Diploid potato populations between a primitive cultivated species, <I>Solanum phureja</I>, and a weedy species, <I>S. chacoense</I>, were used to examine the segregation of microsatellite markers and three traits in backcrosses. Two of the traits, anther culture competence and 2<I>n</I> pollen production, originated from <I>S. phureja</I> whereas the third, leptine production (a specific glycoalkaloid known to convey resistance to the Colorado potato beetle) originated from <I>S. chacoense</I>. Using CP2, a self-incompatible F₁ hybrid originating from a cross between <I>S. chacoense</I> clone 80-1 and <I>S. phureja</I> clone 1-3, three populations were developed: 1-3 x CP2 (PBCp), CP2 x 1-3 (PBCc), and CP2 x 80-1 (CBC). For the microsatellite study, four simple sequence repeat (SSR) primer pairs that amplified fragments within potato sequences found in the GenBank were used to look at segregation ratios in our backcross populations and to eliminate possible spurious genotypes bearing non-parental alleles in these populations. Seventeen spurious genotypes were discarded from PBCp; none was found in PBCc or CBC. Two SSR loci showed skewed segregation in PBCp (favoring transmissnion of the allele originally found in 80-1), PBCc showed normal segregation at all loci, and CBC showed distorted segregation at one locus (revealing a deficiency of homozygotes). In the study of anther culture, three components of ACR were investigated in a preliminary study: 1) embryos produced per anther (EPA), 2) embryo regeneration rate and 3) percentage of monoploids (2<I>n</I>=1<I>x</I>=12) among regenerants. CP2 was intermediate, 80-1 was low, and 1-3 was high for ACR. Only EPA was selected for further characterization in our populations. PBCp (78 genotypes) and CBC (57 genotypes), were characterized for anther culture response ACR/EPA in a series of studies. Nine high and ten low selections were identified in CBC, and ten high and ten low selections were identified in PBCp. EPA selections were used for bulk segregant analysis (BSA) using 214 RAPD primers. Two bands, one amplified by OPQ-10 and another by OPZ-4 were linked in coupling and in repulsion, respectively, to ACR in PBCp. One band amplified by OPW-14 primer was linked in coupling to ACR in CBC. One-way ANOVAs for data from remaining genotypes of the populations verified linkage of the markers to ACR/EPA. For 2n pollen production, a total of 77 PBCp genotypes was characterized; 80-1 produces low % 2<I>n</I> pollen, and 1-3 produces high % 2<I>n</I> pollen. Pollen samples were stained with propidium iodide and examined by flow cytometry. The frequency of 2n pollen varied continuously from 1.7 % to 40.6 % among the 41 genotypes that flowered sufficiently to allow three separate pollen collections. Variation due to the environment was observed where the frequency of 2n pollen appeared greater over a range of genotypes on single collection days. BSA could not be used due to limited population size and a low number of selections at the extremes of the distribution of phenotypes. The continuous variation for 2<I>n</I> pollen production suggests multigenic control of the trait. In the study of leptine content in reciprocal backcross populations, 87 genotypes within PBCp, and 42 genotypes within PBCc were characterized using gas chromatography of leaf samples. CP2 was intermediate, 1-3 had zero, and 80-1 was high for leptine content in the foliage. Leptines were present in low levels in 43 of 87 genotypes in PBCp, indicating simple genetic control. In PBCc, only 7 of 42 genotypes expressed leptines, generally at a higher level than in PBCp, indicating cytoplasmic inheritance. Ten high and ten nil selections within PBCp, and seven high and eight nil selections within PBCc were used for BSA using 214 RAPD primers. Three primers OPQ-2, OPT-16 and OPT-20 amplified bands segregating with high bulks in both populations. These markers were linked in coupling to leptine content in PBCp. Linkage was verified by ANOVAs for leptine content in the entire population. / Ph. D.

