European Journal of Phycology
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Ultrastructure and taxonomy of the genus Endophyton (Ulvales, Ulvophyceae)
P. I. Leonardi , J. A. Correa & E. J. Cáceres
To cite this article: P. I. Leonardi , J. A. Correa & E. J. Cáceres (1997) Ultrastructure and taxonomy of the genus Endophyton (Ulvales, Ulvophyceae), European Journal of Phycology, 32:2, 175-183, DOI: 10.1080/09670269710001737109 To link to this article: https://doi.org/10.1080/09670269710001737109
Published online: 03 Jun 2010.
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Ultrastructure and taxonomy of the genus Endophyton (Ulvales, Ulvophyceae)
; P.I.LEONARDI1, J.A.CORREA2 AND E.J.CACERES1
" Laboratorio de Plantas Avasculares, Departamento de BiologıTa y BioquıTmica, Universidad Nacional del Sur, PeruT 670, 8000 BahıTa Blanca, Argentina # Departamento de EcologıTa, Facultad de Ciencias BioloT gicas, Pontificia Universidad CatoT lica de Chile, Casilla 114-D, Santiago, Chile
(Received 15 July 1996; accepted 15 October 1996)
Fine structure of vegetative cells, development and morphology of sporangia and general morphology of zoospores were studied for the first time in the genus Endophyton Gardner. Vegetative cells contained one parietal perforate chloroplast with 2–4 ulvophycean pyrenoids and transverse cell walls without plasmodesmata. Sporangial walls were formed by two distinct electron-dense and fuzzy layers which contained a contiguous electron-translucent, fibrillar material. Zoospores were naked, with an anterior dense aggregation of vesicles and a cup-shaped chloroplast with one eyespot. The flagellar apparatus showed 180m rotational symmetry and counterclockwise absolute orientation of its components. Microtubular roots had a cruciate pattern, in which d l 2 and s l 4 with 3\1 arrangement. Electron-dense bilobed terminal caps covered the proximal end of each basal body. These observations support the suggested close relationship of Endophyton to the allied genus Entocladia and other ulvalean algae, and reinforce the criteria for including the genus in the class Ulvophyceae, order Ulvales, family Ulvellaceae.
Key words: Endophyton, taxonomy, ultrastructure, Ulvophyceae, Ulvales, Ulvellaceae, zoospore
Introduction with Entocladia and Acrochaete, Endophyton was formerly considered a member of the family Chaetophoraceae Endophyton The chlorophycean genus Gardner includes (Taylor, 1957; Nielsen, 1979; O’Kelly & Yarish, 1981; filamentous plants with an endophytic habit. The type South, 1984). The most definitive treatment of the E ramosum species . Gardner was originally described from taxonomic position of Endophyton was that of O’Kelly Mazzaella flaccida San Francisco, California, infecting (1982), who maintained the genus in the Chaetophoraceae et Chondracanthus (Setchell Gardner) Fredericq and while at the same time pointing to a possible relationship corymbiferus Iridaea laminarioides (Ku$ tzing) Guiry (as and with the Ulvaceae. O’Kelly (1982) implied that Endophyton Gigartina radula , respectively; see literature reviews in was most closely related to Acrochaete and Entocladia. Later E ramosum O’Kelly, 1982). Subsequently, . has been O’Kelly & Floyd (1984a) included Endophyton in the reported from the northeast Pacific as an endophyte in Ulvellaceae in the order Ulvales, along with Entocladia and Callophyllis Halymenia species of the red algal genera , and Acrochaete. Lokhorst (1984), when assigning Endophyton to Dilsea Lessoniopsis Egregia and the brown algal genera and the Ulotrichales, noted that ultrastructural information (Doty, 1947; Dawson, 1965; Silva, 1979; Josselyn & was lacking. Since ultrastructural characters are important West, 1985). Working with several host species belonging in the assessment of affinities within the green algae to the Gigartinaceae, also from the northeast Pacific, (O’Kelly & Floyd, 1984b; Watanabe & Floyd 1989; Lewis E ramosum O’Kelly (1982) obtained . in laboratory