Species are kept separate in plants and animals through barriers to
gene flow. However, the exact mechanisms of speciation have only been
explained within the last 20 years. Scientists found that one mechanism,
hybrid necrosis, is associated with a plant defense gene. Different forms of
these rapidly evolving genes in parent plants can cause autoimmune responses
leading to offspring inviability and may represent a molecular pathway to
speciation unique to plants.
One of the basic tenets of evolution is speciation in which
populations of the same species become so genetically and morphologically
variable that they can be classified as two different species. Individuals
of these species may be capable of mating, but they may not produce
offspring, and if offspring are produced, they will be sterile or so
defective that they die before they are able to reproduce.

Although speciation has been observed and studied since Darwin and
Wallace first proposed their theory, the complex molecular mechanisms
responsible are not yet fully known. One of these molecular mechanisms,
hybrid necrosis, was studied by Dr. Detlef Weigel and his colleagues at the
Max Planck Institute for Developmental Biology in Germany. Dr. Kirsten
Bomblies will present their results at the President's symposium at the
annual meeting of the American Society of Plant Biologists on July 11.
Bomblies and Weigel observed hybrid necrosis in crosses of thale cress,
Arabidopsis thaliana, a member of the mustard family, and found that it is
associated with plant genes that respond to pathogen attack.

Plants must frequently cope with environmental stresses such as heat,
cold, high acidity or salinity, or attack by pathogens such as viruses or
insect predators. Such stresses mobilize defense genes that initiate
physiological responses that help the plants to survive. One such response
is programmed cell death, which occurs in response to invasion by viruses or
bacteria.

The cells invaded by the pathogens are quickly marked by the plant for
death so that the microbe cannot use them to replicate and spread to the
rest of the plant. These types of genes have been shown to evolve rapidly,
giving plants the capability to adapt to changing conditions and pathogens.
Bomblies and Weigel found that the same type of gene is involved in hybrid
incompatibility in Arabidopsis. Because these genes evolve so rapidly, there
are likely to be different forms present in the population, and when two of
these are joined in a hybrid, they can cause fatal defects in the hybrid
offspring.

A biological species is defined as a population of individuals that
can interbreed among each other freely, but not with members of other
species. What finally establishes two populations as different species is
that gene flow between them stops. However, this does not happen suddenly.
Rather, it is a gradual process in which one barrier after another is raised
between two species, including inviable embryos and defective and sterile
adults, as well as genetic incompatibilities that prevent even the formation
of an embryo. The hybrid incompatibility identified by Bomblies and Weigel
is an example of the kind of genetic incompatibility that can result in
speciation.

Because plant reproduction often requires an outside agent like a
pollinator or the wind, which spreads pollen far from the parent plant, the
offspring can be hybrids between parents from two different populations or
even from two different although closely related species. Such hybrid
offspring can be successful but may also be prevented or defective because
some of the parents' genes are not compatible. In their survey of 900 first
generation hybrid offspring among 293 strains of thale cress, Bomblies and
Detlef found that 2% of the offspring were severely defective. They call
this phenomenon "hybrid necrosis" or "hybrid weakness," and identified the
gene responsible for the incompatibility as a disease resistance gene that
has different forms in the two parents.

Some of the molecular mechanisms that prevent hybridization between
species are well-known in both animals and plants. There are a number of
gene flow barriers in plants that are similar to those of animals--among
them are ecological factors such as reproductive season, morphological
differences, and hybrid sterility.

However, hybrid necrosis produced by autoimmune responses due to
pathogen resistance genes has not been observed in animals and may represent
a molecular pathway to speciation unique to plants. Knowledge of these
mechanisms is important not only in the study of the evolutionary history of
plants but can also provide tools for ensuring the safety of genetically
engineered crops. If incompatibility genes can be bred into a GE crop, it
might be possible to prevent the formation of superweeds and to lessen the
probability that harmful genes can be spread to other species.


[www.aspb.org]