Phenotypic plasticity in modular organisms

Chief Investigators: Alistair Poore, Stephen Bonser 
Organisms whose growth involves repeated addition of component parts (e.g., plants, algae, colonial invertebrates) differ fundamentally from unitary organisms with a predetermined body plan (e.g., mammals, insects). A modular growth form allows great variation in final morphology, with strong environmental influences on how organisms allocate resources to growth. At UNSW, we use terrestrial plants and their functional equivalents in marine environments (macroalgae) to understand the evolution of growth form, and how selection may promote plasticity in growth form.

 
The evolution of plasticity in macroalgae

Macroalgae commonly display high levels of phenotypic plasticity, with morphology being dependent on light, nutrient levels, or the degree of wave action. Using a fast growing red alga, Asparagopsis armata, we have tested how morphology varies with environmental conditions (colour and intensity of light), and how selection shapes the level of observed plasticity in morphology. Like terrestrial plants, Asparagopsis produces “phalanx” phenotypes, in which modules are close together, in high resource environments, and “guerrilla” phenotypes, in which modules are widely spaced, in low resource environments (right). These foraging stategies are under both genetic and environmental control, offering opportunities to further understand the evolution of plasticity.

The red alga Asparagopsis armata grown in low (left) and high light (right) environments.

 

Further reading

Monro, K and AGB Poore. 2004. Selection in modular organisms: is intraclonal variation in macroalgae evolutionarily important? The American Naturalist 163: 564-578.

Monro, K and AGB Poore. 2005. Light quantity and quality induce shade-avoiding plasticity in a marine macroalga. Journal of Evolutionary Biology 18: 426-435.

Monro, K, AGB Poore and R Brooks, in press. Multivariate selection shapes environment-dependent variation in the clonal morphology of a red seaweed. Evolutionary Ecology.

 
Growth form plasticity in plants

Different patterns of plant form and function are fundamentally a product of patterns of meristem (bud) allocation (right). Meristem allocation to vegetative and reproductive fates determines the schedule of growth and reproduction, respectively. We have conducted studies demonstrating the functional significance of plasticity in meristem allocation in herbaceous plants, and how patterns of meristem allocation are associated with growth form strategies across environments.
 

Meristem allocation to reproductive and vegetative functions
Plant growth form is also a product of genetically fixed patterns of organ and module placement (the plant’s body plan). We have conducted studies examining how rapid evolution in the placement of leaves is related to shifting habitat specialisation across light gradients (right). We showed that the evolution of habitat specialisation does not limit the expression of phenotypic plasticity. We are continuing research on the relationship between plasticity and the evolution of habitat specialists and generalists.
 

Rosette growth form / body plan mutants of Arabidopsis thaliana

Further reading

Bonser, SP and LW Aarssen. 2003. Allometry and development in herbaceous plants: Functional responses of meristem allocation to light and nutrient availability. American Journal of Botany 90: 404-412.

Bonser, SP and MA Geber. 2005. Growth form evolution and shifting habitat specialization in annual plants. Journal of Evolutionary Biology 18: 1009-1018.

 
Plasticity in leaf form and function

The universal importance of leaf structure and function to plant ecology and evolution has generated great interest in developing a general understanding of leaf trait strategies. We are currently examining plasticity in leaf form using heterophyllous Acacia species to understand the expression of leaf trait strategies across complex environments. These species typically produce thin, productive true leaves early in development and thick, relatively unproductive modified petioles (phyllodes) late in development (right).
 

Heterophylly in Acacia implexa

 
 

The relative allocation to these leaf types, and timing of the developmental switch between leaf types changes predictably across environments (right). For example, shaded plants delay the shift from true leaves to phyllodes. Our experiments suggest that developmental plasticity in leaf form is a major component of plant growth strategies in Acacia species.

Further reading

Bonser, SP 2006. Form defining function: interpreting leaf functional variability in integrated plant phenotypes. Oikos 114: 87-90.

Forster, MA and SP Bonser. in review. Sclerophyllous responses to complex environments in the heterophyllous Acacia implexa.

 

Shade treatments not only induce shade avoidance responses (increased height) but a delay in the developmental shift in leaf form.