Rutaceae is a cosmopolitan family mainly distributed in tropical and subtropical regions of the world (Groppo et al., 2008; Kubitzki et al., 2011). It comprises 150 to 170 genera and nearly 2040 species (Kubitzki et al., 2011; Morton & Telmer, 2014). Engler (1931) divided the family into seven subfamilies, amongst them Rutoideae, which includes the tribe Xanthoxyleae with five subtribes: Evodiinae, Lunasiinae, Choisyinae, Pitaviinae, and Decatropidinae. Decatropidinae comprises three genera: Decatropis Hook. f., Polyaster Hook. f., and Megastigma Hook. f. On the other hand, Amyris P. Browne was included in the subtribe Amyridinae (subfamily Toddalioideae, tribe Toddalieae) with two other genera: Stauranthus Liebm. (distributed in America) and Vepris Comm. ex A. Juss. (Africa, Arabia, Madagascar, and India).

Amyris is an American genus with ca. 40 species distributed from the southern United States to northern South America. It is characterized by tree or shrub habit, imparipinnate or rarely unifoliolate leaves, lateral or terminal cymose inflorescences, stamens twice as many as the petals with a gland on the connective, a meloniform to lobulate (never conical) nectariferous disk, a unilocular ovary, and fleshy fruits with abundant glands and only one seed (Gereau, 1991; Rebman & Chiang, 2005; Hernández-Barón, 2016).

Megastigma is also an American genus and comprises five species distributed in Mexico, Guatemala, Honduras, and Nicaragua. Members of the genus are trees or shrubs with imparipinnate leaves, paniculate to umbellate, terminal, subterminal and/or lateral inflorescences, stamens twice as many as the petals and without a gland on the connective, a conical nectariferous disk, a bilocular, bilobed, didymous ovary (Hooker, 1862), a gynobasic style with a globose and conspicuous stigma, and a fleshy, didymous, fruit with one seed per locule; frequently with only one carpel ripening and the undeveloped carpel deciduous or sometimes persistent on the disk (Pool, 2001; Kubitzki et al., 2011; Jiménez Ramírez & Cruz Durán, 2015 Cuevas-Guzmán et al., 2016).

Megastigma differs from Amyris by having a conical nectariferous disk, a bicarpelar, bilobed, bilocular, didymous ovary with a gynobasic style and a globose and conspicuous stigma, the stamen’s connective without a gland, and a fleshy, didymous fruit frequently with only one carpel ripening and the undeveloped one deciduous or persistent on the disk.

As part of our ongoing taxonomic and floristic investigation of the Mexican members of Amyris, we discovered that A. conzattii Standl. differs from the other members of the genus by some traits of the ovary and fruit. A meticulous examination of the original descriptions and type material of all species of Amyris allowed us to determine that A. conzattii actually belongs to Megastigma, which we discuss below.

For a long time, Amyris conzattii was known only from the type specimen, kept at the United States National Herbarium (US), which has only fruits (Fig. 1). This specimen was described as a member of Amyris due to its possession of several characters of that genus, such as imparipinnate leaves and a fleshy, apparently unicarpellate fruit.

Fig. 1
figure 1

A. Holotype of Amyris conzattii. B. Apex of inflorescence showing a mature fruit and (at red arrow) an undeveloped carpel of a bilobed, bilocular, didymous ovary. C. Apex of leafy twig with mature fruit. (From C. Conzatti 4556, US; scales = 1 cm.)

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When Conzatti collected the type material, he also collected seeds, which he grew to maturity and from which he prepared, in May of 1935, herbarium specimens with flowers (Fig. 2). Eighty years later, in 2015, a duplicate of the same gathering was deposited at the Herbario Nacional de México (MEXU 1390680) (Fig. 2A). Meticulous examination of the flowers of the specimen revealed the presence of a bilobed, bilocular and didymous ovary with a gynobasic style, a globose and conspicuous stigma (Fig. 2E), and an eglandular anther connective (Fig. 2D). Therefore, we concluded that A. conzattii actually represents a species of Megastigma and therefore we make the necessary new combination here.

Fig. 2
figure 2

Megastigma conzattii. A. Specimen made from a cultivated plant grown from seeds of the original collection. B-F. Floral dissection. B. Sepals. C. Petals. D. Stamens. E & F. Didymous ovary, the arrows indicates the stigma (S), ovary (O) and nectariferous disk (N). (From C. Conzatti 5066, MEXU; scales = 1 mm.)

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Taxonomic treatment

Megastigma conzattii (Standl.) Hern.-Barón, Espejo & López-Ferr., comb. nov. Amyris conzattii Standl., J. Wash. Acad. Sci. 13:6. 1923. Type: Mexico, Oaxaca: Los Sabinos, entre [San Pedro] Juchatengo y Santa Ana, 1000 m, 29 Dec 1921 [fr], C. Conzatti 4556 (holotype: US Herb. #1110840 (Bar code # US101736!)).

