SISTEMÁTICA Y TAXONOMÍA DE PLANTAS VASCULARES

Micromorphological characters supporting the removal of Senecio series Otopteri from Senecio (Asteraceae, Senecioneae)

Adriana Riva, Raúl Pozner & Susana E. Freire

Instituto de Botánica Darwinion, Labardén 200, CC 22, B1642HYD San Isidro, Buenos Aires, Argentina; sfreire@darwin.edu.ar (autor corresponsal).

Original recibido el 21 de agosto de 2009;
aceptado el 19 de noviembre de 2009.

Abstract. Recent molecular studies suggest that Senecio otites belonging to Senecio sect. Senecio ser. Otopteri should be removed from Senecio so that this large and taxonomically complicated genus would be monophyletic. In order to evaluate the taxonomic position of Senecio otites and the placement of series Otopteri, 13 species belonging to Senecio sect. Senecio ser. Otopteri, including S. otites, were examined for micromorphological characters of the style branches (stigmatic surface, stylearm apices) and anthers (filament collar, anther bases, anther appendages). Three states of these micromorphological characters were present in nearly all species of Senecio sect. Senecio ser. Otopteri: 1) the style-arms are apically convex or subconvex, papillate, and surrounded by a crown of hairs of different lengths; 2) the stigmatic surfaces have a cleft configuration (banded to cleft in S. attenuatus); and 3) the anthers are auriculate. These results provide new morphological support for the placement of Senecio otites in Senecio sect. Senecio ser. Otopteri and the future removal of that series from Senecio proper.

Keywords. Asteraceae; Compositae; Micro-morphology; Section Otites; Senecio; Series Otopteri; South America.

Resumen. Caracteres micromorfológicos que apoyan la segregación de Senecio serie Otopteri de Senecio (Asteraceae, Senecioneae).

Estudios moleculares recientes sugieren que Senecio sect. Senecio ser. Otopteri debería ser removido de Senecio para que este género extenso y taxonómicamente complicado sea monofilético. Con el propósito de evaluar la posición taxonómica de Senecio otites y la ubicación de la serie Otopteri, 13 especies, incluyendo S. otites, fueron examinadas en cuanto a los caracteres micromorfológicos de las ramas estigmáticas (superficie estigmática, ápice de las ramas estigmáticas) y de las anteras (collar del filamento, base de las anteras, apéndice de las anteras). Tres estados de estos caracteres micromorfológicos están presentes en todas las especies estudiadas de Senecio sect. Senecio ser. Otopteri: 1) ramas estilares apicalmente convexas o subconvexas, papilosas, rodeadas por una corona de pelos de distinta longitud; 2) superficies estigmáticas de configuración hendida (bandeada a hendida en S. attenuatus); y 3) anteras auriculadas. Estos resultados proveen apoyo morfológico nuevo para la ubicación de Senecio otites en Senecio sect. Senecio ser. Otopteri y la futura remoción de esta serie de Senecio L.

Palabras clave. Asteraceae; Compositae; Micro-morfología; Sección Otites; Senecio; Serie Otopteri; Sudamérica.

