Patterns of character evolution of potentially taxonomically important characters are shown in Figures 3–9. The morphological comparison of monophyletic units in Sabicea s.l. allows to recognize them by a combination of characters (Table 3) and delimit four subgenera within Sabicea. Further details are given in the discussion.
Table 3. Table comparing morphological characters in the subgenera of Sabicea s.l. (sensu this paper)
Characters Sabicea subg. Anisophyllae Sabicea subg. Sabicea Sabicea subg.
Stipularia
Sabicea subg.
Stipulariopsis
Growth habit creeping or sarmentose usually twining, sometimes creeping or sarmentose
Stipule type entire or multifid entire or multifid entire entire
Bracts free usually small, not
Inflorescence unbranched, sessile branched or unbranched, sessile or pedunculate
unbranched, sessile unbranched and sessile
Ovary 2–locular 2–5 locular 3–5 locular 2–locular
Distribution Central and East Africa Pantropical West and Central
Africa
Central Africa (Lower Guinea Domain)
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DISCUSSION
HEKISTOCARPA-TAMRIDAEA-VIRECTARIA CLADE
In our analysis, Hekistocarpa, Tamridaea, and Virectaria form a clade moderately supported, sister to Sabicea s.l. (Fig. 2). This is in agreement with Dessein et al. (2001a) and Bremer &
Eriksson (2009), but in conflict with Khan et al. (2008a, b), who retrieved Hekistocarpa as sister to all other Sabiceeae genera, and Kainulainen et al. (2013) who retrieved Tamridaea and Virectaria as sister to the remaining Sabiceeae. The position of Hekistocarpa seems to be ambiguous, pending for further study. In our study, the monophyly of the Hekistocarpa-Tamridaea-Virectaria clade is entirely based on the rps16 sequences and it should be confirmed by additional molecular data. Morphologically, this clade differs from Sabicea s.l.
by the dry fruits. Fruit characters, among others (see key to the genera), can also be used to separate the three genera: fruits indehiscent or tardily splitting into two mericarps in Hekistocarpa, capsules with two persistent valves in Tamridaea and capsules with one caducous valve and the other remaining attached to the pedicel in Virectaria (Dessein et al., 2001a, b). Within this clade, the African genus Virectaria is fully supported as sister to Tamridaea, a genus endemic to Socotra, which is in agreement with previous molecular studies. The two genera are characterized by terminal inflorescences, a character not found in other members of Sabiceeae.
SYSTEMATIC VALUE OF INDIVIDUAL MORPHOLOGICAL CHARACTERS
The results from our study of morphological character evolution demonstrate that many characters used to hypothesize relationships within Sabicea s.l. are labile and do not delineate the genera as proposed by earlier studies. Several characters were judged as uninformative for generic and subgeneric delimitation after preliminary morphological examination, because they exhibited either very little information (e.g, pollen morphology, size and shape of the disk) or considerable intraspecific variation (external indumentum of vegetative parts, dimensions of leaves and stipules). These were excluded from further analysis.
Our Sabiceeae molecular phylogeny provides an independent hypothesis to evaluate taxonomically important morphological characters such as growth form (Fig. 3), anisophylly (Fig. 4), stipule type (Fig. 5), bracts (Fig. 6), inflorescence type (Fig. 7), corolla-throat indumentum (Fig. 8), and the number of ovary locules (Fig. 9) (see Appendices 1 and 2).
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Growth form (Fig. 3)—Sabicea s.l. shows a remarkable range of variation in the growth habit.
In the present study, we summarized the habits in four major groups: Erect shrub or herb (0);
creeping (1); twining (2), i.e. with stems climbing in spiral around a support; and sarmentose (3), i.e. more or less lianescent but not climbing in spiral. In the creeping category we only included the small species (< 30 cm tall) with the stems appressed to ground or shortly ascending at the apex. There are some ambiguous cases where the stems are creeping at base and twining at apex (S. pilosa, S. bullata) or creeping at base and sarmentose at apex (S. sp.-SD2149); these were considered as twining or sarmentose, respectively. The basal lineages within Sabiceae s.l. are all erect (Hekistocarpa, Tamridaea, Virectaria, and Sabicea subg.
Stipularia), so the erect habit is presumably ancestral in the tribe. Most species of subg.
Sabicea are twining, and this habit presumably evolved a single time. The sarmentose habit occurs in subgenus Anisophyllae and in a few species of subgenus Sabicea (the Malagasy S.
acuminata and S. diversifolia and the former Pseudosabicea species), presumably as a result of two independent evolutions. The creeping habit also evolved several times independently in subg. Anisophyllae and subg. Sabicea, either derived from twining or sarmentose.
