Homothallic Species Research Articles

Homologies ofNeurospora homothallic species using repeated and nonrepeated DNA sequences

DNA:DNA hybridization studies of the homothallic species ofNeurospora showed that the repeated DNA sequences provided no means of distinction among them. Hybridization with nonrepeated DNA sequences, however, showed that theN. terricola species was quite unlike the others. These studies suggest that heterothallism might have evolved from homothallism inNeurospora.

Heterothallism in the basidiomycetous yeast genusSporidiobolus Nyland

The sexual speciesSporidiobolus salmonicolor andSporidiobolus pararoseus spp. nov. are described as heterothallic species in the genusSporidiobolus, a genus consisting of two homothallic species,Sporidiobolus johnsonii andSporidiobolus ruinenii. The four species are compared and a key to the genus is presented. As a taxonomic correctionSporobolomyces pararoseus nom. nud. is reported as a synonym ofSporobolomyces shibatanus.

Hormonal Regulation of Sexual Reproduction in Phytophthora

Induction of oospore formation by the complementary strain occurred in both cases when A1 and A2 isolates of Phytophthora parasitica were separated by polycarbonate membranes and up to 3 mm of agarose. The number of oospores produced increased as separation distance decreased. The homothallic species P. heveae induced oospore formation in both the A1 and A2 isolates of P. parasitica. Phytophthora megasperma var. sojae stimulated oospore formation in the A2 but not the A1 isolate of P. parasitica, while P. cactorum and P. katsurae stimulated the A1 but not the A2 isolate. Sexual reproduction of homothallic species of Phytophthora may thus also be controlled by α hormones. Sixteen types of chemically regulated sexuality among the members of Phytophthora, differing in hormone production and responsiveness to hormones, are postulated.

Genome Analysis of Neurospora Terricola

The homothallic species Neurospora terricola Gochenaur & Backus (1962) is different from the other known homothallic species, N. africana Huang & Backus, N. dodgei Nelson & Novak, N. lineolata Frederick & Vecker, and N. galapagosensis Mahoney & Backus. The ascospores of N. terricola germinate at least five times slower than other homothallic species when grown under similar conditions. Neurospora terricola has one germ pore in the ascospore, instead of two in the other species, and its growth pattern is also different. We were interested to see if the genome of N. terricola (haploid DNA content) would show any unique characteristics. The success in this laboratory in distinguishing minute DNA differences of heterothallic, pseudohomothallic, and other homothallic species (Dutta et al., 1976) by analysis of their DNA genomes prompted us to use these procedures for analysis of the genome of N. terricola along with the DNA characteristics of another homothallic species, N. galapagosensis. This second species was chosen because of morphological similarities to other homothallic species, N. africana, N. dodgei, and N. lineolata except for its pattern of ascospore sculpturing (Mahoney et al., 1969). DNA was isolated from exponentially growing mycelia before melanin deposition, as described earlier (Dutta, 1976). The appearance of melanin deposited on hyphal clumps, especially those exposed at the top of the 4-liter flask in which they were grown, marked the end of the growth phase. Molecular studies of the DNAs isolated from N. galapagosensis and N. terricola showed similarities with the DNAs of other Neurospora species (Dutta et al., 1976). The mean G + C (guanine and cytosine) mole percent of all the DNAs tested was between 53 and 57. The thermal denaturation profiles were similar, and the quantities of repeated DNA sequences, 15-20% of the total genome, fell within close range (Dutta and Schwartz, 1973). These DNAs showed two fractions, a major high G + C content comprising 70-80 mole % of the genome and a minor low G + C content of 20-30 mole %. The Tm (temperature at which 50% of the double-stranded DNA of the major fraction dissociates into single strands) for these two species was in the same range, 89-91 C, as others studied, These data show that N. terricola and N. galapagosensis have DNA characteristics similar to N. lineolata (TABLE I). 663

Sexual mutants of a heterothallic Mucor species, Mucor pusillus

The Mucorales consist of both homothallic and heterothallic species. Both types can be found in the same species but this is not a common phenomenon. A few homothallic strains are found in the predominantly heterothallic species, Mucor pusillus . It was interesting to see how readily homothallic mutants could be induced in this species. Spores of CBS 253.53(+) were irradiated with gamma rays. Homothallic and (−) mutants were isolated. An albino (+) and a yellowish-brown (+) mutant were also found. The two pigment mutants were further mutated, and again both (−) and homothallic mutants were isolated. Morphological characteristics of M. pusillus such as branching of sporangiophores, size of sporangia, and height of turf are retained by the mutants. The only obvious change is the color of the pigment of some of them. Like the original strain, they grow in the absence of thiamine. The development, structure, and size of the zygospores are typical of M. pusillus . The dark pigment of the zygospores is lacking in the albino homothallic and in crosses between albino (+) and albino (−), but in all other cases the characteristic color is retained. Homothallic mutants which otherwise are identical to their parent are readily induced by mutation of a heterothallic strain. The mere presence or absence of zygospores cannot be used as a classification criterion in this species which contains both hetero- and homothallic members.

