p and some species develop multicellular pseudohyphae, which

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THE
first
yeast
is thought to have originated
hundreds of millions of years ago and
about 1,500
species of
yeasts
are currently identified. These
unicellular organisms
most likely evolved
from multicellular ancestors,1
and
some species develop
multicellular
pseudohyphae, which
is connected bud strings.

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Snowflakes

William
C. RatcliffIn holds
the view that yeasts might have evolved from multicellular organisms.
He
showed that Saccharomyces
cerevisiae
needs
only
less than 60 days to evolve into many-celled clusters that behaved as
individuals and
they even
developed a primitive division of labour
2.
So
the easiness of shifting into multicellularity means that they might
have diverged from multicellularity long ago and this memory is kept.

They
cultured yeast in a tube of liquid, and the yeast clusters settled
more quickly than single cells. Later they selected
cultured
only the cells that sunk. After
many repeats and selection over 60 days, they were able to see the
newly evolved ‘snowflakes’ that consists of dozens of cells. So
gravity acted as the selective pressure here. So they basically
evolved multicellularity in a single-celled organism. The fascinating
part is, unlike other single-celled organisms that form aggregation
of genetically distinct cells, these snowflakes were genetically
similar. So this points out that this kind of ‘divide-and-stick’
feature may have triggered multicellularity. There is hence ambiguity
in this study because it
does not say exactly whether yeast is evolved from multicellular
organisms or vice versa.

These snowflakes were
wonderfully behaving like true multicellular organisms because they
had a juvenile stage and when they become ‘adults’ with certain
size, they divided into larger parent and smaller daughter flakes.
The name snowflake comes after its amazing resemblance to original
snowflakes.(Check out the video of yeast snowflakes from
reference1)Ratcliff
also went further by cultivating the ones that settled faster, and he
got larger ones that grew bigger before dividing. Hint is, natural
selection was acting on the entire flake rather than individual cells
in them.”They
survive as a whole, or they die as a whole. Selection shifts to the
multicellular level,” says Ratcliff.

One
view that Ratcliff holds is since they evolved from multicellular
organisms, they could very easily re-create their ancestor lifestyle.
And the reason for yeast becoming unicellular may be because they
loosing the genes for multicellularity billions of generations ago.

Mating

MATa
and MAT? are the
non-homologous alleles that determines the mating type of
Saccharomyces cerevisiae, the budding yeast3-5.
They code the regulators for the two distinct haploid (n=16) mating
types. By conjugation they form diploid (32 chromosomes). They are
able to switch the mating types by a site-specific homologous
recombination that can
replace MATa sequence with MAT?
or
vice versa. HML?
and HMRa are two intact unexpressed genes at the heterochromatic
loci, that helps in this process and is located at the opposite ends
of the same chromosomes where MAT sequences lie.
Ratcliff does not say anything about these mating types. So checking
whether his snowflakes consist of either one of the mating-type or
both, would be something interesting to look at.When
two yeast cells of the opposite types are in vicinity of each other,
they will start to form structures called Shmoos.
Shmoo
is
nothing but the extension of the cytoplasm towards the mate and when
they touch, conjugation takes place. Shmoo is named after a cartoon
character resembling these mating projections.Sporulation

Sporulation
occurs in diploid yeast due
to nitrogen starvation and in the presence of depleted carbon source.
After meiosis, S.cerevisiae
packages the haploid nuclei into spores. When the daughter cells are
formed inside a yeast cell, two cellular structures are synthesized,
one being the membrane compartments that will give rise to plasma
membrane of the new spores and the other one being the larger and
tough spore wall that prevents environmental damages6-9.Cytokinesis
and Polarity

Polarization
and cytokinesis results in asymmetric cell division, and it in turn
give rise to cell diversity10.
From
the figure one can see the difference in polarization of budding and
mating yeasts. The actin that is selectively present at the region
that tries to Shmoo or bud off, invites secretions and results in
selective growth of that part 11.
Actin also helps in partitioning the cell organelles between parent
and daughter cells 12.
During
mating, astral microtubules move the nucleus to the tip of the mating
projection in preparation for nuclear fusion, which follows cellular
fusion 13,14.Cell
wall formation

The
wall of S. cerevisiae performs many functions
like defines cell shape during various life cycle events, osmotic
integrity and so on. The
wall consists of b1,3- and b1,6-glucans, a small amount of chitin,
and many different proteins. The
wall components can vary and the cross-linking among them also differ
in different conditions. For instance if the stress on the wall is
altered, it can change accordingly its cross-links to account for it.
Also the composition and
degree of cross-linking vary during growth and development 15,16.

One
other function of the wall is that it presents agglutinins and
flocculins to other yeast cells. So again coming back to the
snowflakes, one need to check whether a snowflake contains only MATa
or MAT?,
and whether they have the same agglutinins and flocculins or not.
This can give some flashes regarding the
development and evolution of multicellularity alongside
sex in unicellular organisms.Filamentous
growth

S.
cerevisiae undergoes filamentous growth. Mitogen activated protein
kinase (MAPK) is one of the signalling pathways that trigger
filamentous growth. Rat sarcoma/protein kinase A (RAS/PKA), sucrose
nonfermentable (SNF), and target of rapamycin (TOR) are also other
pathways involved in this duty. And they all
share some of the
factors among them. So it means that the
filamentous growth is a highly co-ordinated activity of many
different signalling pathways 17-19.

But
one question that can likely be raised is, what calls the filamentous
growth instead of snowflake formation. Both are multicellular but
their morphology is highly variant.Cell
Death Pathways

Is
there any programmed cell death in yeast? Yes! Experiments have shown
that yeasts do have programmed cell death though it is not activated
in most cases. Since yeast is a unicellular organism what is the
advantage of dying.. This happens
because in case of nutrient depleted conditions, the
dying cells
activates the programmed cell death pathway mediated by
mitochondrial superoxide, and
thereby nourishing the others with their ‘debris’.
This is an evolutionary advantageous
process ensuring the survival of the species because lack of
nutrients can very easily wipe out unicellular organisms. So
yeasts are altruistic!20-22Yeasts
are thus very useful organisms for research purpose. With the
potential to form multicellularity occasionally, also acting
altruistic, and being a unicellular eukaryote,
it becomes an
extremely handy model organisms for biologists to manipulate their
ideas. With
over 1500 species of yeasts existing, we realize that there can be
still more mysteries concealed within this group of marvelous
creatures.Figure
7:
Death pathways in mammals and yeasts. A) Mammals
and S.
cerevisiae
encode orthologs of the dynamin-like GTPase Drp1/Dnm1, Fis1, AIF,
caspase-like factors, and EndoG (endonuclease G, not yet examined in
yeast). Drp1 promotes cytochrome c
release from mitochondria in mammals, but this has not been
demonstrated in yeast. Reactive oxygen species (ROS) produced by
mitochondria play an important role in promoting both mammalian and
yeast cell death ; B)
The induction of early (programmed) death by yeast correlates with
the ability of yeast cultures to ultimately survive following
environmental stresses that cause elevated ROS levels.J.Marie
Hardwick and Wen-Chih Cheng, 2004