Q: How do the Target Health Global articles get selected and posted on this Blog?

A: We try to keep up with the fast pace of science and medical research by reading daily.  We realized that others might also be interested in the same material and decided to share these articles by creating this Blog.  The articles posted are read by us, or intended to be read by us. Even when we have not had the time to finish reading everything on our daily list, we post it anyway for our readers, who now visit this Blog on a daily basis.

In addition, we read a great deal, to fill in the huge gaps in our knowledge.  We figure others are like us and want to know more….we share it all with our readers.  When you read below, the postings for Thursday, you will see that we could not tear ourselves away from the excitement of reading about evolution on the microbiological level.  We end the postings with several videos by the great Carl Sagan, who brings science to the viewer as a spiritual quest.

The intention of this Blog is to pass on interesting cutting edge information in science, medicine and technology.  Occasionally, we post a photograph taken in New York (and elsewhere), which is where we’re located, simply to share what we love or find humorous.  Hope you enjoy it all!

Great Oxygenation Event

The Great Oxygenation Event (GOE), also called the oxygen catastrophe or oxygen crisis or Great Oxidation, was the appearance of free oxygen (O2) in Earth’s atmosphere. This major environmental change happened around 2.4 billion years ago.

Photosynthesis was producing oxygen both before and after the GOE. The difference was that before the GOE, organic matter and dissolved iron chemically captured any free oxygen. The GOE was the point when these minerals became saturated and could not capture any more oxygen. The excess free oxygen started to accumulate in the atmosphere.

The rising oxygen levels may have wiped out a huge portion of the Earth’s anaerobic inhabitants at the time. From their perspective it was a catastrophe (hence the name). Cyanobacteria were essentially responsible for what was likely the largest extinction event in Earth’s history. Additionally the free oxygen combined with atmospheric methane, triggering the Huronian glaciation, possibly the longest snowball Earth episode ever.

The amount of oxygen in the atmosphere has fluctuated ever since.

Prokaryotic and Eukaryotic Cells

Fluorescent Stained Eukaryotic Cells – NIH pub dom

There are only two basic types of cells, primitive prokaryotes and the more complex eukaryotes. Here are the main features that distinguish these cell types.

What Is a Cell?

Living things are constructed of cells and can be unicellular (one cell) or multicellular (many cells).

Limits on Cell Size

Cells size is limited because cells must be able to exchange materials with their surroundings. In other words, surface area relative to the volume decreases as size of cell increases, and this limits the size of cells.

Cell Theory

Only a few hundred years ago it was believed that living things could spontaneously generate from non-living matter — abiogenesis. We now know better. Cell theory lays out the basic rules that apply to the smallest unit of life. This cell doctrine states that:

  • All organisms are composed of one or more cells.
  • Cells are the basic unit of structure and function in organisms.
  • All cells come only from other cells.

Two Basic Types of Cells

All cells fall into one of the two major classifications: prokaryotes or eukaryotes.

Prokaryotic Cells

Prokaryotes are evolutionarily ancient. They were here first and for billions of years were the only form of life. And even with the evolution of more complex eukaryotic cells, prokaryotes are supremely successful. All bacteria and bacteria-like Archaea are prokaryotic organisms.

Eukaryotic Cells

Eukaryotic cells are more complex, evolving from a prokaryote-like predecessor. Most of the living things that we are typically familiar with are composed of eukaryotic cells; animals, plants, fungi and protists. Eukaryotic organisms can either be single-celled or multi-celled.

Features of Prokaryotes

Pro = “before”, karyon = “nucleus”

Prokaryotes, the first living organisms to evolve, are primarily distinguished by the fact that they lack a membrane-bound nucleus. In fact, the only membrane in prokaryotic cells is the plasma membrane–the outer boundary of the cell itself. Their genetic material is naked within the cytoplasm, ribosomes their only type of organelle.

