Modular control of time and space during vertebrate axis segmentation

How temporal and spatial control of developmental processes are linked remains a fundamental question. Do underlying mechanisms form a single functional unit or are these dissociable modules?

We address this question by studying the periodic process of embryonic axis segmentation, using genetic crosses of inbred medaka fish strains representing two species,Oryzias sakaizumiiandlatipes. Our analysis revealed correlated interspecies differences with regard to the timing of segmentation, the size of segments and of the presomitic mesoderm (PSM), from which segments are periodically formed. We then did interspecies crosses and real-time imaging quantifications, which revealed extensive phenotypic variation in ∼600 F2 embryos. Importantly, while the F2 analysis showed correlated changes of PSM and segment size, these spatial measures were not correlated to the timing of segmentation. This shows that the control of time and space of axis segmentation can, in principle, be decoupled. In line with this finding, we identified, usingdevelopmentalquantitative trait loci (devQTL) mapping, distinct chromosomal regions linked to either the control of segmentation timing or PSM size. We were able to validate thedevQTL findings using a CRISPR/Cas9 loss-of-function approach on several candidate genesin vivo.

Combined, this study reveals that a developmental constraint mechanism underlies spatial scaling of axis segmentation, while its spatial and temporal control are dissociable modules. Our findings emphasise the need to reveal the selective constraints linking these modules in the natural environment.