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1 December 2002 Thoreauea (Apocynaceae: Apocynoideae), a New Genus from Oaxaca, Mexico
Justin K. Williams
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Recent studies of Mexican Apocynaceae have uncovered a new species. The taxon is here viewed as generically distinct and accordingly the name Thoreauea paneroi J. K. Williams, gen. et sp. nov. is proposed. The species is from montane pine-oak cloud forests of the Santiago Juxtlahuaca area of northwestern Oaxaca, Mexico. Its relationship to Thenardia H.B.K. and other genera is discussed.

Recently, a specimen of Apocynaceae from Oaxaca, Mexico was provided to me by one of the collectors, José L. Panero, for identification. After close examination, I determined that the specimen does not key out to any of the genera recognized in a key to the Mexican genera of Apocynaceae (J. K. Williams, 1996). This specimen keys out most favorably to Thenardia H.B.K., however, it possesses novel characters not found in Thenardia (e.g., dissected corona at the corolla mouth). A cladistic analysis (Fig. 5) based on morphological evidence indicates that if the new taxon were included in Thenardia, the genus would become paraphyletic, no longer representing a monophyletic lineage delimited by a shared consensus of characters. Thus, the problematic specimen is best regarded as representing a new genus.

Thoreauea paneroi J.K. Williams, gen. et sp. nov. (Figs. 1 and 2).

  • Type: MEXICO. OAXACA: Mpio. Santiago Juxtlahuaca, Dist. San Sebastián Tecomaxtlahuaca, 1.8 km N of the road Tecomaxtlahuaca-“Coicoyán, de Las Flores” along the road to Escopeta (17° 18′ 27.1″ N, 98° 07′ 51.5″ W), 4 Mar 1995, J. L. Panero with I. Calzada and J. Kuijt 5583 (Holotype: IZTA!; Isotype: TEX).

  • Thenardia affinis sed corollis urceolatus (vice rotatis) et corona corollae praesenti (vice carenti) et antheris inclusis (vice exsertis) differt.

    VINE, twining, latex milky. STEMS terete, 3–3.5 mm in diameter, light green, glabrous, lenticellate with age; interpetiolar ridge moderately prominent. LEAVES opposite to subopposite, petiolate, membranous; petioles 20–23 mm, with a solitary bract and 2–4 colleters at base; colleters 0.8–1.0 mm long, linear lanceolate, dark brown when dried; leaf blade elliptic, apex acuminate with extended tip, base obtuse, margin entire, glabrous on both surfaces, chartaceous when dry, 10.5–14.0 cm long, 4.3–5.0 cm wide, without colleters, dark green above, light green below, midrib prominent below, slightly obscure above, lateral secondary veins 5–18, conspicuous, impressed alternate. INFLORESCENCE an axillary pedunculate, trichotomously-branched subumbellate cyme, glabrous; primary peduncle 23 mm long, 1.0–1.3 mm diameter; secondary and tertiary peduncles 5–18 mm long; bracts linear-lanceolate, 1.0–4.0 mm long, 0.2–0.4 mm wide, straight; pedicels 7.0–11.0 mm long. FLOWERS 20–25 per inflorescence, tightly clustered, pentamerous, actinomorphic, perfect. CALYX lobes equal, 0.9–1.0 mm long, separate nearly to the base, triangular, erect, glabrous; colleters ca. 0.5 mm long, opposite the sepals, solitary, thin, dentiform. COROLLA fused into a moderately Holotype of Thoreauea paneroi.A. erect tube, urceolate, aestivation dextrorse, creamy white; tube 5–6 mm long, 2.7–3.1 mm wide, slightly constricted at the base and at the distal four-fifths of tube, glabrous, mouth of tube surrounded by a deeply dissected corona annulus, linear-lanceolate, both opposite and alternate the corolla lobes; lobes 1.0–2.0 mm long, 0.6–0.8 mm at widest point, triangular, erect; limb 3.0–4.0 mm in diameter. STAMENS 5.5–6.0 mm long, included; anther tips slightly below the corolla mouth to occasionally exserted ca. 0.2 mm above the rim, filaments 3.0–3.3 mm long, bending inward, and closely encircling the style head, pubescent; anthers 2.5–3.0 mm long, yellow, base sagittate, fertile in the upper part, the lower part enlarged, sterile and equipped with sclerenchymatic guide rails on the ventral face firmly agglutinated to the style-head secretions by thick brushes of hairs, forming a pseudo-gynostegum, in addition thecae agglutinated to the upper slopes of the style-head, forming five separate pollen chambers. PISTIL 3.0–3.5 mm long; ovary of two fused carpels united into a common style, superior, ovoid, glabrous, 0.8–1.5 mm long; style head 1.5–2.0 mm, spool-shaped, slender in the middle and greater in diameter at the base, with developed membranous collar at base; stigmattic zone located on underside of style head beneath collar; nectaries five, free, pressed closely together, tightly surrounding the ovary, as long or slightly shorter than the ovary. FRUIT unknown.

