Trainor, P. A. | bioRxiv Science

bioRxiv2021

Specification of axial identity by Hoxa2 distinguishes between a phenotypic and molecular ground state in mouse cranial neural crest cells

Hox genes play a key role in head formation by specifying the axial identity of neural crest cells (NCCs) migrating into embryonic pharyngeal arches. In the absence of Hoxa2, NCC derivatives of the second pharyngeal arch (PA2) undergo a homeotic transformation and duplicate structures formed by first arch (PA1) NCCs. Current models postulate that PA1 represents a NCC ground state and loss of Hoxa2 causes a reversion of PA2 NCCs to the PA1 ground state. We use bulk and single-cell RNAseq to investigate the molecular mechanisms driving this phenotypic transformation in the mouse. In Hoxa2-/- mutants, PA2 NCCs generally maintain expression of the PA2 transcriptional signature and fail to strongly upregulate a PA1 transcriptional signature. Our analyses identify putative HOXA2 targets and suggest that subsets of NCCs may respond to HOXA2 activity in distinct manners. This separation of phenotypic and molecular states has significant implications for understanding craniofacial development.

developmental biology↗

bioRxiv2021

Ocular elongation and retraction in foveated reptiles

BackgroundPronounced asymmetric changes in ocular globe size during eye development have been observed in a number of species ranging from humans to lizards. In contrast, largely symmetric changes in globe size have been described for other species like rodents. We propose that asymmetric changes in the three-dimensional structure of the developing eye correlate with the types of retinal remodeling needed to produce areas of high photoreceptor density. To test this idea, we systematically examined three-dimensional aspects of globe size as a function of eye development in the bifoveated brown anole, Anolis sagrei. ResultsDuring embryonic development, the anole eye undergoes dynamic changes in ocular shape. Initially spherical, the eye elongates in the presumptive foveal regions of the retina and then proceeds through a period of retraction that returns the eye to its spherical shape. During this period of retraction, pit formation and photoreceptor cell packing are observed. We found a similar pattern of elongation and retraction associated with the single fovea of the veiled chameleon, Chamaeleo calyptratus. ConclusionsThese results, together with those reported for other foveated species, support the idea that areas of high photoreceptor packing occur in regions where the ocular globe asymmetrically elongates and retracts during development. Key FindingsO_LIThe eyes of the brown anole, Anolis sagrei, and veiled chameleon, Chamaeleo calyptratus undergo dynamic asymmetrical changes in ocular shape during development. C_LIO_LIIn both species, asymmetric elongation and retraction of the ocular globe is associated with fovea morphogenesis. C_LIO_LIPit formation and photoreceptor cell packing in the foveal area occur when the corresponding region of the ocular globe is retracting relative to adjacent regions. C_LI

developmental biology↗