Prokaryotes are most always single-celled, except when they exist in colonies. These ancestral cells, now represented by members of the domains Archaea and Eubacteria, reproduce by means of binary fission, duplicating their genetic material and then essentially splitting to form two daughter cells identical to the parent.

Features of Eukaryotes

Eu = “true”, karyon = “nucleus”

The most noticeable feature that differentiates these more complex cells from prokaryotes is the presence of a nucleus, a double membrane-bound control center separating the genetic material, DNA (deoxyribonucleic acid), from the rest of the cell.

In addition to the plasma membrane, eukaryotic cells contain internal membrane-bound structures called organelles. Organelles, such as mitochondria and chloroplasts, are both believed to have evolved from prokaryotes that began living symbiotically within eukaryotic cells. These vital organelles are involved in metabolism and energy conversion within the cell. Other cellular organelles within eukaryotic cell structure carry out the many additional functions required for the cell to survive, thrive, grow and reproduce.

Eukaryotic cells can reproduce in one of several ways, including meiosis (sexual reproduction) and mitosis (cell division producing identical daughter cells).


Cyanobacteria bloom
Image: Wikimedia commons, Lamiot

Cyanobacteria are arguably the most successful group of microorganisms on earth. They are the most genetically diverse; they occupy a broad range of habitats across all latitudes, widespread in freshwater, marine and terrestrial ecosystems, and they are found in the most extreme niches such as hot springs, salt works, and hypersaline bays. Photoautotrophic, oxygen producing cyanobacteria created the conditions in the planet’s early atmosphere that directed the evolution of aerobic metabolism and eukarotic photosynthesis. Cyanobacteria fulfil vital ecological functions in the world’s oceans, being important contributors to global carbon and nitrogen budgets.

Scientific classification

Kingdom: Bacteria
Phylum: Cyanobacteria

Cyanobacteria ( /saɪˌænoʊbækˈtɪəriə/; also known as blue-green algae, blue-green bacteria, and Cyanophyta) is a phylum of bacteria that obtain their energy through photosynthesis. The name “cyanobacteria” comes from the color of the bacteria (Greek: κυανός (kyanós) = blue).

The ability of cyanobacteria to perform oxygenic photosynthesis is thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. According to endosymbiotic theory, chloroplasts in plants and eukaryotic algae have evolved from cyanobacterial ancestors via endosymbiosis.

Cyanobacteria can be found in almost every conceivable environment, from oceans to fresh water to bare rock to soil. They can occur as planktonic cells or form phototrophic biofilms in fresh water and marine environments, they occur in damp soil, or even temporarily moistened rocks in deserts. A few are endosymbionts in lichens, plants, various protists, or sponges and provide energy for the host. Some live in the fur of sloths, providing a form of camouflage. Aquatic cyanobacteria are probably best known for the extensive and highly visible blooms that can form in both freshwater and the marine environment and can have the appearance of blue-green paint or scum. The association of toxicity with such blooms has frequently led to the closure of recreational waters when blooms are observed. Marine bacteriophage are a significant parasite of unicellular, marine cyanobacteria. When they infect cells they lyse them releasing more phages into the water.

Blue-green algae, photosynthetic prokaryotes also known as cyanobacteria, first appeared in the fossil record almost 2.5 billion years ago, and have since populated most of the world in a variety of unicellular and multicellular forms. Using gene sequences from 1,254 species of modern cyanobacteria, a team of researchers led by Bettina Schirrmeister of the University of Zurich
created over 11,000 different phylogenetic trees that helped pinpoint when multicellularity evolved in this lineage.

Their first surprise was that this transition happened numerous times in the distant past more than 2 billion years ago, an unexpected finding if the appearance of multicellularity is a complex evolutionary phenomenon. The researchers also found that many modern unicellular cyanobacteria are descended from multicellular predecessors, suggesting that once multicellularity evolves, it doesn’t always stick around.

