USA Biology Olympiad (USABO) Practice Exam

How does the cleavage pattern differ between protostomes and deuterostomes?

1. Protostomes: Indeterminate, Deuterostomes: Determinate

2. Protostomes: Spiral, Deuterostomes: Radial

3. Protostomes: Radial, Deuterostomes: Spiral

4. Both exhibit the same cleavage pattern

The correct answer is: Protostomes: Spiral, Deuterostomes: Radial

The cleavage pattern in embryonic development is a significant distinguishing feature between protostomes and deuterostomes. In protostomes, the cleavage is typically spiral, meaning that the cells divide at an angle to the vertical axis, resulting in a twisting arrangement. This spiral cleavage is generally determinate, where the fate of each blastomere is established early in development. In contrast, deuterostomes exhibit radial cleavage, where the cells divide parallel or perpendicular to the vertical axis, resulting in a more symmetrical arrangement of cells. This type of cleavage is often indeterminate, meaning that the developmental outcome for each blastomere is not fixed at the cleavage stages, allowing for more developmental flexibility. Thus, the distinction between spiral cleavage in protostomes and radial cleavage in deuterostomes is essential for understanding their developmental biology and evolutionary relationships. The developmental pathways followed by these two groups reflect deep evolutionary divergences in how their embryos are formed and how they develop into complex organisms.