Acetabularia mediterranea (Croatian: klobučić) During her canoe trips along the southern shore of the island of Brač, June 2011, my daughter photographed underwater colonies of tiny sea plants. For half a year, in spite of multiple con- tacts and internet picture browsing, we were unable to identify the plant. Finally, thanks to Dr Maja Novosel of the Biology Division, University of Zagreb, a week ago, we have it: Acetabularia mediterranea.
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Acetabularia, a genus of green algae, is among the largest single-celled organisms (5 to 10 mm), having also a remarkably large nucleus. It has three anatom- ical parts: (1) bottom rhizoid which resembles a set of short roots that contain the nucleus and anchor the cell to fissures in a substrate; (2) median stalk, which accounts for most of its length; and (3) top umbrella of branches that may fuse into a cap, like a smooth, disc shaped cap of A. mediterranea. There are usually several whorls of hair-like appendages close to the cap. The single nucleus of Acetabularia, located in the rhizoid, allows the cell to regenerate completely if its cap is removed. The caps of two Acetabularia may also be exchanged, even from two different species. Although a single cell, Acetabularia exhibits a remar- kably complex shape and has therefore long been a model organism for studying gene expression and morphogenesis. During sexual reproduction, the nucle- us undergoes multiple rounds of mitosis, forming many daughter nuclei all within one nuclear membrane. These nuclei undergo meiosis and are transported to the tips of the branches, the sporangia, where they are released as gametes. The name, Acetabularia, derives from the Latin word acetabulum, a broad, shallow cup used for dipping bread; the upturned cap of Acetabularia resembles such a cup. For this reason, it is also sometimes called mermaid's wineglass. |
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