If organisms always evolved towards greater complexity, as evolutionary biologists have traditionally thought, the unicellular cyanobacteria should be more similar to each other than to the multicellular cyanobacteria, Schirrmeister said. But this isn’t what they found. Instead, the relationship between uni- and multi-cellular cyanobacteria is a complicated evolutionary web.

But even multicellular cyanobacteria are relatively simple organisms, Knoll said, and there are still a lot of open questions about the evolution of complex multicellular life in eukaryotes. (See The Scientist‘s recent feature on this topic.)

Even so, Schirrmeister said, cyanobacteria are some of the most ancient life forms on Earth, “so to understand how multicellularity evolved in cyanobacteria will help us understand how complexity evolves in organisms generally.”

Read more: Multicellular evolution not linear – The Scientist – Magazine of the Life Sciences http://www.the-scientist.com/news/display/58023/#ixzz1EpVtNatd

Characteristics of Cyanobacteria

Cyanobacteria include unicellular and colonial species. Colonies may form filaments, sheets or even hollow balls. Some filamentous colonies show the ability to differentiate into several different cell types: vegetative cells, the normal, photosynthetic cells that are formed under favorable growing conditions; akinetes, the climate-resistant spores that may form when environmental conditions become harsh; and thick-walled heterocysts, which contain the enzyme nitrogenase, vital for nitrogen fixation. Heterocysts may also form under the appropriate environmental conditions (anoxic) when fixed nitrogen is scarce. Heterocyst-forming species are specialized for nitrogen fixation and are able to fix nitrogen gas into ammonia (NH3), nitrites (NO−
2) or nitrates (NO−
3) which can be absorbed by plants and converted to protein and nucleic acids (atmospheric nitrogen cannot be used by plants directly). Rice crops utilize healthy populations of nitrogen-fixing cyanobacteria in some rice paddy fertilizers.

Many cyanobacteria also form motile filaments, called hormogonia, that travel away from the main biomass to bud and form new colonies elsewhere. The cells in a hormogonium are often thinner than in the vegetative state, and the cells on either end of the motile chain may be tapered. In order to break away from the parent colony, a hormogonium often must tear apart a weaker cell in a filament, called a necridium.

Each individual cell of a cyanobacterium typically has a thick, gelatinous cell wall. They lack flagella, but hormogonia and some species may move about by gliding along surfaces. Many of the multi-cellular filamentous forms of Oscillatoria are capable of a waving motion; the filament oscillates back and forth. In water columns some cyanobacteria float by forming gas vesicles, like in archaea. These vesicles are not organelles as such. They are not bounded by lipid membranes but by a protein sheath.

Some of these organisms contribute significantly to global ecology and the oxygen cycle. The tiny marine cyanobacterium Prochlorococcus was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean. Many cyanobacteria even display the circadian rhythms that were once thought to exist only in eukaryotic cells (see bacterial circadian rhythms).

A cyanobacteria bloom near Fiji

Cyanobacteria account for 20–30% of Earth’s photosynthetic productivity and convert solar energy into biomass-stored chemical energy at the rate of ~450 TW. Cyanobacteria utilize the energy of sunlight to drive photosynthesis, a process where the energy of light is used to split water molecules into oxygen, protons, and electrons. While most of the high-energy electrons derived from water are utilized by the cyanobacterial cells for their own needs, a fraction of these electrons are donated to the external environment via electrogenic activity. Cyanobacterial electrogenic activity is an important microbiological conduit of solar energy into the biosphere.

Cyanobacteria have an elaborate and highly organized system of internal membranes which function in photosynthesis. Cyanobacteria get their name from the bluish pigment phycocyanin, which they use to capture light for photosynthesis. Photosynthesis in cyanobacteria generally uses water as an electron donor and produces oxygen as a by-product, though some may also use hydrogen sulfide as occurs among other photosynthetic bacteria. Carbon dioxide is reduced to form carbohydrates via the Calvin cycle. In most forms the photosynthetic machinery is embedded into folds of the cell membrane, called thylakoids. The large amounts of oxygen in the atmosphere are considered to have been first created by the activities of ancient cyanobacteria. Due to their ability to fix nitrogen in aerobic conditions they are often found as symbionts with a number of other groups of organisms such as fungi (lichens), corals, pteridophytes (Azolla), angiosperms (Gunnera) etc.

