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David G. Mann - One of the best experts on this subject based on the ideXlab platform.
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Vegetative cells and auxosporulation in Biremis sp. A, B.
2014Co-Authors: Andrzej Witkowski, David G. Mann, Frederik Barka, Jascha L. F. Weisenborn, Matt P. Ashworth, Krzysztof J. Kurzydłowski, Izabela Zgłobicka, Sławomir DoboszAbstract:Two focuses of a vegetative cell in girdle view. Each cell contains two chloroplasts either side of the centre, each of which comprises two plates (one is shown for each chloroplast in Fig. 7A, the other being out of focus beneath, on the opposite side of the cell) connected by a narrow bridge containing the pyrenoid (e.g. p). C, D. Two paired gametangia, each containing two rounded, rearranged gametes. The gametangia were paired with their girdles adjacent, the cell shown in Fig. 7D lying immediately below that in Fig. 7C. E. Two paired gametangia, unusual in being in contact only via their valves. Each Gametangium contains a single subspherical zygote. Two nuclei are visible in the left-hand cell (arrows) and two of the four chloroplasts in the right-hand cell. F, G. Two focuses of a Gametangium containing a zygote on the point of transformation into an auxospore. Note the slight central inflection of the zygote's outline, marking the deposition of the primary transverse perizonial band (cf. Fig. 7H, arrowhead). The two rows of foramina on the valves can be seen in Fig. 2G (arrowheads). H. Expanded auxospore containing the initial epivalve (in section at arrow). The auxospore is encased in a well developed perizonium, containing a primary transverse band flanked by several secondary bands (see in section: see also Figs 7I, J). I. Peripheral focus of an expanded auxospore containing the initial epivalve. The two rows of foramina on one of the Gametangium valves can be seen (arrowheads). The end of the auxospore is covered by a siliceous cap (arrow). J. Expanded auxospore containing a completed initial cell. The initial hypovalve (in section at h) lies at a distance from the perizonium, as a result of a strong contraction of the protoplast immediately before its formation; the initial epivalve lies opposite, directly moulded by the interior of the perizonium. The auxospore casing can be seen to consist of a perizonium of transverse bands (e.g. at white arrows) and two silicified hemispherical caps (e.g. at black arrow). [Scale bar 10 µm].
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sexual reproduction and auxospore structure in diploneis papula bacillariophyta
Phycologia, 2013Co-Authors: Masahiko Idei, Tamotsu Nagumo, Shinya Sato, Tsuyoshi Watanabe, David G. MannAbstract:We give a detailed account of sexual reproduction and auxospore development in the diatom genus Diploneis, principally from clonal cultures of the marine benthic Diploneis papula. Sexual reproduction of D. papula was apparently homothallic. After pairing side to side, cells entered meiosis, and each Gametangium produced two gametes. Fertilization was physiologically anisogamous, and both gametes of one Gametangium were active and those of the other Gametangium passive so that the two zygotes were formed within or close to the ‘passive’ Gametangium. Each gamete contained a single chloroplast. The zygote became surrounded by delicate incunabula that contained circular or elliptical scales; this confirmed recent observations that scales (or apparently homologous strips or plates) are quite commonly formed by the zygotes of raphid diatoms. Subsequently, a robust transverse perizonium was built up as the auxospore expanded, and the perizonium was comprised of a closed primary band at the centre and open secondary bands towards each pole. The ends of the secondary bands, which met along one side of the auxospore to form a suture as in other pennate diatoms, were curved inwards towards the centre of the auxospore. Novel features were the extension of the transverse perizonium to cover the tips of the auxospores (in other diatoms it stopped short of the poles, leaving an apical dome covered only by the incunabula) and a differentiation of the ends of the transverse bands into truncated and prolonged variants, which alternated along the suture. The longitudinal perizonium possessed the same highly conserved configuration as in other pennate diatoms, with a wide bifacial central band flanked by two other bands, which here differed in shape and structure. All the perizonial bands were fimbriate. The possible homology of the longitudinal perizonium to the thecae of vegetative cells was discussed. During formation of the initial cell, the protoplast contracted away from the perizonium, but the latter nevertheless moulded the outline shape of the initial valves, which were more strongly constricted than preauxospore cells and gametangia. Other Diploneis species produced either one or two gametes per Gametangium.
