Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Asexual generation and reproduction

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infected person with food implements or medical equipment,

as examples.

As with other hemorrhagic fevers, treatment consists of

stabilizing the patient. Avaccinefor the Junin virus, which

consists of living but weakened virus, has been developed and

has been tested in a small cohort of volunteers. The results of

these tests have been encouraging. Another vaccine, to the

Lassa virus, consists of a protein component of the viral enve-

lope. Tests of this vaccine in primates have also been encour-

aging to researchers.

Currently, the human illnesses caused by arenaviruses

are best dealt with by the implementation of a rodent control

program in those regions that are known to be sites of out-

breaks of arenavirus illness. Because the elimination of

rodents in the wild is virtually impossible, such a program is

best directed at keeping the immediate vicinity of dwellings

clean and rodent-free.

See alsoHemorrhagic fevers and diseases; Virology, viral

classification, types of viruses; Zoonoses

AArmillaria ostoyaeRMILLARIA OSTOYAE

Armillaria ostoyaeis a fungus, and is also known as the honey

mushroom. The species is particularly noteworthy because of

one fungus in the eastern woods of Oregon that is so far the

biggest organism in the world.

Armillaria ostoyaegrows from a spore by extending

filaments called rhizomorphs into the surrounding soil. The

rhizomorphs allow access to nutrients. The bulk of the fungus

is comprised of these mycelial filaments. The filaments can

also be called hyphae. The fungal hyphae can consist of cells

each containing a nucleus, which are walled off from one

another. Or, the cells may not be walled off, and a filament is

essentially a long cell with multiple nuclei dispersed through-

out its length.

For the giant fungus, using an average growth rate of the

species as a gauge, scientists have estimated that the specimen

in the Malheur National Forest in Oregon has been growing

for some 2400 years. The growth now covers 2200 acres, an

area equivalent to 1665 football fields.

Analysis of the genetic material obtained from differ-

ent regions of the fungal growth has shown the DNAto be

identical, demonstrating that the growth is indeed from the

same fungus. The weight of the gigantic fungus has not been

estimated.

As the giant fungus has grown the rhizomorph growth

has penetrated into the interior of the tree. The fungus than

draws off nutrients, suffocating the tree. As well, the mycelia

can extend as deep as 10 feet into the soil, and can invade the

roots of trees. When viewed from the air, the pattern of dead

trees looks remarkably like a mushroom. The outline of the

fungal boundary is 3.5 miles in diameter.

Scientists are studying the fungus because of the tree-

killing ability it displays. Understanding more of the nature

of this effect could lead to the use of the fungus to control

tree growth.

The bulk of the gigantic fungus is some three feet

underground. The only surface evidence of the fungus are

periodic displays of golden mushrooms that are present in

rainy times of the year.

Although not as well studied as the Oregon giant,

another Armillaria ostoyaefound in Washington state is even

larger. Estimates put the area covered by the Washington state

fungus at over 11000 acres.

See alsoFungi

ASEXUAL GENERATION AND

REPRODUCTIONAsexual generation and reproduction

Sexual reproduction involves the production of new cells by

the fusion of sex cells (sperm and ova) to produce a geneti-

cally different cell. Asexual reproduction, on the other hand, is

the production of new cells by simple division of the parent

cell into two daughter cells (called binary fission). Because

there is no fusion of two different cells, the daughter cells pro-

duced by asexual reproduction are genetically identical to the

parent cell. The adaptive advantage of asexual reproduction is

that organisms can reproduce rapidly, thus enabling the quick

colonization of favorable environments.

Duplication of organisms, whether sexually or asexu-

ally, involves the partitioning of the genetic material (chromo-

somes) in the cell nucleus. During asexual reproduction, the

chromosomes divide by mitosis, which results in the exact

duplication of the genetic material into the nuclei of the two

daughter cells. Sexual reproduction involves the fusion of two

gamete cells (the sperm and ova), each of which has half the

normal number of chromosomes, a result of reduction division

known as meiosis.

Bacteria, cyanobacteria, algae, protozoa, yeast, dande-

lions, and flatworms all reproduce asexually. When asexual

reproduction occurs, the new individuals are called clones,

because they are exact duplicates of their parent cells. Mosses

reproduce by forming runners that grow horizontally, produce

new stalks, then decompose, leaving a new plant that is a clone

of the original.

Bacteria reproducing asexually double their numbers

rapidly, approximately every 20 minutes. This reproduction

rate is offset by a high death rate that may be the result of the

accumulation of alcohol or acids that concentrate from the

bacterial colonies.

Yeasts reproduce asexually by budding; they can also

reproduce sexually. In the budding process a bulge forms on

the outer edge of the yeast cell as nuclear division takes place.

One of these nuclei moves into the bud, which eventually

breaks off completely from the parent cell. Budding also

occurs in flatworms, which divide into two and then regener-

ate to form two new flatworms.

Bees, ants, wasps, and other insects can reproduce sex-

ually or asexually. In asexual reproduction, eggs develop with-

out fertilization, a process called parthenogenesis. In some

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