    Thoreauea is a member of the subfamily Apocynoideae as evidenced by its anthers agglutinated to the style head, dextrorse aestivation of the corolla bud, and triporate pollen grains. Within the Apocynoideae Thoreauea belongs to the tribe Echiteae, as delineated by Endress and Bruyns (2000). Members of the Echiteae are characterized by the thecae agglutinated to the style head at two levels and by the spool-shaped style head that is slender in the middle and greater in diameter at the base.

  • Distribution and Ecology: Thoreauea paneroi is a moderately sized liana known only from the type collection from the cloud forests of the district of Santiago Juxtlahuaca area of northwestern Oaxaca, Mexico. The species is found in a mesophytic habitat of montane pine-oak forest at 2625 m elevation. Flowers were collected in March but its phenology is unknown.

    Thoreauea was included, together with 24 additional genera, in an unpublished morphological cladistic analysis of the Apocynoideae (Williams, 1999). A portion of this analysis (Fig. 5) is discussed below. Because additional results of the analysis are outside the scope of this paper, the full tree is not included or discussed.

  • Fig. 1.

    Holotype of Thoreauea paneroi.

    Fig. 2.

    Thoreauea paneroi. A. Flower. B. Longitudinal section of open flower. An = anthers. C = dissected corona around the mouth. F = filaments. Black bar represents 5 mm.


    Collection of Data. With the exception of selected species of Parsonsia (see below), a representative specimen is deposited at the Plant Resources Center for each of the species examined in the morphological cladistic analysis. Observations and data were collected from material borrowed from or observed at the following herbaria: BM, BRIT, CHAPA, F, FLAS, G, GH, K, MA, METPEC, MEXU, MO, NY, P, SHST, TAMU, TEX, US, WIS.

    The pollen of all genera was studied using a light microscope as well as a scanning electron microscope (Philips 515). All genera were examined and measured under the SEM at the Cell Research Center of the University of Texas at Austin.

    A total of 37 taxa, representing 25 genera, were included in the original cladistic analysis (Williams, 1999). Character measurements and states for the data matrix were obtained from living material and herbarium specimens for all of the representative species included in this study. Subsequently, three species of Parsonsia (P. latifolia (Benth.) S. T. Blake; P. praeruptis Heads & de Lange; P. purpurascens J. B. Williams) have been added to the study in order to represent better the diversity of Parsonsia (a genus with many superficial similarities to Thenardia). Morphological data for the three species of Parsonsia were obtained from literature descriptions (J. B. Williams, 1996; Heads & de Lange, 1998)

    Selection of Characters. Fortyfive characters and 119 character states (Table 1) were utilized in this study. Informative character states were selected from those utilized in previous studies (Endress et al. 1996; Sennblad et al. 1998; Struwe et al. 1994; Potgieter and Albert, 2001). New characters not included in the above works, but uncovered during the course of this study were also included. A discussion of the characters utilized in this study is provided in Williams (1999). Table 2 lists the characters and character states for each of the taxa shown in this analysis.

