[Ultrastructural study of host-parasite relations: early phases of Venturia inaequalis installation on susceptible and resistance apple leaves]. / Etude ultrastructurale de la relation hote-parasite: phases précoces de l'installation de Venturia inaequalis sur les feuilles de pommiers sensibles et résistants.

The penetration mechanism study of Venturia inaequalis is undertaken on leaves of scab-susceptible and scab-resistant apple cultivars. The penetration through the cuticle occurs whatever the host phenotype. The resistance will be expressed when the infectious hyphae is in contact with the epidermal cell wall. The epidermal cell of susceptible variety does not show any modification. The resistant variety reaction is very fast, since twenty hours after penetration many wall appositions are setting, the cytoplasm strongly degenerates and the parasite hyphae are already necrotic. The contact between the host and the parasite is always very complex in the case of resistant host. The rapidity and the important cytological modifications of the host parasite interaction connect this reaction to an hypersensitive reaction.

Two methods of haploidization in pear, Pyrus communis L.: greenhouse seedling selection and in situ parthenogenesis induced by irradiated pollen.

Seedlings of 12 crosses involving pear varieties or hybrids were observed for the presence of haploid plants. On the basis of phenotypic characteristics, 17 plants corresponded to the haploid condition and, of these, 12 were determined by chromosome counting to be haploid (2n=x=17). In addition, and in order to induce in situ parthenogenesis, several pear varieties were pollinated with a selected clone carrying a homozygous dominant marker gene for the colour of red. This pollen had previously been irradiated with γ-rays of cobalt 60 at 0, 200, 250 and 500 Grays. The immature embryos were cultured in vitro, whereby 1 haploid and two mixoploid plants were obtained. Numerous diploid plants with the maternal phenotype were also obtained, and their genetic origin was subsequently studied by means of isozyme analysis.

Inheritance of pollen enzymes and polyploid origin of apple (Malus x domestica Borkh.).

Electrophoresis of 7 pollen enzymes was applied to 5 progenies from controlled crosses and one self-progeny of apple. Segregation data were examined according to three kinds of hypotheses: monogenic disomic, bigenic disomic and tetrasomic inheritance Twenty codominant alleles and a recessive null were identified. Results provided evidence of bigenic disomic inheritance in most cases: 6 pairs of homoeologous loci carrying identical homoeoalleles were revealed; only 2 enzymes exhibited a simple monogenic control. Preferential pairing between pairs of homologous chromosomes in meiosis can be postulated. These results indicated an allopolyploid origin of apple genome. Fixed heterozygosity occurred for several enzymes, which is a typical feature of allopolyploidy. Loss of duplicate gene expression can account for the monogenic control of 2 of the enzymes.