Tree or shrub with thin, greyish, glabrous branches. Leaves imparipinnate, alternate, 5–12.5 cm long; petioles 4–10 mm long, rachis thin and finely pubescent, blades 4.5–11.5 cm long; pubescent; petiolules 0.3–1.2 mm long, pubescent; leaflets 21 to 29, coriaceous, rhombic to ovate-rhombic, 5–13.2 mm long, 3.5–8 mm wide, obtuse to rounded at apex, oblique at the base, entire or inconspicuously crenulate, glabrous or sometimes sparsely puberulent on the adaxial surface, with numerous glands. Inflorescences lax terminal pauciflorous panicles, 2.4–4.4 cm long. Flowers < 20, tetramerous, rarely pentamerous; sepals 0.6–1 mm long, 0.2–0.4 mm wide, apiculate, connate at the base; petals 1.7–3.5 mm long, 1.2–2.6 mm wide, ovate; stamens twice as many as the petals, the filaments inserted below the nectariferous disc, 1.3–4.5 mm long, the anthers 0.60–0.90 × 0.44–0.59 mm; the nectariferous disc slightly conical and lobed; pistil up to 1.5 mm long, the ovary bicarpelar, bilobed, bilocular, didymous, 0.3–0.5 mm long, each lobe 0.5 mm in diameter, the style gynobasic, ca. 0.3 mm long, the stigma 0.5 mm long × 0.5 mm in diameter. Fruit drupaceous, didymous, black, frequently with only one carpel developed ca. 10 mm long, 0.7 mm wide.

Distribution.—Megastigma conzattii is known only from the type locality.

Phenology.—The flowering collection was gathered in May, the fruiting collection in December.

Additional specimen examined. MEXICO. Oaxaca: Obtenido en Oaxaca de semilla procedente de los Sabinos, veáse el #4556, 20 May 1935 [fl], C. Conzatti 5066 (MEXU Herb. # 1390680).

Megastigma conzattii is most similar to M. chiangii J. Jiménez Ram. & Cruz Durán, and to M. skinneri Hook. f. because the three species exhibit imparipinnate leaves with over 15 leaflets. With M. chiangii it also shares glabrous branches and coriaceous leaflets. However, M. chiangii has the petioles and rachis glabrous and the inflorescences racemose to paniculate with more than 40 flowers; whereas M. conzattii has the petioles and rachis pubescent and the inflorescences paniculate with less than 20 flowers.

Megastigma conzattii and M. skinneri both have the petioles and rachis pubescent and the flowers tetramerous. However, M. skinneri has 15 to 21 membranaceous leaflets and umbellate inflorescences with less than 12 flowers; while M. conzattii has 21 to 29 coriaceous leaflets and paniculate inflorescences with less than 20 flowers. There are clear differences in the vegetative and reproductive characteristics of these three species, which are compared in Table 1.

Table 1 Comparative characters of Megastigma conzattii, M. chiangii, and M. skinneri.
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With the new combination made here, Megastigma comprises six species: M. balsense Chiang & Jiménez Ram., M. conzattii, M. galeottii Baillon, M. chiangii, M. morenoi Cuevas & Guzmán-Hern., and M. skinneri. The first five are endemic to Mexico, and the last one is distributed from Chiapas to Nicaragua.

Key for the identification of the species of Megastigma

1. Leaflets 5–7.

2. Leaves 1–2.5 cm long; leaflets 0.45–1 × 0.5–0.9 cm; rachis of young leaves narrowly winged; inflorescence umbellate; ovary 2-carpelar (valley of Tehuacán-Cuicatlán, Puebla and Oaxaca, Mexico)........................................................................................................................M. galeottii

2. Leaves 3.5–9 cm long; leaflets 3.3–3.6 × 1.2–2.6 cm; rachis of young leaves unwinged; inflorescence racemose, rarely paniculate; ovary 2(−4)-carpelar (oriental basin of Balsas river, Guerrero, Mexico)...................................................................................................................................M. balsense

1. Leaflets 9–29.

3. Flower pentamerous; petioles 25–35 mm long; leaflets 1.8–3.7(−4.2) × (0.5–)0.9–1.7 cm; inflorescences 6–8.5 cm long, with more than 40 flowers............................................................M. chiangii

3. Flowers tetramerous, rarely pentamerous; petioles 4–12 mm long; leaflets 0.3–2.7 × 0.25–1.5 cm; inflorescences 2–4.4 cm long, with 3–16 flowers.

4. Inflorescences umbellate; petioles up to 0.5 mm long; leaflets membranaceous............ M. skinneri

4. Inflorescences racemose to paniculate; petioles 4–12 mm long; leaflets coriaceous.

5. Leaflets (9-)11–15 (Colima, Mexico)........………..................................…....…….M. morenoi

5. Leaflets 21–29 (Oaxaca; Mexico).....………………………….................………M. conzattii