    Cladistic Analysis. The characters and character states (Table 2) used in the analysis were entered into a data matrix using MacClade 3.0 (Maddison & Maddison, 1992). A phylogenetic analysis was then performed in PAUP 3.1 (Swofford, 1993). A heuristic search by stepwise addition of random trees was performed with 100 random addition sequences. The heuristic search was performed with the ACCTRAN, MULPARS and TBR options in effect. Taxa with multi-state characters were recognized as polymorphic for those characters. Characters were treated as unordered and of equal weight. At the end of the analysis the stored trees were rooted, with both the outgroup and ingroup directed as monophyletic. A majority rule consensus tree of the stored trees was then produced. Bootstrap values were calculated using 100 replications.


    The heuristic search yielded a total of 337 equally parsimonious trees with 185 steps. The large number of trees is attributed to the fact that the characters used in this analysis are informative mainly at the generic level. Examination of a majority of the parsimonious trees indicated that the most stable branches were the terminal ones, and that the unstable branches were the basal ones. This is acceptable considering that the main focus of this study was to test the monophylly of problematic genera of Mexican Apocynoideae, in this case Thoreauea. One clade in the majority rule consensus tree (Fig. 5) includes Thoreauea and its relatives, Forsteronia G. Mey., Laubertia A. DC., Parsonsia R. Br., Prestonia R. Br., Echites R. Br. and Thenardia and will be referred to here as the “Prestonia” clade. To date no cladistic analysis has included all of the above genera. In their study, Sennblad et al. (1998) included only Parsonsia and Prestonia. Their tree (based on morphology and molecular evidence) supports the results presented here of a relationship between the two genera. Sennblad and Bremer (2002) presented a second phylogenetic analysis of the Apocynaceae based on molecular evidence. This study again included only Parsonia and Prestonia and, as before, their results showed the two genera as sister to one another. Potgieter and Albert (2001) included Forsteronia, Parsonsia, Prestonia and Echites in their combined morphological and molecular analysis. Their results support the relationship between Parsonsia, Prestonia and Echites, however, Forsteronia appeared in a separate clade paired with Cycladenia. An examination of the tree presented here indicates that Thoreauea is the sister group to a clade comprised of Parsonsia, Thenardia and Forsteronia with all four genera monophyletic.

    Table 1.

    Characters and character states used in the cladistic analysis.




    Table 2.

    Data matrix of the 45 informative characters used in the phylogenetic analysisa,b presented in this study.

    Diagnostic Features and Generic Placement

    Table 3 presents a list of taxonomically useful characters for distinguishing Thoreauea from other closely related genera. Based on these characters and the results of the cladisitic analysis (Fig. 5), Thoreauea appears most related to Thenardia,, sharing a trichotomously branched, cymous inflorescence, relatively small flowers, and triangular corolla lobes. Thoreauea differs from Thenardia in its possession of an annular, corolline corona in the mouth of the corolla, urceolate corollas, and included stamens (vs. rotate and exserted). Laubertia and Prestonia are two New World genera that also possess an annular corolline corona in the mouth of the corolla. However, they differ from Thoreauea in that their corona is continuous (vs. dissected; Figs. 3 and 4, respectively). Laubertia and Prestonia also differ from Thoreauea by their racemiform cymous inflorescences and salverform corollas. In addition, Laubertia and some species of Prestonia (P. mexicana A. DC.) possess an epistaminal corona abaxial to the anthers. In Laubertia the lobes of the second corona appear as five separate swollen calluses each abaxial to the point of divergence of each filament (Figure 3). In Prestonia portobellensis (Beurling) Woodson, the epistaminal corona is comprised of five separate linear protuberances each abaxial to the anther (Figure 4) resembling a staminode.

