Thymus vulgaris is a gynodioecious species (in which females and hermaphrodites coexist) with a highly variable frequency of females among natural populations (5–95%) and a high average female frequency (60%). Sex determination involves both cytoplasmic genes responsible for male sterility, i.e. the female phenotype, and specific nuclear factors responsible for the restoration of male fertility, and thus a hermaphrodite phenotype. In this study, molecular markers of the mitochondrial genome have been used to quantify the cytoplasmic diversity in 11 clumps of individuals observed in four recently founded populations. The very low diversity within patches in conjunction with the strong diversity among patches strongly suggests that clumps of individuals are the result of single matrilinear families. In clumps that contain mainly females, all the analysed females showed the same cytoplasmic pattern. This pattern differed from that shown by neighbouring hermaphrodites, indicating that the determination of sex is locally cytoplasmic. A comparison of genetic diversity before and after fire in one population showed that disturbances may cause a reduction in genetic diversity and a concurrent induction of local cytoplasmic determination of sex. Such cytoplasmic determination of sex in colonizing populations, together with the greater seed set of females, may largely improve the colonizing ability of the species.

Nucleocytoplasmic determination of male sterility in Thymus vulgaris L. has been assumed in all papers attempting to explain the remarkably high frequencies of male steriles found in natural populations of this species. This paper provides strong evidence that both nuclear and cytoplasmic genes are involved in the determination of male sterility of this species, giving a complex inheritance. Interpopulation and intrapopulation crosses have shown that the ratio of females versus hermaphrodites among offsprings varied widely from 1∶0 to 1∶1. Furthermore, interpopulation crosses consistently yielded a higher frequency of females than intrapopulation crosses. Nucleocytoplasmic inheritance was demonstrated by an absence of male fertiles in backcrosses and asymmetrical segregation in reciprocal crosses. Molecular analysis of the mitochondrial DNA of some of the parents used in crosses suggested the involvement of different cytoplasms in the inheritance of male steriliy.

A model of evolution of reproductive systems, when sex is determined by both nuclear and cytoplasmic genes, is presented. Such a control of sex is known to facilitate the occurrence of female individuals in hermaphroditic populations, thus leading to gynodioecy. A two-cytotypes two-nuclear loci (two alleles at each nuclear locus) model for gynodioecy has been developed previously. Such gynodioecious systems are usually considered as stable, i.e. not leading to dioecy. In order to find out if the presence of females can select for male individuals when sex determination is nuclear-cytoplasmic, we followed the evolution of alleles responsible for female sterility. These alleles can be at the preceding loci or at a third locus. We show that male individuals can be selected. Dioecy evolves in less restrictive conditions than under nuclear sex determination. The same also holds for trioecy (coexistence of females, hermaphrodites and males). Nuclear-cytoplasmic polymorphism can be maintained in these reproductive systems.

SUMMARYTwo families of Helianthus lines, T (low desiccation rate) and T+ (low desiccation rate), with contrasting osmotic adjustment (OA), obtained from a divergent selection programme, were compared for several morphological traits, ecophysiological parameters and molecular markers. The physiological analysis in a glasshouse experiment showed that the two families were identical in irrigated conditions except for a single trait, relative water loss (RWL: water loss rate of excised leaves). RWL could be used for these genotypes as a physiological marker of OA. A molecular anah^sis of the two contrasting lines, using differential screening of cDNA libraries, led to the isolation of three specific cDNAs, one of which, DRS26, showed high sequence homology with a mammalian amino-acid transporter. Subsequent RFLP and STS studies, using DRS26 and RAPD bulked analysis, enabled the identification of putative molecular markers of RWL and OA in T":T+ sunfiower.

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Life on the earth is highly dependent on the properties and functions of water. In front of water limitation, herbaceous, woody and epiphyte plants have developed a wide diversity of drought tolerance mechanisms at the molecular, metabolic and physiological levels. The strategies of adaptation to drought have been listed in regard of the level of organization: molecules, cells, whole plant. Root development and water uptake, transpiration and micro-and macromorphological adaptations, and water status and osmotic adjustment have important consequences on drought adaptation. The relationship between these characters and mechanisms and the productivity of cultivated plants are the basis of the breeding for drought tolerance. r71