
To review:
The difference in the micromere GRN of a starfish and sea urchins, in relation to the evolutionary development.
Introduction:
The developmental and evolutionary biology depends upon the anatomy encoded by the genome. The elucidation of genetic mechanism needs the analysis of biological information at genomic, cellular and tissue levels. The study of skeletogenesis in sea urchin provides the key elements to reveals the evolutionary trends of the heritable genomic regulatory programs which helps in determining the morphological characteristic of related species. These evolutionary changes depends upon GRNs (gene regulatory network), which helps to determine the specification of embryonic skeletogenic cells known as primary mesenchyma cells (PMCs).

Explanation of Solution
The two echinoderms, sea urchin and starfish are diverged from their common ancestor, around 520-480 million years ago. Both of these animals possess a similar endomesodermal map, except the skeletogenic cell lineage present in sea urchin which produces a prominent skeleton, entirely lacking in starfish. An important element of GRN is found to be retained in both echinoderm lineage since the Cambrian Period. The evolutionary changes in the GRNs mainly found to occur in few cis-regulatory elements, while other present in unaltered form. The alteration in the cis-regulatory interaction, impacts the architecture of GRNs. The GRNs are actually responsible for the expression of downstream genes which specifies the primary mesenchyma cells (PMCs) responsible for the embryonic skeleton.
Thus it is concluded that few changes in the cis regulatory elements of GRN architecture among sea urchin and starfish, is the reason for the their divergence related to GRN and morphological evolution.
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Chapter 10 Solutions
Developmental Biology
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