Definition Of A Fungus

A startling finding from comparisons of sequences of 18S rRNA—a component of the small subunit of ribosomes found in all organisms—was that some species, for example, the long-studied potato late blight fungus Phytophthora infestans, are quite distinct from other species of fungi. These species, termed the "pseudo fungi," were given the status of kingdom named Straminipila (or Stramenopila) and distinguished from the "true fungi" placed in the kingdom Eumycota. Other differences too came to light: the vegetative phase in Straminipila is predominantly diploid whereas in Eumycota it is haploid. Moreover, Straminipila reproduce by means of motile biflagellate zoospores, suggesting that they evolved from an alga that lost chloroplasts (Cavalier-Smith, 2001). Their cell wall is composed of cellulose and not of p-(1-3)-glucan or mannan and chitin as in Eumycota.

The Straminipila and Eumycota represent separate domains of life—a situation reminiscent of some forms once included in "Bacteria" but were later separated and grouped in the domain Archaea based on distinct DNA sequences and biochemical features. Because of these advances in our knowledge of the organisms possessing hypha, an issue of debate is how a fungus should be defined, based on the evolutionary origin or on a unique structure? In this book, the fungi are non-photosynthetic, generally multicellular and multinucleate, filamentous eukaryotes encased in a multilayered cell wall. The yeasts, although they are typically unicellular, are included in fungi because their cell wall and reproductive structure (ascus) is very similar to filamentous eukaryotes placed in Phylum Ascomycotina.

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