Flagellar basal apparatus and its utility in phylogenetic analyses of the porifera

A broadly based comparative study was initiated to assess components of the flagellar basal apparatus as a character set in phylogenetic analyses of poriferans. The flagellated (monociliated) epidermal cells of sponge larvae were selected for study. Taken together, they create a field of locomotory cells analogous to a multiciliated surface. Larvae of six species in four orders of the Demospongiae were examined by transmission electron microscopy. Results are compared with findings taken from the literature on larvae of five additional species of demosponges and four species of calcareans. Data were assembled on six components of the basal apparatus: (1) basal body, (2) basal foot, (3) accessory centriole, (4) transverse cytoskeletal system, (5) longitudinal cytoskeletal system, and (6) association with Golgi body. Where evidence permits assessment, all have Type II basal bodies. Basal feet are diverse and are subdivided into three categories based on structural complexity. The most anatomically intricate (Type III) is found only in larvae of Mycale spp. Accessory centrioles are present or absent depending on the species, but their occurrence is without overall taxonomic pattern. When present, accessory centrioles are oriented perpendicularly to the long axis of the basal body, but as ascertained from relationship to the anterior-posterior axis of the larvae they are without consistent orientation with regard to the plane of effective beat of the flagellum. Transverse and longitudinal cytoskeletal systems are also diverse among larvae. The existence of cross-striated rootlets is convincingly established only in larvae of calcareans, and such rootlets are present in larvae of all four calcareans studied to date. Three apparently new rootlet structures are described: lateral arms of the transverse cytoskeletal system from larvae of Aplysilla sp. and Haliclona tubifera; laminar sheets of the longitudinal system from larvae of Aplysilla sp. and M. cecilia; and paraxial rootlet in larvae of H. tubifera. A robust similarity in structure of the basal appartus is observed among the three species of halichondrids reported here for the first time. In comparison with the flagellar basal apparatus found in adults, those of larvae are more complex and more diverse. Review of studies on adult sponges that include information on the basal apparatus reveals the absence of a longitudinal rootlet system in all cases. Additionally, there exists a high degree of concordance between properties of the basal apparatus in the one sclerosponge and the one hexactinellid studied to date. These basal apparatus are also the simplest in construction of those found in sponges. Conversely, the basal apparatus of demosponges are varied. Although consistent presentation of the basal apparatus is evident in certain taxa, any discernable systematic pattern in their overall configuration remains obscure. Finally, we conclude that the flagellar basal apparatus of sponges is more similar to that found in choanoflagellates than it is to that observed in eumetazoans. © 1995 Wiley-Liss, Inc.