Introduction

The Guiana Shield, located in northeastern South America, consists of a granite and igneous base that was later covered with a layer of sandstone called the Roraima Formation (Figure 1). Over time, areas of the Shield have been periodically uplifted by tectonic events and eroded by water and wind to form a scattered group of "tepui", flat-topped mountains with sheer cliffs rising out of lower-elevation forests (Figure 2). Prominent in the vegetation of the tepui are the Mutisieae, a tribe in the flowering plant family Compositae. The tribe is non-monophyletic and has been divided into 10 clades based on information from DNA sequencing, however, the sister group relationships among these clades is not resolved. Traditionally, the tepui taxa have been placed in two groups based on flower morphology; the Gongylolepis clade and the Stenopadus clade.

Research Questions

(1) Does the current distribution of members of the Mutisieae reflect the geologic history of the Guiana Shield?

(2) Is the distribution reflected in the phylogeny of the tribe?

Materials and Methods

A database was assembled using information from 874 Mutisieae specimens. These specimens were geo-referenced and climate and elevation layers were taken from WorldClim.com. Maps were generated using ArcGIS (Esri 2005). A branching diagram of the tepui was generated based on distances among the tepui and the size and number of rivers separating the tepui. Table 1 was constructed using the GIS maps and published information to determine the range of each taxon. The distribution pattern of each taxon was compared to the cladogram of the tribe to determine whether it reflected the phylogenetic divisions.

Results

The branching diagram of the tepui splits the high-elevation points into two major groups: eastern and western. The distribution maps show that most of the taxa reflect this grouping.

Genus Level

Seven of the thirteen genera display either an eastern or a western pattern, three have species that span both groups, and three have widespread lowland distributions. All species in the genera Achnopogon, Chimantaea, and Quelchia are only found on eastern tepui whereas all Duidaea, Neblinaea, Glossarion, and Eurydochus are only found on western tepui. Chaptalia, Gochnatia, Lycoseris, and Stifftia each have one or two species in the Shield area but they are lowland taxa that are part of clades centered in Brazil or the northern Andes and were not considered further.

Species Level

Nine species of Gongylolepis have high elevation western tepui distributions but two species, G. fruticosa Maguire, Steyerm., & Wurdack and G. benthamiana R.H. Shomb., are endemic to the eastern tepui and another species, G. martiana (Baker) Steyerm. & Cuatrec. has a western lowland distribution. Stomatochaeta has five species, two only in the west and three only in the east. Stenopadus has eight species, four endemic to the east and four to the west. Additionally, Stenopadus talaumifolius S.F. Blake, was distributed on most of the major tepui groups and Stenopadus campestris Maguire & Wurdack, has a western lowland distribution. At the species level, all but one species are restricted to either the eastern or the western tepui.

Conclusions

Based on the molecular data Chimantaea, Quelchia, Stenopadus, and Stomatochaeta belong to the Stenopadus group, and Achnopogon, Duidaea, Eurydochus, Glossarion, Gongylolepis, and Neblinaea are placed in the Gongylolepis group. The Stenopadus and Gongylolepis groups are no longer considered sister groups. Thus, the basic east-west split between the distributions of these genera reflects a similar split in the phylogeny of the Mutisieae. Several possibilities could account for exceptions to the pattern. Achnopogon, the only eastern-endemic genus in the Gongylolepis group, is considered most closely related to Gongylolepis, which is the only other genus in the group that contains eastern-endemic species. Gongylolepis may have been widespread before erosion separated the tepui or perhaps the eastern-endemics originated from a dispersal event and Achnopogon later branched off from these taxa.

The six taxa endemic to individual tepui suggest speciation occurred after the plateau was eroded. The two species of Gongylolepis and one species of Stenopadus that have widespread, lowland distributions may have originated through dispersal from tepui to the lowlands. Taxonomic re-groupings may in part explain the widespread distributions of some genera. Stenopadus talaumifolius - the only widespread, high-elevation group in any of the genera - contains what used to be six distinct species, suggesting that this species needs to be studied on the population level.

Future Research

Future studies will involve the large species-rich genera and the possibility that the species limits may need to be revised. This study has helped to pinpoint which taxa need to be studied. Climate patterns and dispersal events may also explain the variations to the basic eastern or western distribution patterns and need further study.

Acknowledgements

We would like to thank Sara Alexander for her help databasing specimens, Dan Cole for lending his expertise on ArcGIS, Tom Hollowell for extensive technical support with regard to ArcGIS and the Microsoft Access database, and Carol Kelloff for help with georeferencing and all-around support throughout the completion of this project.