INTRODUCTION

Senecio L. s.l. is one of the largest genera of Asteraceae (Compositae) with ca. 3000 species. It is highly diverse in the mountainous areas and deserts of the Americas, Africa, and Asia, whereas it is poorly represented in the wet tropics (Cabrera, 1949, 1957; Jeffrey et al., 1977). The variable generic and sectional concepts applied by different authors to Senecio have long been recog-nized as a major taxonomic problem in the genus. In the past, some authors (e.g. Bentham & Hooker, 1873; Hoffmann, 1894) tried to give the genus an expanded concept, even including within it the genera Gynoxys Cass. and Emilia (Cass.) Cass., respectively. More recently, several authors removed numerous species from Senecio, and placed them in new genera, or raised to generic rank taxa previously recognized at the sectional level within Senecio (Robinson & Brettell, 1973a, 1973b, 1973c; Robinson, 1974; Nordenstam, 1978; Jeffrey & Chen, 1984; Jeffrey, 1986, 1992). Most of the new taxa were established based on micromorphological characters, such as the configuration of the endothecial cells in the filament collar of the anther, the distribution of the stigmatic area on the style branches, the presence of calcium oxalate crystals in the ovary, and the cellular structure of the carpopodium (Wetter, 1983; Vincent, 1996). As circumscribed by Nordenstam (2007) the genus Senecio s.str. includes about 1250 species of cosmopolitan distribution; it was distinguished principally by its truncate, penicillate stylar tips, separated stigmatic lines, and the obtuse or rounded anther base with a balusterform filament collar.
On the basis of molecular studies, Pelser et al. (2007) proposed a new monophyletic delimitation of Senecio containing ca. 1000 species. In this new delimitation, six genera previously excluded from Senecio are now included in the generic concept of Senecio, and eight species assemblages formerly placed in Senecio s.l. are removed from Senecio. There is a need to examine the morphology of each of the taxa involved in these changes to determine if they can be supported by data other than those generated by the molecular studies. One of these groups is the South American Senecio section Senecio series Otopteri (including Senecio section Otites Cabrera (Cabrera, 1949; Jeffrey, 1992). When Cabrera originally described this section he recognized only one species, Senecio otites Kuntze ex DC., which was restricted to Chile (Cabrera 1949). Later, Cabrera (1985) submerged his section Otites into Senecio section Otopteri (Cabrera) Cabrera (1939), because of its petiolate leaves and auriculate petiole, and placed the group at serial rank, i.e. Senecio sect. Senecio ser. Otopteri (Cabrera) Cabrera (1985). It then contained 14 South American species: Senecio attenuatus Sch. Bip. ex Rusby, S. bangii Rusby, S. belenensis Griseb., S. deferens Griseb., S. herrerae Cabrera, S. kosterae Cabrera, S. lorentzii Griseb., S. otites Kuntze ex DC., S. otopterus Griseb., S. pensilis Greenm., S. pseudotites Griseb., S. sepium Sch. Bip. ex Rusby, S. sinapoides Rusby, and S. tucumanensis Cabrera (Cabrera et al., 1999). This group was defined as being perennial and leafy stemmed plants, leaves petiolate with petiole basally auriculate or decurrent and limb ovate-lanceolate or deltoid, usually 2-9 cm long, capitula usually radiate and several, arranged in corymbose synflorescence, anthers basally obtuse, and style branches apically truncate with short sweeping hairs (Cabrera 1985, Cabrera et al. 1999).
The objective of this study was to test the results of the Pelser et al. molecular phylogenetic study of tribe Senecioneae (2007) in which they suggested that Senecio otites and its closest relatives are too distantly related to Senecio s. str. to remain included in this genus. By examining in detail some micro-morphological characters of the style branches (stigmatic surface, style-arm apices) and anthers (filament collar, anther bases, anther appendages) we wanted to determine whether or not Senecio series Otopteri could be supported as a distinct genus based on morphology.

MATERIALS AND METHODS

Taxa

The study was based on herbarium material from SI and LP. All species of series Otopteri were included in the analysis, except S. tucumanensis; no herbarium material was located for this taxon. In order to examine the taxonomic value of the characters, specimens of the type species of Senecio, S. vulgaris L., were included in the present study. A list of examined specimens is provided in Appendix 1.

Light Microscopy

Mature, fully developed disc florets (al least 3 florets per specimen) were selected for analysis. Herbarium samples were rehydrated in warm soapy water and fixed in FAA. Anthers and styles were dissected, clarified with diluted chlorine, and stained with basic fucsin. Observations were done with LM (Light Microscope) Nikon Microphot-FXA, equipped with a photo-graphic camera.

Scanning Electron Microscopy

Observations with SEM (scanning electron microscopy) were done on herbarium material. Samples were rehydrated with warm soapy water and fixed with FAA. Disc florets were dehydrated in a graded series of ethanol and critical-pointdried (BAL-TEC CPD 030) with liquid CO₂. Dried samples were partially dissected, mounted on stubs, sputtercoated with gold/palladium (Termo VG Scientific SC 7620) and observed with SEM (PHILIPS XL30 TMP New Look).

RESULTS AND DISCUSSION

Style-Arm Apices

In all species of ser. Otopteri the style-arms are apically convex or subconvex, papillate, surrounded by a crown of hairs that differ in length. The hairs can be rather short and few as in Senecio bangii (Fig. 1A) or more commonly long and many in the remaining species (Fig. 1B-E). The outer surfaces of the stylearms may be minutely papillate to papillate in the distal half (Fig. 1A). Senecio vulgaris, the type species of Senecio, has typically truncate stylar tips with only a few hairs that are different in length, and outer surfaces glabrous (Fig. 1F). The fundamental importance of the shape of the style branches in the taxonomy of Senecio has long been recognized. For instance, many species with aberrant stylar branches, were removed from Senecio [e.g. Pseudogynoxys (Greenm.) Cabrera, Paragynoxys (Cuatrec.) Cuatrec.]. However, molecular studies of Pelser et al. (2007), indicate that taxa without precisely penicillate style tips, should be transferred to Senecio (e.g. Aetheolaena Cass., Iocenes B. Nord.).

Fig. 1. Style branches (A-D, SEM; E-G, LM): A-D, Cleft stigmatic configuration. A, S. bangii. Style-arm showing subconvex apex surrounded by a crown of short and few hairs and outer surface papillate. B-D, Style-arm apex sub-convex surrounded by a crown of many and differently long hairs. B, D, S. otites. C, S. otopterus. E, S. attenuatus, banded stigmatic configuration at the tip with basal cleft, style-arm apex subconvex, surrounded by a crown of many and differently long hairs. F, S. vulgaris. Stylearm showing truncate apex with few hairs, and outer surface glabrous.

