Ref ID : 7679
Barry J. Wicklow; The Discocephalina (N. Subord.): Ultrastructure, Morphogenesis and Evolutionary Implications of a Group of Endemic Marine Interstitial Hypotrichs (Ciliophora, Protozoa). Protistologica XVIII(3):299-330, 1982
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Using light optical microscopy of living and protargol stained specimens and both scanning and transmission electron microscopy, I describe cortical morphogenesis through cell division, interphase cortical ultrastructure, and polymorphism in Discocephalus ehrenbergi, as well as structure and morphogenesis in Psammocephalus borrori n. gen., n. sp., Psammocephalus faurei n. comb., and Amphisiella marioni. The developmental pattern of D. ehrenbergi is divergent from that of other hypotrichs. Morphogenesis begins in a single, latitudinal proliferative zone. Frontal primordia develop in close association with the oral primordium, then form a series of oblique streaks which differentiate 4 ranks of procirri: transverse cirri compose the first (posteriormost) rank; accessory transverse cirri compose the second rank; a migratory cirrus and a longitudinal series of midfrontal cirri compose the third rank; right frontal cirri compose the fourth rank. From a left marginal cirral primordium differentiate 2 kinds of cirri: 2 anterior marginal cirri and a group of posterolateral marginal cirri. Caudal cirri are sets of cirri derived from rightmost dorsal kineties. Along with a paroral cirrus a unique differentiation - the peristomial cirrus- forms from the anterior paroral membrane and functions as part of the oral apparatus. The membranelles are paramembranelles with postciliary microtubules contributing to a postmembranellar fiber. The paroral membrane is a polystichomonade; the endoral membrane is a stichomonade. The complex microtubular cytoskeleton is continuous with cortical protrusions (membrane-bound bundles of microtubules) that extend dorsally and laterally from the cephalized region of the cell. Dorsal bristle complexes comprise 2 kinetosomes: the anterior is ciliated, the posterior is not. Postciliary microtubules are associated with triplet number 9 of both kinetosomes; transverse microtubules are associated with the anterior kinetosome while a kinetodesmal fiber is associated with the posterior kinetosome. The bristle complex is set within a cortical pit and is surrounded by a unique, basket-like, fibrillar framework. The developmental pattern of P. faurei is of the Discocephalus type. It includes formation of a midfrontal ciliature from different frontal streaks as well as formation of left anterior and posterolateral marginal cirri from a single marginal cirral anlage. In contrast, development in A. marioni proceeds as in sporadotrichine hypotrichs such as Gastrostyla: 5 frontal streaks appear as a fan-like array; the median cirral row is derived from a single frontal streak. I describe a new discocephalid genus with 2 new species: Psammocephalus borrori n. g., n. sp. and Psammocephalus dragescoi n. g., n. sp. Because of morphological and morphogenetic differences between P. faurei and A. marioni, I remove P. faurei (and P. lithophora) from Amphisiella and place them in the genus Psammocephalus. The new discocephalid species serve as intermediates in an evolutionary series that demonstrates a trend in discocephalids toward increased cytoskeletal complexity, cirral reduction and hypertrophy, and ovoid cell shape. Because of close similarities between "primitive" discocephalids and urostylines such as Holosticha discocephalus, I propose the 2 groups may share a close common ancestry. I suggest, due to apparent morphological and developmental homologies, that the euplotine hypotrichs may have diverged from the discocephalid lineage, thereby providing a hypothesis for a monophyletic origin of the highly differentiated Euplotina within the order Hypotrichida. I consider the Discocephalus - like hypotrichs as a distinct evolutionary assemblage sufficiently divergent to warrant separation into their own suborder: the Discocephalina. I present a classification of discocephalines that includes 2 families: Discocephalidae (Discocephalus, Prodiscocephalus, Marginotricha, and Psammocephalus) and Erionellidae (n. fam.) (Erionella). The discocephalines represent an adaptive radiation within a single adaptive zone; I consider cephalization and the development of a complex microtubular cytoskeleton as specializations for existence within interstices of marine sands on exposed beaches. The adaptive value of microtubular systems lie in their high degree of functional versatility, their capability of being rapidly dismantled and resorbed, and their potential for rapid assembly during morphogenesis; they are non-restrictive to cell growth and provide structural integrity without impeding motility. Morphogenetic and ultrastructural information permits recognition of an unsuspected evolutionary diversity within the Hypotrichida.