The three hypotheses concerning the evolution of segmentation
- The acquisition of segmentation in the three phyla was independently achieved.
- The evolution of segments among the chordates was independent and led to homologous segments that are located within protostomes.
- The Bilateria have homologous segments.
Evidence that supports or contradicts each of the three hypotheses
Hypothesis 1
The author evaluated segmentation in insects and non-insect arthropods. In all the insects, the author found that embryos start as a syncytium. The nuclei then form blastoderm after moving to the periphery in which cellularization takes place. After the early expression, the successive embryogenesis shows variation among the different insects. For instance, distinctions are noticeable in short-germ insects and long-germ insects. An example of a long-germ insect is the Drosophila. The pattern of segments in the long-germ insects is located in the blastoderm. Among the short-germ insects, the pattern of the head segments is located in the first blastoderm. Examples of the short-germ insects include Tribolium and Schistocerca. The formation of segments of the abdomen and the thorax progresses from the posterior growth zone. In the intermediate germ insects, the segmentation is mainly in the cellular environment. Despite the differences, some intermediate and short-germ insects, segmentation is added progressively in a cellular environment.
The author analyzed crustaceans that make up the category of non-insect arthropods. In the category, the development of segments of the abdomen and thorax was found to take place in a cellular environment. The author noted that the growth is from the posterior zone and takes place after gastrulation. The expression resembles the development of segments in short-germ insects.
Hypothesis 2
In order to test the hypothesis, the author evaluated the engrailed gene of annelids. In the case of the Helobdella triserialis, the author noted that the segment pattern expressed by the gene is consistent. The expression of the gene engrailed in Theromyzon rude occurs after the separation of ganglionic primordial. Thus, the expression points to variations that do not resemble the process dictated by the engrailed gene in the Helobdella triserialis segmentation process. The variations rule out the existence of homologous segmentation. To establish the basis of segmentation, there is a need for further analysis of the various annelid groups.
Hypothesis 3
The author evaluated the hypothesis in relation to chordates. The author established that vertebrates’ segmentation is similar to that of short-germ insects. The expressions happen after the occurrence of morphological segmentation. In addition, the gene expressions are limited to some muscle cells found within the somite. For example, Amphioxus has engrailed transcripts. In the first eight somites, the transcripts are localized in the posterior. The expression comes before morphological segmentation. However, the author noted that the observation of the stripes might not be adequate. For example, random analysis of cDNA from Drosophila found that reiterated stripes constituted 87% of the segment patterns. Transcripts of hers1, in zebrafish, correspond to alternating somites. The findings imitate the pair-rule pattern; hence, the conclusion that Bilateria segmentation is homologous. However, there exists an issue on whether the her1 is a true orthologue of Drosophila hairy.
The final conclusion that the author puts forth
The author concluded that distantly related organisms have similarities in segment patterning. However, the similarities cannot be used as the basis for ascertaining the existence of homologous segmentation in the various phyla. There is a need for more analysis of data and genes in order to establish the basis of segmentation across the various phyla.