cultures et al., 1992), we have undertaken such an approach to for the first time. He reported on various aspects of the provide these data for Endophyton. Thus, the main goal of structure, reproduction and life history of the endophyte. this work was to assess whether the fine structure of E ramosum Endophyton More recently, . (as sp.) was vegetative cells, the development and morphology of reported from the southeast Pacific, as an endophytic sporangia and the general morphology of zoospores of Mazzaella pathogen causing the green patch disease in Endophyton agree with similar features of ulvalean algae, laminarioides et al (Bory) Fredericq (Correa ., 1994). The especially the allied genus Entocladia (O’Kelly & Floyd, E southeast Pacific alga was formally recognized as . 1983). ramosum in a host-specificity study reported by Sa! nchez et al . (1996). Materials and methods To date, there is no general agreement regarding the evolutionary affinities of the genus Endophyton. Together Plants of Endophyton ramosum used in this study came from unialgal stock cultures, isolated from diseased, wild Correspondence to: P. I. Leonardi. Mazzaella laminarioides collected in Matanzas, Chile Published online 03 Jun 2010 P. I. Leonardi et al. 176
(33m 58hS). Transfer to fresh medium induced these isolates strata: an outer layer representing the vegetative cell wall to sporulate abundantly. Zoospores, which were bi- or and the inner layer or capsule (Fig. 10). Subsequently, the tetraflagellate, germinated and initiated filamentous sporangium contents underwent simultaneous cytokinesis germlings. Sexual reproduction was not observed in our which divided the cytoplasm into uninucleate portions, cultures (see also Correa et al., 1994), although it was the apical vacuole remaining intact during these events reported by O’Kelly (1982), who described anisogametes (Fig. 11). Numerous closely packed small vesicles with in material from Washington and British Columbia. translucent contents appeared in the cytoplasm of the Plants were fixed at 5 mC in (i) 2% glutaraldehyde in developing zoospores (Fig. 12). Zoospore maturation was 0n2 µm filtered culture medium and postfixed in 1% not synchronous in all the sporangia from a given filament, osmium tetroxide or (ii) 3% acrolein and 5% glutaral- even if they had developed from contiguous cells (Fig. 12). dehyde in 0n2 µm filtered culture medium and postfixed in The sporangium cell wall exhibited two distinct electron- 2% osmium tetroxide. In both cases the material was dense fuzzy layers (Fig. 13). The sporangium was filled dehydrated in an acetone series and embedded in Spurr with mucilage, which appeared fibrillar in electron micro- resin following the flat embedding method (Reymond & graphs (Fig. 13), and in which mature zoospores remained Pickett-Heaps, 1983). Sections were cut with a diamond immersed until liberation (Fig. 14). A thick stratum of knife and stained with uranyl acetate and lead citrate. condensed mucilage covered the inner sporangial cell Sections were examined in a JEOL 100 CX-II electron wall. Immediately prior to zoospore release, a large microscope of the Centro Regional de Investigaciones amount of electron-translucent, condensed mucilage Ba! sicas y Aplicadas de Bahı!a Blanca (CRIBABB). became apparent in the upper apical part of the spor- angium (Figs 14, 15). Later, the sporangium wall dis- sociated and the mucilage plug produced as a result of the earlier apical condensation was released prior to zoospore Results liberation (Fig. 16). Non-liberated zoospores were often seen germinating inside intact sporangia (Fig. 17). Vegetative structure Fig. 18 shows a diagrammatic reconstruction of a Vegetative cells contained one nucleus with a large biflagellate zoospore of Endophyton ramosum. The cell nucleolus (Figs 1, 2, 8) associated with a prominent body and flagellar surface of biflagellate and tetraflagellate paranucleolar body of similar appearance (Fig. 2, arrow). In zoospores