The key points of the article can be enlisted as follows. The process of segmentation in arthropods, annelids, and chordates is yet not proved to date back to a unified ancestor. The three hypotheses on the issue include segmentation as a process diverse for all phyla, as a process equal for all protostomes, and as a process equal for all Bilateria. The arguments in favor of each of the hypotheses appear relevant, albeit based on hasty assumptions. Although similarities do exist, more extensive research is needed to resolve the controversy (Davis and Patel 68-72).
The most optimal approach to the investigation of the evolution of segmentation
The approach that can be deemed optimal is to study the orthologue patterns of Drosophila since this particular arthropod is studied in the most detail by now. The patterns are to be compared to those of other organisms, particularly the genes of segment-polarity and pair-rule genes. The reason is that the way these genes express themselves can be compatible with how segmentation manifests itself morphologically.
Comparison of the segmentation patterns across the phyla
To adequately estimate the common points and the points of diversity in the segmentation processes that different phyla exhibit, it is needed to estimate how segmentation varies on each of them. It is evident that different organisms within one and the same phyla diversify in their segmentation modes, which is why cross-phyla segmentation analysis would be rushing the events.
The three hypotheses concerning the evolution of segmentation
The three hypotheses for the evolution of segmentation are indicated as follows. The first one asserts the segmentation was acquired independently in three periods. The second hypothesis suggests the protostomes are segmented homologically, which means they were required to deviate from the segmented state. The third hypothesis states Bilateria are segmented homologically, meaning they are prone to lose the state further still.
Evidence that supports or contradicts each of the three hypotheses
The first hypothesis is supported by the fact that the mechanisms of somitogenesis in leeches and Drosophilae expose great diversity, both molecularly and morphologically. Such diversity has resulted in assumptions that segmentation was an independent threefold process within the evolutionary timeline. On the other hand, the models of segmentation shown by both annelids and arthropods are evolutionarily derived. Also, their phyla are not represented on the molecular level. To evaluate the second hypothesis, the authors review the molecular development of leeches. He states that the engrailed gene found in these animals expresses itself in a way that might be in agreement with its part in segmentation. On the other hand, the engrailed is seriously influenced by the separation of ganglionic primordia, which questions its significance as a segmentation facilitator. As to the third hypothesis of the homology of Bilateria, the authors review the similarities of segments in arthropods and chordates. He asserts that observing somitic stripes can lead to non-evidential and inadequate assumptions. In addition, the her-1 and hairy genes of Zebrafish and Drosophila hairy that have been once deemed as having a common ancestor does not provide enough evidence to ground the supposition of the homology of the process.
The final conclusion
In conclusion, the authors state that more profound research is needed to establish the homology of segmentation. He recommends a wider database of genes and species studied. Apart from the recommendations, he suggests the homology of segmentation cannot be established as yet, due to the lack of evidence.
Davis, Gregory K., and Nipam H. Patel. “The origin and evolution of segmentation.” Trends in Genetics 15.12 (1999): 68-72. Print.