Charles G. Elliott

Sterols in fungi: their functions in growth and reproduction.

Publisher Summary This chapter focuses on the Phytophthoru cuctorum, which grows on a simple medium containing sucrose, asparagines, mineral salts and thiamin, but on this medium it remains purely vegetative. On the other hand, when grown on oatmeal agar, oospores are produced in abundance. The factors in oats responsible for the difference have been found to be sterols. Addition of sterols to the simple basal medium changed the mode of growth from vegetative to reproductive. The role that sterols play in controlling reproduction in ascomycetes is not yet clear. The importance of sterols as essential structural components of membranes and their consequent importance in efficient metabolism are well established in yeast, but their precise functions are still hardly understood. In the zygomycetes, control of reproduction by trisporic acid is well understood. It is surprising that more fungi cannot synthesize sterols, as they mostly inhabit environments where sterols would be expected to occur. The sterol content of spores of Plusmodiophoru brassicue varies significantly according to the sterol composition of the host, suggesting at least partial heterotrophy.

The cytology of Aspergillus nidulans

In haploid strains of Aspergillus nidulans , asci arise from croziers. The two nuclei of the young ascus (the penultimate cell of the crozier) fuse, and the zygote nucleus immediately undergoes meiosis. At diakinesis and first metaphase eight bivalents are seen: three large (one, Chromosome 2, with a satellite), two medium sized (Chromosomes 4 and 5), two small (Chromosomes 6 and 7) and one very small (Chromosome 8). The perithecia of haploid strains are packed with asci and have very few sterile hyphae. Diploid strains (heterozygotes made by Ropers technique) are very different from haploids in that the perithecia contain many sterile hyphae with little cytoplasm and coiled hyphae with dense cytoplasm made up mainly of uninucleate cells, and there are few asci in a perithecium. Croziers are absent: some of the cells of the coiled hyphae become asci. At diakinesis and first metaphase, the same chromosome configurations are seen as in a cross between the haploid strains from which the diploid was synthesized; the chromosomes are bivalents. The young ascus thus has a single (diploid) nucleus which undergoes meiosis, and asci develop apogamously. No evidence of nuclear fusion in the young ascus and of a tetraploid meiosis was obtained. Cultures from ascopores isolated by micro-manipulation from perithecia of diploids were all haploid. Meiosis appears to proceed normally to first metaphase, but typical later stages, and asci with spores, are rarely seen. First anaphase frequently fails to occur, and the chromosomes clump together. In the strain ad 2 y , obtained from a normal strain by X-irradiation, nine bivalents are present, one a very small fragment. In crosses between ad 2 y and a normal strain, and in a diploid made from them, the fragment pairs with Chromosome 6. Asci in the cross between ad 2 y and a normal haploid are highly irregular, often with less than the usual eight spores.

Genetical evidence on the life-history of Phytophthora

Zoospores of Phytophthora cactorum were treated with the mutagen N-methyl-N′-nitro-N-nitrosoguanidine. Cultures derived from treated zoospores had the phenotype of the wild-type parent strain, but the sexual (oospore) progeny of some of them segregated in Mendelian fashion for mutant characters which must have been induced in the treated spores in heterozygous condition. The segregation pattern observed for one of the mutations through three generations is consistent with a life-history in which the vegetative nuclei are diploid with meiosis occurring in the gametangia.

Environmental effects on the distribution of chiasmata among nuclei and bivalents and correlation between bivalents

Environmental effects on the distribution of chiasmata among nuclei and bivalents and correlation between bivalents

Analysis of wood-decaying ability of monokaryons and dikaryons of Serpula lacrymans

The wood-decaying abilities of 40 monokaryons of Serpula lacrymans have been compared with those of 138 dikaryons synthesized from them. This ability was less than in either of the component monokaryons in 57% of dikaryons, intermediate in 33% and greater in 10%. The character is controlled by a relatively simple polygenic system. One monokaryon introduced an unexplained genetic interaction; otherwise there was no evidence of dominance.

Stages in oosporogenesis of Phytophthora sensitive to inhibitors of calmodulin and phosphodiesterase

Addition of trifluoperazine (100μ m ) to cultures of Phytophthora cactorum prior to appearance of oogonia inhibits oogonium initiation. Later additions have little effect on numbers of oogonia formed, but severely disrupt their further development. Addition of theophylline (2.5 m m ) inhibits oogonium production, but has little effect on development of those already formed at the time of its addition. This suggests calcium promotes and cAMP inhibits oogonium initiation, while calcium has a continuing role throughout oospore development.

Control of sexual reproduction in Pythium sylvaticum

The pattern of reproductive interaction between five strains of the heterothallic species, Pythium sylvaticum has been studied. When male and female strains are separated by a polycarbonate membrane, the female strain induces development of antheridial hyphae in the male strain, and the male strain induces development of oogonia in the female; hyphal contact is not required. This implies the existence of hormones analogous to antheridiol and oogoniol of Achlya. The antheridial hyphae appear in the male before the oogonia in the female. The numbers of oogonia increase as pairing continues from 8 to 24 h. Male strains, induced by pairing with a female strain, themselves produce oogonia on subsequent separation from the female strain, implying ability to respond to the oogoniol-like hormone in the absence of a female strain. Cultures are competent to reproduce sexually when they first achieve maximum weight.

Non-localised negative interference in Aspergillus nidulans.

WHEN the incidence of recombination in regions adjacent to a very short segment of a chromosome (i.e. one less than i unit in length) is measured (a) in a whole population of meiotic products and (b) in that small proportion of the population in which there has been recombination between the marker genes delimiting the short segment, it is found that the values for (b) are higher than for (a): there is negative interference between exchanges in the short segment and the adjacent regions (Pritchard, 1955 1960; Calef, in Aspergums nidulans; Streisinger and Franklin, 1956, in phage T2, and Chase and Doermann, 1958, in phage T4). In practice, the two sets of recombination fractions, for (a) and (b), are estimated in two different samples from the population of meiotic products. One, called in the present paper the unselected sample, is a random sample of the whole population. The other, called the selected sample, consists only of recombinants between the closely linked markers. The short segment, the rare recombinants in which comprise the selected sample, will be called the selective