M. Brent Hawkins

A cloud formation that resembles the chevron shape of vertebrate segmented muscle blocks

Myomeres in the sky

Thank you Aurelie!!

Thanks Mathieu! This reminds me that i have a gill mutant I need to show you sometime

Thank you Neha!

Thank you Min Ya! This means a lot coming from someone with truly beautiful DV patterning work, although in a very different context!

Thank you Graham!

Thanks Ale!

Thanks! 🙌

Thank you Marian! It has been a pleasure

Thanks Jan!

🫡

Thanks Mathi!

Thanks Seth!

Matthew Harris @fish4walking.bsky.social and myself at Boston Children’s Hospital performed the CRISPR-based functional genetic manipulations in zebrafish and analyzed the microCT cypriniform data available through Morphosource (www.morphosource.com). Thank you! fin. 🧪🧬🐟

Jacob Daane at the University of Houston analyzed the evolution of the LARM elements across cypriniform phylogeny and identified putative variants responsible for the adaptive variation seen in hillstream loach fins

David Jandzik of Comenius University in Slovakia with Dan Medeiros at the University of Colorado obtained rare lamprey material at the ammocete stage and performed in situ hybridization to confirm the ancestral, pan-vertebrate nature of posterior unpaired fin lmx1b expression

Nicolas Cumplido and Sarah McMenamin at Boston College in the US did the meticulous anatomical analysis of the fin musculo-tendonous-skeletal system made possible with expert clearing of imaging of the fins

Juan Tena and Silvia Narajno at CSIC-Universidad Pablo de Olavide in Spain performed the extensive reporter transgenic assays using many vertebrate LARM orthologs and provided the beautiful zfish HCR data

Marian Ros @marianros.bsky.social at the Universidad de Cantabria in Spain led the effort to identify LARMs outside of the tetrapods with Miguel Juliá Manuel, and with Sofia Zdral generated the “fishified” mice harboring zfish LARM elements.

This evolutionary interrogation of DV patterning with functional genetics, bioinformatics, expression analysis, anatomical assessment, & comparative genomics was only possible through a wonderful international collaboration of investigators. It has been a delight & I think the best way to do science

The conserved LARM1s core might serve as a “tent-pole” under which regulatory motifs can appear and disappear, resulting in lineage-specific sequence and activity while retaining core functionality

Our study also shows that regulatory function can be conserved even in the absence of sequence identity. Only the LARM1s element showed strong sequence conservation in teleost species, even though a larger surrounding region with divergent sequence retained the regulatory function.

The embellishment of DV polarity during the fin-to-limb transition occurred in the context of this prepattern, and likely involved the evolution of targets downstream of Lmx1b that await investigation.

Altogether, our findings show that determination of dorsal fates by LARM-mediated Lmx1b is an ancestral feature of vertebrate paired appendages that arose independently from the median fin program.

These results suggest that the LARM might act as a hub which can facilitate the evolution of novel DV anatomies.

The skeleton of a true loach pectoral fin with its conspicuous dorsal ornament

True loaches also exhibit modifications to the dorsal aspect of the pectoral fin, and males develop large ornaments on the fin rays that exhibit species-specific morphology. They also develop dorsal hook-like flanges in the posterior fin region.

A phylogenetic tree of the cypriniform fishes upon which is plotted a blue color, the intensity of which indicates accelerated sequence evolution

Additionally, we also detected accelerated sequence evolution within the LARM2 element in the lineage leading to the hillstream loaches as well as the true loaches (family Cobitidae).

A plot showing the evolutionary relationships of cypriniform fishes and a DNA sequence alignment showing LARM1x deletions in the hillstream loaches

However, we detected a deletion specifically within the hillstream loach lineage in the LARM1x element. This deleted region is predicted to contain transcription factor binding sites and its removal could impact Lmx1b regulation

Plot of sequence conservation across an alignment of the LARM locus

We asked if the hillstream loach lineage exhibited variation in the LARM architecture that correlated with apparent modification of DV pattern. Genomic alignments revealed that the various LARM elements are largely conserved across the cypriniforms, which include zfish & the loaches amongst others

On top, a CT reconstruction showing the ancestral cypriniform fin condition with ventral flanges (arrow) and simple dorsal aspect. On bottom, a CT reconstruction of a hillstream loach with derived ectopic dorsal flanges (arrowheads)

Using microCT datasets from Morphosource, we found that hillstream loaches naturally develop the ectopic dorsal flanges we observe in holo-LARM deletion mutant fins. This phenocopy is a derived condition, as most cypriniform fishes have a configuration similar to WT zfish with ventral flanges only