Prasiola Meneghini, 1838, nom. cons.

Holotype species: Prasiola crispa (Lightfoot) Kützing

Original publication and holotype designation: Meneghini, G. (1838). Cenni sulla organographia e fisiologia delle alghe. Nuovi Saggi della [Cesarea] Regia Accademia di Scienze, Lettere ed Arti di Padova 4: 325-388.

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Description: Thalli forming monostromatic blades generally expanded above and narrowing to short stipitate region at base. Blade shape varies from orbicular, to irregular or spatulate to long linear; terrestrial and marine species generally 0.5 to 20 mm in high, freshwater forms usually much larger. Vegetative cells squareto rectangular, moslty alligned in distinct vertical and horizontal files, about 5-10 (-20) µm in diameter, gamete producing cells even smaller. Marginal or basal rhizoids may be present. Blades become polystromic in regions of gamete production. Spore and gamete formation occur at blade apices. Cell walls prominent, comprising up to ca. 50% of thallus volume.

Cells uninucleate with single, stellate chloroplast and prominent central pyrenoid. Cell walls mostly xylomannan.

Asexual reproduction commonly by aplanospores. In some species propagation by fragmentation or release of vegetative cells as akinetes. Some species entirely asexual with blades recycling by spores (P. furfuracea), or spores develop into aplanosporangia, the products of which regenerate the blade form (P. calophylla).

Sexual reproduction oogamous and either monoecious or dioecious with biflagellate, drop-shaped sperm 2-7 µm long, and small, non-motile, rounded to irregular shaped eggs 3-6.8 µm long; sperm only flagellated stage in life history. Sperm attachment by the absorption of one flagellum into the egg results in planozygote that settles and absorbs entire sperm cell. Zygotes grow into new diploid individuals although zygotic meiosis inferred for one species. In P. stipitata, asexual, aplanospore-producing thalli produce spores that form similar plants or develop into gamete forming blades. Zygotes may develop into either blade type. During maturation of gamete-forming thalli meiosis occurs in vegetative cells near apices and these cells continue to divide, first in plane of thallus and then in three dimensions, forming dense packets of cells. These divisions result in mosaic of male and female gametangial areas. Lighter colored cells and portions of blade produce sperm, whereas darker colored regions produce eggs. Gamete release involves dissolution of cell walls; in P. meridionalis bladder-like structure is formed in which gametes mixed. A variety of environmental cues implicated in induction of sexual reproduction. Details of life history remain problematic for many species. Chromosome numbers of n=3, 6 or 8 have been reported.

Information contributed by: D.J. Garbary & M.D. Guiry. The most recent alteration to this page was made on 2024-04-22 by M.D. Guiry.

Taxonomic status: This name is of an entity that is currently accepted taxonomically.

Gender: This genus name is currently treated as feminine.

Most recent taxonomic treatment adopted: John, D.M. (2002). Order Prasiolales. In: The Freshwater Algal Flora of the British Isles. An identification guide to freshwater and terrestrial algae. (John, D.M., Whitton, B.A. & Brook, A.J. Eds), pp. 473-475. Cambridge: Cambridge University Press.

Comments: Prasiola species are cosmopolitan in arctic to temperate climates in terrestrial, freshwater and marine conditions. Marine species mostly in high intertidal and adjacent splash zone habitats where they grow in great densities, often associated with bird rocks. Freshwater species in rapid streams, terrestrial forms on rock or damp soil. The supralittloral species, P. crispa, grows at salinities from 0.35-17.5 ppt. Flagellar apparatus with 9 + 1 microtubule configuration in flagellar axoneme, 180° rotational symmetry and counterclockwise absolute orientation of major components. Flagella naked with an elongated hairpoint. Some species form regular associations with the ascomycete, Kohlmeyera complicatula. These associations (mycophycobioses) usually referred to Prasiola borealis and P. tessellata in northern and southern hemispheres, respectively, however, this infection is also reported for P. crispa subsp. antarctica. This symbiosis may also be referred to as the lichen Mastodia. The taxonomic status of these entities remains unclear, i.e., are they distinct taxa or simply populations infected with the fungus?

Prasiola is the most mophologically elaborate genus in the Prasiolaceae and has the most species. Ultastructure of flagellar apparatus indicates close relationship with Rosenvingiella, which has been confirmed by sequencing.

The direction of evolution in Prasiola is thought to be into marine habitats rather than from marine to terrestrial/freshwater environment.

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Some of the descriptions included in AlgaeBase were originally from the unpublished Encyclopedia of Algal Genera, organised in the 1990s by Dr Bruce Parker on behalf of the Phycological Society of America (PSA) and intended to be published in CD format. These AlgaeBase descriptions are now being continually updated, and each current contributor is identified above. The PSA and AlgaeBase warmly acknowledge the generosity of all past and present contributors and particularly the work of Dr Parker.

Descriptions of chrysophyte genera were subsequently published in J. Kristiansen & H.R. Preisig (eds.). 2001. Encyclopedia of Chrysophyte Genera. Bibliotheca Phycologica 110: 1-260.

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M.D. Guiry in Guiry, M.D. & Guiry, G.M. 22 April 2024. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway.; searched on 16 July 2024

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