Stigmatic Surface of the Style Branches

Some authors (e.g. Robinson & Brettell, 1973c; Nordenstam, 1977; Jeffrey et al., 1977) have found the extension of the stigmatic area on the style branch to be a useful diagnostic character, e.g., the bands are fused in "Cacalioid" Senecioneae and discrete or separate in non "Cacalioid" Senecioneae. In all except one species of series Otopteri examined in this study the stigmatic surface has a narrow, longitudinal, medial cleft running the length of the style branches, without any apparent morphological distinction between the cells of the ridges and those in the cleft (Fig. 1A-C). Wetter (1983) used the term "cleft" for this configuration. The exception is Senecio attenuatus as its stigmatic surface presents a noticeable morphological distinction between the cells of the groove and those of the bands at the terminal half of the style branch (Fig. E). This type of stigmatic surface represents an intermediate state between the cleft configuration and the one referred as "banded" by Wetter (1983). In the banded confi-guration the different morphology of the groove cells is appreciable all the long of the style branch since the bands look more separated. The typical banded form is present in S. vulgaris (Wetter, 1983). Consequently, the absence of clearly banded stigmatic surface supports the removal of species of series Otopteri from Senecio.

Filament Collar

The filament collars (Koyama, 1967; Drury, 1973; Nordenstam, 1978) or anther collars (Robinson & Brettell, 1973d; Jeffrey et al., 1977) have also been used to separate "Cacalioid" Senecioneae from "non Cacalioid" Senecioneae, having a "balusterfom" collar and "cylindrical" collar, respectively. All but two members of series Otopteri and Senecio vulgaris have an elongated filament collar that is dilated or somewhat dilated towards the base, and broader than the filament (Fig. 2C-F). This type of filament collar was called "balusterform" by Drury (1973) and its occurance supports a relationship with Senecio. Only Senecio kosterae and S. bangii, have filament collars that are very elongated and uniformly thick with some cells slightly thicker than the filament (Fig. 2A, B); this condition is similar to the "cylindrical" filament collar described by Drury (l.c.). Therefore, "cylindrical" filament collars distinguish Senecio bangii and S. kosterae from the remaining species of ser. Otopteri and S. vulgaris. Recently, Nordenstam et al. (2009) includedSenecio bangii in a new genus segregated fromSenecio, Lomanthus B. Nord. & Pelser. According to these authors, Lomanthus has filament collars that are elongate-balusterform with larger basal cells. Our study shows that Senecio bangii has an elongate-cylindrical and uniformly thick (not balusterform) filament collar.

Fig. 2. Anthers (LM): A-F, Anther collars and anther bases. A, B, Cylindrical collar. C-F, Balusterform collar. A-E, Auriculate bases. F , Obtuse or rounded bases. A, S. kosterae B, S. bangii. C, S. attenuatus. D, S. otites. E, S. deferens. F, S. vulgaris. G-I, Anther appendages narrower than the anther apex.G , S. otopterus. H, S. deferens. I, S. vulgaris.

Anther Bases

Obtuse or rounded anther bases (ecaudate anthers), are features of Senecio s.str. that occasionally occur in other Senecioid genera (e.g., Dendrophorbium Cuatrec.). Our results confirm that ecaudate anthers are present in S. vulgaris (Fig. 2F). However, all species of series Otopteri examined in this study have auriculate anthers (Fig. 2A-E), which separate them from Senecio. Graphystilis B. Nord., one of the sister genera of S. otites in the molecular cladogram (Pelser et al., 2007), also has auriculate anthers showing a close relationship to series Otopteri. However, there are exceptions, for instance S. deferens Griseb. (Fig.2 E) appears within Senecio s.str. in Pelser et al. (2007), even having auriculate anthers (vs. obtuse or rounded anthers in Senecio s. str.).

Anther Appendages

Jeffrey (1980) mentioned the taxonomic value of the shape of the anther apical appendages but it has not been emphasized by many other authors. All species of series Otopteri examined in this study and Senecio vulgaris have anther appendages that are more narrow than the anther apex (Fig. 2G-I) linking this series with Senecio.

CONCLUSIONS

According to the present study, the segregation of series Otopteri from Senecio seems justified by the concurrence of the following micromorphological characters: (1) style-arms that are apically convex or subconvex, papillate or minutely papillate, surrounded by a crown of hairs that are different in length; (2) members of the group usually with a cleft configuration of the stigmatic surface; and (3) auriculate anthers present in all of the group. These results suggest two possible future taxonomic and nomenclatural changes: 1) removal of series Otopteri from Senecio s. str., and 2), its recognition as a distinct genus. However, neither of these changes are presently advisable until more morphological and molecular data are available for more species of Senecio s. lat. and those already analized.

Appendix I. Studied specimens of Senecio sect. Senecio ser. Otopteri (Cabrera) Cabrera.

ACKNOWLEDGMENTS

We are very grateful to Vicki Funk and Cecilia Ezcurra for their comments and suggestions that significantly improved the manuscript. Thanks also to the curators of SI and LP from which specimens have been borrowed for this study. Support for this work by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina, is gratefully acknowledged.

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