Many cyanobacteria are able to reduce nitrogen and carbon dioxide under aerobic conditions, a fact that may be responsible for their evolutionary and ecological success. The water-oxidizing photosynthesis is accomplished by coupling the activity of photosystem (PS) II and I (Z-scheme). In anaerobic conditions, they are also able to use only PS I — cyclic photophosphorylation — with electron donors other than water (hydrogen sulfide, thiosulphate, or even molecular hydrogen just like purple photosynthetic bacteria. Furthermore, they share an archaeal property, the ability to reduce elemental sulfur by anaerobic respiration in the dark. Their photosynthetic electron transport shares the same compartment as the components of respiratory electron transport. Their plasma membrane contains only components of the respiratory chain, while the thylakoid membrane hosts both respiratory and photosynthetic electron transport.

Attached to thylakoid membrane, phycobilisomes act as light harvesting antennae for the photosystems. The phycobilisome components (phycobiliproteins) are responsible for the blue-green pigmentation of most cyanobacteria. The variations to this theme is mainly due to carotenoids and phycoerythrins which give the cells the red-brownish coloration. In some cyanobacteria, the color of light influences the composition of phycobilisomes. In green light, the cells accumulate more phycoerythrin, whereas in red light they produce more phycocyanin. Thus the bacteria appear green in red light and red in green light. This process is known as complementary chromatic adaptation and is a way for the cells to maximize the use of available light for photosynthesis.

A few genera, however, lack phycobilisomes and have chlorophyll b instead (Prochloron, Prochlorococcus, Prochlorothrix). These were originally grouped together as the prochlorophytes or chloroxybacteria, but appear to have developed in several different lines of cyanobacteria. For this reason they are now considered as part of the cyanobacterial group.

Epithilial Cells (eukaryotic), nucleus green.


How and Why did Single Cells Evolve into Multicellular Organisms?

“There are lots and lots of transitions from single cells to organisms that have more than one cell, but as far as evolving into complex multicellular organisms with cellular differentiation—that only happened a handful of times.”
—Matthew Herron,
University of British Columbia

Given the complexity of most organisms—sophisticated embryogenesis, differentiation of multiple tissue types, intricate coordination among millions of cells—the emergence of multicellularity was ostensibly a major evolutionary leap. Indeed, most biologists consider it one of the most significant transitions in the evolutionary history of Earth’s inhabitants. But single-celled organisms have stuck together or assembled to spawn multicellular descendants more than two dozen times, suggesting that maybe it’s not such a big leap after all.

“The transition from unicellularity to multicellularity is critical for explaining the diversity of life on Earth,” says evolutionary biologist Casey Dunn of Brown University. “We tend to think of it as quite special, but maybe it’s not. Maybe this is an easier transition than we think.”

To understand how and why it happened, scientists are utilizing the recent explosion in genomics data to assemble more accurate phylogenies and piece together each step in the transition to multicellular life. Despite their efforts, however, the origins of this intriguing phenomenon remain shrouded in mystery. Evolution and extinction over hundreds of millions of years have blurred the details of the transition, and the answers provided by genome sequencing only lead to more questions.

“New studies are always pushing the envelope on our thinking,” says evolutionary biologist Mansi Srivastava of the Whitehead Institute for Biomedical Research in Massachusetts. Since scientists began studying a much wider array of animals, far afield from the classic model systems of fruit flies and mice, “our thinking about what having certain kinds of genes means to being an animal has shifted.”

