Related Species
Nepenthes dubia belongs to a group of closely related montane Sumatran species that includes N. flava, N. inermis, N. jacquelineae, N. jamban, N. talangensis, and N. tenuis. These species are characterised by infundibular upper pitchers and highly viscous pitcher fluid.
Nepenthes dubia is thought to be most closely related to N. inermis. It shares with this species the general morphology of its pitchers and leaves. It differs in having a well-developed peristome, more glands on the underside of the lid, and the lid being reflexed by more than 180 degrees. The lid of N. dubia is unique in this respect and is almost always reflexed beyond this angle, unless surrounding objects prevent it from assuming such a position.
In their description of N. tenuis, Joachim Nerz and Andreas Wistuba included a table of morphological characteristics that distinguish it from related species, including N. dubia:
| Character | N. bongso | N. dubia | N. talangensis | N. tenuis |
|---|---|---|---|---|
| Shape of upper pitchers | tubulate - infundibulate | tubulous in the lower part, infundibulate above the middle | tubulous to narrow infundibuliform in the lower half, ovate in the upper half | wide infundibulate, contracted below the mouth |
| Lid | orbiculate | narrow cuneate | broad-ovate | very narrow elliptical |
| Length/width ratio of upper pitchers | 3.3 | 1.9 | 2.3 | 1.75 |
In 2001, Charles Clarke performed a cladistic analysis of the Nepenthes species of Sumatra and Peninsular Malaysia using 70 morphological characteristics of each taxon. The following is part of the resultant cladogram, showing "Clade 1", which has 51% bootstrap support. Its most strongly supported subclade is the sister pair of N. inermis and N. dubia, having 95% support.
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