Choanoflagellate Lorica Construction and Assembly: The Tectiform Condition. Volkanus costatus (=Diplotheca costata)

The tectiform choanoflagellate Volkanus costatus (= Diplotheca costata ) possesses a lorica comprising an outer layer of 14-18 longitudinal costae and three transverse costae. Six categories of costal strips can be distinguished on the basis of morphology. Costal strips are deposited intracellularly, upside-down, starting with the transverse costae, posterior to anterior, and then longitudinal costae, again posterior to anterior. Strips are exocytosed from the cell anteriorly and stored horizontally at the top of the collar. Cytokinesis involves inversion of the juvenile cell which is then pushed backwards into the accumulation of strips thereby restoring the normal relationship between costal strips and cell. Once free of the parent, the juvenile assembles its lorica by a forward and left-handed rotational movement. Whilst individual stages in costal strip production, storage and lorica assembly are the same in tectiform and nudiform species, nevertheless the overall procedure in tectiform species is complicated by inversion of the strip producing apparatus and cytokinesis. One explanation for this increase in complexity is that a single regulatory change has caused the coincidental inversion of costal strip production, storage and cytokinesis. The consequences of this change bequeathed tectiform cells with two important evolutionary advantages. One was the immediate inheritance by the juvenile of a lorica following division and the second was the development of transverse costae, which are mechanically superior to the helical costae of nudiform species. The outcome of these changes probably accounts for the major radiation of tectiform species within the marine environment.