When things get nasty in a particular environment, organisms that call it home have got to come up with ways to cope. Animals are mobile and tend to migrate to an environment where the proverbial grass is greener. However, if you are a sedentary organism - a plant for example - you must find other ways of managing. This week a study by two American ecologists suggests that some plants get maternal help in coping with their environment.
In general, plants have little choice in terms of where they live. Seeds tend to germinate where they fall and even though many plants use elaborate schemes to insure that their seeds are spread great distances there is no way to guarantee that they will touch down in a hospitable location. To compensate for their lack of geographical decision making, plants have evolved a great deal of developmental plasticity in order to succeed in a variety of environmental conditions.
In some cases, particularly those in which the seed doesn’t fall far from the tree, it appears that offspring may get a head start from lessons that their mother has learned about the environment. This week’s Science features an article by Laura Galloway of the University of Virginia and Julie Etterson of the University of Minnesota that elegantly characterizes a system in which the maternal effects lead to an adaptive advantage in their offspring.
The authors focus on Campanulastrum americanum (American Bellflower) which can either have an annual or biennial life cycle depending on when and under what sort of environmental conditions its seeds germinate. Bellflower usually grows in the shady forest understory, exposed to little direct light. However, in some cases bellflower may grow in “light gaps” - holes in the forest canopy left after a tree fall - and receive as much as ten times more light than plants growing in the understory. Shade grown plants tend to have a biennial life cycle whereas those growing in light gaps are usually annuals. Galloway and Etterson propose that maternal effects may play a role in life cycle fate.
To test this hypothesis, they fertilized genetically similar mother plants in the two light environments with pollen from the same father. Seeds from these mothers were planted in either understory or light gaps and the fitness of the progeny was assessed based on germination rate. In general, offspring fared better when grown in the same light environment as their mother. For example, offspring of an understory mother germinated better in understory conditions than in light gaps. In addition, the offspring typically express the maternal life cycle - annual or biennial - regardless of light conditions. These results indicate that the maternal plant is passing something on to her “children” that gives them an adaptive advantage in her environment. For a species, like this one, that does not disperse its seed over a long distance this provides an evolutionary advantage - essentially priming its’ offspring to deal with the environmental conditions that they are most likely to face.
This paper is impeccably written and is an excellent example of communicating one’s research clearly and concisely. Galloway and Etterson state their experimental questions, their hypothesis and the experiments that they will use to test this hypothesis. Their experiments are well designed and controlled and the conclusions that they draw are the simplest explanation of the results. Sounds straightforward, but you would be surprised how many papers are written in a style that can only be described as Byzantine and attempt to draw revolutionary conclusions from skeletal evidence.
For me, the most exciting implications of this paper are left unstated by the authors. My training is in epigenetics, which refers to the study of heritable and reversible changes in gene expression that do not result from genetic mutations. One of the things that 
attracted me to this paper was that they seem to be studying a phenomena that is by definition epigenetic. The maternal plant is undergoing what is presumably a change in the expression of a number of genes which it then passes on to the progeny. This maternal effect is not likely to be due to genetic mutation and bears some functional resemblance to genomic imprinting (my friend Matt is an author on this review). Imprinting, the preferential expression of one parent’s genes over the others, is known to be important in seed and early embryo development in plants.
What makes this particularly interesting to a student of epigenetics is that it is a potential “real life” example of maternal imprinting in action. Most epigenetics research is done in model systems under artificial conditions. Under these conditions a fair bit about the molecular mechanisms involved in epigenetic phenomena is learned at the expense of knowledge about biological implications. However, as Galloway herself acknowledges, working in a natural system has both benefits and drawbacks. “This (paper) reveals both the strength and weaknesses of a non-model system. The natural history of the system provided motivation as to why maternal effects might be important but once I’ve found them, it is not clear how to get at the mechanism.” Perhaps the best approach would be one similar to that taken by Kirsten Bomblies and her colleagues in a recent PLOS Biology paper. They found a variety of the model species Arabidopsis thaliana that allowed them to recreate a naturally occuring phenomenon that they were interested in studying in genetically amenable system. Perhaps the adaptive maternal effect observed in C. americanum is also employed in Arabidopsis varieties or other closely related crucifers.
Imprinting is not the only possible explanation for the adaptive maternal effect. For example, in Drosophila RNA is maternally deposited into the embryo allowing progression through the first phases of embryogenesis. Perhaps C. americanum uses a related mechanism or something else entirely. Galloway and Etterson’s research opens the door into further study of the role that Mom plays in plant development.
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2 responses so far ↓
1 mogLi // Nov 21, 2007 at 11:32 pm
Definitely a big disadvantage to use a non-model organism. And I assume that the genome of this plant has not been sequenced yet? Otherwise it would have been possible to design microarray chips, and perform expression studies between the offsprings of the same maternal type, i.e. offspring of maternal (A) grown in light gaps vs offspring of maternal (A) grown in understory. In this way, one can see which genes are the fitness determinants, and proceed from there to check the underlying epigenetic mechanism, as you suspect it to be.
2 arizaphale // Nov 25, 2007 at 11:12 am
Can’t pretend to completely follow the technical aspects of model vs natural systems but an interesting post anyway. (Go the mothers!!!)
Was reading recently about the Wollomi Pine and the fascinating discovery that all the trees, in both sites, are genetically identical. Has there been any further developments in finding out why this is so? Just thought you might have some inside info.
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