The Muchhala Lab
University of Missouri - St. Louis
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Does biodiversity beget biodiversity?  Alfred Wallace suggested that high tropical diversity is due to the “complex influences of organism upon organism”: an increased number of biotic interactions increases the number of specialized niches available, accelerating adaptation and speciation in a positive feedback loop.  The Muchhala Lab conducts research in evolutionary ecology addressing the role of such interactions, especially mutualism and interspecific competition, in structuring communities and driving diversification.  We focus on plants pollinated by bats and hummingbirds, and integrate various approaches including molecular phylogenetics, mathematical modeling, and field experiments. Three specific questions we are particularly interested in include:

     1) What selective pressures favor specialization in pollination systems?  

     2) What are the evolutionary and ecological consequences of competition for pollination?

     3) How do pollinators influence plant speciation?  

Specialization in Pollination Systems

     A common theme in angiosperm diversification involves repeated evolution of suites of traits, termed pollination syndromes, that adapt a flower to a certain type of pollinator (see Lagomarsino et al. 2017, Clark et al. 2015).  But why specialize to a given pollinator, when generalizing ought to allow many more opportunities for successful pollination per unit time?  Work with the genus Burmeistera (Campanulaceae) showed that most species are specialized to either bats or hummingbirds (Muchhala 2006a), and experiments demonstrated that an adaptive tradeoff in floral form (wide flowers fit bat heads better while narrow flowers fit hummingbird bills better) is strong enough that generalist flowers are suboptimal (Muchhala 2007).  We are continuing to explore the traits that adapt flowers to bat pollination, such as increased pollen production (e.g. Muchhala et al. 2010), and olfactory, visual, and echo-reflectance cues.

Burmeistera borjensis
flower with
Anoura geoffroyi (A) and B. rubrosepala with Adelomyia melanogenys (B)
Anoura fistulata w/ tubeAnoura fistulata w/ Centropogon nigricans

Anoura fistulata
drinking honey-water from a glass tube (top) and pollinating Centropogon nigricans (bottom)

Specialization in pollination systems (continued)

     The majority of flowers are pollinated by more than one species, and pollinators typically visit many species of flowers, thus pollination syndromes are examples of either one-sided evolution (plants evolving to pollinators) or, at best, diffuse coevolution.  True reciprocal coevolution can only occur in highly specialized interactions.  One example of this is the nectar bat Anoura fistulata (Muchhala et al. 2005), which can launch its tongue 1.5 times its body length (double that of other bats and longer than any other mammal). Unique adaptations allow it to store its tongue in its rib cage (Muchhala 2006b).  Experiments suggest that this pair is involved in a coevolutionary race with the 9-cm-long flowers of Centropogon nigricans; longer tongues allow bats to reach more nectar, while longer flowers maximize pollen transfer (Muchhala & Thomson 2009).  We plan to continue to study the geographic mosaic of the interaction between this bat and the flowers it pollinates throughout its range, as well as the ‘trigger’ behind the coevolutionary race – that is, why exactly do long flowers receive more pollen?  

Competition for pollination

     A given species of plant occurs within a community of many other plants.  How does an individual ensure that it will receive conspecific pollen and that its pollen reaches conspecifics?  The costs of competition may include loss of pollen to foreign flowers or blockage of stigmas with foreign pollen (Muchhala & Thomson 2012). 
Hummingbird-pollinated Iochroma demonstrate one solution to this problem: co-occurring species evolve differences in flower color to encourage hummingbirds to stay faithful to one species rather than switching between species in their foraging bouts (Muchhala et al. 2014). Bat-pollinated Burmeistera demonstrate a another solution: co-occurring species have evolved different lengths of the staminal column, thus placing pollen on different regions of bat’s heads (Muchhala & Potts, 2007).  Ongoing work in the lab is exploring whether pollen transfer between Burmeistera species results in hybridization, and thus introgression, and is further detailing the fitness costs for male and female reproductive functions.

Floral character displacement in pollen placement:  pollen transfer experiments
(top) and bat with pollen from by two different Burmeistera species (bottom)

Preliminary phylogenetic hypothesis for Burmeistera (top) and a sampling of floral diversity across the genus
Plant speciation

     What evolutionary innovations allowed the remarkable and rapid radiation of flowering plants?  One long-standing paradigm holds that coevolution with pollinators is the key (reviewed in Armbruster & Muchhala 2009), because speciation rates are directly amplified via pollinator-mediated reproductive isolation.  Post-pollination processes have received much less attention.  To begin to understand the relative importance of different isolating barriers for angiosperm speciation, they must be compared among multiple sister-species pairs of various ages; in this way we can distinguish those that directly contributed to speciation from those that only arose after speciation was complete.  We are currently developing molecular phylogenies for Burmeistera (e.g. Uribe-Convers et al. 2017) and related genera (Lagomarsino et al. 2017), and will use these in combination with cross-pollination experiments to explore the importance of various pre- and post-zygotic reproductive barriers in the diversification of these clades.

Video of Anoura fistulata visiting Centropogon nigricans ( National Geographic Channel, 2012)

Comparison of the tongue morphology of
A. fistulata and a typical glossophagine nectar bat