In fact, the vast majority of all microbes, have yet to be grown in monoculture. Even if it were possible, studying the properties of an organism in monoculture could be misleading, since interactions with other organisms can dramatically alter their behavior.

The properties of a biofilm are not necessarily a simple function of its inhabitants.  A biofilm is more than the sum of its parts.

Biofilms mature over time. They have a natural progression, like a forest.

There is colonization, maturation and climax, each characterized by specific intrabiofilm and biofilm-host interactions.

Not all of the organisms within a biofilm are necessarily "friendly" towards one another, and many antibiotics are the result of interfilm competition.

Making metazoans:  Biofilms are communities or microecologies composed of distinct organisms living in various states of interdependence and competition.

An early step on the path to true organismic multicellularity is the development of different cell types of the same organism.

One example of multiple cell types is found in the bacteria Caulobacter crescentus. Under certain conditions it will produce stalk cells, that attached to a surface, and swimming cells that can migrate away and colonize new surfaces.  In a sense, the stalk cell is sacrificing itself to form a swarming cell, that has a better chance at survival.

A higher level of integration occurs in the eukaryotic slime molds, in particular the cellular slime molds, such as Dictyostelium discoideum, which can form temporary multicellular organisms.  

Cellular slime molds live in soil and eat bacteria - they are predators.  Most of the time they are small, amoeba-like, haploid cells.

Upon starvation, however, they undergo a dramatic aggregation process.
(slime mold introduction )(aggregation)(signaling)

Aggregation is triggered by the release from isolated cells of pulses of cyclic adenosine monophosphate (cAMP). This leads to the streaming aggregation of cells towards the cAMP source.

Between 10,000 to 100,000 amoebae aggregate but remain as distinct cells. They adhere to one another to form a slug.

The slug can migrate and eventually will differentiate, forming a fruiting body that rises above the soil.

Some of the cells of the slug differentiate to form the stalk.  These cells are terminally differentiated and die without reproducing themselves. They sacrifice themselves for the good of the aggregate.

Others form the fruiting body, which differentiates to form spores. When released, the spores can be widely dispersed and go on to form single celled amoebae.

The slime molds are temporary metazoans.

How is the "self-sacrificing" behavior of the stalk cells possible?  The answer lies in inclusive fitness.

The purpose of the slug and stalk are to enable Dictyostelium cells to escape a hostile environment and colonize new, hopefully more hospitable environments. 

Since Dictyostelium cells do not migrate far, most of the cells in any particular region, that is the cells that combine to form a slug, are likely to be closely related to one another - they are part of a clone.  The sacrifice of stalk cells is more than made up for by the increased chance that the spore cells will survive and prosper, that is, from an evolutionary perspective, have lots of offspring. 

Of course there is a danger that some cells have diverged (through mutation), and will try to cheat, that is, avoid becoming stalk cells.  Such cheating has been observed in wild type Dictyostelium [link][link], and cheating is a challenged faced by all multicellular systems.  We deal with it more in the next reading. 

The steps to true metazoans and metaphyta (multicellular animals and plants) remain to be completely defined, but important events were the development of permanent adhesions and distinct cell types.

There are a number of colonial eukaryotes that contain multiple cell types; specifically cells that possess flagella and those that do not.

The flagellum is a motile organelle used to move the organism through space, or to move fluid over the surface of the organism.

Alternatively, the cells can be more regularly organized, as in a Volvox.

Volvox can reproduce asexually, by forming daughter colonies that develop within the mother.

There is also a sexual cycle, mediated by the formation and fusion of gametes.

These colonial organisms appear closely related to higher plants or metaphyta.

The animals or metazoa appear to have been derived from a different eukaryotic lineage.

Their sister group (that is their closest unicellular relatives) appears to be the choanoflagellates.

Sponges (porifera) are among the simplest of the metazoans. They contain only a few different types of cells. These include

• Pinococytes, which form the outer layer of the organism
• Porocytes, which form the pores or ostium in the organism's outer layer.
• Sclerocytes secrete and form the skeletal system of the sponge, the spicules. Spicules can be soft and spongy or mineralized and rigid.
• Archaeocytes appear to function in digestion, gamete production, tissue repair and regeneration.
• Choanocytes move fluid through the sponge. The fluid enters through the ostium and exits from the top, the osculum.  These cells are located in the interior layer of the sponge.

The next level of metazoan complexity is represented by hydra and related organisms, the hydrozoa, which include the jellyfish.

Some of these organisms alternate between a sessile and benthic or floating lifestyle.

They can swim and can move in subtle ways.

The hydrozoa contain more distinct cell types than the porifera.  The most dramatic difference is their ability to produce coordinated organismic movement.

While sponges are like sieves, the hydrozoa have a single distinct mouth and an internal stomach-like cavity, specialized to digest prey.

Their mouth also serves as their anus, through which wastes are released.

Hydrozoan movement is coordinated by a network of cells, known as a nerve net, that acts to regulate contractile muscle cells.

Together the nerve net and muscles cells generate coordinated movements. There is no central brain, which in its simplest form is just a dense mass of nerve cells.

Nevertheless, a hydra can display movements complicated enough to capture and engulf small fish!

1. How might a biofilm alter the response of bacteria to an antibiotic?
2. Why might an organism grow well in a biofilm but not in an isolated (monoculture)?
3. In the case of the cellular slime molds, what is the advantage of multicellularity?
4. Why do the cells of the stalk "sacrifice themselves" so that the cells of the fruiting body can produce offspring?
5. What are the advantages of a closed gut versus a sieve?  
6. What are the evolutionary values of coordinated movement?
7. How would active swimming evolve? 
8. What types of evidence suggest that choanoflagellates and sponges are related?  
9. Why is the presence of highly specialized cells evidence for common ancestry?
10. In terms of cell types and functions, how do a hydra and a sponge differ from one another?
11. What kind of evidence, in modern organisms, might lead you to conclude that the last common ancestor of plants and animals had flagella?

• Does this make sense in evolutionary terms?
• Are the advantage(s) of multicellularity the same for plants versus animals ?
• Why might you think that the last common ancestor of plants and animals had flagella?