How can conserved gene expression allow for variation? Lessons from the dorso-ventral patterning gene muscle segment homeobox

Arthropods are common in marine, freshwater, terrestrial, and even aerial environments. The arthropod nervous systems must be adjusted to the highly diverse behaviour and requirements of the individual arthropod species. This raises the question of how the underlying patterning mechanisms have changed during arthropod evolution to produce the characteristic axonal scaffold on the one hand and allow for variations in neuronal networks on the other hand. Here we show that the overall number of the neural precursor groups/neuroblasts as well as their spatial arrangement in rows and columns is similar in all four arthropod groups indicating a common origin of this pattern. Furthermore, we demonstrate differences in the expression pattern of the columnar gene muscle segment homeobox and both differences in the expression and regulation of the neural subtype specific genes even-skipped and islet. This variation may underlie the evolutionary variations in neural identity in the individual arthropod groups. Furthermore, we discuss to what extent the stereotyped pattern of neural precursors is required for the conserved axonal scaffold and thus might have been constrained along with the underlying patterning mechanisms.