A hybrid Phlebodium (Polypodiaceae, Polypodiophyta) and its influence on the circumscription of the genus.

ABSTRACT.--The fern genus Phlebodium is traditionally described as having a row of costal areoles lacking included veins, with the sori located in extra-costal areoles and each sorus served by two veinlets. The discovery of a hybrid between Phlebodium pseudoaureum and Polypodium pleurosorum raises questions about the limits of Phlebodium and necessitates a revised taxonomic circumscription of the genus.

KEY WORDS.--ferns, hybrid, Mexico, Phlebodium

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The fern genus Phlebodium has a Neotropical distribution and has been thought to comprise three species: P. aureum (L.) J. Sm., P. decumanum (Willd.) J. Sm., and P. pseudoaureum (Cav.) Lellinger [syn. P. areolatum (Humb. & Bonpl. ex Willd.) J. Sm.] (see e.g., Proctor, 1989; Nauman, 1993; Mickel and Smith, 2004). When first recognized at generic rank, Phlebodium (R. Br.) J. Sm., based on Polypodium sect. Phlebodium R. Br., was a superfluous name because it included sect. Pleopeltis Humb. & Bonpl. ex Willd., an older name that should have been adopted under current rules (see Smith, 1981). Article 52.3 (McNeill et al., 2005; see also its Ex. 15) is applicable to this matter. Since Phlebodium is based on a name-bringing synonym (in other words, it has a basionym, i.e., Polypodium sect. Phlebodium R. Br., that is legitimate), Phlebodium is not illegitimate. Because Smith's genus was a stat. nov., Art. 7.4 dictates that the type of R. Brown's section must also be the type of Phlebodium. Art. 10.2 establishes that the type must be either P. aureum or P. decumanum, given that these were the only two species included in sect. Phlebodium by Brown. Phlebodium was lectotypified by Phlebodium aureum (L.) J. Sm. (Smith, 1875), and this choice has been reaffirmed by several authorities (e.g., Copeland, 1947; Tryon and Tryon, 1982).

Phlebodium has usually been characterized by venation that is highly reticulate (but free near margins), with 1 to 4 rows of fertile costal polygonal areoles and two or three rows of alternate marginal sterile areoles (without free included veinlets) (Fig. 1J). The costal areoles include one secondary areole that extends laterally from secondary vein to secondary vein, with two included excurrent veinlets meeting at apices. The genus is further characterized by having pinnatifid to pinnatisect blades (Fig. 1G).

Often, Phlebodium aureum has been treated in a broad sense (e.g., by Tryon and Stolze, 1993), to include also Ph. pseudoaureum and segregates of that species. Tryon and Tryon (1982) placed Phlebodium aureum s.1. and Polypodium lowei C. Chr. [= Po. pleurosorum] in with a group of Mexican and Mesoamerican species related to Polypodium plesiosorum Kunze, P. subpetiolatum Hook., and several other species. The Po. plesiosorum group is now thought to be closely related to true Polypodium (type: Po. vulgare L.), and less closely related to Phlebodium (Schneider et al., 2006; Tejero-Diez, 2005).

In 2002, the first author (JDTD) discovered in Chiapas, Mexico, a specimen (Fig. 1 A-C) that appears to be a hybrid between the most common species of Phlebodium in Mexico, Ph. pseudoaureum (Figs. 1G-J), and a simply pinnate species of Polypodium, Po. pleurosorum Kunze ex Mett. (Figs. 1D-F). The plant has blades that are pinnate proximally and pinnatifid distally, a mixture of sori each served by a single vein or by two veins, and differential development of secondary costal sterile areoles (Figs. 1A and C). Its sori have abundant sporangia and mostly malformed spores (Fig. 2H). Some authorities have considered Phlebodium and Polypodium as only distantly related (e.g., Copeland, 1947, who thought Phlebodium to be derived from Pleopeltis), while others have thought them to be more intimately related (e.g., Tryon and Tryon, 1982, p. 691). Closer examination was made to see if Polypodium pleurosorum might in fact belong to Phlebodium. Moore (1855), in his description of Polypodium pleurosorum (under the name Phlebodium inaequale T. Moore) wrote: "The sori are large, round, situated in a single series near the midrib; sometimes seated on the apex of a veinlet within a costal areole, which is characteristic of Goniophlebium; sometimes on a veinlet exterior to the costal areole, sometimes at the point where two or more veins unite, which is the normal condition of Phlebodium. It is consequently an osculating species between the genus Goniophlebium and Phlebodium." He also noted that it resembles Phlebodium aureum but has truly pinnate fronds. Examination of herbarium specimens of Polypodium pleurosorum shows that although most of the sori are located in costal areoles and served by a single vein, there are occasional sori, especially distally, that are served by two veins.

