Chemotaxis, or movements directed by chemicals in the environment, can be seen from immune cells talking to one another with cytokines during an infection to 20-somethings intoxicated by pheromones at a bar on Friday night (right: bacteria require extensive chemotaxis for cell-to-cell communication to create these intricate growth patterns during swarming, on agar plates).
I think it’s safe to say that all multicellular organisms owe their existence to cell-to-cell communication by chemotaxis, but what about unicellular organisms? If chemotaxis is so important for us to grow, live & survive, shouldn’t it be important for our smaller brothers from another mother?
I say we should shy away from bacteria today (it’s hard for me to do this) and talk about eukaryotic unicellular organisms, yeast.
Yeast are eukaryotic cells and have a defined nucleus & double-stranded linear DNA. Yeast are very interesting characters, and not because they make your bread rise, but they can be both a diploid and haploid. A diploid is a cell that has two homologous, or identical, copies of each chromosome (one from yo mama & one from yo papa) – we are diploids, however our eggs & sperm are haploid. A haploid cell contains only one copy of the chromosome. In humans this means that when an egg & sperm come together they make a diploid (in a very interesting, yet too complicated to explain here way).
Since yeast can be both diploid and haploid they have two different types of replication. Both types can reproduce by mitosis, with daughter cells budding off of mother cells. However, haploid cells can mate!
In these mating yeast cells there are 2 populations, a-cells and α-cells (very creative naming in yeast research). a-cells can only mate with α-cells, and vice versa – and both must have the mating gene MATa or MATα, respectively.
Why would chemotaxis be important for yeast?
The basis of yeast mating is detection of pheromone gradients that lead a-cells to α-cells and α-cells to a-cells (think the 20-somethings at the bar). a-cells secrete a-factor pheromone that stimulates α-cells, and vice versa. a-factor & α-factor can manipulate gene transcription, phosphorylation pathways, and morphological changes in their mating partner.
When our haploid yeast are exposed to large quantities of these chemotactic factors the cell in response is arrested in growth and form the “schmoo” morphology. When the haploids are exposed to intermediate levels of these factors the cells elongate in the direction towards the increasing concentrations of pheromone.
These yeast are serious about their mating.
1) Yeast haploids chit-chatting.
2) Schmoo formation.
3) Diploid formation. Happy little yeast.
Oddly enough, a-cells secrete the protein Bar1, an enzyme that degrades α-factor and lowers the mating response, but Bar1 is required for mating.
Why would a-cells produce an enzyme to degrade a pheromone required for their mating? (Darwin is confused).
It was previously hypothesized that Bar1, despite degrading important pheromones, can promote mating by limiting diffusion of the α-factor, therefore creating local pheromone gradients that are better aligned with the nearest α-cell hottie.
Meng Jin, et al. explored this hypothesis using experimental and computational approaches. They showed that the secretion of Bar1 from a-cells allowed cells of the same mating type to avoid one another, therefore minimizing the competition (Figure 1).
Bar1 provides a self-avoidance mechanism. Simulated cell growth demonstrated that a-cells avoid one another during chemotropic growth in a gradient chamber. White arrows indicate the direction of growth (the color bar at right shows the color scale for pheromone concentrations in nM, within the computational domains).
They also show that Bar1 allows a-cells to find unique mating partners (Figure 2). Additionally, they finish their paper by showing Bar1 amplifies pheromone gradients during chemotactic growth – allowing for a-cells to orient their elongation towards the incoming α-factor, and adjusting their elongation if their initial growth is not toward a mating partner.
Bar1 allows a-cells to find unique mating partners. When there is presence of a uniform concentration of Bar1 there is competition by two a-cells (empty) with the α-cell (yellow). When a-cells locally release Bar1, both cells find a unique mating partner (the color bar at right shows the color scale for pheromone concentrations in nM, within the computational domains).
This group clearly showed that Bar1 promotes mating efficiency, despite it’s function to degrade specialized pheromones required for yeast mating. Taking a step back, it’s pretty insane that we can see cells reshaping their environment to avoid less-than-desirable situations, or in this case non-reproductive cell-to-cell interactions. It is also interesting to think that cells are well-tuned and dynamic systems that can respond with discrete and efficient planning.
Meng Jin, Beverly Errede, Marcelo Behar, Will Mather, Sujata Nayak, Jeff Hasty, & Henrik G. Dohlman, Timothy C. Elston (2011). Yeast Dynamically Modify Their Environment to Achieve Better Mating Efficiency Science Signaling , 4 (186)