17 Figure 3. Evolution of growth form in Sabiceeae.
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Anisophylly (Fig. 4)—Some species of Sabicea s.l. are anisophyllous, and Hallé (1963) used this character to diagnose Pseudosabicea sect. Anisophyllae. There is a good range of variation in the level of anisophylly: leaves slightly unequal with one about twice larger than the other (S. tersifolia), strongly unequal with one normally developed and the other vestigial (e.g. S. aurifodinae), or, at the extreme, a single leaf per node (S. batesii, S. sthenula). Our study shows that unequal leaves appeared two or three times in Sabiceeae: once or possibly twice in subg. Sabicea (S. sp.-OL1446 from Gabon, S. acuminata and S. diversifolia from Madagascar) and once in subg. Anisophyllae. Sabicea becquetii with equal or slightly unequal leaves is sister to the remainder of the Anisophyllae clade. This character, being homoplasic, cannot be used alone to delimit subg. Anisophyllae.
19 Figure 4. Evolution of anisophylly in Sabiceeae.
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Stipule type (Fig. 5)—Stipules are often useful in distinguishing Rubiaceae genera (Gentry, 1993). The stipules in Sabiceeae are interpetiolar and persistent; in Sabicea subg.
Anisophyllae they are often connate with the base of the petioles. They vary in shape from entire or shortly bidentate at apex, to multifid (divided in many segments). Multifid stipules are found in some (but not all) species of Sabicea subg. Anisophyllae, but also in the Malagasy taxa S. acuminata and S. diversifolia. Consequently, this character has to be considered homoplasic.
21 Figure 5. Evolution of stipule type in Sabiceeae.
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Bracts (Fig. 6)—The genus Stipularia has been separated from Sabicea (e.g. Hallé, 1966) based on its peculiar urn-shaped involucre surrounding the inflorescence. This involucre consists of 2–4 bracts that are fused for most of their length, and either split in fruit (S.
elliptica) or remain intact (S. africana). However, as pointed out by Hepper (1958), the bracts of Sabicea s.str. show considerable variation: they can be free and inconspicuous (e.g. S.
exellii), free and conspicuously surrounding the inflorescence (e.g. S. capitellata) or shortly fused at the base (S. dewevrei, S. urniformis; in the latter species the bracts split in fruit); an exception is S. urceolata that has bracts fused for most of their length and splitting in fruit, thus much resembling the involucre of Stipularia. This character, therefore, is also homoplasic. In subg. Anisophyllae bracts are free, usually small and inconspicuous (in Sabicea sp.-SD2149, they are larger and enclose the flowers when young, but not the whole inflorescence). In subg. Stipulariopsis, bracts are free and very small, except in S.
gigantostipula, which has 2 large free bracts surrounding the inflorescence.
23 Figure 6. Evolution of the bracts in Sabiceeae.
Inflorescence type (Fig. 7)—In the present study, we summarized inflorescences of Sabiceeae in three major types: (1) branched; (2) unbranched and sessile; (3) unbranched and pedunculate. Our study shows that all inflorescence types occur in subg. Sabicea. The
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inflorescences of subg. Stipularia are always unbranched, but vary from sessile to pedunculate. In subg. Stipulariopsis and Anisophyllae, the inflorescences are always unbranched and sessile (with fruits pedicellate in the former, and usually sessile in the latter).
Unbranched and sessile inflorescences are also found in some species of subg. Sabicea (e.g. S.
bullata, S. diversifolia, S. speciosa), hence, this character is homoplasic as well.
Figure 7. Evolution of inflorescence type in Sabiceeae.
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Indumentum of corolla-throat (Fig. 8)—An important character that has been overlooked in the discussion of Sabiceeae is the nature of the internal indumentum. Verdcourt (1958) examined the trichomes of many Rubiaceae genera and distinguished two groups: the internal indumentum (inside the corolla tube) and the external indumentum (other plant organs).
