Biomedical Engineering Reference
In-Depth Information
form) and Cryptococcus neoformans (the anamorphic form). Clinical
mycologists prefer the asexual name, because it is the asexual form that
grows in human tissues.
5.
Unclassifiable organisms. Many fungi have never exhibited sexual repro-
duction in culture. Many other fungi cannot be cultured. A special pseudo-
class of fungi, deuteromycetes (spelled with a lowercase “d,” signifying its
questionable validity as a true biologic class) has been created to hold
these indeterminate organisms until definitive classes can be assigned. At
present, there are several thousand such fungi, sitting in a taxonomic
limbo, until they can be placed into a definitive taxonomic class [102].
6.
Variable morphologies. Pathogenic fungi grow within human tissues
without reproduction (asexual or sexual). They typically grow in tissues
as an expanding colony of hyphae or yeasts (so-called vegetative
growth). The pathologist who observes fungal infections in human tis-
sues typically reaches a diagnosis on clinical features and the somewhat
restricted morphologic features of the fungus in biopsied tissue. Adding
to the general confusion, fungal specimens grown in culture may have a
different morphology from that of the same fungus growing in human
tissue. This situation is very different from that of bacterial infections,
which have the same morphology in tissues as they have in the culture
dish.
7.
An historical blunder. Class fungi got off to a very bad start when classical
taxonomists mistook these organisms for plants. To this day, academic
mycologists are employed by Botany Departments, and fungal taxonomy
is subsumed under the International Code of Botanical Nomenclature
(ICBN) [102]. Superficially, fungi resemble plants; both classes of organ-
isms have members that emerge from the ground. We now know that
fungi descended from a flagellated organism. Class Fungi was eventually
reassigned to Class Opisthokonta (unikonts with a posterior flagellum),
making Class Fungi a sister class to Class Animalia. A misunderstanding,
based on an incorrect assumption related to the absence of a defining fea-
ture (in this case a flagellum) led to one of the most jarring re-assignments
in the classification of living organisms (see Glossary item, Negative
classifier).
Keeping these seven points in mind, the members of Class Fungi have a
common ancestral lineage, share a set of common biological properties, and
can be divided into four distinctive subclasses: Class Zygomycota
(Chapter 34), Class Basidiomycota (Chapter 35), Class Ascomycota
(Chapter 36), and Class Microsporidia (Chapter 37).
It is ironic that the important clue to the phylogenetic origin of fungi rests
on the presence of a posterior flagellum, as fungi, with only one exception,
lack flagella altogether. The only fungal class with a flagellum is Class
Chytrid, considered to be the most primitive fungal class [73]. The chytrids
