Thread by @evagaresp

Let me tell you today about recent exciting work by my former lab which brings together two things I love: #evolution and the #bacterial world! 🧑‍🔬䷰🦠

Let’s talk about #speciation, chromosome organization and positive selection.

Intro thread! 🧵 1/

For a really long time we have known that the many species of the bacterial kingdom have a common ancestor, and evolution during millions of years have allowed bacteria to be genetically very different and live in many different environments 2/

But, for closer species in the tree of evolution, which share a lot of similar genes, the order of these genes in the chromosome is highly variable…

what evolutionary process could result in these re-arrangements? 3/

First we thought it could be weak selection for the order (i.e.“it’s not that important”) mixed with genetic drift...
but this hypothesis implies that low-frequency events occur without selection & fixate in populations even if they are deleterious

so what's an alternative?🧐 4/

In 2020 Brandis & @diarhugh published an article in @PLOSGenetics proposing a new hypothesis in which chromosomal rearrangements could happen from positive selection during niche adaptation, also called the SNAP hypothesis

Link to original work: doi.org/10.1371/journa…
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Here positive selection drives reorganization during adaptation to new environments, in 4 steps:
1. Duplication a chromosomal part
2. Selection to maintain duplication
3. Inactivation of random genes in either copy
4. Fixation of the new organization, which cannot be reversed
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Although this was a sound and logical hypothesis, we didn’t have experimental proof that this process could work in this way, allowing rapid evolution of the chromosome organization…

But guess what? NOW WE DO!! 😱
(Yes, keep reading!) 👇
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.@diarhugh's lab has now followed up this story with a beautiful elegant article in @MolBioEvol, using the power of experimental evolution and next generation sequencing techniques, proving that the SNAP hypothesis works exactly as predicted (!!)

Link: doi.org/10.1093/molbev…
8/

In this work they take Salmonella and force it to adapt to grow only with malate as carbon source (not their preferred food 🍔 yum!)…

...so, what happened during thousands of generations of evolution in this environment? 9/

First a huge duplication (1/3 of the whole genome!) was selected to increase the amount of a malate transporter (more food coming in), proving the first and second steps of the SNAP process
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Then they could see a step-wise inactivation and deletion of other duplicated genes in the region, including some essential genes.

So yes! this happened just as predicted by the third step of the SNAP hypothesis, which led to…
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An irreversible evolutionary trajectory where the original duplication cannot be reversed (also called segregation) and a new chromosomal organization is fixed in the population adapted to grow on malate.

Final step of the process confirmed!
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Like this, now we have an evolved bacterial strain, after just a few thousand generations, that has rapidly rearranged its chromosome as a byproduct of the positive selection to adapt to a new environment

Isn't this amazing? Love to learn how our rich world has come to be!
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