Conventional thought on evolutionary change has led researchers to believe that genetic innovations underlie the transition. Advances in genomics research, however, are revealing that more and more of the genes associated with complex processes also exist in simpler animals and even in their unicellular cousins. This suggests that the appearance of new genes cannot fully explain the appearance of new traits that are key to multicellularity. Sponges are commonly considered the most basal of all the metazoan (animal) lineages, yet a recently published sponge genome revealed genes known to be involved in the development of a neuromuscular system, which sponges lack.1 “These genes that we previously thought were associated with complex multicellular animals really have to do with basic multicellular functions—to get the simplest multicellular animals, you have to have these genes present,” says Srivastava, who coauthored the analysis.

As some of the most ancient animals, sponges can provide information regarding the evolution of the metazoan lineage, but for true insights about the origin of multicellularity, scientists must look even further back on the evolutionary tree. Choanoflagellates, unicellular organisms that look remarkably similar to the feeding structures of sponges, are the closest living relatives of metazoans. It turns out that they also share a number of genes once thought to be unique to multicellular animals. Tyrosine kinases (TK), for example, enzymes that function in cell-cell interactions and regulation of development in animals, were identified in the choanoflagellates in the early part of this decade, and the first sequenced choanoflagellate genome, published in 2008, revealed that they have more TK genes than any animal—and many other components of the TK signaling pathway as well.2

“So this gene family that was thought to be essentially a trigger that unleashed animal origins, we can now say with great confidence evolved long before the origin of animals,” says evolutionary biologist Nicole King of the University of California, Berkeley, who has been studying choanoflagellate biology for over 10 years.

Scientists have also identified choanoflagellate homologs of cadherins, known to be involved in cell-cell adhesion and signaling in animals. And more recently, a widespread search for genes associated with integrin-mediated adhesion and signaling pathways revealed that the integrin adhesion complex originated much earlier than even the choanoflagellates, dating back to the common ancestor of animals and fungi.3

“It’s pretty surprising to find these adhesion genes in far-flung species,” says Srivastava. “We would have thought that integrin signaling has to do with cells sticking together, but it goes much further back in time than our most recent unicellular cousins.”

The genomic exploration of the evolution of multicellularity is really just beginning, but already, a trend is emerging. “Almost every month now we are seeing genes that were supposed to be exclusive to metazoans that are already present in their single-cell relatives,” says evolutionary biologist Iñaki Ruiz-Trillo of the University of Barcelona. “I think that means co-option of ancestral genes into new functions is important for evolutionary innovations like the origin of multicellularity.”

“Probably the more data we collect, the fewer and fewer animal-specific genes there are going to be,” agrees Dunn. “And we’re going to have to explain the origins of multicellularity in terms of changes in the way these gene products interact with each other.”

We tend to think of [the evolution of multicellularity] as quite special, but maybe it’s not. Maybe this is an easier transition than we think.
—Casey Dunn, Brown University

Unfortunately, because the genomics data are so new, experimental data regarding the functions of these genes in single-celled organisms remains limited. Research by biochemist Todd Miller of Stony Brook University in New York and his colleagues, for example, demonstrated that while tyrosine kinases exist in great numbers in choanoflagellates, they lack the tight regulation found in animal signaling pathways, suggesting regulatory elements may have been key to the evolution of multicellularity. But this idea remains speculative, Miller says, as the targets of these enzymes in the unicellular relatives of animals and the details of their activation are still unknown.

“What we’d really like to be able to do is compare signaling pathways overall and see how they evolved,” Miller says. “We know a lot about the proteins themselves, but it would be great to have a glimpse into a simple pathway where we can begin to unravel what the core elements of the pathway are before layers and layers of additional regulatory elements were added, as we see in metazoan cells.”

To confuse matters more, the vast stretches of time that separate most multicellular organisms from their unicellular cousins—more than half a billion years, in most cases—make for a lot of uncertainty. And as sequencing studies raise more questions, phylogenetic studies are also throwing a shadow of doubt on the animal tree. For example, are sponges really the most basal animals, as has long been thought? A recent phylogenetic study performed by Dunn and his colleagues suggested that perhaps ctenophores (comb jellies) are the earliest diverging extant multicellular animals.4 “Either sponges or ctenophores are sister to all other animals,” Dunn says. “The answer you get depends still on the organisms you include in the analysis, the analysis methods you use, and what genes you look at.”