were naked. Zoospores had a cup-shaped transverse views of apical cells the nucleus was located in parietal chloroplast with numerous large starch grains, one the central part of the cell, separated from the chloroplast or two pyrenoids with the same characteristics of those in by a thin layer of cytoplasm (Fig. 3). The chloroplast was vegetative cells (Fig. 21), and a protruding eyespot (Fig. parietal, perforate, normally with 2–4 pyrenoids. It 22). The nucleus occupied a central position in the included numerous starch grains and spherical lipid concavity of the chloroplast. The anterior third of the cell droplets scattered in the stroma (Fig. 1) and 2–10 undulate contained a dense aggregation of vesicles and lomasome- thylakoid lamellae (Figs 4, 5). The pyrenoid matrix was like structures (Figs 19, 20). Many elongated mito- pyriform (Fig. 4, arrow), penetrated by 2 or 3 single chondrial profiles with tubular cristae were also seen in modified thylakoids and flanked by a variable number of this region (Fig. 20). The basal apparatus occupied a starch plates of different sizes and shapes (Fig. 5). Central conspicuous apical papilla (Fig. 19), and flagella were vacuoles with translucent contents occupied part of the directed toward the posterior end of the cell (Fig. 18). The cell volume (Figs 2, 7). Vacuole diverticula with 1 or 2 flagellar apparatus showed a 180m rotational symmetry, electron-dense inclusions occupied chloroplast perfora- and a counterclockwise absolute orientation of the tions (Fig. 1). In the apical cells the vacuole was reduced overlapped basal bodies and roots when viewed from and the chloroplast occupied most of the cell volume (Fig. above (Fig. 23). Microtubular roots had a cruciate pattern 3). The cell wall was homogeneous, electron dense (Fig. 6) (Fig. 23), in which d l 2 and s l4 with 3\1 arrangement and without plasmodesmatal perforations in the cross (Fig. 24). Root insertions were associated with an walls. Branch initials began as small cell evaginations near amorphous material localized at the anterior-lateral surface transverse cell walls, penetrated by the vacuolar system of basal bodies (Fig. 23). Cross-sections through the and chloroplast (Fig. 7). The nucleus was normally nearby overlapped basal bodies showed the normal counter- (Fig. 8). clockwise imbrication of the triplets when observed from base to tip (not shown). Electron-dense, bilobed terminal caps covered the proximal ends of basal bodies (Fig. 23). The medial proximal end of each basal body was occupied Zoosporogenesis by a cylinder of electron-dense material (Figs 19, 23). Differentiation of zoospores in the sporangial mother cells Proximal sheaths subtended the proximal end of each was initiated by chloroplast condensation accompanied by basal body (Fig. 19). In tetraflagellate zoospores, neither an increase in cytoplasmic vacuolation and, eventually, the terminal caps nor cylinders of electron-dense material and development of a large vacuole (Fig. 9). The cell wall of proximal sheaths were present in the lower pair of basal the sporangium mother cell comprised two electron-dense bodies. Ultrastructure and taxonomy of the genus Endophyton 177
Figs 1–8. Vegetative structure of Endophyton ramosum. Fig. 1. Longitudinal section of a vegetative filament showing the distribution of the principal organelles. Fig. 2. Detail of a nucleus showing the nucleolus and the paranucleolar body (arrow). Fig. 3. Transverse section through an apical cell. The chloroplast occupies most of the cell volume, the nucleus remaining enclosed in the central part of the cell separated from the chloroplast by a thin layer of cytoplasm. Figs 4, 5. Details of chloroplasts. Note the undulating 2–10 thylakoidal lamellae. Fig. 4. Longitudinal median section through a pyriform pyrenoid (arrow), penetrated by two single modified thylakoids. Fig. 5. The pyrenoid matrix is flanked by starch plates of different size and shape. Fig. 6. Detail of the cell wall. Epiphytic bacteria are present on the wall. Figs 7, 8. Detail of branch initials. Fig. 7. Incipient cell evagination intruded by vacuolar system and chloroplast. Fig. 8. Cell evaginations with a nucleus nearby. Abbreviations: C, chloroplast; N, nucleus; Nu, nucleolus; P, pyrenoid; S, starch; T, thylakoid membrane; V, vacuole; VD, vacuolar diverticulum. Scale bars represent: Figs 1, 7, 8, 1 µm; Figs 2–5, 0n5 µm; Fig 6, 0n2 µm. P. I. Leonardi et al. 178
