The article in question dwells upon the evolution of segmentation with specific attention on arthropods, chordates, and annelids. It also unveils some secrets of evolution. The authors provide a brief description of major perspectives existing in the field. The key points of the article include the assumption that the similarities in segmentation can be found in all three phyla and the assumption that these similarities cannot be the evidence of the common ancestry.
What is true segmentation according to Davis and Patel?
True segmentation is “the repetition along the anterior-posterior axis of a structural unit that comprises a suite of characters involving the entire body” (Davis and Patel 68).
What distinction can be found between the embryogenesis modes?
In Drosophila (a long-germ insect), the segments are “patterned simultaneously within the blastoderm” (Davis and Patel 70). In short-germ insects, head segment patterns develop first, and other segments patterns formed after that.
What similarities can be found in the embryogenesis modes in non-insect arthropods and insects?
In many non-insect arthropods, the abdomen and thorax segment patterns develop in the posterior growth zone. The engrailed expression is also rather similar in arthropods and insects. At the same time, the segmentation process is “quite variable” in this group (Davis and Patel 70).
The three hypotheses provided in the article in question dwell upon differences and similarities between segmentation in arthropods, chordates, and annelids. The first hypothesis holds that segmentation developed differently in arthropods, chordates and annelids, which means that the similarities in segmentation will be scarce and quite insignificant. The second hypothesis holds that annelids and arthropods share similar segmentation patterns while segmentation patterns developed independently in chordates. The third hypothesis holds it that all three phyla share common ancestry, and segmentation patterns will be very similar with quite a few differences.
When considering the first hypothesis, the authors look into segmentation in insects and non-insect arthropods. It is noted that long-, intermediate and short-germ modes of embryogenesis are very different in insects. For example, in long-germ insects segmentation patterns are developed simultaneously. In short-germ insects, head segments patterns develop in the initial blastoderm, but other segment patterns develop during later phases. The data provided show that the three phyla have quite different segmentation patterns that outweigh similarities that can be found. To evaluate the second hypothesis, the authors consider segmentation of annelids, leech, in particular. The segmentation in these species is quite similar to that of the arthropods, but it requires further research (Davis and Patel 70). When evaluating the third hypothesis, the researchers focus on the segmentation patterns in chordates. The authors note that there are some similarities between somitogenesis in arthropods and chordates, but these similarities seem quite superficial after a closer analysis. The researchers stress that the differences are quite significant and show that the chordates do not share common ancestry with arthropods or annelids. It is also stressed that found similarities cannot account for homologous segmentation patterns that evolved in the phyla mentioned.
The final conclusion made is that scientists have to collect more data, and further research is necessary to understand whether the three distant phyla share the common ancestry. At present, it is impossible to state that different species have homologous segmentation patterns. The authors also add that the existence of similarities between developmental mechanisms shows the gaps in the field.
Works Cited
Davis, Gregory K., & Nipam H. Patel. “The Origins and Evolution of Segmentation.” Trends in Genetics 15.12 (1999): 68-72. Print.