The genus Thymus is an Old World endemic with the center of
diversity found in the Western Mediterranean. In southern France thyme
occurs in an ecosystem called garrigue, a mixture of oak and pine woodlands
with open areas where thyme and many other labiates are found. For the
past 40 years the most extensive research on Thymus vulgaris
has been conducted in an area north of Montpellier, France and at the
Centre d’Ecologie Fonctionnelle et Evolutive (CEFE) (Thompson
et al. 1998). The CEFE (http://www.cefe.cnrs.fr/)
is just one institute of several on the campus of the Centre National
de la Recherche Scientifique (CNRS) at Montpellier.
 Monoterpene synthesis
in thyme involves a well-documented polymorphism
where individual plants produce one of six monoterpenes: geraniol (G),
a-terpineol (A), thuyanol (U), linalol (L), carvacrol (C), or thymol
(T) as the dominant component of their total monoterpene profile (Vernet
et al. 1986). The genetic control involves an epistatic series of at
least five loci. At each locus, there is a pair of alleles, one dominant
to the other. The epistatic effects are linear, so that: G > A >
U > L > C > T. Therefore, a plant that has at least one dominant
allele at the G locus produces a G chemotype regardless of its genetic
composition at the other loci. A plant that is homozygous recessive
at the G locus (i.e. gg) but with at least one dominant A allele will
have chemotype A, again regardless of the genetic composition of the
remaining loci in the series. The series continues in this manner. A
plant which is homozygous recessive at all five loci has chemotype T.
These various chemotypes have been shown to have differential effects
on many other community memebers (Linhart et al. 2005, Linhart and Thompson
1995, 1999).
Thompson, J.D.,
Manicacci, D., Tarayre, M., 1998. Thirty-five years of thyme: A tale
of two polymorphisms. Bioscience 48, 805-815.
Vernet, P., Gouyon,
P.H., Valdeyron, G., 1986. Genetic control of the oil content in Thymus
vulgaris L.: a case of polymorphism in a biosynthetic chain. Genetica
69, 227-231.
Linhart, Y.B.,
Keefover-Ring, K., Mooney, K.A., Breland, B., Thompson, J.D., 2005.
A chemical polymorphism in a multitrophic setting: Thyme monoterpene
composition and food web structure. Am. Nat. 166, 517-529.
Linhart, Y.B.,
Thompson, J.D., 1995. Terpene-based selective herbivory by Helix
aspersa (Mollusca) on Thymus vulgaris (Labiatae). Oecologia
102, 126-132.
Linhart, Y.B.,
Thompson, J.D., 1999. Thyme is of the essence: Biochemical polymorphism
and multi-species deterrence. Evol. Ecol. Res. 1, 151-171.
Papers
from the thyme-space continuum:
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