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Paedogamy and auxosporulation in Nitzschia sect. Lanceolatae (Bacillariophyta)
Phycologia, 2013Co-Authors: David G. Mann, Shinya Sato, Laia Rovira, Rosa TrobajoAbstract:Paedogamy (fusion of gametes produced within the same Gametangium following meiosis) has rarely been reported in diatoms, with fewer than 10 confirmed examples. One of these, reported by L. Geitler, was in a diatom from Illmitz, Lake Neusiedl (Austria), identified as ‘Nitzschia frustulum var. perpusilla'. We observed uniparental auxosporulation in two Nitzschia clones isolated from the lower Ebro River (Catalonia, Spain), morphologically similar to Geitler's material and belonging to the N. inconspicua species complex. We established that the auxospores were formed paedogamously by Feulgen staining of the nuclei and time-lapse microscopy of living cells. However, reinvestigation of Geitler's original cytological preparations revealed differences between the Illmitz and Ebro material with respect to the length of the initial cells, the structure of the perizonium, and the timing of degeneration of superfluous haploid nuclei during gametogenesis, indicating a genetic and possibly a taxonomic separation. Sca...
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Mating system, auxosporulation, species taxonomy and evidence for homoploid evolution in Amphora (Bacillariophyta)
Phycologia, 2010Co-Authors: David G. Mann, Aloisie PoulíčkováAbstract:Abstract Mann D.G. and PoulIckova A. 2010. Mating system, auxosporulation, species taxonomy and evidence for homoploid evolution in Amphora (Bacillariophyta). Phycologia 49: 183–201. DOI: 10.2216/09-08.1 Cytological characteristics of the mitotic cycle, sexual reproduction and auxospore formation are described for three members of the type group of Amphora: A. ovalis, A. copulata and A. minutissima. All have a single lobed ventral chloroplast, nuclei with granular heterochromatin and a single nucleolus (unusually large in A. ovalis). Amphora copulata is heterothallic, with two mating types that do not differ in gamete behaviour. All three species reproduce allogamously via fusion of two rearranged gametes per Gametangium to produce elongate zygotes enclosed within the volume delimited by the gametangial thecae. Pairing is strictly via the ventral sides of the cells, with tight apposition of the cells, and expansion of the auxospores is strictly perpendicular to the Gametangium long axes. Comparisons are m...
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Heterothallic auxosporulation, incunabula and perizonium in Pinnularia (Bacillariophyceae)
European Journal of Phycology, 2007Co-Authors: Aloisie Poulíčková, Victor A. Chepurnov, Shigeki Mayama, David G. MannAbstract:The cytology and life cycle of Pinnularia cf. gibba was examined in nine clones from three Scottish localities. This freshwater epipelic diatom is heterothallic and produces two isogametes per Gametangium in type IC auxosporulation (Geitler's classification [1973]). The zygote undergoes a highly unusual metamorphosis before beginning expansion, becoming shortly linear-lanceolate; this is accompanied by formation of a complete covering of thin, oxidation-resistant strips and scale-like structures (at the poles), which are quite separate from the perizonium formed during auxospore expansion. Observations of similar incunabular structures in P. acidojaponica show that these elements are siliceous. The P. cf. gibba perizonium also has unusual features, including a remarkably wide primary band. Trikaryotic and haploid auxospores are sometimes formed and haploid 'zygotes' mature and expand like diploids, but do not develop into mature initial cells. Several phases of mucilage secretion take place, from the gametangia, zygotes and auxospores. Triplets of gametangia and polyspermy occurred with high frequency; this and the systematic significance of variation in auxospore, incunabula and perizonium structure, are discussed. Aspects of the taxonomy of the P. gibba group are treated in supplementary material provided on the European Journal of Phycology website.