“In order to understand evolutionary transitions, you need to have a robust phylogenetic framework,” says Ruiz-Trillo. The more genomes are sequenced, the better the phylogenies get, and the more similarities and differences are recognized between multicellular organisms and their unicellular cousins. “It’s a really exciting time” for studying the evolution of multicellularity, King says. With so many open questions and more and more sequenced genomes available each year, “there’s a lot of low-hanging fruit.”

A multicellular model?

Animals aren’t the only multicellular organisms, of course, and thus not the only system applicable to the study of multicellularity’s origins. In fact, multicellularity is believed to have evolved as many as 25 different times among living species. So while the search for metazoan origins may be riddled with uncertainty, perhaps scientists can draw inferences from the study of multicellularity in other lineages.

Comparing brown algae to their unicellular diatom relatives, for example, researchers saw an increase in membrane-spanning receptor kinases, a protein family known to play a role in cellular differentiation and patterning in both animals and green plants.5 The independent evolution of more kinase genes in each of these lineages suggests that this family of proteins may have been key to this transition.

Of all the multicellular lineages, however, the volvocine green algae represent the best-studied and most tractable system for teasing out the evolution of multicellularity. In contrast to most other origins of multicellularity, which likely arose close to a billion years ago, the change to multicellularity in these algae may have occurred as little as 200 million years ago—possibly limiting evolution’s mark on their genomes. Furthermore, between the unicellular Chlamydomonas species and the most-derived multicellular Volvox there are several extant intermediate species, some of which appear to have changed little since their divergence from their unicellular ancestors. While recent evidence indicates a complicated evolutionary history, including multiple origins of some traits and reversals,6 this lineage nonetheless presents a phylogenetic road map by which step-by-step transitions can be inferred. (See illustration)

Volvocine algae are aquatic, flagellated eukaryotes that range in complexity from unicellular species to a variety of colonial forms to multicellular Volvox, some of which boast up to 50,000 cells. This transition involved a series of key innovations, including cell-cell adhesion, inversion, and differentiation of somatic and germ cell lines. Two species in particular have become models for the evolution of multicellularity—the single-celled Chlamydomonas reinhardtii and the 2,000-or-so–celled Volvox carteri.

As with animals, comparisons of their recently sequenced genomes have revealed that there are few striking differences among the genetic codes of these organisms that could explain the drastic differences in their morphology. “It was pretty disappointing at first,” admits developmental biologist Stephen Miller of the University of Maryland, Baltimore County, who helped to analyze the Volvox genome (the sequence was published last summer). “We were hoping to see differences that would point to explanations for why Volvox is so much more developmentally complex than Chlamydomonas, but that certainly wasn’t the case.”

Not only do the genes exist in Chlamydomonas, they are so similar to the Volvox versions that they appear to be able to stand in for missing or mutant copies in their multicellular cousins. Volvox’s glsA gene, for example, codes for an essential component of asymmetric division; glsA mutants can only divide symmetrically, resulting in adults comprised entirely of small somatic cells, with none of the large germ cells, known as gonidia, that normally give rise to the next generation. While the homologous protein in Chlamydomonas is only about 70 percent identical to glsA’s protein, it can restore asymmetric cell division when the gene is transformed into glsA mutants. “Its ortholog in Chlamydomonas is perfectly capable of carrying out the same function,” Miller says.

Similarly, invA is essential to the process known as inversion, which gives adult Volvox their spherical shape, with the gonidia on the inside and the small, flagellated somatic cells around the exterior. In invA mutants, inversion fails to occur due to the cells’ inability to move relative to the cytoplasmic bridges that connect them, and the gonidia are exposed on the surface of the spheroid. Just like glsA mutants, however, this phenotype can be rescued by the transformation of the Chlamydomonas ortholog, known as IAR1.

