Question

Problem 1. Molecular Genetics

A sea urchin mutation results in an unusual positioning of the mitotic spindle during the very first cleavage division:instead of the zygote dividing along the animal-vegetal axis, the cell is now dividing perpendicular to the A-V axis (as if a globe were divided in halves at the equator plane). You decide to repeat Hans Driesch’s 19th century experiments, carefully separate the resulting blastomeres and let them develop further.

True or false?

Since sea urchins are known to have highly regulative earlydevelopment, you expect to see normally developing embryos. Explain youranswer.

Answer 1

In most species the sexes are separate, and indirect development produces a free-swimming larva that then undergoes metamorphosis, at which point the juvenile emerges with the characteristic fivefold symmetry and grows into the adult. About one-fifth of all sea urchin species follow a direct development strategy. Heliocidaris erythrogramma, for example, gastrulates and reaches the juvenile stage 3 days after fertilization. Such direct developing species generally have large eggs, allowing them to bypass the larval feeding stage. Direct developing strategies arose relatively recently in several groups, and because of this it is likely that molecular remnants of the transition from indirect to direct development will be found in the not too distant future. The lifespan of species varies from a few years to more than 200 years . Most echinoderms also have the capacity to develop asexually. Sea stars, for example, can regenerate a complete animal from an isolated arm or half. Furthermore, sea urchin larvae have recently been found to ‘clone’ themselves as another form of asexual reproduction. Sea star development is similar to sea urchin development, although the bipinnaria and brachiolaria larvae are not the same shape as the pluteus larva of the sea urchin . In addition, there is no internal skeleton initially. Furthermore, some sea stars species brood their embryos until metamorphosis.