Biomedical Engineering Reference
In-Depth Information
do not form dikaryons (Class Zygomycota, Chapter 34 and Class
Microsporidia, Chapter 37). Dikaryons occur exclusively in the ascomycotes
and basidiomycotes (i.e. in no other earthly organisms).
A dikaryon is a cell with a double nucleus, composed of two haploid nuclei
that came to occupy the same cell, through conjugation, and without fusion of
the two nuclei (i.e. two cells fused, but the nuclei within the cells do not fuse).
A dikaryon is a dividing cell, wherein both nuclei divide synchronously, and
both nuclei are metabolically active. The somatic cells of hyphae are haploid
(from the Greek, “haplous,” meaning single), having a complete set of
unpaired chromosomes. Likewise, the gametes of fungi are haploid. Dikaryons
can be formed by the fusion of haploid cells from two physically adjacent
compatible mycelia (i.e. hyphae), or from the sexual fusion of two gametes, or
from the fusion of a gamete with a haploid somatic cell. The dikaryotic state
may be very short, or relatively long, but it eventually leads to a fused, diploid
state. Diploid cells can yield, through meiosis, two haploid spore cells.
The two subclasses of Class Dikarya (Class Basidiomycota and Class
Ascomycota) each have their own characteristic sexual and asexual bodies
that produce cells that leave the organism and enter the environment, often
as airborne spores. In Class Basidiomycota organisms reproduce sexually
using a club-shaped structure, called a basidium, that produces basidiospores.
They can also reproduce asexually by producing hardened spores from
specialized cell structures (conidiophores), extending from hyphae.
A deep understanding of dikaryon biology and sexual reproduction is not
particularly relevant to clinical mycology, as the tissue growth of virtually
all infectious fungi is vegetative (i.e. yeast dividing by a budding process, or
hyphae forming mycelium). Suffice it to say that the air we breathe, and, to
some extent, the food we eat and the water we drink, carries the sexual and
asexual spores of Class Basidiomycota.
Basidiomycota (Chapter 35)
Agaricomycotina
Tremellomycetes
Tremellales
Tremellaceae
*Cryptococcus
Ustilaginomycotina
Exobasidiomycetes
Malasseziales
Malasseziaceae
*Malassezia
Only two genera account for the human infections caused by members of
Class Basidiomycota: Genus Cryptococcus and Genus Malassezia.
Cryptococcus species grow in tissues exclusively as yeasts (i.e. hyphal
mycelia are never observed). Cryptococcus neoformans causes cryptococcal
