Hell yea

Excellent ep!

Eukaryotes are just weird archaea

Honestly, me too! I think the concept isn’t wrong, but is maybe 4th or 5th on my list of challenges to overcome.

It's interesting to think about the economics of reproduction as a stronger driver of unicellular bottlenecks rather than, say, the familiar concept of cheating, though a new collaboration with @jeffmaltas.bsky.social shows it is far, far more impactful during MC origins.

Evolution should really care about these costs! As a field, we made a big deal out of the 2-fold cost of sex back in the day (which is a cost for the exact same reasons, with males being like soma), but the costs from soma are much higher in most organisms.

Thanks Inaki!

I agree, it will be cool to see how this is tested down the road. I think it *should* apply to all MC organisms that satisfy our key assumptions: unicellular genetic bottlenecks in the life cycle and somatic cells contribute to growth (but do not become propagules).

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Yea, it’s confusing terminology. But germlines are rare. Reproductive specialization, often called germ soma, is pretty common.

Like how plants do it- terminal differentiation (which can occur late in ontogeny)

Dude let’s chat- would love to collaborate!

If this holds up...one could in theory write another small paper about it, expand it, considering it from multiple angles, and thinking about the overall implications

The other reasons unicellular bottlenecks are important is because of

1) Clonality is a simple and durable solution to social conflict
2) It maximizes partner fidelity, which is critical for evolving complex development

But now we have another reason this is beneficial...right?

And that is awesome because it provides yet another reason that unicellular bottlenecks are special for the evolution of complex MC.

You maximize the impact of the size scaling law we describe, which favors larger size

JEFF! You just blew my mind. Had I had this insight, I might have opted for a full sized paper not a brief comms (which I gravitate towards with theory as I like to make a point simply and then be done!)...

I love this generalization- you are right, it is ln/k, it removes overall size from denom

hahahaha, you just made my day Kyle

25/25 As always, thanks for sticking around to the end. Code and data are available on GitHub. We'd love to hear your thoughts!

24/25 By reducing the costs of specialization, larger size removes a key constraint on reproductive division of labor during evolutionary transitions in individuality. This helps explain one of the broadest patterns in the evolution of complex life: complex organisms tend to be large!

23/25 Most work on germ-soma differentiation has focused on the benefits of division of labor, and rightly so. Our contribution is on the other side of the ledger: quantifying the costs that those benefits must overcome, and showing that these costs depend critically on size.

22/25 We can even do a rough quantitative test. V. aureus has about 2,040 somatic cells and only 8 germ cells. Our model predicts it should grow 3.7-fold slower than if every cell were germ, which is close to the 3-4 fold difference between V. aureus and its unicellular relative Chlamydomonas.

21/25 What does this scaling mean? Despite vastly different levels of somatic investment, the proportional cost to growth rate is roughly constant across species. Larger volvocine algae haven't used their size advantage to grow faster, they've used it to afford more soma.

20/25 Across 18 volvocine species with germ-soma differentiation, the proportion of germ cells scales as pg ∝ N^(-1.2) with an r² of 0.95. This holds within the genus Volvox alone (r² = 0.86), spanning organisms from 2,000 to nearly 50,000 cells.

19/25 The volvocine green algae offer a nice empirical lens for examining these dynamics. This clade ranges from small, undifferentiated species like Gonium to large, highly differentiated species like Volvox carteri, spanning orders of magnitude in both size and somatic investment.

18/25 This creates a ratchet: larger organisms can afford more soma, which favors further increases in size, which further reduces the cost of soma. The model predicts a positive correlation between size and somatic investment.

17/25 Because smaller organisms have shorter life cycles, so the cost of soma compounds more frequently over any given period of time. It's not just that you pay the cost more often, it compounds geometrically. Larger organisms escape this compounding.

16/25 Even cooler: the model reveals a positive feedback loop. Once organisms start investing in soma, getting bigger actually increases fitness, even without any additional benefits to large size from the survival function. Why?

15/25 This means that for any given frequency of environmental stress, there exists some organism size above which it pays to start investing in soma. Size opens the door to specialization.

14/25 Something really interesting falls out of this cost-benefit model. There's a critical frequency of stress events above which investing in soma becomes advantageous. And this threshold depends on organism size: larger organisms need less frequent stressors to favor somatic investment.

13/25 But we wanted to go beyond just costs. What happens when somatic cells provide a compensating benefit? We built a simple model where soma increases survival against periodic stressors.

12/25 Here, contour lines show the fold reduction in lineage exponential growth rate for different organism sizes and germ cell proportions. You can see that as organisms get bigger (moving right), the same proportion of germ cells becomes much less costly. More on the volvox data in post 20!