While Verdcourt (1958) considered the trichome structure of external indumentum as a useful secondary character, the systematic importance of the inner indumentum has not been extensively discussed in the literature. The inner indumentum mostly consists of unicellular thin-walled trichomes, which are flat and ribbon-like or sometimes moniliform (Robbrecht, 1988). In Sabicea, the internal indumentum consists of thin white multicellular trichomes of variable length. Of particular systematic value are the trichomes occuring around the corolla throat, which are short (c. 0.5 mm) and sparse in the subgenera Stipulariopsis and Stipularia, and in most species of subg. Sabicea. In subg. Anisophyllae (except for S. becquetii) these trichomes are long (1‒2 mm) and form a dense beard around the throat, sometimes extending to the basal part of the lobes. However, the two Madagascan species (S. acuminata and S.
diversifolia) also display a long and dense corolla throat indumentum, so this character appears to be homoplasic.
26 Figure 8. Evolution of indumentum corolla-throat in Sabiceeae.
Number of ovary locules (Fig. 9)—Hallé (1963) considered the number of ovary locules as an useful character to separate Ecpoma and Pseudosabicea (2-locular) from Sabicea (5-locular), while Wernham (1914) judged this character as quantitative, and therefore of little taxonomic significance. In subg. Sabicea, most species are 5-locular, however, the former Pseudosabicea
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species nested in Sabicea and also one new species from Gabon (Sabicea sp.-OL1446) are bilocular, and so are the members of subg. Stipulariopsis and subg. Anisophyllae. Therefore, this character seems to be homoplasic.
Figure 9. Evolution of ovary in Sabiceeae.
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NEW SUBGENERIC CLASSIFICATION FOR SABICEA S.L.
The synonymization of Ecpoma, Pseudosabicea and Stipularia under Sabicea as proposed by Khan et al. (2008a) is accepted in the present study. However, Khan et al. (2008a) did not propose a new subgeneric classification for Sabicea s.l., indicating that a better sampling was needed for taxa previously placed in Ecpoma, Pseudosabicea, and Stipularia. With the sampling currently presented, several moderately to well-supported clades can be identified within Sabicea s.l., allowing us to propose a new subgeneric classification (see key to the subgenera). We choosed to use subgenera instead of sections as four well supported lineages were found within Sabicea s.l..Moreover, although, additional studies are needed to delimitate monophyletic groups within Sabicea sensu Hallé, some previous sections could be clearly found within this latter (sections. Floribundae, Laxae, Sphaericae).
The first clade branching off consists of S. africana and S. elliptica and is highly supported.
This clade is characterised by the erect habit, and the peculiar urceolate involucre surrounding the inflorescences. On account of the latter character, both species were previously referred to the separate genus Stipularia (Palisot-Beauvois 1807; Hallé 1963, 1966). Since the clade is sister to the rest of Sabicea s.l., the generic status of Stipularia could be retained. However, since none of its diagnostic characters is synapomorphic (the erect habit also occurs in subg.
Stipulariopsis, and the urceolate involucre in S. urceolata), we agree with Hepper (1958) and Khan et al. (2008a) to include Stipularia in Sabicea.
A second well-supported lineage within Sabicea s.l. corresponds to Pseudosabicea sect.
Anisophyllae (Hallé, 1963). This group is well supported and comprises six described species and one variety (Table 4), as well as two still undescribed taxa (S. sp.-SD1865, S. sp.-SD2149). Hallé (1963) characterized section Anisophyllae by anisophyllous nodes combined with sessile inflorescences. However, both characters are also found in the two Madagascan species of subg. Sabicea, which differ by their 5-locular ovaries. The present study shows that this clade lacks a unique synapomorphy and that it can only be circumscribed by a combination of characters, i.e. bilocular ovaries, and inflorescences constantly unbranched and sessile. Species of this group also usually show strong anisophylly and long trichomes in the corolla-throat. However, Sabicea becquetii (and S. arborea, not sequenced here and morphologically similar) are aberrant in having leaves equal or only slightly unequal and a corolla throat with short trichomes. A case could be made for placing them in a separate
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subgenus (especially if we consider the basal position of S. becquetii in the group), but due to lack of sequence data for S. arborea, we refrain from doing so at this moment (Table 4).
Table 4. Subgeneric composition and name changes in Sabicea. An asterisk (*) denotes species included in the present study. Type species are marked in bold.
Pseudosabicea sect. Anisophyllae N.Hallé Sabicea subg. Anisophyllae (N.Hallé) Zemagho, Sonké, Dessein
& Liede Pseudosabicea arborea (K.Schum.) N.Hallé
Pseudosabicea aurifodinae* N.Hallé Pseudosabicea batesii* (Wernham) N.Hallé Pseudosabicea becquetii* N.Hallé
Pseudosabicea medusula* (K.Schum. ex Wernham) N.Hallé
Pseudosabicea mildbraedii* (Wernham) N.Hallé Pseudosabicea sthenula* N. Hallé
Sabicea arborea K.Schum.
