No Predictable Relationship Between Presacral Vertebral Number and Hes7 Intron Number Across Mammals

Abstract

In mammals, the number of vertebrae and the somites they derive from is highly limited. Nevertheless, there are some lineages that have an increased number of presacral vertebrae and thus an elongated trunk. This suggests that somitogenesis, the process of somite formation in early development, is altered in these lineages. According the ‘clock and wavefront’ model of somitogenesis, temporal information of somite boundary formation is generated by a traveling wave of cyclic expression of oscillator genes. Hes7 has been suggested to be a key oscillator gene of this molecular segmentation clock. A previous study showed that reducing the number of introns within the Hes7 gene results in a more rapid tempo of Hes7 oscillation and an increased number of presacral vertebrae. Variation in Hes7 intron number could therefore be a potential evolutionary mechanism for varying vertebral number across mammals. In order to test this hypothesis, Hes7 intron number is here compared to presacral vertebral number across a variety of mammals.No significant relationship between both metrics could be detected as their variation across the mammalian phylogeny is fundamentally different. Integrating our data in the previously published mathematical model of Hes7 oscillation confirms the finding that variation in intron number does not predict variation in presacral vertebrae, rendering a direct causal relationship unlikely. However, our data support the previous suggestion that at least two introns are required for Hes7 pace making function of the segmentation clock.