Meiotic nuclear behavior and ascospore formation in five homothallic species of Neurospora

Meiotic nuclear behavior and chromosome numbers have been examined, using a propionic–iron–hematoxylin staining method, in Neurospora crassa and in five homothallic species, N. africana, N. dodgei, N. galapagosensis, N. lineolata and N. tcrricola. Ascus development, haploid chromosome numbers (n = 7), and morphology in all five homothallic species resemble the heterothallic N. crassa. The following observations have not been reported previously for any Neurospora species, although some have been described in other fungi. Most observations apply to all five homothallic species and to N. crassa except where otherwise indicated. (1) Chromosomes elongate considerably between karyogamy and the beginning of synapsis, and leptotene and zygotene stages can be identified. (2) The ascus tip flattens, and an apical pore begins to form during diplotene. (3) After telophase III, when all eight nuclei line up in single file, the nuclei are always arranged perpendicularly to the ascus wall with all spindle plaques on the same side. All nuclei then tilt relative to the ascus base with the spindle plaques at the lower end; consequently, the spores become obliquely arranged. (4) Ascospores of N. terricola are initially spindle shaped and occupy the entire ascus but later become compact and ovoid with spaces between them. (5) Maturing ascospores grow two to four times their initial compacted size. (6) In the young ascus, the nucleolus usually orients toward the base. (7) In N. terricola, the nucleolus is frequently hemispherical, especially in the young asci. (8) Half-moon-shaped polar caps are sometimes visible, one on each side of the prophase I nucleus; these are most prominent in N. terricola. (9) Cytological preparations are greatly improved if hydrolyzed perithecia are thoroughly washed in a Carnoy's solution containing chloroform.

Morphological Variation in the Genus Neurospora: Ascospore and DNA Studies

DUTTA, S. K., SHEIKH, I. & CHOPPALA, J. 1976. Morphological variation in the genus Neurospora: ascospore and DNA studies. Trans. Amer. Micros. Soc., 95: 462-469. The genus Neurospora shows remarkable morphological variations based on mating type compatibility and cellular variations caused by mutations. Scanning electron microscopical and cytological studies of ascospore development were carried out on three homothallic (8-ascospore ascus) species, N. africana, N. dodgei, and N. lineolata, and one pseudohomothallic (4-ascospore ascus) species, N. tetrasperma. Although N. tetrasperma ascospores were the largest in size quantitatively, the highest yield of pure ascospores was obtained from N. africana culture, and the shape of ascospores of all species studied were elliptical, though those of N. tetrasperma were more elongated. N. africana ascospores matured within 15-20 days, whereas ascospores of other species matured after 21 days when grown on agar medium. Pictures were taken by SEM to study patterns of wall sculpturing of ascospores of different species. DNAs isolated from all of these species showed biomodal thermal denaturation curves typical for DNAs of heterothallic species N. crassa; however, minor fractions of ascospore DNA were significantly smaller, quantitatively, in comparison with those of mycelial and conidial DNAs.

DNA homologies among homothallic, pseudo-homothallic and heterothallic species of Neurospora.

Highly purified DNAs from three homothallic species Neurospora africana, N. dodgei and N. lineolata; three reference strains representing authentic heterothallic species, N. crassa, N. intermedia and N. sitophila; and two strains of pseudo-homothallic species N. tetrasperma were characterized by spectrophotometry and DNA reassociation using hydroxyapatite chromatography. All of these known species are closely related on the basis of DNA characteristics such as base composition and thermal denaturation profiles of major DNA components. Minor components of ascospore DNA was, however, only 5-7% of total DNA instead of 15-20% minor component DNA shown by mycelial DNA. Species belonging to same group were not distinguishable morphologically, but all of these species were distinguishable by DNA:DNA homology studies. Greater DNA homology was noticed between DNAs of heterothallic species and DNAs of pseudohomothallic species than DNA of true homothallic species. Difference on DNA-nucleotide sequences among homothallic species was very little. Pseudo-homothallic species N. tetrasperma was found to be distinctly different from homothallic species but closer to heterothallic species based on such studies.

Pichia lindnerii sp. n., a new methanol assimilating yeast from soil (author's transl)

A new undescribed species of yeast could be detected in forest soils in the area of Berlin, Germany. A description of the new homothallic species including latin diagnosis is given.