Recent phylogenetic studies based on DNA molecular characters (Schneider et al., 2004; Schuettpelz and Pryer, 2007) show that Phlebodium pseudoaureum and P. decumanum are sister to a clade comprising species of Pecluma. Sampled are Pe. alfredii (Rosenst.) M. G. Price, Pe. eurybasis (C. Chr.) M. G. Price, and Pe. ptilodon (Kunze) M. G. Price and two Mexican/Mesoamerican species of Polypodium, Po. hartwegianum Hook. and Po. longepinnulatum E. Fourn. the last two species, as well as some others, are probably better referred to Pecluma, but these transfers await more comprehensive sampling in the Pecluma clade. The Phlebodium + Pecluma clade is in turn sister to a large group (75+ spp.) of scaly polypods, the Pleopeltis clade (Otto et al., in press), including scaly species usually included in Polypodium s.1. The true Polypodium clade, comprising Po. vulgare L. and allies (Haufler and Ranker, 1995), is yet more distantly related to Phlebodium. Phlebodium inaequale has now also been sampled for DNA (Schneider, unpubl, data), and nucleotide sequence data show that Phlebodium, as redefined here and including the newly transferred P. inaequale, is monophyletic, with strong bootstrap and Bayesian support, sister to the Pecluma alliance (Schneider, pers. comm.)

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Proctor (1989) reported that in Puerto Rico, where Phlebodium aureum, Ph. pseudoaureum, and Ph. decumanum occur together, both P. pseudoaureum and P. decumanum appear to be diploid, Phlebodium aureum s.s. is their fertile, allotetraploid hybrid, and at least one sterile, triploid backcross hybrid was reported. Chromosome counts for Phlebodium include three counts of 2n = 74 (diploid, based on x = 37) for Ph. decumanum from Trinidad (Walker, 1985), three counts of n = 74, 2n = 147 for Ph. aureum from Trinidad and Tobago (Walker, 1985), and four counts n = 37, 2n = 74 of Ph. aureum s.1. from Jamaica and Mexico (Walker, 1966; Mickel and Smith, 1977, reported as Po. araneosum M. Martens & Galeotti, now considered a synonym of Ph. pseudoaureum). These last diploid counts likely pertain to the species now called Ph. pseudoaureum, and not the true Ph. aureum, which appears to be tetraploid. Walker (1985) reported spontaneous, sterile, triploid hybrids between what he called Po. aureum s.1. and Po. decumanum in Trinidad. There is also an early report of a hybrid called Phlebodium x schneideri, reputed to be the hybrid between Po. aureum s.1. and Po. vulgate L. (Schneider, 1894). The parentage of this hybrid now seems in doubt, because of the relatively distant relationship between Phlebodium and Polypodium, as currently defined.

In an attempt to verify hypothesized relationships among species of Phlebodium, Caruso (1985) studied living plants of Phlebodium aureum, Ph. pseudoaureum, and Ph. decumanum growing in the greenhouses of the New York Botanical Garden. Although cytological studies were unsuccessful, measurements of spores and stomatal guard cells showed significant differences, with the tetraploid, Ph. aureum having the larger measurements.

The rarity of the Tejero-Diez collection (4362) and its morphological intermediacy suggest that it is a hybrid, and with its significant bearing on the circumscription of the genus Phlebodium, we hereby give it a hybrid name.

Phlebodium x hemipinnatum Tejero, Mickel and A. R. Smith, hyb. nov. TYPE.--MEXICO: Chiapas, Mpio. San Cristobal de las Casas, Km 67 de la carretera federal 190, Tuxtla Gutierrez a San Cristobal de las Casas (16[degrees] 42' 23" N, 92[degrees] 46' W), bosque de Pinus-Quercus, 2440 m, 6 Ago 2002, Tejero-Diez 4362 (Holotype: MEXU; isotypes: IEB, IZTA, NY, UAMIZ). Figs. 1A-C.

Phlebodio pseudoaureo atque Polypodio pleurosoro proxima, sed laminis hemipinnatis, id est basis pinnatis apiceque pinnatifidis, plane differt.

Rhizomes long-creeping, 4-6 mm diam. (excluding scales), pruinose, densely scaly; rhizome scales 8-12 x 2-4 mm, ovate, long-attenuate, yellowish brown, each with enlarged, round, peltate base, darker at point of attachment, margins denticulate to short-ciliate and erose throughout, with short to long, flexuous, contorted, hairlike tips; fronds (55) 60-70 cm long; stipes 1/3-1/2 the frond length, brown, glabrous; blades ovate-deltate to broadly-oblong, 26-35 cm wide, 1-pinnate at middle basal part, becoming pinnatifid above the middle, terminal segment subconform, 5-16 cm long; pinnae (segments) 8-12 pairs, 12-30 mm wide, linear-oblong to linear-lanceolate, some falcate, acuminate, glabrous, green-yellowish, margins entire to repand; veins netted, free near margins, with 1 row of fertile costal polygonal areoles, each with a single simple or bifurcate, excurrent included veinlet or 2 veinlets that form a secondary areole and meet at their tips, 2-3 rows of similar areoles closer to pinna margins, these mostly without included veinlets; sori round, 2-3 mm diam., submedial, one row on each side of the costa; spores mostly malformed, bilateral, monolete, (33)39(45) x (22)26(33) [micro]m, tuberculate, tubercles dome-shaped, somewhat overlapping, amber.