N. A. Davidovich - One of the best experts on this subject based on the ideXlab platform.
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Sexual Reproduction of the Black Sea Diatom Climaconeis scalaris (Brébisson) E.J. Cox
Moscow University Biological Sciences Bulletin, 2019Co-Authors: O. I. Davidovich, N. A. Davidovich, R. Gastineau, A. WitkowskiAbstract:— The paper reports a process of sexual reproduction in the Black Sea diatom Climaconeis scalaris (Brébisson) E.J. Cox, 1982, initiated in laboratory conditions. A crossing system of this species allows both homo- and heterothallic reproduction. The fertilization is allogamous. Diploid mother gametangial cells secreted mucilage, forming a mucilage track, while actively gliding relative to each other. Each Gametangium in a pair produced two elongated, cylindrical round-ended haploid gametes, which were largely morphologically and behaviorally isogamous. The division of the protoplast of the gametangial cell occurred in the transapical plane without the rearrangement of gametes. Growing auxospores were located parallel to the parent frustules. The research produced data on changes in the cell size, number of chloroplasts, and the position of cardinal points in the life cycle of the examined species.
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Sexual Reproduction of the Black Sea Diatom Climaconeis scalaris (Brébisson) E.J. Cox
Moscow University Biological Sciences Bulletin, 2019Co-Authors: O. I. Davidovich, N. A. Davidovich, R. Gastineau, A. WitkowskiAbstract:— The paper reports a process of sexual reproduction in the Black Sea diatom Climaconeis scalaris (Brébisson) E.J. Cox, 1982, initiated in laboratory conditions. A crossing system of this species allows both homo- and heterothallic reproduction. The fertilization is allogamous. Diploid mother gametangial cells secreted mucilage, forming a mucilage track, while actively gliding relative to each other. Each Gametangium in a pair produced two elongated, cylindrical round-ended haploid gametes, which were largely morphologically and behaviorally isogamous. The division of the protoplast of the gametangial cell occurred in the transapical plane without the rearrangement of gametes. Growing auxospores were located parallel to the parent frustules. The research produced data on changes in the cell size, number of chloroplasts, and the position of cardinal points in the life cycle of the examined species.
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Sexual reproduction in the newly-described blue diatom, Haslea karadagensis.
Fottea, 2012Co-Authors: N. A. Davidovich, Pierre Gaudin, R. Gastineau, O. I. DavidovichAbstract:For decades, the diatom genus Haslea has been known to comprise both "colorless" species and one species containing a blue pigment, the latter being known as H. ostrearia. Recently, a new blue diatom named H. karadagensis has been isolated from the Black Sea. The mating compatibility of the two species has been tested, and their reproductive isolation confirmed. We provide a detailed description of the heterothallic sexual reproduction process in this new species. Cells from clones which are sexually compatible arrange gametangial pairs. Each Gametangium in the pair produces two gametes, which to a large extent are morphologically and behaviorally isogamous. No mucilage or mucilage structures are observed. Zygotes and auxospores have no contact with parental frustules, and their orientation is rather irregular though they tend to lie parallel to each other. Evaluation of the position of cardinal points in the life cycle of the species, and the rate of cell size decrease in culture are presented. In the breeding system of this species both homo- and heterothallic ways of reproduction are realized. The latter is basic and predominant. Among the 36 clones investigated, 16 were sexually compatible with the other 20. Homothallic behavior was extremely rare; only one clone displayed a homothallic mode of reproduction.
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Heterothallism in the pennate diatom Haslea ostrearia (Bacillariophyta)
European Journal of Phycology, 2009Co-Authors: N. A. Davidovich, Jean-luc Mouget, Pierre GaudinAbstract:Twelve clonal cultures of the marine pennate diatom Haslea ostrearia (Gaillon) Simonsen were used to study sexual reproduction and auxosporulation. After crossing, compatible gametangia paired girdle to girdle, with no visible surrounding mucilage. Two gametes were formed per Gametangium; these were not attached to the thecae of the parental frustule and gametes from one Gametangium fused with the gametes from the other Gametangium. Sexual reproduction was isogamous, resulting in the production of two zygotes per paired gametangia. The pattern of reproduction in H. ostrearia can be classified as type IB2a according to Geitler's system. The expansion of auxospores, which was bipolar, was accompanied by the formation of a transverse perizonium. Superfluous nuclei resulting from meiosis were not visible. The upper limit of the cell size range suitable for sexualization was c. 68 µm, i.e. about half the maximum species-specific size of about 140 µm. Sexual reproduction has been shown by previous authors to oc...
