Research interests

My research interests broadly encompass the evolutionary development of flower form and function, plant systematics, taxonomy as well as biodiversity and conservation. My broad interests allow me to combine a variety of scientific approaches, such as DNA sequencing, field observations and surveys, and manipulative experiments, to attempt to explain the evolutionary history and ecology of various plant groups. I am interested in the taxonomy of the flowering plant family Gesneriaceae in the Neotropics (especially Ecuador, Peru, Bolivia, and Colombia) where I have worked in various biodiversity and floristics projects since 1994.

Since the time of Darwin naturalists and biologists used patterns of morphological variation among species to make inferences about relationships, and evolutionary processes such as natural selection and adaptive radiation. The visual comparison between flowers and their potential pollinators infers that their interaction may have resulted from coevolution.

Figure 1. Variation in flowers of Episcieae A. Glossoloma sp. nov. showing lateral view of resupinate flower; B. Glossoloma sp. nov. showing lateral view of non-resupinate flower; C. Crantzia cristata showing front view of resupinate flower; D. Drymonia sp. showing front view of non-resupinate flower; E. Pouched flower of Paradrymonia hypocyrta; F. Pouched flower of Neomortonia nummularia.

My research aims to understand floral diversification from a phylogenetic, ecological, and molecular context. To address these areas I combine field work, molecular sequencing, genetics, and collections-based research.

My own fascination with floral diversification resulted from four years of living in the tropics and attempting to understand the relationships, classification, and biology of common plants that either had no name, or no intuitive generic boundaries, and from a desire to understand the interaction of the animals that visited them. One of the discoveries that I made in graduate school is that flowers of some species of Gesneriaceae are inverted, or resupinate, in orientation compared to those of related taxa (Fig. 1A-D). Although inverted flowers have been mentioned prominently in the literature about orchids since Linnaeus (1780) and Darwin (1892), there re few phylogenetic based studies on the evolution of this peculiar floral feature.

That some gesneriads have inverted flowers compared to most of the other members of the family is important because it provides a morphological context for clades that had previously been proposed based only on molecular data.

Figure 2. A. Summary of strict consensus tree from analysis of nrITS+cp trnH-psbA+morphology for tribe Episcieae (Gesneriaceae); bootstrap values above branches. Arrows mark the hypothesized origin of resupination. From Clark et al., in press.

Discovery of inverted flowers in gesneriads has revealed other phenomena that invite further study, such as niche partitioning between sympatric species, floral adaptation to hummingbird pollination, morphological and anatomical mechanisms of floral inversions, and developmental genetics of corolla morphology. The switch to inverted flowers is apparently independently derived in three gesneriad lineages (Fig. 2) and I am pursuing projects that address the ecological implications, structure, and development of floral inversions. Furthermore, it is also surprising that inverted flowers are known in several flowering plant families (e.g., Heliconiaceae, Acanthaceae, Lobeliaceae, Polemoniaceae), yet flower orientation has not been adequately explored in most of these groups.

Another floral feature that is poorly understood is the presence of pouches or hypocyrtoid corollas (see Fig. 1E-F). Pouches are independently derived in nearly every major Gesneriaceae clade from Central and South America. I am interested in using manipulative experiments to test whether a pouch plays a role in attracting secondary pollinators by functioning as an overflow chamber. It wasn’t until recently (Clark et al., 2006) that adequate taxon sampling and a well-resolved species-level phylogeny made it possible to consider the multiple origins of pouched corollas and from this background I will be able to further pursue projects that address this peculiar floral feature.

Selected publications

Roalson, E. H. and J.L. Clark. 2006. Phylogenetic patterns of diversification in the Beslerieae (Gesneriaceae). In: A.K. Sharma and A. Sharma (eds.), Plant Genome: Biodiversity and Evolution, Phanerograms 1C. Science Publishers, Inc., Enfield, New Hampshire.

Clark, J.L., P.S. Herendeen, L.E. Skog, and E.A. Zimmer. 2006. Phylogenetic relationships and generic boundaries in the Episcieae (Gesneriaceae) inferred from nuclear, chloroplast, and morphological data. Taxon 55: 313-336.

Clark, J.L. 2005. A Monograph of Alloplectus (Gesneriaceae). Selbyana 25(2): 182-209.

Clark, J.L. 2005. The Andes. Pp. 69-71. In: G.A. Krupnick and W.J. Kress (eds.). Plant conservation: A natural history approach. Smithsonian Institution, Washington, DC

Clark, J.L., I. Salinas, and L.E. Skog. 2005. Novae Gesneriaceae Neotropicarum XIV: four new species of Alloplectus (Gesneriaceae) from South America. Novon 15: 70-79.

Kvist, L.P., L.E. Skog, J.L. Clark, and R.W. Dunn. 2004. Biological extinction in western Ecuador as exemplified by the plant family Gesneriaceae. Lyonia 6(2): 127-151.

Clark, J.L., and E.A. Zimmer. 2003. A preliminary phylogeny of Alloplectus (Gesneriaceae): implications for the evolution of flower resupination. Systematic Botany 28(2): 365-375.