PARATYPE.--MEXICO: Chiapas, Mpio. Tenejapa, a 3.5 km al NE del paraje Balum Canal (16[degrees] 48' 05" N, 92[degrees] 31' 50" W), Acahual derivado de bosque de Pinus-Quercus, 2200 m, 8 Mar 1995, Ramirez-Marcial & Hernandez-Rojas 654 (MEXU!, ECOSUR--herbarium of the Colegio de la Frontera Sur, Chetmul, Quintana Roe, Mexico).

HABITAT.--Epiphytic in pine-oak forests and adjacent disturbed areas; 2200--2500 m.

DISTRIBUTION.--Mexico, Chiapas, montane areas.

The existence of this new hybrid, with characters intermediate between Phlebodium pseudoaureum and Polypodium pleurosorum, causes us to conclude that the latter species can once again be included in the genus Phlebodium, with the earliest available name, Ph. inaequale T. Moore. Impetus for the recircumscription of polypod genera has been given by several other recent phylogenetic studies on Polypodiaceae, most importantly the one by Schneider et al. (2004), outlining a global phylogeny for the family. Subsequently, several other papers directed toward the placement of problematic Neotropical polypods have appeared (e.g., Krier et al., 2007; Schneider et al., 2006; Tejero-Diez, 2005), are in press (Krier et al., 2008), or have been submitted for publication (Otto et al., in press). The redefinition of Phlebodium also recalls the recent recircumscription of the polypod genus Microgramma, necessitated by the finding of a new and radically different species of the genus in coastal Brazil (Saline et al., in press).

Cladistic analysis of morphological characters in species of Polypodium and related taxa (Tejero-Diez, 2005) suggests that the critical characters separating Phlebodium from its sister group (Pecluma, and Mexican/Mesoamerican species allied to Pecluma but still placed in Polypodium; Schneider et al., 2004; Schuettpelz and Pryer, 2007) are: a) spores with tuberculate ornamentation (Fig. 2A-H; b) small size of spore body (33) 38 (45) gm; and c) the presence of several rows of marginal sterile polygonal areoles.

Of the aforementioned characters, the spore ornamentation in Phlebodium and the smaller spore size are unique in Polypodiaceae, but the ornamentation is somewhat similar to spores of Polypodium arcanum Maxon and some species of Serpocaulon (Tryon and Lugardon, 1991; Tejero-Diez, 2005). It is clear that the taxonomic limits of Phlebodium cannot be governed by the way in which the internal veinlets of the main costal areoles are organized.

The species of Phlebodium and the newly described hybrid can be separated by the following key:

1. Blades 1-pinnate, at least proximally; sori each at the end of a
   simple or bifurcate veinlet; secondary costal areoles absent or
   irregularly so.
   2. Blades pinnate throughout their length              P. inaequale
   2. Blades pinnate proximally but pinnatisect or
      pinnatifid distally                            P. x hemipinnatum
1. Blades pinnatifid or pinnatisect; sori each at the end of two
   veinlets; secondary costal areoles regularly present.
3. Sori in 1 row on each side of costae;
   (170-)550-2500 m                                     P. peudoaureum
3. Sori in 2 or more rows on each side of costae;
   0-500 m.
   4. Sori on 3 or more rows on each side of
      costae                                              P. decumanum
   4. Sori on 2 (infrequently 1) rows on each side
      of costae                                             P.  aureum

The use of the name Phlebodium inaequale T. Moore for what has been called Polypodium pleurosorum Kunze ex Mett. requires a brief explanation. The former name was published first by Moore (1855), but when treated as belonging in Polypodium cannot be used because of the existence of an earlier homonym, Polypodium inaequale Link, published in 1833 (Mickel and Smith, 2004).

ACKNOWLEDGMENTS

We thank Harald Schneider for permission to use unpublished information on the phylogenetic relationships of Phlebodium inaequale. We also thank John Wiersema for nomenclatural advice on the legitimacy of Phlebodium. Spore images were obtained by Rafael Emiliano Quintanar-Zuniga. using a scanning microscope Jeol 6380 LW at the Facultad de Estudios Superiores Iztacala of the Universidad Nacional Autonoma de Mexico. Haruto Fukuda prepared the line drawings in Fig. 1.

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J. DANIEL TEJERO-DIEZ

Universidad Nacional Autonoma de Mexico, Facultad de Estudios Superiores Iztacala, Carrera de Biologia, Apartado Postal 314, Tlalnepantla 54090, Mexico, Mexico.

JOHN T. MICKEL

The New York Botanical Garden, Bronx, NY 10458-5126, U.S.A.

ALAN R. SMITH

University Herbarium, University of California, Berkeley, CA 94720-2465, U.S.A.