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Arjuna Rajakumar PhD

@arjunarajakumar.bsky.social

1.8K followers 1K following 20 posts

EvoDevo biologist - Post Doc Lehmann lab Whitehead Institute ants + flies germ cells, eusociality & endosymbisis

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Arjuna Rajakumar PhD @arjunarajakumar.bsky.social · 19/09/2026

Always a pole cell never a pgc! Transcription may be essential to segregate healthy and defective pole cells. Pole cells that never activate their zygotic genome and retain what mommy gave them, die. Pole cells that activate their zygotic genome too much also die. Life is a balance! 8/8

Stage 10b embryo highlighting our two populations of germ cells, ones that retain a maternal transcriptome (yellow) and ones that activate zygotically (magenta)

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Arjuna Rajakumar PhD @arjunarajakumar.bsky.social · 19/09/2026

Lost children are the first germ cells to die during embryogenesis, they fail to undergo trans-epithelial migration and are lost in the gut. When we rescue their death with chk2 mutants they still retain maternal transcripts and do not express zygotic transcripts. 7/8

Stage10 b embryos showing two clusters of germ cells, ones with low level expression of the maternal transcript pgc (yellow) and ones with high. Using gut markers we find these high pgc germ cells remain stuck in the posterior midgut

Lost children die between stage 9-11, when we rescue their death using chk2 mutants HR-mediated cell death, we can rescue the lost children death. However, these germ cells still retain maternal transcripts when their successfully migrating counterparts do not!

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Arjuna Rajakumar PhD @arjunarajakumar.bsky.social · 19/09/2026

Finally, our data uncovered a second unexpected cluster of germ cells that we term lost children. These germ cells are isolated from the 9 to 12 hour AEL window as coalesced germ cells. These germ cells completely retain their maternal transcriptome and never activate zygotic gene expression. 6/8

Batch correction of scRNA-seq data using Harmony results in the lost children clustering within the early timepoints, indicating that their transcriptome resembles that of newly formed germ cells despite their late developmental stage.

Feature plots of nanos and amos expression on the Harmony batch-corrected UMAP. Despite their late developmental age, the lost children nestle among the early timepoint germ cells, retaining the maternal transcript nanos. However, unlike the surrounding germ cells, the lost children do not express the Class II gene amos.

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Arjuna Rajakumar PhD @arjunarajakumar.bsky.social · 19/09/2026

As It turns out, nanos actually inhibits Zelda protein levels in pole cells! Now we find that pole cells in nanos mutants have a dramatic increase in Zelda protein. Nanos -> Zelda -> class II genes. This sheds new light for how nanos protects germline identity and prevents its somatization. 5/8

Top panels: nanos heterozygotes showing limited levels of Zelda protein in pole cells.
Bottom panels: nanos mutants showing increased levels of Zelda protein in pole cells, veering into soma level territory!

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Arjuna Rajakumar PhD @arjunarajakumar.bsky.social · 19/09/2026

We found that while these class II genes are expressed in pole cells, their translation is inhibited by nanos. Unexpectedly, when we examined class II transcription in nanos mutants we found a dramatic increase in their transcription!! 4/8

Top panels: Control embryos showing that translation of the class II gene Amos is limited to a handful of pole cells.
Bottom panels: nanos mutant embryos showing a significant increase in Amos translation

top panels: nanos heterozygous embryos (controls) showing limited early transcription of the class II genes elba1 and sisA (gcl is a germline marker mark the pole cells)
bottom panels: nanos mutants showing dramatic increase in the levels of class II transription

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Arjuna Rajakumar PhD @arjunarajakumar.bsky.social · 19/09/2026

Instead, their transcription is dependent on the pioneer factor #Zelda, where loss of Zelda results in loss of early pole cell transcription. 3/8

Images showing Zelda protein expression in stage 4 pole cells

Top panels show control embryos zygotic transcription in the germ cells
Bottom panels show Zelda RNAi which results in loss of pole cell transcription

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Arjuna Rajakumar PhD @arjunarajakumar.bsky.social · 19/09/2026

We discovered unexpected transcription during a developmental period when pole cells were long thought to be transcriptional quiescent. These class II genes are almost all intron-less and RNApol2 independent. 2/8

Unexpected early zygotic transcription in germ cells during a period where germ cells were long thought to be incapable of transcribing (due to pgc inhibition of RNApol2)

Stage 4 Drosophila embryo showing transcription of Taf12l in the absence of RNA polII serine 2 phosphorylation (transcription is supposed to be inhibited....)

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Arjuna Rajakumar PhD @arjunarajakumar.bsky.social · 19/09/2026

So proud of my lab mate @sagrill.bsky.social (whose on the job market 👀) for this magnum opus from @remlehmann.bsky.social lab @whiteheadinstitute.bsky.social ScRNA-seq of 🪰 germ cells reveals new biology, answers age old questions, and proposes new paradigms! 1/8 www.biorxiv.org/content/10.6...

Drosophila stage 10b embryo w/HCR staining marking two germ cell clusters. One will successfully colonize the gonad and the other, like tears in the rain, will be lost forever

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Arjuna Rajakumar PhD @arjunarajakumar.bsky.social · 15/07/2026

After a brutal year from a horrific shoulder injury I am SO excited for #2026SDB in Las Vegas next week! @socdevbio.bsky.social It will be my first time going to the national SDB and can't wait to reconnect and meet new scientists! I will be presenting about #ants at the #HildeMangold symposium!

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