PHOTOPHYSIOLOGY OF Phycomyces blakesleeanus

Influence of parental strains on the germination ofPhycomyces blakesleeanuszygospores

Phycomyces blakesleeanuswild-type NRRL1555( − ), the standard strain, when crossed with UBC21( + ), another wild type, gives zygospores that germinate in 50–60 days. By backcrossing to UBC21 and selecting for shorter dormancy we have isolated a ( − ) strain, A803, and a ( + ) strain A804, which when crossed give zygospores that germinate in 32 days, the shortest dormancy period found inPhycomyces.The same result was obtained when A803 was crossed with UBC21. Zygospore dormancy decreased as the parental strains became more isogenic with UBC21, but the number of zygospores giving germsporangia with viable germspores also decreased to zero in the third backcross. The existence of germspore-killer alleles in the strain UBC21 is postulated. The strains of shortest dormancy can be used as helper strains (Orejaset al.1985) in sexual crosses. Tetrad analysis of the cross NRRL1554 × S102, a two-factor cross, showed 90% of reciprocal ditypes plus tetratypes in the progeny, indicating that the ( + ) wild-type strain NRRL1554, when crossed with the standard strain, gives regular meiosis and, contrary to current beliefs, may be used inPhycomycesgenetic analysis.

Mutations that affect circadian rhythms in Neurospora crassa can alter the reduction of cytochromes by blue light

We have examined membrane fractions from mutant strains of Neurospora crassa that have altered responses to blue light or have altered circadian rhythms. Using an in vitro assay, we assessed whether the mutations affected the levels of photoreducible cytochromes. Three of the mutant strains, prd-1, rib-1, and wc-1, were not qualitatively different from the wild type. The poky strain was found to have high concentrations of photoreducible cytochrome c. After removal of this cytochrome, however, the photoreducible cytochromes in the plasma membrane and endoplasmic reticulum were also similar to those of the wild type. The most significant differences were found in strains mutated at the frq locus, which affects circadian rhythms. In the frq-9 strain, the cytochrome in the endoplasmic reticulum was not detectably reduced by blue light. The frq-1 mutation caused a significant shift in the spectrum of blue-light-reduced cytochrome in the endoplasmic reticulum.

Photomorphogenesis in Physarum polycephalum. Temporal expression pattern of actin, alpha- and beta-tubulin

Photo-induced fruiting-body formation of the slime mold Physarum polycephalum can be divided into two stages. The first stage (I) starts with the beginning of illumination (0 h) and ends with the formation of nodules (12 h). The second stage (II) is characterized by culmination, sporangiophore formation and melanization of sporangiophore heads (13-17 h). We investigated the expression of actin, alpha- and beta-tubulin during this differentiation process using Northern blot analysis and run-off transcription in isolated nuclei. Whereas actin mRNA is irreversibly lost during stage I, we observed two peaks in mRNA concentration for alpha-tubulin and one peak for that of beta-tubulin during stage I and a coordinated alpha/beta-tubulin mRNA induction during stage II. Stage II induction appears to be related to a presporangial mitosis. Transcriptional activity of the three genes studied shows two maxima, namely one in the middle of stage I and the other at the end of stage II. Our data suggest that the expression of the three cytoskeletal proteins investigated follows a distinct temporal pattern comprising changes in both mRNA synthesis and decay. We also propose a novel function of tubulin in Physarum which is not related to mitosis.

System analysis of Phycomyces light-growth response with Gaussian white noise and sum-of-sinusoids test stimuli

The sporangiophore (fruiting body) of the fungus Phycomyces modulates its elongation rate in response to changes in blue light intensity. This light-growth response of wild-type and behavioral mutant strains has been studied extensively by two methods of nonlinear system identification employing Gaussian white noise and sum-of-sinusoids test stimuli. Both methods are in the framework of the Wiener theory of nonlinear systems. The light-growth response is well described by the first-order Wiener G-functional; addition of the second-order functional improves the precision. The Wiener kernel of first-order resembles the light-growth response to a nonsaturating pulse stimulus. The second-order kernel indicates the nonlinear property of rectification. The kernels have been interpreted by system analysis methods in the frequency domain. A nonlinear dynamic model of the light-growth response has been developed from the kernels obtained by both methods. The model includes a nonlinear dynamic subsystem, including a squarer, followed by a linear dynamic subsystem (which, by itself, accounts for the first-order kernel). This parametric model has been used to evaluate light-growth response kernels under various conditions (viz. wavelength, temperature, and genetic strain). The kernels of single and double mutants have been analyzed jointly with a nonparametric model to reveal interactions among the products of eight genes that influence the light-growth response. The extent of interactions found suggests that these gene products function together in a molecular complex.

A genetic map of Phycomyces blakesleeanus

Complementation tests among Phycomyces auxotrophic strains revealed the existence of four genes with mutants requiring riboflavin, three genes with purine auxotrophs, two with nicotinic acid auxotrophs, and two with lysine auxotrophs. A total of 134 sexual crosses between strains carrying mutations affecting phototropism (madA-madE), carotenoid biosynthesis (carA), auxotrophy (ribA-ribD, purA-purC, lysA and lysB, nicA and nicB, and leuA) and resistance to 5-fluorouracil (furA and furB) were studied; mating type (sex) was also included as a marker. The results from random spore analysis, tetrad analysis, and gene-centromere distances shows that these markers are distributed into 11 linkage groups.

Characterization of Phycomyces blakesleeanus mutants temperature-sensitive for heat-shock induced germination

Sixty-nine mutants of the fungus Phycomyces blakesleeanus, temperature-sensitive for heat-shock induced germination, have been characterized. All of them show a low viability at 26 degrees C and normal viability at 16 degrees C. Eleven mutants recover the wild type phenotype if yeast extract is added to the minimal medium; the mutant phenotype of eight of these mutants is also suppressed by the addition of putrescine or other polyamines. The majority of the mutants are affected very early in germination. Spontaneous, heat-shock and acetate induced germination are not equally impaired by some of the mutations, so specific and independent steps seem to be involved in part of the activation mechanism of germination.