Article

Life cycle and sexuality of the freshwater Raphidophyte Gonyostomum semen (Raphidophyceae)

Lund University, Lund, Skåne, Sweden
Journal of Phycology (Impact Factor: 2.84). 07/2006; 42(4):859 - 871. DOI: 10.1111/j.1529-8817.2006.00240.x

ABSTRACT

Previously unknown aspects in the life cycle of the freshwater flagellate Gonyostomum semen (Ehrenb.) (Raphidophyceae) are described here. This species forms intense blooms in many northern temperate lakes, and has increased in abundance and frequency in northern Europe during the past decades. The proposed life cycle is based on observations of life cycle stages and transitions in cultures. Viable stages of the life cycle were individually isolated and monitored by time-lapse photography. The most common processes undertaken by the isolated cells were: division, fusion followed by division, asexual cyst formation, and sexual cyst formation. Motile cells divided by two different processes. One lasted between 6 and 24 h and formed two cells with vegetative cell size and with or without the same shape. The second division process lasted between 10 and 20 min and formed two identical cells, half the size of the mother cell. Planozygotes formed by the fusion of hologametes subsequently underwent division into two cells. Asexual cyst-like stages were spherical, devoid of a thick wall and red spot, and germinated in 24–48 h. Heterogamete pairs were isogamous, and formed an angle of 0–90° between each other. Planozygote and sexual cyst formation were identified within strains established from one vegetative cell. The identity of these strains, which was studied by an amplified fragment length polymorphism analysis, was correlated with the viability of the planozygote. Resting cyst germination was described using cysts collected in the field. The size and morphology of these cysts were comparable with those formed sexually in culture. The excystment rate was higher at 24°C than at 19 or 16°C, although the cell liberated during germination (germling) was only viable at 16°C. The placement of G. semen within the Raphidophyceae family was confirmed by sequence analysis of a segment of the 18S ribosomal DNA.

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    • "Cysts of Gonyostomum have been observed in old cultures (Drouet and Cohen, 1935; Cronberg et al., 1988). Figuerora and Rengefors (2006) observed not only asexual cyst stages in Gonyostomum in culture, but also division of motile cells, sexual cyst formation, and formation of hologametes and heterogametes. "
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    ABSTRACT: The Xanthophyceae, Eustigmatophyceae, and Raphidophyceae are three independent classes of stramenopile algae (Heterokontophyta or Ochrophyta); they are not closer related with each other. Most Xanthophytes are unicellular or colonial coccoid algae, others from multicellular filaments and or exhibit thalli composed of multinucleate siphons. The Eustigmatophyceae comprises only coccoid members which are very difficult to distinguish from the coccoid Xanthophytes. Freshwater Raphidophytes are rather distinct, because they form flagellated vegetative stages. The color of Xanthophytes and Eustigmatophytes is yellowish green due to the absence of the brown fucoxanthin, present in Raphidophytes and other stramenopile algae. Only the Eustigmatophytes lack chlorophyll c. Many Xanthophytes and Eustigmatophytes share that they predominately occur in terrestrial habitats, e.g. soil, representing a small group of terrestrial algae with their plastids obtained from an ancestral red alga by secondary symbiosis. For Raphidophytes only three genera are recognized in freshwater yet and they are observed within the plankton.
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    • "While asexual reproduction will not alter the genetic diversity, sexual events are important in creating diversity through recombination and by allowing the spreading of favorable alleles [50]. G. semen alternates between asexual and sexual reproduction, with a mainly asexual phase during the growing season followed by sexual reproduction at the termination of the bloom [51]. In G. semen, the low gene diversity observed in the different populations might be explained by a bottleneck effect resulting from colonization by few individuals, which was maintained in the populations due to the life cycle characteristics involving vegetative growth. "
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    • "Anabaena macrospora and Woronichinia compacta are also common in the northern temperate zone (Komárek & Anagnostidis, 1999; Komárek & Zapomelova , 2008). Gonyostomum semen is a well-known nuisance alga with widespread distribution in Northern Europe (Figueroa & Rengefors, 2006) and has been recorded as increasing in Scandinavian (Willén, 2003; Figueroa & Rengefors, 2006; Trigal et al., 2011) and Baltic soft water lakes (Rakko et al., 2008). "
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