GENE DEPENDENCY OF VEGETALIZATION IN SEA URCHIN EMBRYOS TREATED WITH LITHIUM

¹ The Wenner-Gren Institute, University, Stockholm V A, Sweden

1. Embryos of Paracentrotus lividus were raised in sea water (C₀); in sea water with 12 µg/ml. actinomycin D (C_act); in sea water with 0.033 M lithium (Li₀) and in sea water with both 0.033 M lithium and 12 µg./ml. actinomycin D (Li_act). The embryos were transferred into the media at the 8-cell stage and exposed to these for 5.5 hours; thereafter they were returned to normal sea water. The embryos or larvae were fixed after different intervals of time as detailed in Table I. Living specimens were also observed. The lithium concentration and time of exposure used gave a weak but definite vegetalization in a high percentage of larvae (Table I, D, Li₀). The effect of lithium either involved merely a reduction of the animal-oral region (oral lobe) or was manifested also as an enlargement of the archenteron, in particular of its more animal (posterior) region. Accessory archentera or ectodermic bulges were often found in the border region between ecto-and entoderm. The differentiation seems here to be in a "fluctuating" state.

2. Previous results of Runnström, Ranzi and Lallier indicate that the primary effect of lithium increases inter- and intramolecular bondings in certain cytoplasmic components. The animalizing agents are assumed to be particularly sensitive to these changes which may block their diffusion. In this way, the normal animal-vegetal balance is disturbed in favor of the vegetalizing agents which are less affected by the structural changes.

3. The vegetalizing agents may directly or indirectly induce the activation of genes concerned with the vegetal anabolic pathways. When the action of these agents is enhanced in the state of unbalance caused by lithium, the predominance of the vegetal anabolic pathways extends in animal direction. If, however, the transcriptions of genes are blocked by actinomycin D this extension of the region of vegetal pathways is inhibited (for a diagrammatic representation of lithium action on one side and the combined action of lithium and actinomycin D on the other, see Figure 13, Li₀ and Li_act). The blocking effect of actinomycin D does not depend on a direct reaction with the vegetalizing agents. as follows from data presented by Giudice and Hörstadius (1965). The inference is that gene activation is a link in the vegetalization following exposure to lithium.

4. Exposure to actinomycin D alone (Fig. 13, C_act) causes a tendency to animalization which is a secondary effect of retarded gastrulation. The development of the embryos becomes more balanced after exposure to the combined action than after that to actinomycin alone. A weak lithium effect balances the slight tendency to animalization which follows upon treatment with actinomycin D. Even after combined treatment, however, a certain inhibition prevails, manifested particularly in subnormal outgrowth of skeleton rods and arms.