However, in all of these cases the patterns of gene expression define a single segment periodicity, rather than the double segment periodicity observed in Strigamia and in Tribolium.
It is also expressed later during Drosophila segmentation, with single segment periodicity.
eve2 shows a similar pattern of primary stripe formation, but expression fades completely without ever resolving to single segment periodicity.
The precise mechanism of the oscillator appears to vary between species [ 27], but in all vertebrates examined, the oscillation is of single somite periodicity, and is limited to the mesoderm, whereas in arthropods primary segment patterning involves the ectoderm.
Alternatively, given that the short-bodied centipede Lithobius appears to make its segments singly, the oscillating double-segment periodic mechanism in Strigamia may have been acquired in the trunk on top of an ancestral single segment periodicity mechanism.
The rate of appearance of morphologically defined segments, which has been constant throughout stage 4 [ 14], continues unchanged for a period after the cessation of oscillation, while the remaining primary bands resolve from double to single segment periodicity.
A fascinating observation is the molecular signature of this switching of gears: the genetic network underlying segmentation appears to shift from an oscillatory to a non-oscillatory mode that correlates with the transition from a double to single segment periodicity.
In some arthropods (for example, the myriapods Lithobius and Glomeris, the spider Cupiennius), this initial pattern appears to have a single segment periodicity, as defined by the subsequent expression of engrailed and other segment polarity genes in register with these primary stripes, and by the appearance of definitive segment morphology shortly afterwards [ 6- 8].
Such arrangements might have the effect (with potential advantage to the organism) of allowing a single underlying periodicity in some regions of the genome to constrain incremental sliding of nucleosomes in response to lateral forces, while potentially increasing nucleosome dissociation in response to such forces.