Stimulation of Sexual Reproduction in Phytophthora by Damage

Summary: Mechanical damage to hyphae of some A2 compatibility-type Phytophthora spp., by cutting with a cork borer or scalpel, can induce formation of pairs of gametangia followed by oospores. The A1 compatibility types examined did not respond significantly to damage. Two homothallic species of Phytophthora responded to mechanical damage by forming increased numbers of gametangia. Induction of gametangia in A2 Phytophthora cinnamomi following damage depends on the composition of the medium. Injury produced by H2O2 or ether also induced sexual reproduction in A2 P. cinnamomi. It is concluded that the mechanism by which gametangia are induced in A2 Phytophthora strains by Trichoderma viride may involve damage and that production of oospores in response to damage may be significant in the survival of Phytophthora.

Trisporic Acid Synthesis in Homothallic and Heterothallic Mucorales

SUMMARY: The synthesis of trisporic acids, the sexual hormones, proceeds in a similar way in mated cultures of the heterothallic fungi Mucor mucedo and Blakeslea trispora. In both species precursors are secreted by both mating types and these can be converted to trisporic acids by the sexual partner. Precursor production is strongly stimulated by trisporic acids. In homothallic species the presence of biologically active substances can be demonstrated, which have characteristics similar to those found in heterothallic strains. Moreover, precursors of trisporic acids isolated from Mucor mucedo are readily converted to trisporic acids by homothallic species. These results suggest a close relationship between the hormonal systems of homothallic and heterothallic species.

Scanning Electron Microscope Studies on Ascospores of Homothallic Species of Neurospora

SUMMARYThe surface and subsurface topography of ascospores of homothallic species of Neurospora has been studied with the scanning electron microscope. Rib surfaces of the ascospore walls, as viewed under SEM, are smooth and free of any fibrillar pattern. Intercostal veins are revealed as pitted furrows in the perisporium aligned with circular to elongate depressions. Height of ribs, depth and continuity of the veins, and distinctiveness of the pitting of the ascospore walls vary according to species. Topographic features of the walls of N. dodgei are the most prominent and those of N. lineolata are the least prominent Surface features of the three other homothallic species fall within these two extremes. SEM views of ascospores after removal of the perisporium show that the germinal pore is an opening in the episporium and that the perisporium is not perforated.

Scanning Electron Microscope Studies on Ascospores of Homothallic Species ofNeurospora

The surface and subsurface topography of ascospores of homothallic species of Neurospora has been studied with the scanning electron microscope. Rib surfaces of the ascospore walls, as viewed under...

Observations on the sexual mechanism in Phytophthora palmivora and related species

Twelve out of twenty-three isolates of Phytophthora palmivora (Butl.) Butl. and related species formed oospores in single culture when freshly subcultured from paraffin oil stock culture, demonstrating their bisexual nature. This ability declined with further subculturing, and the isolates were apparently self-sterile under normal conditions. They fell into two compatibility groups in the classical manner on the basis of oospore formation in paired cultures. More oospores were formed in pairings between isolates from black pepper than in pairings between isolates from black pepper and isolates from other host plants. This situation is discussed. A relationship between ability to form sex organs in single culture and in paired cultures is suggested. The possibility of chemical stimulation of sexual reproduction between compatible isolates of P. palmivora was investigated. No evidence of this was obtained from experiments involving liquid culture filtrates, but single isolates from both compatibility groups were stimulated to form sex organs when separated by a cellophane membrane from an isolate of the homothallic species Phytophthora heveae Thompson in agar culture. An isolate of Trichoderma viride Pers. ex S. F. Gray stimulated sex organ formation in a single isolate of P. palmivora . These results are considered possible evidence of chemical stimulation of reproduction in P. palmivora . Alternative mechanisms of sexuality in heterothallic Phytophthora species are discussed.

Zygospore formation by the heterothallic Mortierella elongata and a related homothallic species, M. epigama sp.nov.

Zygospores between heterothallic strains of Mortierella elongata Linnem. were discovered independently in Athens, Georgia, U.S.A., and Wageningen, The Netherlands. Homothallic strains with similar sporangiophores and zygospores were isolated from composted municipal refuse in Germany. In both cases, one suspensor is strongly swollen and the outer wall of the mature zygospore shrinks in a honeycomb-like pattern. In both cases zygospores are formed at temperatures up to 30°C, with optimum between 20 and 25°C. For heterothallic strains SAB-sucrose-agar, hay extract agar, Pablum extract agar and defatted hempseed agar gave the best results, whereas for homothallic strains 2 % malt extract agar was found to be most suitable. Moreover differences were observed in shape of the sporangiospores, shape of the swollen suspensor and absence of chlamydospores which justify the description of the homothallic strains as a new species, Mortierella epigama . Since the swollen suspensor can originate from both sexes, A and a, this mating pattern is considered as physiological heterothallism, being determined by incompatibility factors only.