    Table 3 indicates that the Paleotropical genus Parsonsia also shares many similar characters with Thoreauea. Both genera have similar stamen architecture (filaments along the style and inserted at the base of the corolla), corolla color, and pollen diameter. Thoreauea differs from Parsonsia mainly in the number of calycine colleters (1 vs. many), corolla aestivation (dextrorse vs. valvate), and geography (Neotropics vs. Paleotropics). The cladistic analysis presented in Fig. 5, indicates that Forsteronia is sister to Parsonsia. The relation between Parsonsia and Forsteronia is supported by their shared rotate corolla and numerous calycine colleters. Endress and Bruyns (2000), however, included Forsteronia in a different tribe (Apocyneae Rchb.) from Parsonsia (Echiteae Bartl.). In addition, molecular evidence presented by Potgieter and Albert (2001) clearly showed Forsteronia and Parsonsia distinct from one another, with Parsonsia more closely related to the “Prestonia” clade. Table 3 also shows many similarities between Thenardia and Parsonsia. This relationship was suggested by Baillon (1890), and is supported by the cladistic analysis (Fig. 5) that shows Thenardia sister to the clade containing Parsonsia and Forsteronia.

    Table 3.

    Morphological comparisons of the genera Thoreauea, Thenardia, Prestonia, Laubertia, Forsteronia and Parsonsia, Echites.

    Fig. 3.

    Longitudinal section of a flower of Laubertia contorta. Note how the epistaminal corona is reduced to a callus ridge. A = anthers. C = epistaminal corona. Co = corona around the corolla mouth. P = petals. S = style. Black bar represents 5 mm.

    The surface morphology of pollen grains in the Apocynoideae is essentially uniform. The majority of genera have grains that are spherical, triporate (occasionally 4–5; Telosiphonia) and with smooth perforate surfaces (Erdtman, 1952; Nilsson, 1990; Sampson and Anusarnsunthorn, 1990; Roubik and Moreno, 1991; Nilsson et al., 1993; Williams, 1999). Huang (1989) showed that the number, arrangement and shape of the pores are occasionally useful diagnostic characters for circumscribing genera. Williams (1998) and Roubik and Moreno (1991) also showed that species of a genus can occasionally be identified by their pollen diameters. It is speculated that pollen diameter may also be a useful character in resolving generic relationships. Measurements of the pollen grains of Thoreauea paneroi (under light microscopy and SEM) show the diameter to be between 3045 µm. Interestingly, the pollen grain diameter of Thenardia chiapensis J. K. Williams falls within this range. The second species of Thenardia, T. floribunda H.B.K., in this study has grains 45–60 mm wide (Williams, 1998).

    Fig. 4.

    Longitudinal section of a flower of Prestonia portobellensis. Note how the epistaminal corona appears as a linear protuberance. A = anthers. C = epistaminal corona. Co = corona along the corolla mouth. F = filaments. S = style. Black bar represents 5 mm.

    Fruiting specimens of Thoreauea will certainly aid in its taxonomic positioning within the Apocynoideae. Observations of fertilized ovaries suggest that the follicles, as in Thenardia, will be fused to one another.

    The combination of vegetative and floral characters in this new species is so unique that including it in a currently recognized genus is not justified. Consequently, the new genus Thoreauea is proposed. I have not located any types or specimens of Neotropical Apocynaceae in the following herbaria that resemble this new species (Williams, 1999): BM, BRIT, CHAPA, F, FLAS, G, GH, K, MA, METPEC, MEXU, MO, NY, P, SHST, TAMU, TEX, US, WIS.

    It is an honor to name this new genus after Henry David Thoreau (1817–1862), noted essayist and naturalist (Angelo, 1985; Egerton and Walls, 1997). His loving, and often unrecognized, commitment to botany inspired me to undertake the subject.

    Fig. 5.

    Prestonia” clade of majority rule consensus tree calculated from 337 most parsimonious trees (length = 185, CI = 0.53, RI = 0.76, RC = 0.40). Numbers above the lines indicate branch length and the numbers below the branches are bootstrap values greater than 50%.

    The species epithet honors José L. Panero, Professor of Integrative Biology at the University of Texas at Austin and collector of the type specimen.


    I thank José L. Panero for discussions on the location and habitat of the type specimen. I also thank Mary Endress, William Lutterschmidt, Kurt Potgieter, Beryl Simpson, B.L. Turner, Tom Wendt and an anonymous reviewer for comment on the manuscript.

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    Justin K. Williams "Thoreauea (Apocynaceae: Apocynoideae), a New Genus from Oaxaca, Mexico," Lundellia 2002(5), 47-58, (1 December 2002).
    Published: 1 December 2002
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