O. I. Davidovich - One of the best experts on this subject based on the ideXlab platform.
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Sexual Reproduction of the Black Sea Diatom Climaconeis scalaris (Brébisson) E.J. Cox
Moscow University Biological Sciences Bulletin, 2019Co-Authors: O. I. Davidovich, N. A. Davidovich, R. Gastineau, A. WitkowskiAbstract:— The paper reports a process of sexual reproduction in the Black Sea diatom Climaconeis scalaris (Brébisson) E.J. Cox, 1982, initiated in laboratory conditions. A crossing system of this species allows both homo- and heterothallic reproduction. The fertilization is allogamous. Diploid mother gametangial cells secreted mucilage, forming a mucilage track, while actively gliding relative to each other. Each Gametangium in a pair produced two elongated, cylindrical round-ended haploid gametes, which were largely morphologically and behaviorally isogamous. The division of the protoplast of the gametangial cell occurred in the transapical plane without the rearrangement of gametes. Growing auxospores were located parallel to the parent frustules. The research produced data on changes in the cell size, number of chloroplasts, and the position of cardinal points in the life cycle of the examined species.
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Sexual Reproduction of the Black Sea Diatom Climaconeis scalaris (Brébisson) E.J. Cox
Moscow University Biological Sciences Bulletin, 2019Co-Authors: O. I. Davidovich, N. A. Davidovich, R. Gastineau, A. WitkowskiAbstract:— The paper reports a process of sexual reproduction in the Black Sea diatom Climaconeis scalaris (Brébisson) E.J. Cox, 1982, initiated in laboratory conditions. A crossing system of this species allows both homo- and heterothallic reproduction. The fertilization is allogamous. Diploid mother gametangial cells secreted mucilage, forming a mucilage track, while actively gliding relative to each other. Each Gametangium in a pair produced two elongated, cylindrical round-ended haploid gametes, which were largely morphologically and behaviorally isogamous. The division of the protoplast of the gametangial cell occurred in the transapical plane without the rearrangement of gametes. Growing auxospores were located parallel to the parent frustules. The research produced data on changes in the cell size, number of chloroplasts, and the position of cardinal points in the life cycle of the examined species.
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Sexual reproduction in the newly-described blue diatom, Haslea karadagensis.
Fottea, 2012Co-Authors: N. A. Davidovich, Pierre Gaudin, R. Gastineau, O. I. DavidovichAbstract:For decades, the diatom genus Haslea has been known to comprise both "colorless" species and one species containing a blue pigment, the latter being known as H. ostrearia. Recently, a new blue diatom named H. karadagensis has been isolated from the Black Sea. The mating compatibility of the two species has been tested, and their reproductive isolation confirmed. We provide a detailed description of the heterothallic sexual reproduction process in this new species. Cells from clones which are sexually compatible arrange gametangial pairs. Each Gametangium in the pair produces two gametes, which to a large extent are morphologically and behaviorally isogamous. No mucilage or mucilage structures are observed. Zygotes and auxospores have no contact with parental frustules, and their orientation is rather irregular though they tend to lie parallel to each other. Evaluation of the position of cardinal points in the life cycle of the species, and the rate of cell size decrease in culture are presented. In the breeding system of this species both homo- and heterothallic ways of reproduction are realized. The latter is basic and predominant. Among the 36 clones investigated, 16 were sexually compatible with the other 20. Homothallic behavior was extremely rare; only one clone displayed a homothallic mode of reproduction.
Marina Montresor - One of the best experts on this subject based on the ideXlab platform.
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Genetic characterization and life cycle of the diatom Fragilariopsis kerguelensis
European Journal of Phycology, 2013Co-Authors: Nike Fuchs, Eleonora Scalco, Wiebe H. C. F. Kooistra, Philipp Assmy, Marina MontresorAbstract:The planktonic diatom Fragilariopsis kerguelensis plays an important role in the biogeochemical cycles of the Southern Ocean, where remains of its frustules form the largest deposit of biogenic silica anywhere in the world. We assessed the genetic identity of 26 strains, from cells collected at various sites in the Southern Ocean, using three molecular markers, LSU and ITS rDNA and rbcL. The LSU sequences were identical among the tested strains, ITS sequences were highly similar, and only one base pair difference was detected among the rbcL sequences. These results, together with a large number of successful mating experiments demonstrated that the strains belong to a single biological species. We investigated the mating system and life cycle traits of F. kerguelensis. Cell size diminished gradually in clonal strains. Gamete formation only occurred when strains of opposite mating type – within a cell size range of 7–36 µm – were mixed together. Two binucleate gametes were formed in each Gametangium and ga...
