In the world of microbiology, it’s always the most outrageous microbes that get all the buzz. Prokaryotic microbes, like Escherichia coli are usually the first examples of microbiology that come to most people’s mind. When you think about eukaryotic microbiology, yeasts like Saccaromyces cerevisiae involved in brewing and baking are the model examples. Or some insane fungi that can grow in and out of a host – I’m looking at you, Cordyceps.
Then there are the microbes that just do some weird, but
awesome stuff by themselves. Like the warnowiids.
Warno-what?
The warnowiids are a little bit like the cousin you hear
about at family reunions – they’re always doing something cool, but you’re not quite sure how they’re doing it, and you never really see them either. So what makes warnowiids so cool? They’re unicellular eukaryotic microbes that are flagellate protists, and have developed a complex eye structure called the ocelloid.
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| Figure 1. Microscopy and Illustrations of theOcelloid. Light micrograph (left), illustration (middle), and electron micrograph (right) of the ocelloid. The parts of the ocelloid and their evolutionary origins are labeled on the illustration. |
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| Figure 2: Analogous structures of the ocelloid andhuman eye. The ocelloid (left) has structures that are similar in function to structures in the human eye (right). H is the hyalosome, C is the crystalline lens, and R in both illustrations is the retina or retinal body. |
These discoveries bring up some important food for thought. What
role does multicellularity really play in the development of specialized
structures such as the eye, if a single celled microbe can develop an analogous
structure in the ocelloid? How did this development even happen? Why favor a
complex ocelloid over a simpler eyespot? What are the evolutionary forces that
drove the warnowiids to adapt this structure?
However, answering these questions wasn’t so easy due to how
warnowiids grow and where they grow. Warnowiids are unable to be cultivated in
a lab, making it difficult to grow and keep these cells viable for experiments.
So why not just find isolates in the environment and just use those instead?
The other problem is that warnowiids are also rare in the wild. They are
typically found on plankton in marine environments, but they are found with
rates of only one or two cells isolated from plankton per year (1). One or two cells! You’d have
better luck finding affordable college education at that rate. Despite these obstacles,
a few studies have come out in the last ten years detailing the structure and
function of the ocelloid, as well as its evolutionary history.
In one study, researchers used electron microscopy and
tomography to investigate the structure of the ocelloid (1). Electron microscopy uses
electrons and tomography uses an ion beam to obtain images of the ocelloid. The
authors also used genomics, both isolated organelle and single cell genomics in
order to study the makeup of the ocelloid. What built the structures of the
ocelloid?
It turns out that the ocelloid might be derived from different
organelles. Previously, the retinal body was reported to have thylakoid-like
structures. Typically, thylakoids are found within chloroplasts, and usually
are the site of the light dependent reactions of photosynthesis. By using
electron microscopy, the researchers found that during cell development, the
retinal body appeared to be derived from a plastid-like structure with double
stacked thylakoids (1). Plastids are organelles that
usually house pigments used in photosynthesis, so this discovery brings into
question what the function of the retinal body is if it contains these light
harvesting proteins.
The researchers also used electron microscopy to examine the
lens of the ocelloid, the hyalosome. They found that the lens contained a sheet
of individual mitochondria that were connected to the cytoplasmic mitochrondria
nearby. With the discovery of both thylakoid-like structures and mitochondria
present in the ocelloid, the authors concluded that the ocelloid was composed
of different organelle components. Mitochondria, similar to plastids, are thought
to have an endosymbiotic relationship with warnowiids.
The researchers suggested that the presence of these
plastid-like structures with thylakoids might indicate the ancestor of
warnowiids were photosynthetic, helping to possibly narrow down what the
evolutionary path of warnowiids might have been. There is also the question of
where these plastids were derived from in the first place. In dinoflagellates, which
warnowiids are a part of, plastids are thought to originate from endosymbiosis
with red algae (1). Coupled with the mitochondria
sheets found in the lens, these conclusions suggest that the ocelloid
incorporates different endosymbiotic organelles together in order to form this
complex structure.
Whoa. So instead of undergoing cell differentiation or
similar mechanisms by which our own multicellular eyes develop, the warnowiids instead
create a hodgepodge of different organelles in the form of mitochondria and
plastids to achieve complexity. In some ways, the warnowiids have adopted a
“fake it ‘till you make it” mentality when it comes to producing a complex
structure of their own analogous to a multicellular structure found in our
eyes.
So we’ve elucidated what we think the structure looks like,
what its composed of, but what does it actually do? In another study, the
researchers sought to determine the function of the ocelloid by determining if
the structure was photoreceptive (3). The researchers examined the
ocelloid using electron microscopy in two different light conditions, in a
light adapted state and a dark adapted state (3). They found that the retinal
body in the light adapted state had thicker lamellae compared to the dark
adapted state. That is, the total surface area of the retinal body became
larger in the dark adapted state, suggesting that the ocelloid does play a role
in responding to light and dark conditions as a larger surface area has more
chance to gather light. The researchers also found that the hyalosome can also
play a role in light sensitivity by concentrating dim light (3). With these conclusions, the
researchers determined that the ocelloid does play a role in sensing light and
functions as an ocular structure.
As we figure out methods to cultivate organisms that were
previously only able to be isolated from the wild, more experiments can be done
on warnowiids to further investigate the evolutionary path that these organisms
took in order to develop the ocelloid structure. This is important in two ways.
One, the fact that ocelloids developed not out of multicellularity, but instead,
a chimeric combination of different organelles provides another way to how complexity
can be achieved in organisms. Second, the evolutionary forces that drove
warnowiids to develop ocelloids might serve as an example of driving forces for
other evolutionarily mysterious structures found in different organisms.
Warnowiids might not be in the public eye (no pun intended) as
much as yeast, or fungi, or even other protists, but their unique development
of a complex structure without multicellularity provide insights on how
evolution could have driven this process, as well as how different
photoreceptive structures in organisms are made and what their function is. Due
to the difficulties in obtaining warnowiids, not many experiments have been
done on these unique organisms, but in the next couple of years, we might be
seeing more of these guys as cultivation techniques improve.
Keep an eye on them.
References:
1.
Gavelis GS, Hayakawa S,
White Iii RA, Gojobori T, Suttle CA, Keeling PJ, Leander
BS. 2015. Eye-like ocelloids are built from different endosymbiotically
acquired components. Nature 523:204–207.
2. Hoppenrath
M, Bachvaroff TR, Handy SM, Delwiche CF, Leander BS.
2009. Molecular phylogeny of ocelloid-bearing dinoflagellates (Warnowiaceae) as
inferred from SSU and LSU rDNA sequences. BMC Evol Biol 9:116.
3. Hayakawa
S, Takaku Y, Hwang JS, Horiguchi T, Suga H, Gehring
W, Ikeo K, Gojobori T. 2015. Function and evolutionary origin
of unicellular camera-type eye structure. PloS One 10:e0118415.
