The car1 gene of the filamentous fungus Podospora anserina was cloned by complementation of a mutant defective for caryogamy (nuclear fusion), a process required for sexual sporulation. This gene encodes a protein that shows similarity to the mammalian PAF1 protein (Zellweger syndrome). Besides sequence similarity, the two proteins share a transmembrane domain and the same type of zinc finger motif. A combination of molecular, physiological, genetical, and ultrastructural approaches gave evidence that the P. anserina car1 protein is actually a peroxisomal protein. This study shows that peroxisomes are required at a specific stage of sexual development, at least in P. anserina, and that a functional homolog of the PAF1 gene is present in a lower eucaryote.

The frequency of mutations affecting the normal course of meiosis which have been described for several species of lower and higher Eucaryotes as well as the rather high number of meiotic mutants (see definition in Sandler et al. 1968) obtained whenever they have been sought out systematically (review in Baker et al. 1976a; Heywood and Magee 1976; Golubovskaya 1979), speak for the fact that many steps in the meiotic process are under genetic control. Analysis of a large spectrum of mutants deficient in the various functions involved in normal meiotic progression and haploid progeny differentiation, thus provides a fruitful approach to the study of meiotic-specific events and their regulation.

Twenty-seven genes necessary for sporulation were identified inPodospora anserina. The structural characteristics of these sporulation-deficient mutants (spo) compared to wild type were examined by light and electron microscopy. Several features of the interplay of sporulation events emerge from this study: (1) Any disturbance of the postmeiotic mitosis (PMM) spindle arrangement leads to abnormal sporulation. (2) Initiation and development of the double ascospore-delimiting membranes are independent from nuclear division and spindle plaques, but their lack leads to the occurrence of supernumerary PMM. In wild type, PMM are inhibited from the end of the first PMM until ascospore maturity. (3) Most mutants have a pleiotropic phenotype suggesting that some altered functions involved in the sporulation process are also involved in vegetative growth and/or meiosis. The order and dependence of the respective gene functions of some of the mutations were determined by the behavior of double-mutant strains. The data obtained indicate thatspo mutants are useful tools for probing the genetic control of developmental events.

RésuméLa recherche systématique de mutants de méiose chez Podospora anserina a permis la mise en évidence d'au moins 12 étapes essentielles à la prophase de première division et de 7 gènes impliqués dans la mise en place de l'appareil cinétique. Deux mutants de caryogamie permettent de poser le problème de l'induction de la méiose. Les résultats obtenus à partir des mutants de recombinaison génétique laissent supposer que cette dernière est indispensable au bon déroulement de la méiose. Le phénotype pléiolrope d'un grand nombre des mutants isolés signifie que les fonctions indispensables à la méiose sont également impliquées au cours de la mitose et de la croissance végétative.

Two mutants, mei1 and mei2, were isolated by screening for deficiencies occurring in the meiotic process. The sensitivity of mei1 and mei2 mutant strains to UV irradiation showed a significant increase as compared with that of the wild-type stock, whereas the sensitivity to gamma-rays remained unchanged. The double-mutant strains were no more sensitive than each single mutant. The data indicate that both mei1 and mei2 loci are probably involved in the same pathway of excision-repair of UV-induced lesions.

Forty mutants affecting meiosis and two affecting caryogamy were isolated in Podospora anserina. Growth and mitosis of these mutants were normal. Eighteen of them were studied by both light and electron microscopy so as to determine precisely how the behaviour of the chromosomes and/or the structure of the kinetic apparatus were altered.