Revision of Spathacanthus
Phylogenetic Analysis
Thomas F. Daniel
Department of Botany
California Academy of Sciences
Golden Gate Park
San Francisco, CA 94118
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Revision of Spathacanthus Thomas F. Daniel |
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Phylogenetic Analysis Generic Description & Key to Species Acknowledgments & Literature Cited
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A cladistic analysis of Spathacanthus using PAUP 3.1.1 (Swofford 1993) was conducted to reveal possible phylogenetic relationships of the species. Chalarothyrsus was designated as the outgroup and ten characters were used (Table 1), four of which (2--5) were potentially informative. A branch-and-bound search was performed using equal weighting and unordered character states. Both ACCTRAN (accelerated transformation) and DELTRAN (delayed transformation) optimization methods were utilized. A discussion of each character, which includes information from other Odontoneminae (if known), follows: 1. Calyx color: 0 = green; 1 = yellow. Colored calyces are rare among Acanthaceae in general and among Odontoneminae in particular. The bright yellow calyx exhibited by S. parviflorus when the corolla is mature is highly unusual and probably attracts floral visitors to the relatively small flowers of this species. 2. Calyx division: 0 = spathaceous and with major divisions equal; 1 = spathaceous and with major divisions unequal; 2 = not spathaceous and equally 5-parted. The feature from which the genus takes its name is a calyx that is fused proximally and comprises two large, spathelike segments distally. This type of calyx is unique to Spathacanthus among Odontoneminae. Whether the divisions of the major segments are equal or unequal varies in the genus. The pentamerous and deeply divided calyx of Chalarothyrsus is typical of other Odontoneminae. This multistate character could be treated as two binary characters: calyx lobes fused into two major segments vs. calyx lobes not fused into two major segments, and major calyx segments equally divided vs. major calyx segments unequally divided. The latter character would apply solely to the ingroup. 3. Corolla length: 0 = short (15--29 mm long); 1 = long (46--97 mm long). Corollas of both S. hahnianus and S. hoffmannii are exceptionally large among Odontoneminae. 4. Corolla color: 0 = white; 1 = yellow; 2 = red. These and other colors are known for corollas among Odontoneminae. Because this character is likely very labile and because corolla color of the outgroup is not represented among species of Spathacanthus, it is not possible to determine directionality of the states represented a priori. 5. Length of lobes of lower lip: 0 = short (1--6 mm long); 1 = long (8--20 mm long). It is possible that this character is linked to corolla length (character 3). However, using the measurement ranges noted herein, there are taxa that have short corollas with long lobes (e.g., Mexacanthus mcvaughii T.F. Daniel) and long corollas with short lobes (e.g., Dicliptera anomala Leonard) elsewhere in the Odontoneminae. 6. Number of apertures in pollen: 0 = 3; 1 = 3--4. Triaperturate pollen is widespread among Odontoneminae. See discussion above concerning pollen of Spathacanthus. 7. Polar diameter of pollen: 0 = 54--67 µm; 1 = 39--51 µm. In Chalarothyrsus the polar diameter of pollen varies from 54--59 µm and overlaps measurements of this diameter in S. hahnianus. Although large pollen might be correlated with large corollas within Spathacanthus, this is obviously not the case in the outgroup where the corollas are relatively small and the pollen is relatively large. 8. Exine surface: 0 = reticulate; 1 = foveolate to verrucate to rugulate. Exine of interapertural surfaces in Chalarothyrsus and most other Odontoneminae is conspicuously reticulate. In species of Spathacanthus, the exine varies from foveolate to fossulate to verrucate to rugulate. 9. Capsule length: 0 = short (4--30 mm long); 1 = long (40--90 mm long). Capsules of Spathacanthus are the longest known among American Odontoneminae (which otherwise vary in length from 4 to 27 mm). Capsules of Chalarothyrsus vary from 12--18 mm in length. 10. Seed fusion: 0 = seeds not fused to capsule valves; 1 = seeds fused to capsule valves. Fusion of the seed coat to the inner wall of the capsule, as observed in Chalarothyrsus, has been reported previously in the family only in the unispecific Mexican genus Aphanosperma (Daniel 1988). This latter genus is also treated in the Odontoneminae, but it has pseudopapilionaceous corollas and an androecium of two stamens. Therefore, it does not appear to be particularly closely related to Chalarothyrsus. The phylogenetic analysis resulted in a single most parsimonious tree (Fig. 2) of 12 steps (CI = 1.0, HI = 0, RI = 1.0, RC = 1.0). No differences in character state changes resulted from the ACCTRAN vs. DELTRAN optimization. Even if one of the possibly linked characters noted above (i.e., 3 and 5) is deleted from the analysis, a tree with the same topology (but with one less step) is obtained. Dividing character 2 into two binary characters, as noted above, also results in a single tree with the same topology. On the basis of character states in other Odontoneminae, most characters at the base of the tree in Fig. 2 can be unambiguously mapped onto one of the clades. Given their absence elsewhere in the subtribe, the exceptionally long capsules (9), the two-parted and spathelike calyces (2), and the non-reticulate exine of pollen grains (8) likely are synapomorphic for Spathacanthus. Permanent retention of seeds in the capsule valves (10) would appear to represent a derived condition in Chalarothyrsus. As noted above, corolla color cannot be polarized on the basis of character states occurring in other Odontoneminae. The change to red corollas in Chalarothyrsus (as mapped) could with equal likelihood have been shown as a change to white corollas in Spathacanthus. Within Spathacanthus, S. hahnianus and S. hoffmannii comprise a clade defined by long corollas with long lobes on the lower lip. Spathacanthus parviflorus, which has white corollas like S. hoffmannii but spathaceous calyces with unequal segments like S. hahnianus, is sister to this clade.
TABLE 1. Data matrix for species of Spathacanthus
and the outgroup Chalarothyrsus.
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