Figs 9–12. Development of sporangia and zoosporogenesis in Endophyton ramosum. Fig. 9. Longitudinal section of a sporangial mother cell. Initial differentiation into zoospores is recognized by chloroplast condensation accompanied by an increase of cytoplasmic vacuolation. Fig. 10. Detail of the cell wall of the sporangium mother cell, formed by two electron-dense strata: the outer one representing the vegetative cell wall and the inner one the capsule. Fig. 11. Sporangium with its contents during the simultaneous cytokinesis which divides the cytoplasm into uninucleate portions. Note the apical vacuole. Fig. 12. Two contiguous sporangia both showing zoospores at different stages of maturation. Those of the upper, elongate one display numerous small vesicles. Abbreviations: C, chloroplast; ICW, inner cell wall; Mu, mucilage; N, nucleus; OCW, outer cell wall, P, pyrenoid; Ve, vesicles; V, vacuole. Scale bars represent: Figs 9, 11, 12, 1 µm; Fig. 10, 0n3 µm. Ultrastructure and taxonomy of the genus Endophyton 179
Figs 13–17. Development of sporangia, zoosporogenesis and zoospore germination in Endophyton ramosum. Fig. 13. Detail of sporangial wall exhibiting two distinct electron-dense and fuzzy layers containing contiguous mucilage. Fig. 14. Flask-shaped mature sporangium with a wide base, just before the liberation of zoospores. Fig. 15. Detail of the apical portion of the sporangium seen in Fig. 14. Mucilage is condensed in the upper part of the sporangium. Fig. 16. Zoospore liberation. The sporangium wall dissociates and the mucilage plug produced as a result of the earlier apical condensation is released prior to the spores. Fig. 17. Zoospore germination inside the zoosporangium. Abbreviations: CW, cell wall; ICW, inner cell wall; Mu, mucilage; OCW, outer cell wall. Scale bars represent: Fig. 13, 0n3 µm; Figs 14, 16, 1 µm; Figs 15, 17, 0n5 µm. P. I. Leonardi et al. 180
two-membered roots, which fold over and cover a small part of the proximal end of the basal body (Sluiman et al., 1980; Hoops et al., 1982; Floyd & O’Kelly, 1984; O’Kelly et al., 1984; O’Kelly & Floyd, 1984a). Terminal caps in zoospores of Endophyton and swarmers of Entocladia viridis, however, are typically bilobate (O’Kelly & Floyd, 1983) – a feature which favours the inclusion of both genera in the Ulvales sensu O’Kelly & Floyd (1984a) rather than in the Ulotrichales. Furthermore, bilobate terminal caps have been described in the motile cells of several members of the Ulvales, including Ulva lactuca (Melkonian, 1979), Enteromorpha sp. and Enteromorpha flexuosa (Stuessy et al., 1983; Leonardi & Ca! ceres, 1990) and Ulvaria obscura (O’Kelly et al., 1984). Additional support for assigning Endophyton to the Ulvales rather than to the Ulotrichales comes from the proximal sheaths present in the basal bodies of the zoospores. In Endophyton these sheaths, even though they looked prominent in longitudinal view (Fig. 19), did not display the typical wedge-shaped appearance observed in members of the Ulotrichales (O’Kelly & Floyd, 1984a; Lokhorst & Star, 1986). Furthermore, proximal sheaths in Endophyton resemble in shape those of other Ulvales (O’Kelly & Floyd, 1984a). O’Kelly & Floyd (1984a) recognized two families within the order Ulvales. The Ulvellaceae is characterized by filamentous or pseudoparenchymatous thallus Fig. 18. Diagrammatic reconstruction of a biflagellate zoospore organization and absence of rhizoplasts in motile cells. of Endophyton ramosum. Abbreviations: D, dictyosome; E, The Ulvaceae, on the