There are exceptions to this pattern, however, such as the appearance in Volvox of many new genes that encode cell wall or extracellular matrix (ECM) proteins, with a dramatic increase in the number and variety of Volvox genes in two major ECM protein families, as compared with Chlamydomonas. While Volvox carteri have only a couple thousand times as many cells as Chlamydomonas, they can grow to more than 100,000 times larger thanks to a dramatic increase in the amount of ECM, which constitutes more than 99 percent of the volume of a mature Volvox.

Another significant genetic change in Volvox becomes evident when examining the mating locus—a region on one chromosome containing sex-specific genes that dictate whether the organism will be male or female during the sexual part of the volvocine life cycle. A notable difference in the sexual strategies of Chlamydomonas and Volvox is the size of their gametes. While the sperm and the egg of Chlamydomonas are nearly indistinguishable and are produced in similar quantities, Volvox eggs are significantly larger than its sperm, and there are far fewer of them. This transition to oogamy, as it’s called, appears to be a hallmark of multicellularity.

“It’s a remarkably conserved trait,” says cell and evolutionary biologist James Umen of The Salk Institute. “When you look at any lineage that becomes multicellular and has sex, it almost invariably goes from having mating types where the gametes are same size to having a sperm/egg system of some sort.” And in contrast to most of the innovations associated with the asexual reproductive phase of the Volvox life cycle, new genes do seem to be a big part of the evolution of dimorphic gametes. The mating locus of Volvox is greatly expanded to more than 500 percent of the size of Chlamydomonas’ 200–300 kilobase mating locus, and contains many genes that fall outside the mating locus in Chlamydomonas, as well as at least 13 new gender-specific genes.7

“Overall the two genomes are very similar, but the mating locus of Volvox kind of exploded in terms of size and context,” says Umen. “In general, things related to sex don’t follow the normal rules regarding evolution; innovation seems to be a really important part of sex.”

But how much can scientists learn about the evolution of the complex multicellularity exhibited by animals and other lineages from studying the volvocine algae? According to some, not much. Volvox represents a relatively simple form of multicellularity, with only two cell types and no organized tissues or organs.

“I think it’s dangerous to generalize too much,” says Stephen Miller. “Because [multicellularity] has evolved independently in each of these cases, there don’t have to be similarities in how it evolved. But I would guess there might end up being some common themes.”

One emerging idea is that complex multicellularity, such as that of animals, plants, and fungi, may have evolved only a handful of times, and that it almost always resulted from the division of a single cell into the components of the larger organism, King says. In contrast to slime molds, for example, which form via aggregation of neighboring cells, the earliest multicellular animals were likely to have evolved by failing to disperse after the mother cell divided.

Evidence of this comes from a recent study out of King’s lab that found choanoflagellates fail to form colonies when cell division is inhibited.8 If the earliest ancestors of animals were anything like modern-day choanoflagellates, this suggests that animal development from a single-celled embryo is core to our evolution, and not a secondary development.

Similarly, the volvocine algae all divide via multiple fission, where the nucleus divides many times before the cytoplasm splits to generate that number of daughter cells. “It’s a way of producing a large number of genetically identical cells all at once,” says evolutionary biologist Matthew Herron of the University of British Columbia. “The only thing you need to do to produce an eight-cell colony [is have] them to stick together.”

The Evolution of Volvox
The volvocine algae are a model system for studying the evolution of multicellularity, as the group contains extant species ranging from the unicellular Chlamydomonas to a variety of colonial species and the full-fledged multicellular Volvox varieties. Comparing the biology of these organisms, evolutionary developmental biologist David Kirk of the Washington University in St. Louis proposed 12 steps that were key to this transition. Here are some highlights that were key to this transition (BioEssays, 27:299-310, 2005).