Mefenoxam has been a premier compound for Phytophthora disease control in the nursery industry for 30 years. The primary objectives of this research were to examine whether Phytophthora species have developed resistance to this compound and to investigate fungicide resistance management strategies. Phytophthora nicotianae, a destructive pathogen of numerous herbaceous and some woody ornamental plants, was used as a model system. P. cinnamomi, a major pathogen of a wide range of tree species and shrub plants, was also included for comparison. Twenty-six isolates of P. nicotianae were highly resistant to mefenoxam with a mean EC50 value of 326.5 µg/ml while the remaining 70 were sensitive with an EC50 of <0.01 µg/ml (Label rate: 0.08µg/ml). All resistant isolates were recovered from herbaceous annuals and irrigation water in 3 Virginia nurseries. Resistant isolates were compared with sensitive ones using seedlings of Lupinus "Russell Hybrids" in the absence of mefenoxam for relative competitive ability. Resistant isolates out-competed sensitive ones within 3 to 6 sporulation cycles. Resistant isolates exhibited greater infection rate and higher sporulation ability than sensitive ones. No mefenoxam resistant isolates were identified in P. cinnamomi. All 65 isolates of P. cinnamomi were sensitive to mefenoxam with an EC50 of < 0.04 ï ­g/ml. Attempts to generate mutants with high resistance to mefenoxam through UV mutagenesis and mycelial adaptation were not successful. However, there were significant reductions in sensitivity to mefenoxam; those slightly resistant mutants carried fitness penalties, which may explain why P. cinnamomi remains sensitive to mefenoxam. The effect of propamocarb hydrochloride on different growth stages of Phytophthora nicotianae was evaluated in search for an alternative fungicide. Propamocarb greatly inhibited sporangium production, zoospore motility, germination and infection. However, it has little inhibition of mycelial growth and infections. Propamocarb can be used as an alternative fungicide to mefenoxam where mefenoxam resistance has become problematic. However, it must be used preventively; i.e. before infections occur. The genetic inheritance of mefenoxam resistance in P. nicotianae was studied using F1 progenies of a cross between resistant and sensitive isolates. The F1 progenies segregated for mefenoxam resistance in ratio of 1R:1S, indicating the mefenoxam resistance is controlled by a single dominant gene. One RAPD marker putatively linked to resistant locus in repulsion phase was obtained by bulked segregant analysis and was converted to the SCAR marker. This marker is capable of differentiating mefenoxam resistant populations from sensitive populations included in this study. / Ph. D.

Une question centrale de la génétique moderne est de mieux comprendre comment la variation génétique présente au sein d’individus d’une même espèce influence la diversité phénotypique et l’évolution. La levure modèle Saccharomyces cerevisiae offre une occasion unique d’apporter des éléments de réponse à cette question à travers la dissection de l’architecture génétique de la variation de tolérance à des stress environnementaux à l’échelle d’une population. Mon étude révèle un niveau supplémentaire de complexité de la relation génotype-phénotype où finalement les caractères supposés les plus simples, dits Mendéliens (déterminisme strictement monogénique) peuvent se révéler être complexes (déterminisme multigénique) selon le fond génétique en raison de l’action de gènes modificateurs, d’interactions épistatiques et/ou de suppresseurs. Toutefois, les processus évolutifs peuvent être bien différents en fonction des espèces. Afin de mieux les décrypter, je me suis également intéressée à Lachancea kluyveri, une levure phylogénétiquement distante de S. cerevisiae. Cette espèce présente une diversité génétique plus élevée et constitue une ressource encore peu exploitée. L’exploration de la diversité phénotypique et la détermination de leurs origines génétiques initiées dans cette étude sont extrêmement prometteuses et apportent de solides fondations pour l’étude à la fois de l’architecture génétique des caractères et de l’évolution de la relation génotype-phénotype au sein de diverses espèces de levures. / A central issue of modern genetics is to better understand how genetic variations between individuals within a species influence the phenotypic diversity and the evolution. The budding yeast Saccharomyces cerevisiae as a model organism offers a unique opportunity to address this issue through the dissection of the genetic architecture of stress tolerance across a population. My study reveals an additional level of complexity of the genotype-phenotype relationship. Indeed, simple Mendelian traits (monogenic determinism) may become more complex (multigenic determinism) depending on genetic background due to the action of modifier genes, epistatic interactions and / or suppressors. However, evolutionary processes can be very different depending on the species. That is why a non-conventional yeast species namely Lachancea kluyveri (formerly S. kluyveri) was also studied. This species distantly related to S. cerevisiae has a higher genetic diversity and remains a relatively unexplored resource. The exploration of the phenotypic diversity and the determination of the genetic origins initiated in this study lay foundations for the analysis of the genetic architecture of traits and the evolution of the genotype-phenotype relationship within diverse yeast species.