James McCulloch

And this is my favourite figure, nicely illustrating diversity and turnover of oligocentromeric satelites. Look at Carol and Sandra - one invaded the other at least 2x, which I find particularly amusing.

Very proud of @jamesimcc.bsky.social to submit his first PhD paper about evolution of genomes and centromeres in taxa with oligocentromeric organisation. Lots of interesting discoveries including possibly the first reported re-evolution of monocentricity.

Chromosomes are often classified as either monocentric (single centromere per chr) or holocentric (centromeric activity spread across each chr).

In reality, there is a continuum in centromere organisation - and cyperids are a fascinating system to explore this in as James shows in this preprint! 🌱

Thank you to my co-authors: @marcelauliano.bsky.social, @charlottewright.bsky.social, @hendersi.bsky.social, Sam Ebdon, @kamilsjaron.bsky.social & Mark Blaxter.
Finally, this would not have been possible without the Darwin Tree of Life sequencing & assembly teams @sangerinstitute.bsky.social! 🧵12/12

There are further tidbits in the preprint. E.g., what's the effect of oligocentromeres on the gene & TE landscape of chromosomes? The same as classical monocentromeres? Spoiler: surprisingly, yes, oligocentromeres are associated with ⬆️ TE & ⬇️ gene density. 🧵11/12

www.biorxiv.org/content/10.6...

In annotating centromeres, we also came across another surprise. Some chromosomes of the sedge Carex myosuroides seem to have a single centromere.

"Monocentricity" is considered ancestral for eukaryotes, but if confirmed, this would represent the first known reversion to this state! 🧵10/12

Breakpoint regions are generally enriched for oligocentromeres. This is more noticeable for fusions as fusion regions are easier to identify at a greater resolution than fission regions.

This is despite fusion & fission breakpoints themselves being enriched for oligocentromeres!

So, having multiple centromeres per chromosome may promote fission & fusion, but having too many also seems destabilising. A balance is needed for rapid chromosomal rearrangement. 🧵9/12

Larger chromosomes have greater oligocentromere share and density. Each line corresponds to a species.

This might be partly explained by our finding that longer chromosomes have centromeric sequence taking up more of their length. Fission could result in disproportion, causing spindle geometry problems.

Meanwhile, the fusion of two centromere-rich chromosomes may increase the risk of merotely. 🧵8/12

The relationship between whether a chromosome is a fission product and the scaled median length of oligocentromeric arrays on that chromosome, with glm fits for each species. Generally, recently-split chromosomes have shorter oligocentromeres.

Inter-specific relationships between oligocentromere organisation and rearrangement rates. Greater fusion rate is associated with sparser oligocentromeres and lower oligocentromeric share of the genome, and greater fission rate with longer inter-array gaps.

We'd expect denser and longer oligocentromeres to correlate with chromosomal fusions, or vice versa depending on the direction of drive.

However, we find that both fissions and fusions seem more stable when there are sparser, shorter oligocentromeres. 🧵7/12

Under our theory of "oligocentromeric drive", chromosome inheritance during asymmetric meiosis could be biased by the number of spindle fibre attachments to a chromosome, which could be modulated via the length & density of oligocentromeres, or via inter-chromosomal rearrangements. 🧵6/12

A phylogeny of the cyperid species for which candidate oligocentromeric arrays could be identified, showing for each species the values of four oligocentromeric arrangement variables: array length, inter-array gap length, oligocentromeric share of the chromosome, and array density. The points correspond to either individual arrays or gaps (array length and gap length) or chromosomes (oligocentromeric share and array density). The rearrangement rates are the rates along the terminal branches of the phylogeny.

They also vary in their organisation. We calculated oligocentromere length, inter-oligocentromere gap length, density, and oligocentromeric share of the chromosome. 🧵5/12

The candidate oligocentromere-associated satellites vary within the cyperid clade. The heatmap shows families of candidate oligocentromeric satellites, defined as repeats with > 0.1 Jaccard similarity using 8-mers. The phylogeny inset illustrates the distribution of each family in the cyperid clade. The arrays of the candidate oligocentromere-associated satellites on chromosomes of a selection of species are illustrated beneath to demonstrate the wide variation in the organisation of oligocentromeres across the clade. The phylogeny was made using iTOL (https://itol.embl.de/)

We identified where the oligocentromeres were in each genome using CAP. There's a lot of turnover - across 35 species, we could classify candidate centromere-related satellite DNA into no fewer than 25 families (named after Superstore characters). 🧵4/12