Lucy Reading-Ikkanda (diagrams and cells); SOURCE: David L. Kirk

Complexity breeds cooperation

Beyond the molecular and developmental logistics of evolving multicellularity, there is the added complication of genetic conflict. An incredible amount of cooperation is required for individual cells to come together and function as one, and with natural selection acting at the level of the individual cell, there will be significant evolutionary pressure to cheat the system and sabotage the success of the multicellular whole.

The collaboration of first a few, then millions of cells to create an entirely new kind of “individual” thus requires a shift in the level of biological organization upon which natural selection acts. In this way, the evolution of multicellularity can be considered what has been termed an “evolutionary transition in individuality” (ETI), where the unit of selection changes from a single cell to a group of cells—the newly evolved multicellular individual. Other ETIs include the congregation of replicating molecules to yield the first prokaryotic cells, the associations of prokaryotic cells to create eukaryotic cells with organelles such as chloroplasts and mitochondria, and the establishment of cooperative societies composed of discrete multicellular individuals, like eusocial insect colonies.

“The general principle is, in any of these kinds of transitions there’s always some form of cooperation that’s needed,” says Herron. “In the example of the ants and bees, it’s the workers that are being cooperative in the sense that they’re sacrificing their own reproduction in order to help the queen reproduce. And in multicellular organisms like us and Volvox, the somatic cells are cooperating in the sense that they’re sacrificing their own reproduction in order to help the reproductive cells reproduce.”

But such transitions are not always smooth, as conflict can arise when selfish mutations result in cheaters that attempt to benefit from the group without contributing their fair share. One of the first cooperative steps required for the evolution of multicellularity in the volvocine algae was the development of the ECM from cell wall components, which can be metabolically costly to produce. The ECM can thus be thought of as a shared resource, and cells that do not contribute to its production may still benefit from its existence, thus gaining a growth or reproductive advantage.

To defend themselves against such cheating, these new kinds of individuals must evolve mechanisms of conflict mediation. One proposed theory for how the volovcine algae defend themselves against ECM cheaters is the evolution of genetic control of cell number. In unicellular Chlamydomonas, the number of cells produced depends on the size of the parent cell, which in turn is contingent on the amount of resources available. Under these circumstances, it is conceivable that a cell in a multicellular organism could benefit from not contributing to ECM production by putting that saved energy to use making more offspring cells. But all volvocine algae that have evolved an ECM have also switched to genetic control over cell number. As a result, cheaters have less to gain because the total number of daughter cells produced by the group is limited.

The differentiation of somatic and germ cells to yield a division of labor between viability and reproduction represents another potential conflict. In essence, somatic cells are giving up their own reproductive output to support the success of the entire colony of cells, presumably by providing enhanced motility. They don’t always cooperate willingly, however, Herron says; mutations still arise that cause some somatic cells in Volvox to try to reproduce on their own rather than support the entire organism’s reproductive success as sterile swimmers.

The somatic regenerator, or regA, gene appears to be one important factor in keeping somatic cells from defecting. In regA mutants, somatic cells develop normally at first, but then they enlarge and develop into gonidia that can divide to yield Volvox offspring. “What it causes is a dysfunctional colony,” Herron says. “In the lab, we can keep these mutant colonies alive, but they sink to the bottom of the test tube. We assume that they would not last long in nature.”

One proposed mechanism of conflict mediation following this transition is the early segregation of the germ line. The Volvox gonidia that will produce the next generation are formed by just a few rounds of asymmetric cell division very early in development, so there is little time for mutations to accumulate in these cells. While somatic cells may still accumulate mutations, these defects will not be passed on. “They are evolutionary dead ends,” Herron says.

These recurrent mutations in Volvox suggest that “the conflict between the individual cells and the interest of colony may still be going on,” he adds. Such conflict may limit the organism’s complexity, as selection on individual cells battles with the whole organism’s attempt to survive and reproduce, suggesting that perhaps the evolution of advanced multicellularity wasn’t so easy after all.

Read more: From Simple To Complex – The Scientist – Magazine of the Life Sciences http://www.the-scientist.com/article/display/57883/#ixzz1Eoi8ErSm