other hand, is characterized by a eyespot; L, lomasomes; Lp, lipid droplets; M, mitochondrial parenchymatous thallus organization and rhizoplasts in profiles; N, nucleus; P, pyrenoid; s, d, roots; S, starch; T, thylakoid membrane; Ve, vesicles; V, vacuole, 1, 2, basal motile cells. In zoospores of Endophyton rhizoplasts are bodies. absent and therefore this genus should be assigned to the Ulvellaceae. Several ultrastructural features of vegetative cells are Discussion consistent with those of reproductive cells, and support Our results provide strong evidence that Endophyton the assignment of Endophyton to the class Ulvophyceae. should be assigned to the class Ulvophyceae. The For example, as is characteristic of the Ulvophyceae sensu counterclockwise 11\5 orientation of the basal bodies of O’Kelly & Floyd (1984a), plasmodesmata are absent in the zoospores of Endophyton is identical with that in transverse cell walls of Endophyton ramosum. The fine Entocladia viridis (O’Kelly & Floyd, 1983) and other genera structural details of maturing sporangia in Endophyton of the class Ulvophyceae such as Ulva (Melkonian, 1980), were similar to those of Entocladia viridis and Acrochaete Ulothrix (Floyd et al., 1980; Sluiman et al., 1980; Lokhorst operculata (O’Kelly & Floyd, 1983; Correa & McLachlan, & Star, 1986), Pseudobryopsis (Roberts et al., 1982), 1994). O’Kelly & Yarish (1980) noted that the capsular Enteromorpha (Stuessy et al., 1983; Leonardi & Ca! ceres, zoosporangial wall and method of sporangial dehiscence 1990), Ulvaria and Monostroma (O’Kelly et al., 1984), (wall dissociation and the production of a mucilage plug) Cladophora and Chaetomorpha (Floyd et al., 1985), Prasiola were characters allying the genus Entocladia closely to (O’Kelly et al., 1989) and Phaeophila (Chappell et al., 1990). members of the Ulvales sensu Stewart & Mattox (1975). Cruciately arranged microtubular s and d roots with s l The electron-dense layer of the sporangial wall of E. 4 with 3\1 arrangement occur in most of the members of ramosum was similar to the layer described as a capsule in the Ulvales and Ulotrichales (Floyd & O’Kelly, 1984). Our sporangia of Ulva mutabilis (Bra/ ten & Løvlie, 1968) and in study supports the inclusion of Endophyton within the gametangia of Enteromorpha intestinalis (McArthur & Ulvales sensu O’Kelly & Floyd (1984a). Segregation of five Moss, 1979). marine genera from the Chaetophorales, Endophyton and Furthermore, the pyrenoids of the vegetative cells and Entocladia among them, and their inclusion in the zoospores of Endophyton share the typical ulvophycean Ulotrichales, was postulated by Lokhorst (1984). The feature of being penetrated by single modified thylakoids ulotrichalean type of terminal cap is a more or less and flanked by starch plates. Nevertheless, they display prominent electron-dense flap located on the anterior some differences: the number of modified thylakoids (2[4]) surface of basal bodies, often near the insertions of the and starch plates (4–6) is greater and the shape of the body Ultrastructure and taxonomy of the genus Endophyton 181
Figs 19–24. Biflagellate zoospores of Endophyton ramosum: general features. Fig. 19. Detail of the anterior part of a zoospore. One basal body is longitudinally sectioned. Its proximal end is occupied by a cylinder of an electron-dense material and subtended by a proximal sheath. The anterior third of the cell contains a dense aggregation of vesicles. Fig. 20. Detail of upper part of the cell showing the vesicles and lomasome-like structures. Mitochondrial profiles are also seen. Fig. 21. Detail of pyrenoid with the same characteristics as those in vegetative cells. Fig. 22. Transverse section through a cell at the level of the intraplastidic protruding eyespot. Fig. 23. Detail of the basal apparatus. Transverse section through a cell in which both basal bodies are longitudinally sectioned. Note the 11\5 counterclockwise disposition of the basal bodies, the cruciate