www.checkbiotech.org ; www.raupp.info ; www.czu.cz

By tracing the 30-million year history of variation in a gene found in
plants such as tomatoes and tobacco, biologists at the University of
California, San Diego have found new evidence to support an old idea ? that
some evolutionary changes are irreversible, January 2006 by Carina Stanton.

Their study, published this week in an early online edition of the journal
Proceedings of the National Academy of Sciences, offers new support for the
idea that the loss of complex traits, like eyes, wings or in this case a
reproductive mechanism, is often irreversible. In other words, once lost,
the traits never revert to their original state.

"This is the strongest evidence yet to support irreversibility,? said Joshua
Kohn, an associate professor of biology at UCSD who headed the study. "If we
had not used the genetic data coding for this reproductive mechanism and
only inferred the pattern of evolution based on the traits of living
species, we would have come to the opposite conclusion and with high
statistical support ? that the trait evolved more than once.?

The scientists examined existing variation in the gene used by many members
of the Solanaceae family, which include tomatoes and tobacco, to recognize
and reject their own pollen, thereby avoiding self-fertilization and the
harmful effects of inbreeding. This ability is sometimes lost, as is the
case for garden tomatoes, which can set seed by self-fertilization.
Apparently, once lost, the ability to reject pollen in order to prevent
self-fertilization is never regained.

Irreversible loss of complex traits, which result from the combined
interaction of several genes, is an old and at times controversial
scientific question. While the late evolutionary biologist Stephen Jay Gould
popularized the hypothesis of irreversibility, known as Dollo's Law, studies
that use current methods to reconstruct the evolution of complex traits
often fail to support it. This is because it is often difficult to
reconstruct characteristics of extinct ancestors with any certainty.

The study contradicts earlier studies of complex trait evolution, which have
tended to favor multiple reappearances of complex traits after these organs
were lost in ancestral species. The authors suggest that traditional methods
for reconstructing the history of trait evolution may be inaccurate.

Discovering irreversible change for this sexual system trait highlights the
importance of considering genetic data underlying the trait when
reconstructing its evolutionary history.

?Our work implies that evidence for such evolutionary change in other cases
may have been missed because the current methods aren?t sufficiently
refined,? said Boris Igic, who conducted the study while a graduate student
at UCSD and is now a postdoctoral fellow at Cornell University.

While lending support to the question of unidirectional evolution, the
biologists? findings also lead to new questions.

?Apparently, plants that have sex exclusively with other plants and not
themselves, enjoy a greater evolutionary advantage,? Igic said. ?Exactly why
is unclear,?

Species capable of rejecting their own pollen in favor of pollen from other
individuals, harbor more genetic variation than those that self-fertilize.

?An intriguing aspect of this study is that the mechanism for ensuring
cross-fertilization is very old, often lost, and never regained,? Kohn said.
?That it is still common despite frequent and irreversible loss implies that
this trait confers an advantage to species that possess it, perhaps in terms
of reduced rates of extinction.?

[ucsdnews.ucsd.edu]

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