Introduction to
DevelopmentMolecular
Microbiology Department, John Innes Centre
Single-cell
Differentiation: Stalk and Swarmer
Single-cell
Differentiation: Endospore Formation
Differentiated
Cells Cooperate: Cyanobacterial Heterocysts
Multicellular
Differentiation: Streptomycetes
Multi-species
Development: Biofilms (Kolter Laboratory) Introduction
to Development
What is meant by development?
In
complex multicellular organisms, development is understood as the process
by which a single fertilized egg transforms itself into a multicellular
organism with differentiated tissues and organs. Development is
marked by these features:
- Cells
of the developing organism maintain the same DNA content as the original
cell. That is why it's possible to clone
a mammal using a somatic cell nucleus. The major exception
to this rule is the immune
system, in which DNA is rearranged in separate clones.
- As
cells reproduce, they change into different kinds of cells with different
complementary functions. This process of change is called differentiation.
Most cells (somatic cells)
lose the ability to reproduce themselves directly; instead they depend
upon germ cells (such
as sperm or egg) to reproduce, with identical copies of their DNA.
- Differentiation
occurs by distinctive regulation
of gene expression, in which different genes and splicing
patterns are turned on in different cell types.
- The
differentiated cells form tissues and organs with different functions
that cooperate with each other for the good of the overall organism.
Some lineages of cells are even programmed to die, to enable maturation
of organs; this programmed cell death is called apoptosis.
In
the evolutionary context, however, multicellular development forms a continuum
with developmental processes in microbes--many of which have major significance
for the environment and for human health. Understanding these forms
of microbial development can enable a deeper understanding of the more
complex systems such as vertebrate animals and vascular plants.
Differentiation
of a Single Cell: Stalk and Swarmer. Top
An
example of a single-celled bacterium that undergoes cellular differentiation
is Caulobacter crescentus. Caulobacter is commonly found
swimming in swamps and sewage outlets.
- Caulobacter
differentiates into two cell types--but they do not stay together
to cooperate.
A motile swarmer cell of Caulobacter finds a nutrient-rich
place to attach and grow. It loses its flagellum, replacing it with
a stalk that attaches to detritus in the pond.
The stalk cell then divides, forming two daughter cells:
a swarmer (flagellated) and a cell with the original stalk. The
swarmer cell swims off to find a new location, avoiding competition
with the stalk cell left behind.
How does the original cell regulate its division to produce
two different types of daughter cell? Regulation involves a complex
cascade
of cell cycle regulators, including activator proteins and proteases
that cleave the activators.
Differentiation
of a Single Cell: Endospore Formation. Top
Some bacteria can form long-lived
dormant forms called endospores.
Examples include Bacillus anthracis
(cause of anthrax), Bacillus
thuringensis (inch-worm insecticide) and Clostridium
botulinum (cause of botulism; source of Botox protein). Endospores
can remain viable for thousands of years.
- Bacillus differentiates
into two cell types--one of which remains viable to reproduce, while
the other supports its development, then dies. This behavior
is analogous to apoptosis, the developmentally programmed death of
helper cells--a common occurrence during vertebrate development.
To form an endospore requires a complex
process called sporulation.
Bacteriology
at U.W. Madison
Step-through
sporulation (Doc Kaiser's Page)
The process of sporulation involves:
- Replication of the bacterial chromosome into two
daughter chromosomes.
- Two sigma factors, sigma A and sigma H, regulate
gene transcription to initiate two different developmental programs
in the two daughter chromosomes.
- Compartmentalization of the cell into the forespore
(regulated by sigma F) and the motherspore
(regulated by sigma E).
- The motherspore assists maturation
of the endospore (regulated by sigma G and sigma K), but its own DNA
ultimately disintegrates.
Multicellular
Differentiation: Streptomyces. Top
Streptomyces species are soil
bacteria that produce many kinds of antibiotics, such as streptomycin.
Streptomyces form mycelial structures that produce spores, in a
life cycle remarkably analogous to that of fungi.
- Streptomyces differentiates into several cell
types within a colony. The cells cooperate to form mycelia,
which generate spores that can start new colonies. This life
cycle is analogous to that of eukaryotic fungi; that is, it is functionally
similar, but evolved independently.
Genome
of Streptomyces coelicolor |
BIOL
114
