lpetrich
December 22, 2003, 08:29 AM
Over at http://pharyngula.org I found Chemical oscillators and somitogenesis (http://pharyngula.org/comments?id=215_0_1_0_C), about how to make segmentation and other sorts of patterns.
PZ Myers, Mr. Pharyngula, started out by mentioning some detailed order produced by simple means, like reaction-diffusion systems such as Belousov-Zhabotinsky. But a favorite model system, Drosophila melanogaster, is an embarrassment. Although it starts out simple, with a gradient of some morphogen, it gets complicated and kludgy, with gap genes being expressed as a function of that gradient and the neighboring gene products, and then pair-rule genes on top of those. He comments that this looks like some custom-made system that had replaced an earlier, simpler system.
And there is evidence for such a system in vertebrates; he described some work on zebrafish, chicken, and mouse embryos in which a chemical oscillator is involved in laying down somites (muscle blocks).
This system may be more widespread; like vertebrates, annelids and many arthropods produce new segments from a read-end growth zone. Insects divide up as:
Short-germ: head simultaneous -- grasshopper Schistocerca
Intermediate-germ: head and thorax simultaneous -- beetle Tribolium
Long-germ: the whole body simultaneous -- fly Drosophila
The more "primitive", three-stage (hemimetabolous) insects are usually short-germ. Longer-germ development typically appears in four-stage (holometabolous) insects, and is likely an adaptation for increased speed of development (no need to wait for all the segments to form if they all form simultaneously).
And judging from the occurrence of the intermediate-germ state, the emergence of long-germ development has gone in the rearward direction; the front end has simultaneous-specification mechanisms that can be copied or build on, and it is the forward segments whose specification would most need to be speeded up.
The evolution of the replacement of segment-specification mechanisms would be interesting to work out; however, doing so may require studying the genetics of development in more species than has usually been done.
It is also a rather blatant violation of Ernst Haeckel's "biogenetic law", in which earlier development features are more conserved than later ones. Segment specification is conserved, but not the mechanism for generating it. However, his "biogenetic law" is already known to be violated by the different sizes and yolk contents of eggs across the animal kingdom.
And if this segment-making mechanism for vertebrates, annelids, and arthropods is conserved, then it is likely to be present in their shared ancestor, an ancestral bilaterian. This is also consistent with how rearward Hox genes override those expressed in more forward locations, which suggests that they have been added in the front-to-rear direction.
Some pieces are starting to fall together...
PZ Myers, Mr. Pharyngula, started out by mentioning some detailed order produced by simple means, like reaction-diffusion systems such as Belousov-Zhabotinsky. But a favorite model system, Drosophila melanogaster, is an embarrassment. Although it starts out simple, with a gradient of some morphogen, it gets complicated and kludgy, with gap genes being expressed as a function of that gradient and the neighboring gene products, and then pair-rule genes on top of those. He comments that this looks like some custom-made system that had replaced an earlier, simpler system.
And there is evidence for such a system in vertebrates; he described some work on zebrafish, chicken, and mouse embryos in which a chemical oscillator is involved in laying down somites (muscle blocks).
This system may be more widespread; like vertebrates, annelids and many arthropods produce new segments from a read-end growth zone. Insects divide up as:
Short-germ: head simultaneous -- grasshopper Schistocerca
Intermediate-germ: head and thorax simultaneous -- beetle Tribolium
Long-germ: the whole body simultaneous -- fly Drosophila
The more "primitive", three-stage (hemimetabolous) insects are usually short-germ. Longer-germ development typically appears in four-stage (holometabolous) insects, and is likely an adaptation for increased speed of development (no need to wait for all the segments to form if they all form simultaneously).
And judging from the occurrence of the intermediate-germ state, the emergence of long-germ development has gone in the rearward direction; the front end has simultaneous-specification mechanisms that can be copied or build on, and it is the forward segments whose specification would most need to be speeded up.
The evolution of the replacement of segment-specification mechanisms would be interesting to work out; however, doing so may require studying the genetics of development in more species than has usually been done.
It is also a rather blatant violation of Ernst Haeckel's "biogenetic law", in which earlier development features are more conserved than later ones. Segment specification is conserved, but not the mechanism for generating it. However, his "biogenetic law" is already known to be violated by the different sizes and yolk contents of eggs across the animal kingdom.
And if this segment-making mechanism for vertebrates, annelids, and arthropods is conserved, then it is likely to be present in their shared ancestor, an ancestral bilaterian. This is also consistent with how rearward Hox genes override those expressed in more forward locations, which suggests that they have been added in the front-to-rear direction.
Some pieces are starting to fall together...