Developmental stages of attachment of in vitro protoplasts in two Mediterranean Valonia species (Siphonocladales, Chlorophyta)

Abstract: In vitro growing protoplasts of two coenocytic green algae, Valonia aegagropila and V. utricularis, developed into small spherules, which behaved as adventitious germlings attached to the substrate. Their developmental patterns are compared, considering that the Mediterranean V. aegagropila is an unattached (pleustophytic) form present in lagoons, whereas V. utricularis is an attached (haptophytic) species of rocky shores. In both species, thin terete branches, able to attach the thallus to the substrate arose from lenticular cells which marked the lower pole of the spherules. Some attachment branches grew as long stolon-like aseptate axes, becoming uniformly green and swelling into secondary vesicular expansions. V. utricularis plantlets usually formed numerous attachment branches, some of which produced other peripheral self-attaching vesicles. On the contrary, V. aegagropila plantlets were fixed by few, weak attachment branches; stolon-like growth was poor and ineffective in extending the attached system. This developmental pattern suggests that, although the Mediterranean V. aegagropila was able to generate attachment branches, these could be easily detached from the substrate by water movement. Therefore, this free floating form typical of lagoons does not originate attached populations as reported for other seas of the world. Some considerations on the taxonomy of Valoniaceae are also made.