoids. The nucleus contained a single small body of heterochromatin at one end, also visible during most of meiotic prophase. During auxosporulation, induced by transfer of stationary‐phase cells to fresh medium and suppressed by high nitrogen (N), an unpaired mother cell produced a single auxospore. Although meiosis II and nuclear fusion were not observed, indirect evidence indicated that auxosporulation was autogamous (rarely reported in pennate diatoms), rather than apomictic; paedogamy was excluded. The protoplast produced after meiosis either (1) matured into a “pseudozygote,” via an asymmetrical contraction after meiosis I to form a single spherical cell at one end of the mother cell (pathway 1); or (2) constricted into two spherical cells (pathway 2). In pathway 2, the “pseudogametes” never fused and only one or none developed into a pseudozygote and then into an auxospore. Pathway 2 could be suppressed by continuous light. During metamorphosis of the spherical pseudozygote into an elongate young auxospore, a complete covering of thin siliceous incunabular strips was formed, separate from the organic wall formed around the pseudozygote when first formed and from the perizonium. Mature auxospores produced via pathway 2 had 60% of the volume as those produced via pathway 1 and had smaller chloroplasts (through loss of fragments during protoplast cleavage), but they achieved exactly the same lengths, suggesting that absolute length is monitored during expansion.