Motile Cell Ultrastructure and the Circumscription of the Orders Ulotrichales and Ulvales (Ulvophyceae, Chlorophyta)

Abstract

We have examined the motile cell ultrastructural features of several green algal species having filamentous or foliose thallus morphology and probable affinities with the Ulvophyceae, and compared them with the structural, reproductive, and life history features known for these taxa. We separate the algae studied into the orders Ulotrichales and Ulvales on the basis of consistent variations in terminal cap and proximal sheath structure that correlate well with life history patterns and certain features of sporangial and gametangial structure and development. Body scales are present only in certain members of the Ulotrichales. Both orders encompass a variety of thallus forms, demonstrating parallel evolution of thallus morphology. Flagellar apparatus features common to all the motile cells examined include 180° rotational symmetry, counterclockwise absolute orientation, the positioning of the basal bodies in an apical papilla, and the presence of one or more sets of striated bands associated with the X rootlets. Additional features that are usually present include basal body overlap and orientation roughly perpendicular to the long axis of the cell during forward swimming, striated distal fibers, and a single, striated, microtuble‐associated component underlying each two‐membered rootlet. These similarities indicate to us that the two groups are closely related members of the Ulvophyceae. We suggest that the Ulotrichales is the most primitive ulvophyceous assemblage known, but that all groups studied have advanced features relative to those supposed to have been present in the ancestral members of the Ulvophyceae.

PHYLOGENETIC SYSTEMATICS OF THE ULVACEAE (ULVALES, ULVOPHYCEAE) USING CHLOROPLAST AND NUCLEAR DNA SEQUENCES1

“…An inclusive list of traits considered in assessing relationships among genera includes sexual life history pattern, sporangial and gametangial structure and development, ultrastructure of motile cells and method of their release, and development and morphology of vegetative thalli. Sexual life history pattern and sporangial and gametangial structure and development have been used to separate genera at the ordinal level (Kornmann 1965, Floyd and O'Kelly 1984). Clades that reflect this separation are resolved by molecular data.…”

“…Ulva species are ubiquitous and abundant in intertidal and nearshore subtidal areas, where they provide habitat and nutrients to marine invertebrates and vertebrates (Lobban and Harrison 1997). Fourteen genera have been allied with Ulva since Thuret (1854) amended the Ulvaceae to include green algae with foliose and tubular morphologies, and classification of the family has undergone numerous changes as a result of culture and electron microscopic studies (Kunieda 1934, Fritsch 1935, Chapman 1952, Papenfuss 1960, Bliding 1963, 1968, Kornmann 1965, 1973, Yoshida 1970, Gayral 1971, Round 1971, Tatewaki 1972, Vinogradova 1974, Abbott and Hollenberg 1976, Kornmann and Sahling 1977, Tanner 1979, Silva 1982, Floyd and O'Kelly 1984, 1990, Bold and Wynne 1985, van den Hoek et al 1995, Gabrielson et al 2000). In recent treatments, four genera are considered members of the family: Chloropelta, Enteromorpha , Ulva , and Ulvaria (Fig.…”

“…They included Percursaria in Ulvaceae sensu stricto , but grouped Blidingia and Kornmannia with the genus Pseudendoclonium ( sensu Nielsen 1980 in an informal “ Blidingia group” in the Ulvales. They considered Capsosiphon to be allied with taxa of the sister order Ulotrichales sensu Floyd and O'Kelly (1984). Further, they expanded the concept of the Ulvales to include microscopic species with branched filamentous thalli, algae traditionally placed in the Chaetophorales.…”

Systematics of the marine microfilamentous green algaeUronema curvatumandUrospora microscopica(Chlorophyta)

“…Urospora has a complex nomenclatural history and the taxonomic position of the genus has long been uncertain (Lokhorst & Trask, 1981). The genus has been placed in the Cladophorales or Siphonocladales based on the multinucleate cells (Wille, 1890;Rosenvinge, 1893;Setchell & Gardner, 1920;Printz 1932) but is now recognised as a close relative of Acrosiphonia J. Agardh and Spongomorpha Ku¨tzing in the ulvophycean order, Ulotrichales, based on morphological, ultrastructural and lifehistory features, and molecular data (Jorde, 1933;Kornmann, 1963;Floyd & O'Kelly, 1984;van Oppen et al, 1995;Jo´nsson, 1999;Lindstrom & Hanic, 2005). The genus includes about 12 species worldwide (Guiry & Guiry, 2009), four of which are common along western European shores (Lokhorst & Trask, 1981).…”

Observations onDilabifilum(Class Chlorophyta, order Chaetophorales sensu stricto) and allied genera

“…Mattox & Stewart (1984) have used ultrastructural features when circumscribing the order Chaetophorales and include it within their class Chlorophyceae. Many taxa traditionally placed in this order lack a chaetophoralean ultrastructure and have been referred to the Ulotrichales and Ulvales as currently conceived (see Floyd & O'Kelly, 1984 pores in the present study lends doubt as to whether its relationship with allied taxa will be resolved without recourse to an examination of its genome using data such as those provided by a study of plastid DNA endonuclease patterns. Lacking any ultrastructural information on Dilabifilum we prefer to refer it to the order Chaetophorales as traditionally defined.…”