Perhaps they rearrange more because they are "oligocentric" - they have multiple discrete centromeres per chromosome. This means both halves will likely retain centromeres when a chromosome splits. Also, the chromosomes are used to having multiple centromeres: fusion becomes less problematic.🧵3/12

Figure 1. Dynamic chromosomal evolution in cyperids. A. Phylogeny of cyperid species analysed. The number of ALGs at the internal nodes and the karyotypes at the tip nodes are represented by the numbered circles. Grey branches lack rearrangements. Rearrangements were not inferred for the black branches given the unreliability of the ALG inference at the base of the cyperid clade. B. A ribbon plot showing the macrosynteny of Carex pendula and C. sylvatica, two sister taxa. Ribbons connect groups of at least 10 syntenic markers, illustrating how roughly similar chromosome numbers in closely related species hide a plethora of independent fissions and fusions. C. The macrosynteny of C. caryophyllea and C. distans, which are less closely related than C. pendula and C. sylvatica and, accordingly, are even more rearranged despite similar karyotypes.

Sedges and rushes are known to have 2 - 114 pairs of chromosomes.

This variation is driven by fissions and fusions, and comparing whole genome sequences allows us to quantify these rearrangements accurately. Many dozens underlie even small chromosome number variation. 🧵 2/12

A sedge. It may look unassuming but its genome has several unusual features.

Why do the chromosomes of cyperids (sedges & rushes) split and fuse so regularly on evolutionary timescales?

Is it because they have so many centromeres?

Our new preprint, the first major paper of my PhD, addresses this question. 🧵⬇️ 1/12

www.biorxiv.org/content/10.6...

I'm very happy to share that a paper from my DPhil thesis has been selected as the Editor's Choice article in Evolution. 😊

You can read it here: lnkd.in/eCnAQfzT 🐜

@journal-evo.bsky.social

#EvolutionaryBiology #SocialEvolution #Insects

How insects’ sex is determined is unlikely to be what makes them so social

Does haplodiploidy - a method of sex determination seen in bees and ants among other animals - promote eusociality?

New research from @rbonifacii.bsky.social and @stuwest.bsky.social enters evidence into this long debate that - despite popular belief - this is not the case 👇
bit.ly/4qEoEWm

Isotomurus only has the macrosetae on the last 3 abdominal segments.

In terms of species, I’d guess that this is Isotoma anglicana given the colour, but I’d need a microscope to be totally sure!

It looks close to Vertagopus, but a subtle detail is that Vertagopus only has a coat of quite short hairs (although it’s a dense coat). Here there are some much longer hairs (macrosetae), and it looks like they’re on all abdominal segments, indicating Isotoma.

It was such a privilege to work with @juliet-turner.bsky.social on this project while I was a research assistant in the West group @biology.ox.ac.uk. It's wonderful to see our paper in its final form! 🐜

Thank you to @nmaybury4.bsky.social for support during the sampling and for accurately translating the original Latin description, and to @jstgerlach.bsky.social for permission to study @peterhousecam.bsky.social's springtail fauna!

Finding a colony of Hypogastrura springtails on this wall in Peterhouse, Cambridge started me on a trail through old papers, concluding that there may be another - morphologically distinguishable! - species in Britain.

The story is in the latest issue of BJENH:

www.researchgate.net/publication/...

I do think you are right with Proisotoma minuta!

Sorry for the delayed response! The mucro does look sufficiently narrow (and the anal spines sufficiently small) for me to be happy for this to be Hypogastrura manubrialis!

Congratulations! 🥳

A photo from the side would help if possible!

Regarding the mouthparts, my technique is usually to clear the specimen using NaOH or KOH, and then squash under a coverslip. Usually then, if the specimen is cleared well enough, you can see the mouthparts through the head without any fiddly work!

You're certainly right that this is in the Hypogastrura/Ceratophysella group! With small anal spines it'll be Hypogastrura, but I'm not 100% confident that the shape of the mucro is definitely manubrialis - it can be hard to judge depending on angle, but in photo 4 it looks a little broader

366 days ago I started my PhD at the @sangerinstitute.bsky.social. No better time to publish my profile on the Sanger website ⬇️

www.sanger.ac.uk/person/mccul...

I tend to have my taxonomic hat on when posting here, but there is a bigger picture!

(S. trinotatus has these setae, while bimaculatus does not)

Finding hyper-specific details like this to back up an identification never fails to make me smile!