This tribe has been problematic as to circumscription for a long time. This group of Rubiaceae is centered in the Neotropics, partiuclarly in the Caribbean region and Mexico, but also ranges widely through the Pacific. Chiococceae includes a relatively large portion of the Rubiaceae generic diversity of the Caribbean region. This group was long identified as systematically complex and has been treated taxonomically very differently by different authors for a long time. When molecular data became available, the complex systematic nature of this group became even clearer, when morphological characters were difficult to correlate with molecular phylograms. At that point, instead of being treated as a tribe or set of tribes these plants were separated as a complex, and its component taxa included both tribes and some genera that were unplaced as to tribe (Bremer et al., 1995; Delprete, 1996; Robbrecht & Manen, 2006). This group for a while took on the acroynym "CCE", for "Catesbaeeae-Chiococceae-Exostema", and usually included Strumpfia (e.g., as sister to the rest of the group; Robbrecht & Manen, 2006) but morphological differences were known that suggested Strumpfia might not be closely related to the rest of the CCE (Igersheim, 1993). The systematics and taxonomy of this group were perhaps more controversial when cladistic techniques and molecular data were introduced, including as to the appropriate methodologies for the analysis (e.g,, Ochoterena-Booth, 2000).
Both this tribe, or these tribes, and the component genera have been circumscribed very differently over time and among authors. Some of the genera in this group have variously been treated as a few genera with numerous species and varied characters, or as numerous morphologically distinct genera that each have only one or two species (e.g., Aiello, 1976; Borhidi, 2003). As elaborated more clearly now in Condamineeae, the Chicocceae complex has extensive variation in reproductive characters, including flowers that range from quite small to some of the largest in the genus; inflorescences of various positions, with one to hundreds of flowers; varied fruit forms, from fleshy drupes with two cylindrical seeds to dry capsules with dozens of flattened winged seeds; in vegetative characters, with at least one genus having raphide crystals; and habitats, from dry seasonal vegetation to wet forests found at sea level to the high Andes. These plants also range across a number of distinct phytogeographic regions. The problem in this group has been, as with Condamineeae, somewhat circular: in order to understand character evolution and find systematically informative features, some understanding of systematic groups is needed; but, finding those groups has been difficult because of the number of different characters and combinations of these in different plants and the incompletely conclusive molecular analyses.
The most well sampled recent studies of Chiococceae are the broad molecular analyses of Paudyal et al. (2014, 2018). They excluded Strumpfia from this tribe, , included Exostema and the related Solenandra within it, included the Catesbaeae wtihin in, and designated all the genera they treated as "Chiococceae sensu Paudyal et al. (2014)". They then (Paudyal et al., 2018) separated "Chiococceae sensu Paudyal" into four groups, three of them designated as sets of genera that were placed together on their molecular phylogram, and the fourth as their "Chiococceae s.str.".Their first group of genera that were not included in Chiococceae s. str. were Coutaportla and Lorencea, both from Mexico and formerly included in the "Portlandia" group of Chiococceeae; their second set of genera were Exostema, Solenandra, Coutarea, and Hintonia, all Neotropical; their third set of genera were Catesbaeeae and the remaining "Portlandia" group. The genera of Chiococceae s.str. were variously Neotropical and Pacific, most represented in their molecular analyses. As they noted, a number of the genera they studied were known to apparently comprise several lineages in need of further study (e.g., Exostema, McDowell et al. 2003; Coutarea, Taylor & Lorence, 2010; Chiococca, Jardim et al. 2015). Based on their molecular results, they separated these lineages as additional genera of Chiococceae and as, similarly to previous workers, comprising one or a few species. Also as with previous workers, they searched for morphological characters to diagnose their genera but were not always able to clearly find these, or separated their genera from each similar genus by different characters. Also several genera they studied had been previously suggested to not be distinct, and their results supported synonymizing them. Their analysis shed significant light on this group, but some of their generic circumscriptions seem inconsistent, for example the grouping of some species one a clade with one well separated into one genus (e.g., Solenandra, found in two different biogeographic regions) but the recognition of two genera in other similar cases (e.g., Coutareopsis and Motleyodendron, both found in one distinctive habitat in one region and separated by number of flowers).
More recently, Greuter & Rankin-Rodríguez (2021) reconsidered the systematics of this group again, based on Paudyal et al's (2018) analysis. They differed from Paudyal et al.'s conclusions in recognizing a broadly circumscribed genus that included Exostema, Solenandra, Hintonia, Paudyal et al.'s (2018) segregate genera, and Coutarea. Based on their view of this group as centered in Cuba and Exostema caribaeum as its presumed most well known species, Greuter & Rankin-Rodríguez (2021) also proposed conservation of the name Exostema over the oldest applicable name, Coutarea, for this entire group. However, Coutarea hexandra is much more widely distributed naturally and in cultivation, and this genus could be argued as a better name for this entire group. If Exostema is eventually conserved against Coutarea, it will only have priority when these two genera are combined. The genus taxonomy of Paudyal et al. (2018) is here followed for now, because the generic circumscriptions there are more comparable to, and so congruent with, the general consensus taxonomy of Neotropical Rubiaceae. The broad generic circumscription proposed by Greuter & Rankin-Rodríguez which encompasses a large number of species with a markedly widely varied morphology, which are found in a wide range of biogeographic regions and habitats. Greuter & Rankin-Rodríguez recognized several of the clades detailed by Paudyal et al. as sections of their large genus, but did not recognize other lineages that Paudyal et al. found to be distinct. Greuter & Rankin-Rodríguez (2021) published nomenclatural transfers to synonymize the previously recognized genera, but some of these are based on the conservation of Exostema that has not yet been recommended by the Nomenclature Committee.
Siemensia was described in the protologue and by Aiello (1979) as having raphide bundles in the leaf tissues, which are evident in the dried leaves of many specimens though these are not evident in the flowers also as in other raphide-bearing taxa. These crystals or crystal groups also differ from raphide bundles in other Rubiaceae genera in their apparently subglobose shape. Aiello noted that the presence of raphides is anomalous in Cinchonoideae and the Portlandia group of genera, and suggested that based on these crystals and the numerous tiny seeds of Siemensia, it may be better classified in Hedyotideae in the Rubioideae. However, Paudyal et al. (2018) found Siemensia deeply nested in their Chiococceae. Paudyal et al. did not mention or discuss the reports of raphides in this genus, and inaccurately reported that Aiello had suggested its relationships were with Portlandia.
And more recently, Torres-Montúfar et al. (2022) began a new review of Chiococceae in Mexico with detailed morphological, anatomical, taxonomic, and field studies. Their project is compiling information from ample new materials collected in the last decades, and re-examinig characters used by Aiello and Lorence and adding new ones to the analysis. As part of this study they re-evaluated the separation between Lorencea and Coutaporta, and concluded by synonymizing these (Torres-Montúfar et al., 2023). They have also begun extensive field surveys of this group in northern Mexico, where it seems to be diverse but has been poorly documented; their work has the potential to change what seem to be the centers of diversity for some of these genera, and their ecological characterizations.
Author: C.M. Taylor.
The content of this web page was last revised on 29 March 2023.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml