Playing Evolution With a Full House

Fitness trade-offs between spores and nonaggregating cells can explain the coexistence

of diverse genotypes in cellular slime molds

Tarnita CE, Washburne A, Martinez-Garcia R, Sgro AE, Levin AS(2015) PNAS 112(9):2776-2781

Joshua Gefen

Say hello to slime molds… Dictyostelium discoideum – a soil-living amoeba

from the phylum mycetozoa

Capable of unicellular - multicellular transition

Capable of asexual and sexual reproduction

Genome was sequenced and mapped in 2005. Consists of 34Mb haploid genome with a base composition of 77% (A+T) and 6 chromosomes

Life Cycle and Reproduction

MoundSlug/Finger

Mexican Hat

Fruiting Body

“Loner” cells left behind after aggregation are fully viable and capable of aggregation during future starvation cycles.

Left behind

Why I decided to study biology

A selfish pause…

The problem with anthropocentrism

Decisions decisions decisions… …Evolution!

Social Behavior Farming

Brock DA, Douglas TE, Queller DC, Strassmann JE (2011). “Primitive agriculture in a social amoeba.” Nature 469 (7330): 393–396

Cooperation and cheatingStrassman JE, Queller DC (2011). “Evolution of cooperation and control of cheating in social microbe.” Proc Natl Acad Sci USA 108 (Suppl 2):10855-10862

Cannibalistic sexual phagocytosisLewis KE, O’Day DH (1994). “Cannibalistic sexual phagocytosis in Dictyostelium discoideum is modulated by adenosine via an A2-like receptor.” Cellular Signalling 6(2): 217-222

Questions How is it that in nature we find a big

genotypic diversity when the main process for spore dispersal is a reducing mechanism?

What are the processes that lead to bet-hedging and long-term optimization strategies: dormancy vs. dispersal, persistence vs. normal growth, and exploitation vs. exploration?

𝑑𝑅𝑑𝑡 =− 𝑐𝑅

𝑅1 /2+𝑅∑𝛼𝑋𝛼

𝑑 𝑋𝛼

𝑑𝑡 = 𝑐𝑅𝑅1 /2+𝑅

𝑋𝛼−𝜇 𝑋𝛼

Methods & Terms

Several clonal natural strains were collected and cultivated. Strains were manipulated to begin the spore/loner function

Michaelis-Menten kinetics were used in order to describe the dynamic of the spore/loner distribution

In order to synchronize between starvation time, spore germination time, aggregating time, etc., there was a need to build an algorithm of numerical simulations (M=25 patches) undergoing desynchronized growth-starvation cycles

Assumptions

2. The model doesn’t test the selection of a stock-to-spore investment ratio or the average length of slug migration

3. The model describes only starvation selective pressure and doesn’t deal with other ecological terms (e.g. soil type, light, moisture, etc.)

1. Even if there is interaction within chimeric fruiting bodies, they are not necessary to explain the existence of a reproductive skew

More Assumptions4. Spores are very resistant to environmental

stress but nevertheless incur a small decay rate (δ), which is smaller than the loner amoeba dying rate (μ)

5. If resources dwindle before the germination period is over, the writers assumed, that the spore returned to dormancy without incurring any cost associated with the abortion of germination process.

Chimeric interactions are not necessary to produce reproductive skew in spores.

Reproductive Skew

Fast-recovery environments select for investment in loners; slow-recovery environments select for investment in spores.

Conclusion

Discussion Chimeric interactions between genotypes are

not necessary to explain the reproductive skew

In a rich ecology context (variable food, recovery environments, weak to moderate dispersal), the loners can provide a better explanation for the great genotypic diversity observed in nature

The viability of loners is consistent with the bet-hedging strategy

The quorum sensing is instrumental in the decision to aggregate. A direct signal mediated quorum can be behind the mechanism of a loner/spore distribution