Biomedical Engineering Reference
In-Depth Information
Class Rhizaria. These organisms can exceed four inches in length. Class Mimiviridae
was thought to contain the largest viruses. Class Mimiviridae viruses can exceed 0.8
microns in length, thus exceeding the size of some bacteria (e.g. Mycoplasma species
may be as small as 0.3 microns). The genome of species in Class Mimiviridae can
exceed a million base pairs, encoding upwards of 1000 proteins. Class Megaviridae is
a newly reported class of viruses, related to Class Mimiviridae, but bigger [128]. The
organism with the largest genome is currently thought to be
Polychaos dubium
(Class
Amoebozoa, Chapter 22), with a genome length of 670 billion base pairs; humans
have a puny 3 billion base pair genome.
Long branch attraction When gene sequence data are analyzed, and two organisms share
the same sequence in a stretch of DNA, it can be very tempting to infer that the two
organisms belong to the same class (i.e. that they inherited the identical sequence from
a common ancestor). This inference is not necessarily correct. Because DNA mutations
arise stochastically over time, two species with different ancestors may achieve the
same sequence in a chosen stretch of DNA. When mathematical phylogeneticists
began modeling inferences for gene data sets, they assumed that most class assignment
errors based on DNA sequence similarity would occur when the branches between sis-
ter taxa were long (i.e. when a long time elapsed between evolutionary divergences,
allowing for many random substitutions in base pairs). They called this phenomenon,
wherein non-sister taxa were assigned the same ancient ancestor class, “long branch
attraction.” In practice, errors of this type can occur whether the branches are long, or
short, or in-between. Over the years, the accepted usage of the term “long branch
attraction” has been extended to just about any error in phylogenetic grouping due to
gene similarities acquired through any mechanism other than inheritance from a shared
ancestor. This would include random mutation and adaptive convergence [24]. See
Non-phylogenetic signal.
Metazoa Class Metazoa is equivalent to Class Animalia. It contains two subclasses: Class
Parazoa and Class Eumetazoa. Class Eumetazoans contain all the animals that develop
from a blastula. Class Parazoa contain two small subclasses: Class Porifera (the
sponges) and Class Placozoa. Class Placozoa contains a single species,
Trichoplax
adhaerens
. Sponges and
Trichoplax adhaerens
are exceedingly simple animals, con-
sisting of a layer of jelly-like mesoderm sandwiched between simple epithelium.
Monophyletic class A class of organisms that includes a parent organism and all its des-
cendants, while excluding any organisms that did not descend from the parent. If the
subclasses of a parent class omit any of the descendants of the parent class, then the
parent class is paraphyletic. If a subclass of a parent class includes organisms that did
not descend from the parent, then the parent class is polyphyletic. A class can be para-
phyletic and polyphyletic, if it excludes organisms that were descendants of the parent
and if it includes organisms that did not descend from the parent. The goal of cladis-
tics is to create a hierarchical classification that consists exclusively of monophyletic
classes (i.e. no paraphyly, no polyphyly).
Mosquito Mosquitoes are members of Class Culicidae. Four genera of mosquitoes are
vectors for human diseases: Aedes, Anopheles, Armigeres, and Culex. Among these
genera, there are hundreds of individual species. Mosquitoes are vectors for biologi-
cally diverse organisms (animals, protists, and viruses). As yet, mosquitoes are not
known to be vectors for bacterial diseases; but this biological oversight may soon be
corrected. It has recently been shown that mosquitoes carry pathogenic bacteria,
