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The evolution of seed dispersal

 
 
Specimens of the tribe Brassiceae, taken from herbarium sheets in the Harvard University Herbaria.
From upper left: Brassica nigra, Muricaria prostrata, Erucaria aegiceras, Erucaria cakiloidia, Didesmus bipinnatus

Dispersal is one of the primary factors that influence the rate and outcome of evolution, and it has long been recognized as such. It also influences population demography, population structure, intra- and interspecific competitive interactions, and community structure. Because of the ecological and evolutionary importance of dispersal, it is of interest to investigate how dispersal itself can evolve. In plants, and in many insects, dispersal is maternally determined. We therefore investigate the evolution of dispersal within the context of maternal character evolution. Maternally determined characters can cause unusual evolutionary and demographic dynamics such as negative responses to selection, evolutionary momentum, and fluctuations in population size.

I have studied the adaptive value of seed dispersal of the Great Lakes sea rocket, Cakile edentula var. lacustris, an annual mustard that grows along the shores of the Great Lakes. In Cakile, plasticity of maternal characters that influence dispersal resulted in a negative maternal effect on dispersal. Therefore, dispersal is predicted to oscillate across generations. Dispersal also strongly influenced both progeny and maternal fitness; a maternal phenotype simultaneously increased progeny fitness through increasing their dispersal, but decreased maternal fitness. The total adaptive value of dispersal, therefore, is much lower when one considers maternal fitness in addition to progeny fitness. Studying seed dispersal as a maternally influenced character can help explain the ubiquity of inefficient dispersal in many systems, even when dispersal is strongly advantageous to the progeny.

Cakile can grow at extremely high density or as isolated individuals,
depending on whether or not seeds were dispersed from the maternal parent.

We have also recently investigated the genetic basis of seed dispersal in Arabidopsis thaliana. As in Cakile, we found strong negative maternal effects on dispersal and strong conflicting fitness consequences of dispersal for maternal parents and progeny. In this research, we addressed how the genetic architecture dispersal varies with environmental conditions, such as density, which in turn varies as a function of dispersal. In this manner, we investigated how dispersal can influence the genetic component of character variation, including variation in dispersal itself.

We have begun to study the evolution of seed dispersal in the Brassiceae in collaboration with Elena Kramer of OEB and Wah-Keat Lee of the Argonne National Laboratory. Post doc Jocelyn Hall is heading a combination of a studies on the systematics of the tribe Brassiceae, morphological and functional studies of fruit anatomy and its relation to seed dispersal, and molecular developmental genetics. We are especially interested in the development of a novel abscision zone, the "joint", which enables the dispersal of protected propagules. We are also interested in the evolution of the corky pericarp, which enables dispersal by water of the maritime species and very likely contributed to the enormous range expansion and subsequent radiation of the genus.

X-ray microtomography images of Cakile and its close relatives. From left to right: Erucaria hispanica, Cakile arabica, Cakile maritima, Cakile constricta. Images taken at the European Synchrotron Radiation Facility by Wah-Keat Lee and Kamel Fezzaa.

 

 

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