A New Species of Neurospora from Soil of West Pakistan

SUMMARY A new homothallic species of Neurospora, N. lineolata is described. The fungus was isolated from soil collected in the vicinity of Lahore, West Pakistan. Of the three other homothallic species with bipolar ascospores, N. lineolata most closely resembles N. africana; it differs, however, from this species by having narrower ascospores with numerous indistinct ribs and intercostal veins, and larger, beaked perithecia. Ascospore germination in N. lineolata is uni- and bipolar. The bipolar pattern appears to predominate. N. lineolata represents the twelfth species now placed in the genus Neurospora. A key to all species in the genus is provided. During the present decade new species of Neurospora have been reported with surprising frequency. Prior to 1960 only 7 species were included in the genus. With the publication of this communication the total number now recognized rises to 12. Five new species have been discovered within a span of less than 7 years. All of the new species are homothallic and have been isolated from soil. This development, however, has not been entirely unsuspected, for when Nelson et al. (5) described the second new species of this decade, Neurospora dodgei Nelson & Novak, they suggested that Neurosporas were probably much more common members of the soil mycoflora than heretofore believed. They speculated that the discovery of additional species from soil should be facilitated by the application of a selective isolation technique developed by Novak and Backus (6). It did not take long for these speculations to be substantiated. Mahoney, Huang and Backus (3) report in a current communication the discovery of two new species, N. galapagosensis Mahoney & Backus and N. africana Huang & Backus, from tropical soil. They employed the selective isolation technique of Novak and Backus (6) in their

New Homothallic Neurosporas from Tropical Soils

SUMMARY Two new 8-spored, non-conidial, homothallic species of Neurospora are described: N. galapagosensis Mahoney & Backus and N. africana Huang & Backus, obtained from soil samples collected on the Galapagos Islands and in Eastern Nigeria, respectively. These organisms were discovered through use of a technique which appears to be highly effective for isolating sordariaceous fungi from soil. The new Neurospora taxa share several important distinctive features with both N. terricola Gochenaur & Backus and N. dodgei Nelson & Novak but appear to be most closely related to the latter. Prior to 1960 no truly homothallic Neurospora was known, nor had any Neurospora been described which did not have a Monilia imperfect stage. This state of affairs was changed, however, with the discovery, early in the present decade, of Neurospora terricola Gochenaur & Backus (5) and N. dodgei Nelson & Novak (8)-two homothallic 8-spored species apparently devoid of both conidia and spermatia. The former had been encountered in Wisconsin lowland soil and the latter in moderately high elevation soils of Puerto Rico. In the present communication two additional specific taxa of the same general pattern are described. One of these was isolated from Galapagos Islands soil, the other from a soil sample from Nigeria. As had been the case with the discovery of N. terricola and N. dodgei, they were found through the use of selective isolation procedures. The discoveries reported here were made in the course of studies on the microfungal flora of selected tropical soils, the general results of which will be reported elsewhere.

Relationship Between Sexuality and Carotene Synthesis in Blakeslea trispora

When stimulated by equivalent amounts of progametangia-inducing hormones, cultures of the minus mating type of Blakeslea trispora produce about the same quantities of carotenoids as mated cultures of the fungus, which suggests that the stimulation of carotene synthesis during the sexual activity of mated cultures is the result of hormonal action. These hormones were isolated and purified. From spectroscopic analysis of purified samples, it appears that the hormones are identical with trisporic acids B and C. When both mating types of B. trispora were cultivated in one vessel but were kept apart by membrane filters, the formation of sex hormones was not inhibited. Physical contact between the mating types is obviously not required for the induction of sexual activity. The sex hormones also formed in combined cultures of B. trispora-plus and Zygorhynchus moelleri (a homothallic species), but not in combined cultures of B. trispora-minus and Z. moelleri. This is evidence for the hypothesis that the hormones are produced by B. trispora-plus only.

CHLAMYDOMONAS GYMNOGAMA, A NEW HOMOTHALLIC SPECIES WITH NAKED GAMETES(1).

The life history of a new homothallic species of Chlamydomonas is described. Sexual reproduction involves the pairing and fusion of naked gametes.

Rape Seed Extract Agar: A New Medium for Production and Detection of Oospores of Heterothallic Species of Phytophthora

The production of oospores of homothallic and heterothallic species of Phytophthora in artificial media has been reported by several workers (1-13). The most common media used are oatmeal (1, 2, 7, 8, 9, 13), lima bean (6, 12, 13), hemp seed extract (3, 11), and V-8 juice agars (4, 5, 10, 12). Although good results have been obtained with these media, the presence of oospores cannot be detected without microscopic examination.