Endosymbionts Still Happening Now

[THEO] Okay, picture this. You're a cell, and inside you there's another, smaller cell. And it's not a parasite you're trying to kill — it's living there, maybe working for you. That nested doll, a cell within a cell, is basically the whole history of complex life.

[DR. MARA] It's how you and I came to exist, mechanistically. Your mitochondria were once free-living bacteria. At some point, a couple billion years ago, one cell took up residence inside another and never left. Chloroplasts in plants — same story, a different ancient bacterium. Endosymbiosis.

[THEO] And the thing that always gets me is that this isn't some frozen event in the deep past. It's still happening. There are organisms all around us right now caught at different stages of becoming someone else's organelle.

[DR. MARA] Which is what makes it a living laboratory rather than a fossil. Today I want to trace how a handful of groups have been reading that process in real time — the cell within the cell, from the molecular handshake all the way to an actual working chloroplast inside an animal cell.

[THEO] Let me set up the vocabulary for anyone outside the field. An endosymbiont is one organism living inside the cells of another. When it's passed straight from parent to offspring — never going out into the environment — we call that vertical transmission. And that matters because an endosymbiont you can't culture, can't isolate, is really hard to study.

[DR. MARA] And the deepest level of this is communication. For two cells to live together, one has to send signals or proteins into the other. Bacteria do this with a type IV secretion system — a T4SS. Think of it as a molecular syringe the cell uses to inject proteins, or DNA, directly into a neighbor.

[THEO] Which is exactly where our story starts. 2017, a group working on Bartonella — these are bacteria that infect mammals, carried by fleas and lice. And they asked a sort of evolutionary origin question: where do the proteins a pathogen injects into its host actually come from?

[DR. MARA] Their answer is elegant. They engineered a sensor to catch protein transfer in the act. It's called CRAfT. You fuse your protein of interest to Cre — the recombinase — in the donor bacterium. The recipient carries a loxP switch that flips on antibiotic resistance only if Cre actually gets delivered inside it. So the recipient lights up only when the protein was physically injected.

[THEO] It's a tripwire. The protein arrives, trips Cre, the lock flips, the colony survives. And the numbers were wild — a Cre fused to the relaxase got delivered at essentially a hundred percent per donor cell.

[DR. MARA] Compared to roughly point-one to one percent for transfer of the plasmid itself. So the protein machinery is vastly more efficient than moving the DNA. And the key finding: a toxin called VbhT, a FicT-family toxin, travels through a conjugative T4SS from one bacterium into another.

[THEO] And here's the evolutionary punchline. VbhT is caught halfway through a career change.

[DR. MARA] Precisely. It began life as half of a toxin-antitoxin module — a bacterial self-regulation system, a poison and its antidote kept in balance. The 2017 work shows it being repurposed into an effector that one cell injects into another. An evolutionary intermediate. You can see a defensive molecule becoming a tool of intimacy.

[THEO] Which to me is the seed of the whole arc. Living inside someone, or alongside them, starts with learning to talk — and that machinery gets borrowed from older conflicts.

[DR. MARA] Now jump to 2021, and the problem shifts from mechanism to a brutal practical wall. There's a group of endosymbionts — feminizing microsporidia — that live inside crustaceans, in this case the amphipod Gammarus roeselii. Strictly vertical. They form no spore you can isolate from the environment. You literally cannot separate the symbiont from the host to sequence it.

[THEO] So how do you read a genome you can't pull out of the host?

[DR. MARA] You stop trying. You sequence both genomes tangled together — host plus symbiont in one soup — and then you deconvolve them computationally. Separate the assemblies afterward by their statistical signatures. They resolved three microsporidia species and a draft of the host.

[THEO] And they used that to answer a lovely convergence question. These microsporidia feminize their host — skew the offspring toward female. And there's a bacterium, Wolbachia, that does the same trick.

[DR. MARA] Independently evolved. A eukaryote and a bacterium both landed on feminizing their host, through different genes. The microsporidian candidates had DNA-binding and membrane-fusion functions. Same manipulation, separate inventions — which tells you the selective pressure to control host reproduction is enormous.

[THEO] Same year, a completely different intimacy problem. Midichloria mitochondrii. The name already tells you where it lives.

[DR. MARA] Inside the mitochondria of ticks. Not in the cytoplasm — inside the organelle that was itself once a bacterium. A bacterium living inside a former bacterium.

[THEO] And the fight here was over how it moves. One model said it behaves like Bdellovibrio — a predatory bacterium that bursts the cell it invades. Lyse the mitochondrion, move on.

[DR. MARA] The 2021 study rejects that. They combined quantitative electron microscopy with network simulation and showed Midichloria travels through the mitochondrial network between organelles, rather than destroying them. And that movement reproduces the power-law pattern of how colonized the tick oocytes get. It's a commuter, not a wrecking ball — explicitly in tension with the Bdellovibrio-like model.

[THEO] So now we've got communication, we've got unreadable genomes solved, we've got the physical lifestyle inside the organelle. And then the arc swings toward tools.

[DR. MARA] It does, and toward the gut — because the most accessible endosymbioses for engineering are our own commensals. Two 2023 entries. First, Akkermansia muciniphila, a gut bacterium that eats mucin, the mucus lining your intestine.

[THEO] A transposon screen — you knock out genes at random across the whole genome and see which knockouts lose the competition — pinned down a system they call MUL. Pili plus a periplasmic complex that actively imports mucin into little internal compartments, mucinosomes.

[DR. MARA] And that import isn't just feeding. MUL lets Akkermansia outcompete other gut microbes, and it represses the host's colonic sterol biosynthesis genes. The symbiont is reaching out and tuning host metabolism.

[THEO] And the companion piece is a review of RB-TnSeq — barcoded transposon libraries — applied to gut commensals. Chemical genomics, host interaction, fitness inside a live animal.

[DR. MARA] Which matters because it's the toolkit maturing. We've gone from catching one protein crossing between cells to systematically mapping which genes let a symbiont live inside a host.

[THEO] And then 2024 just goes for broke. Cyanidioschyzon merolae — a thermophilic red alga — they take its chloroplast, a tidy 243-gene genome, and put it inside CHO cells. Mammalian cells. Hamster ovary cells.

[DR. MARA] And the chloroplast keeps running photosystem II electron transport for at least 48 hours. Photosynthesis, confirmed, inside an animal cell. Measured.

[THEO] Which is the whole arc in one experiment, right? We spent the segment watching evolution build cells inside cells over billions of years — and here's a lab doing the first step by hand.

[DR. MARA] Forty-eight hours is not a stable organelle. No division, no inheritance yet. But it's the opening move, reconstructed deliberately. The question the next decade asks is whether you can make that passenger permanent.

[THEO] From a toxin learning to be a messenger, to a chloroplast catching light inside a hamster cell. That's where we'll leave it — more cells within cells after the break.