Thirty-Five Years of Thyme: A Tale of Two Polymorphisms

The aromatic or wild thyme (Thymus vulgaris) is a small, woody shrub of the Labiatae family that inhabits open areas in the western Mediterranean basin. Two types of polymorphic variation exhibited by thyme have provided for the study of the evolutionary dynamics of genetic polymorphisms. First, wild thyme plants bear either female or hermaphrodite flowers but not both, which makes them gynodioecious. In southern France, populations exhibit female frequencies ranging from 5% to more than 90%, which is much higher than other gynodioecious species. Second, six different genetically determined monoterpene compositions of thyme's oil can be found in southern France. The authors John D. Thompson, Domenica Manicacci, and Michele Tarayre have synthesized 35 years of past research on the gynodioecy and oil compound variation of thyme in reference to the high frequency of females and number of oil compounds. The average female frequency of the thyme populations is approximately 60%, which indicates cytoplasmic inheritance. Although nuclear genes are also involved in the determination of sexual phenotype, cytoplasmic sterility genes may block male function. Field investigations have determined that female frequency is correlated with population age, with young populations bearing a high number of females. The male function of nuclear genes is suspected of being restored from the sterility influence of cytoplasmic genes by appropriate alleles of restorer genes in the nuclear genome. The possibility that restorer alleles are lacking in patches of colonizing populations is suggested by the low levels of mitochondrial DNA and chloroplast DNA diversity in the populations. Studies have also shown that male-fertility restoration is more frequent when the females are pollinated by males of the original populations than for females cross-pollinated by distant populations. Other fitness considerations include a possible inbreeding depression among hermaphrodites, reduced seed production resulting from resource allocation toward pollen, and a variable sexual resource investment in hermaphrodites. Eleven species of the genus Thymus ranging from southeastern Spain to south-central France were studied by T. Adzet and others who found a pattern of geographic variation in their occurrence, with the three chemotypes of cineol, borneol, and camphene found in southeastern Spain; the phenolics of carvacrol and thymol abundant in northern Spain and southern France; and nonphenolics increasing in frequency toward the northern latitudes. The geographical distribution indicates an adaptive strategy whereby phenolic compounds are produced in response to hot, dry climates and nonphenolic compounds are produced in cool, moist habitats. The geographical distribution also occurs at a microhabitat scale according to microclimates and is thought to be in response to drought stress, interspecific competition, and herbivory