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Life cycle, size reduction patterns, and ultrastructure of the pennate planktonic diatom Pseudo-nitzschia delicatissima (Bacillariophyceae)
Journal of Phycology, 2005Co-Authors: Alberto Amato, Luisa Orsini, Domenico D'alelio, Marina MontresorAbstract:Pseudo-nitzschia delicatissima (Cleve) Heiden is a very common pennate planktonic diatom found in temperate marine waters, where it is often responsible for blooms. Recently, three distinct internal transcribed spacer types have been recorded during a P. delicatissima bloom in the Gulf of Naples (Mediterranean Sea, Italy), which suggests the existence of cryptic diversity. We carried out mating experiments with clonal strains belonging to the most abundant internal transcribed spacer type. Pseudo-nitzschia delicatissima is heterothallic and produces two functional anisogametes per Gametangium. The elongated auxospore possesses a transverse and a longitudinal perizonium. The sexual phase was observed to occur over aw ide size spectrum, spanning 19–80l ma nd corresponding to almost the whole range of cell length observed for P. delicatissima .W e also investigated cell morphology, valve ultrastructure and morphometry of parental, F1-generation strains, and the progeny of crosses between parental and F1 strains. Although ultrastructural features match those described for P. delicatissima, variability in cell shape was recorded in the largest cells of the F1 generation as well as in valves with an abnormal arrangement of poroids. As many other diatoms, P. delicatissima undergoes size reduction over its life cycle, and cells of different size showed differences in growth rates and the amount of size reduction per cell cycle. Cells between 60 and 30l mi n length showed the fastest growth and the slowest rates of size reduction per generation. In culture, P. delicatissima cells can decrease to 8lm in length; however, such small cells ( � 30lm) are not recorded in the sea, and this raises interesting questions about the factors that control their survival in the natural environment.
Victor A. Chepurnov - One of the best experts on this subject based on the ideXlab platform.
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Heterothallic auxosporulation, incunabula and perizonium in Pinnularia (Bacillariophyceae)
European Journal of Phycology, 2007Co-Authors: Aloisie Poulíčková, Victor A. Chepurnov, Shigeki Mayama, David G. MannAbstract:The cytology and life cycle of Pinnularia cf. gibba was examined in nine clones from three Scottish localities. This freshwater epipelic diatom is heterothallic and produces two isogametes per Gametangium in type IC auxosporulation (Geitler's classification [1973]). The zygote undergoes a highly unusual metamorphosis before beginning expansion, becoming shortly linear-lanceolate; this is accompanied by formation of a complete covering of thin, oxidation-resistant strips and scale-like structures (at the poles), which are quite separate from the perizonium formed during auxospore expansion. Observations of similar incunabular structures in P. acidojaponica show that these elements are siliceous. The P. cf. gibba perizonium also has unusual features, including a remarkably wide primary band. Trikaryotic and haploid auxospores are sometimes formed and haploid 'zygotes' mature and expand like diploids, but do not develop into mature initial cells. Several phases of mucilage secretion take place, from the gametangia, zygotes and auxospores. Triplets of gametangia and polyspermy occurred with high frequency; this and the systematic significance of variation in auxospore, incunabula and perizonium structure, are discussed. Aspects of the taxonomy of the P. gibba group are treated in supplementary material provided on the European Journal of Phycology website.
