Arjuna Rajakumar PhD

@arjunarajakumar.bsky.social

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

@sagrill.bsky.socialReposted by @arjunarajakumar.bsky.social

This work would not have been possible without the incredible snATAC-seq atlas of the Drosophila embryo from @diegoisworking.bsky.social @jshendure.bsky.social and @eileen-furlong.bsky.social which helped us discover that germ cells retain a totipotent chromatin state throughout development!

@arjunarajakumar.bsky.social

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)
@arjunarajakumar.bsky.social

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 midgutLost 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!
@arjunarajakumar.bsky.social

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.
@arjunarajakumar.bsky.social

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!
@arjunarajakumar.bsky.social

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 translationtop 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
@arjunarajakumar.bsky.social

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....)