microtubular roots and the bilobate terminal caps. Fig. 24. Detail of the ‘s’ microtubular root with 3\1 arrangement. Abbreviations: CM, cylinder of electron-dense material; E, eyespot; L, lomasomes; M, mitochondrial profiles; P, pyrenoid; PS, proximal sheath; s, d, roots; S, starch; TC, terminal cap; Ve, vesicles, 1, 2, basal bodies. Scale bars represent: Figs 19, 21, 24, 0n2 µm; Fig. 20, 0n3 µm; Fig. 22, 1 µm; Fig. 23, 0n1 µm. is pyriform rather than lenticular. With the exception of Floyd, 1983) and Acrochaete (Correa & McLachlan, 1994). the pyriform shape of the matrix, these features are in This is the first study of the fine structure of vegetative agreement with the pyrenoids found in Ulothrix zonata cells, developing and mature sporangia and zoospores in and U. mucosa (Lokhorst, 1985), but curiously they differ Endophyton ramosum. The fine structural details of veg- from those in Entocladia (Floyd & Yarish, 1978; O’Kelly & etative cells and sporangia of this species strongly P. I. Leonardi et al. 182 resemble those of Entocladia viridis and Acrochaete vegetative cells of selected marine Chaetophoraceae (Chlorophyta). J. operculata (O’Kelly & Yarish, 1980; Correa & McLachlan, Phycol., 14 (Suppl.): 32. F, G.L., H, H.J. & S, J.A. (1980). Fine structure of the 1994) and other ulvalean algae (O’Kelly & Floyd, 1984a). zoospore of Ulothrix belkae with emphasis on the flagellar apparatus. The general features of the fine structure of zoospores of Protoplasma, 104: 17–31. Endophyton ramosum are very similar to those of swarmers F, G.L., O’K, C.J. & C, D.F. (1985). Absolute configuration of Entocladia viridis (O’Kelly & Floyd, 1983). Endophyton analysis of the flagellar apparatus in Cladophora and Chaetomorpha motile cells, with an assessment of the phylogenetic position of the Clado- zoospores have a flagellar apparatus with 180m rotational phoraceae (Ulvophyceae, Chlorophyta). Am. J. Bot., 72: 615–625. symmetry, counterclockwise arrangement of the basal H, H.J., F, G.L. & S, J.A. (1982). Ultrastructure of the bodies – proved by the counterclockwise imbrication of biflagellate motile cells of Ulvaria oxysperma (Ku$ tz.) Bliding and the triplets of the overlapped basal bodies when observed phylogenetic relationships among ulvophycean algae. Am. J. Bot., 69: 150–159. from base to tip – and terminal caps. Therefore, they have J, M.N. & W, J.A. (1985). The distribution and temporal dynamics the essential motile-cell features of the class Ulvophyceae of the estuarine macroalgal community of San Francisco Bay. (O’Kelly & Floyd, 1983; Floyd & O’Kelly, 1984). Features Hydrobiologia, 129: 139–152. such as bilobate terminal caps and absence of scales L, P.I. & C! , E.J. (1990). Absolute configuration of the flagellar apparatus of biflagellate zoospores of Enteromorpha flexuosa ssp. pilifera support the assignment of the genus to the order Ulvales. (Ulvophyceae, Chlorophyta). 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The ultrastructure of Ulothrix mucosa. II. The flagellar apparatus of the zoospore. Can. J. Bot., 64: 166–176. Ulvophyceae, order Ulvales, family Ulvellaceae according MA, D.M. & M, B.L. (1979). Gametogenesis and gamete structure to O’Kelly & Floyd (1984a). of Enteromorpha intestinalis (L.) Link. Br. Phycol. J., 14: 43–57. M, M. (1979). Structure and significance of cruciate flagellar root systems in green algae: zoospores of Ulva lactuca (Ulvales, Acknowledgements Chlorophyceae). HelgolaW nder Wiss. Meeresunters., 32: 425–435. M, M. (1980). Flagellar roots, mating structure and gametic fusion This work was partially supported by the Consejo in the green alga Ulva lactuca (Ulvales). J. Cell Sci., 46: 149–169. Nacional de Investigaciones Cientı!ficas y Te! cnicas de la N, R. (1979). Culture studies on the type of species of Acrochaete, Repu! blica Argentina, grant PID no. 33111\92. Funding Bolbocoleon and Entocladia (Chaetophoraceae, Chlorophyceae). Bot. 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