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Congruence of morphological, reproductive and ITS rDNA sequence data in some Australasian Eunotia bilunaris (Bacillariophyta)
European Journal of Phycology, 2007Co-Authors: Pieter Vanormelingen, Victor A. Chepurnov, David G. Mann, Sylvie Cousin, Wim VyvermanAbstract:Clones identified as Eunotia bilunaris were isolated from two freshwater habitats in New Zealand and one in Tasmania. Experimental studies of sexual reproduction showed that the mating system was strictly heterothallic. All clones were morphologically similar and, in mixtures of compatible clones, were able to produce a viable F1, regardless of geographical origin. There was nevertheless some geographically related variation in ITS rDNA sequences, suggesting that dispersal and concerted evolution are unable to prevent allopatric divergence. Isolates from two of the three localities also showed significant ITS polymorphisms. Auxosporulation followed a similar pattern to that in other Eunotia species, being isogamous with a single functional gamete per Gametangium; plasmogamy was effected via two fusing papillae. Details of auxospore structure, initial cell size, and vegetative valve metrics (width, striation densities) show that conspecificity with a recently studied European E. bilunaris is doubtful. The ...
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Auxosporulation, mating system, and reproductive isolation in Neidium (Bacillariophyta)
Phycologia, 2005Co-Authors: David G. Mann, Victor A. ChepurnovAbstract:D.G. MANN AND V.A. CHEPURNOV. 2005. Auxosporulation, mating system, and reproductive isolation in Neidium (Bacillariophyta). Phycologia 44: 335‐350. Three allogamous Neidium demes, referrable to N. ampliatum sensu lato,coexist without intergradation in Blackford Pond, Edinburgh, and some other lakes. A slight change in valve shape has occurred in one deme during the last 20 years. Morphological deme traits persist in healthy clonal cultures, but with time or during senescence, aberrant morphologies and sizes of cell can be produced that have no parallel in nature. Simplification of valve outline occurs as cells get smaller, but the initial cells also have a simplified morphology. The rostrate apices of some N. ampliatum demes develop rapidly after auxosporulation, during the first divisions of the initial cells. The ‘major’ and ‘minor’ demes of N. ampliatum are heterothallic, although some inbreeding occurs. Very rarely, mixed pairs of ‘major’ 3 ‘minor’ are formed, but hybrid auxospores are apparently never produced, so the demes are reproductively isolated. The ‘maternal’ Gametangium has a nongenetic influence on initial cell size; however, the effect is slight and control of initial cell size is very well buffered to variation in Gametangium size, so the concept of ‘cardinal points’ is valid for this species complex. The characteristics of sexual reproduction in clones and seminatural populations (including the effective, though incomplete, suppression of triplets and larger groups during copulation) prompt hypotheses about pairing mechanisms, in particular that a chemoattractant is involved. Preferential polyandry in Sellaphora and theoretical considerations indicate that the chemoattractant is produced by only one mating type. Size selection of mates in the ‘minor’ deme probably reflects progressively easier and more rapid sexualization as cells become smaller.
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Auxosporulation of Licmophora communis (Bacillariophyta) and a review of mating systems and sexual reproduction in araphid pennate diatoms
Phycological Research, 2004Co-Authors: Victor A. Chepurnov, David G. MannAbstract:SUMMARY 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.
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Sexual behaviour, auxosporulation and chloroplast dynamics in a marine Amphora (Bacillariophyceae) studied in culture
Botanica Marina, 2004Co-Authors: Koen Sabbe, Victor A. Chepurnov, David G. Mann, Wim VyvermanAbstract:Chloroplast, mating behaviour, and auxosporulation are described for Amphora cf. proteus Gregory. The interphase chloroplast is butterfly-shaped and lies with its central part, containing a flat discoid pyrenoid, pressed against the ventral side of the girdle. Chloroplast division is longitudinal and begins and ends before mitosis and cytokinesis. Over the cell cycle, no rotations or lateral translocations of the chloroplast were noticed. A. cf. proteus is the first species of this genus in which sexual reproduction and breeding behaviour have been studied in clonal cultures. The cells isolated exhibited strictly dioecious behaviour, reproducing only in mixed cultures of sexually compatible clones. Sexual reproduction in A. cf. proteus is morphologically and behaviourally isogamous. Each Gametangium forms two functional gametes, which do not rearrange within the Gametangium, and two small aborted cells. The aborted cells are produced after an unequal cytokinesis accompanying meiosis II; the second cytokinesis during gametogenesis represents a phenomenon hitherto unknown in pennate diatoms. Auxospore development takes place by bipolar expansion. The expansion is unequal on the two long sides of the auxospore, so that the auxospore becomes curved and dorsiventral. The auxospore forms a transverse and longitudinal perizonium.
