Biometric and Morphogenetic Comparison of the Sibling Species Stylonychia mytilus and S. lemnae, including a Phylogenetic System for the Oxytrichids (Ciliophora, Hypotrichida)

Abstract

Stylonychia mytilus only differs from S. lemnae by its dorsal kineties 1–4, which are more bent apically and have a significantly greater number of cilia. These characters are difficult to recognize in vivo. Thus, we suggest that field biologists ought to employ the designation “member of the Stylonychia mytilus complex”. The cortical divisional morphogenesis is the same in the 4 clones investigated and described in detail for S. lemnae. The development of the dorsal ciliature and the division of the nuclear apparatus do not differ from those known of other Stylonychia species. The stomatogenesis begins with a proliferation of basal bodies close to the uppermost transverse cirrus. Three frontal-ventral-transverse (FVT) anlagen of the opisthe separate from the oral primordium and 3 from the right postoral ventral cirrus. In the proter anlage 1 originates from the parental undulating membranes, anläge 2 from the oral primordium of the opisthe, anlage 3 from the left posterior frontal cirrus, anlage 4 from the right posterior frontal cirrus, and anlagen 5 and 6 evolve from the right postoral ventral cirrus. The anlagen 1–6 generate 1-3-3-3-4-4 FVT-cirri. The phylogenetic sequence of the oxytrichous genera Paraurostyla (Pa), Amphisiella (Am), Gastrostyla (Ga), Histriculus (Hi), Stylonychia (St), Oxytricha (Ox), Tachysoma (Ta), Urosoma (Ur), Urosomoida (Ua), and Gonostomum (Go) is reconstructed using the method of Hennig. For this proposal, 8 morphological and morphogenetic characters were selected. Their ancestral and derived states are discussed. The monophyletic origin of the oxytrichous ciliates is assumed for their longitudinal, fan-like FVT-anlagen, which evolve during the formation of the ventral ciliature. Synapomorphies show the close relationship of oxytrichids “sensu lato” (Pa, Am, Ga) and oxytrichids “sensu stricto” (Hi, St, Ox, Ta, Ur, Ua, Go). Only Urosomoida and Gonostomum can be justified to be “true” sister groups by autapomorphies at the present state of knowledge. This method furnishes additional arguments for the validity of questionable genera, like Urosoma and Urosomoida, and for the transfer of Amphisiella to the Oxytrichidae. The loss of caudal cirri in Amphisiella and Tachysoma is interpreted as parallelism. Synapomorphies relate HistriculusCorliss, 1960 to StylonychiaEhrenberg, 1838, but their lack of autapomorphies suggests an invalidity of Histriculus. However, the 17 FVT-cirri of “Histriculus” muscorum imply a slightly distant position, as compared with “typical” Stylonychia species, which possess 18 FVT-cirri. Stylonychia mytilus and S. lemnae are probably more evolved than S. vorax and S. pustulata, because of their more numerous synapomorphies. The scheme only proves S. mytilus and S. lemnae to be “true” sister species.