Tetrasporophyte

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The second diploid generation of red algae is known as the tetrasporophyte . The development of the three generations in the red algae usually takes place on only two vegetation bodies, occasionally even on a single one.

The tripartite generation change of the red algae

Generation change of the red algae, here in the cartilage wrack

Red algae are the only group of organisms to have a tripartite generation change , consisting of:

  • the haploid gametophyte (literally: "the gamete- producing plant");
  • the first diploid sporophyte (literally: "the spore- producing plant"): This generation of red algae is called carposporophyte for better differentiation ;
  • the second diploid sporophyte: in red algae it is called the tetrasporophyte .

Gametophyte and tetrasporophyte have the same design in most red algae and can hardly be distinguished by laypeople; they form the red to purple colored “ algae plant ” , which is often divided into a basal adhesive organ (“ rhizoid ”) and long, often branched bands ( phylloids ) . The middle generation - the carposporophyte - grows directly on the gametophyte. It is usually much smaller than the other two generations and looks so different in some red algae species that the botanists initially thought it was a real parasite and gave it its own species name.

Life cycle of the red algae

enveloping cystocarp and carposporophytes of Polysiphonia

The gametophyte releases haploid male gametes (sex cells), which are not flagellated and passively driven to the female gametes in the water. The female sex cells are located in a gametangium ("gamete container"), which the algae call Karpogon . In many red algae, a slim conception organ (the trichogyne) protrudes from this, to which a male sex cell can adhere. This creates an opening on both sides at the point of attachment through which the contents of the male sex cells enter the trichogynes.

After the male genome has fused with the egg cell , the zygote germinates directly in the carpogon and develops into the second generation, the carposporophyte. This usually only consists of inconspicuous, microscopic cell threads and grows directly on the gametophyte. In many species it is additionally covered by a protective tissue of the gametophyte (the cystocarp). The carposporophyte forms naked, diploid, ungulmized carpospores through mitosis ; they are also called mitospores because of their formation through mitosis.

Only these carpospores germinate to the third generation, the diploid tetrasporophytes, which again form an autonomous "algae plant". The tetrasporophyte forms four haploid spores (meiospores), which are also called tetra (meio) spores, from each spore mother cell with reduction division . These eponymous tetrameiospores, which are also not flagellated, are drifted by the water and finally germinate into new gametophytes.

Deviations from the normal type

In some species, including the " frog spawn alga " ( Batrachospermum ) that occurs in fast-flowing streams , the tetrasporophyte grows on the carposporophyte, so that all three generations remain connected to one another throughout their life.

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