Sabicea aurifodinae (N.Hallé) Razafim., B.Bremer, Liede & Saleh A.Khan
Sabicea batesii Wernham
Sabicea becquetii (N.Hallé) Razafim., B.Bremer, Liede & Saleh A.Khan
Sabicea medusula (K.Schum) Wernham Sabicea mildbraedii Wernham
Sabicea sthenula (N.Hallé) Razafim., B.Bremer, Liede & Saleh A.Khan
Ecpoma K.Schum. Sabicea subg. Stipulariopsis Wernham
Ecpoma apocynaceum K. Schum.
Ecpoma cauliflorum (Hiern) N.Hallé Ecpoma geanthum (Hiern) N.Hallé
Ecpoma gigantostipulum* (K. Schum.) N.Hallé Ecpoma hiernianum* (Wernham) N.Hallé &
F.Hallé
Sabicea apocynacea (K.Schum.) Razafim., B.Bremer, Liede &
Saleh A.Khan
Stipularia P. Beauv. Sabicea subg. Stipularia (P.Beauv.) Zemagho, Sonké, Dessein &
Liede Stipularia africana* P.Beauv.
Stipularia elliptica* Schweinf. ex Hiern Stipularia mollis Wernham
Sabicea africana (P. Beauv.) Hepper
Sabicea elliptica (Schweinf. ex Hiern) Hepper Sabicea lanata Hepper
Sabicea Aublet Sabicea subg. Sabicea
43 spp. New World 43 spp. New World
ca. 75 spp. Old World ca. 75 spp. Old World
Schizostigma Arn. ex Meisn.
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Schizostigma hirsutum* Arn. ex Meisn. Sabicea ceylanica Puff Pseudosabicea N. Hallé sect. Sphaericae N.Hallé
Pseudosabicea nobilis* (R.D.Good) N.Hallé Sabicea nobilis R.D. Good Pseudosabicea N. Hallé sect. Floribundae
Wernh. ex. N.Hallé
Pseudosabicea floribunda* (K. Schum.) N.Hallé Pseudosabicea proselyta* N.Hallé
Pseudosabicea sanguinosa N.Hallé Pseudosabicea segregata* (Hiern) N.Hallé
Sabicea floribunda K.Schum.
Sabicea proselyta (N. Hallé) Razafim., B. Bremer, Liede & Saleh A.Khan
Sabicea sanguinosa (N.Hallé) Razafim., B.Bremer, Liede & Saleh A.Khan
Sabicea segregata Hiern
A third moderately well-supported lineage within Sabicea s.l. groups the former Ecpoma species (E. gigantostipulum and E. hiernianum) together with Sabicea urbaniana and S. xanthotricha. The latter two species were not included in Ecpoma by Hallé (1963) presumably because he had not seen the material. This clade is mophologically homogeneous and can be characterized by the erect habit, bilocular ovaries, and unbranched sessile inflorescences with free (and usually small) bracts. It could be retained as a genus, but this would render Sabicea paraphyletic, unless subg. Anisophyllae is also elevated to generic level.
This group largely corresponds with Wernham’s (1914) subgenus Stipulariopsis, so this subgenus is here reinstated. Two aberrant species placed in this subgenus by Wernham have already been excluded from Sabicea: S. bicarpellata K.Schum. is a Bertiera (Hallé, 1970), while the Neotropical S. umbrosa Wernh. is now placed in Amphidasya (Standley, 1936).
Following Art. 11 of the International Code of Botanical Nomenclature, the name Stipulariopsis has priority over Ecpoma at the infrageneric rank, because the latter was described as a genus.
The fourth clade comprises all species of Sabicea s.str., plus Schizostigma, Pseudosabicea sect. Floribundae, and Sabicea nobilis (the type species of the genus Pseudosabicea and of sect. Sphaericae). Despite the generally low resolution, the three latter groups are deeply and unambiguously nested in this clade (Table 4). Therefore, we propose to recognize this large group as Sabicea subg. Sabicea. This name is choosed instead of subg.
Eusabicea to respect the rule of priority of the International Code of Botanical Nomenclature.
It comprises approximately 75 species in the Old World and 43 species in the New World (Khan, 2007). With the deeper nodes unresolved or only weakly supported, the relationships
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within this subgenus remain unclear and will be subject of further phylogenetic studies including more taxa and/or characters.