Biometry of oospores and oogonia of Pythium (Oomycetes): the independent taxonomic value of calculated ratios

SHAHZAD, S., COE, R. & DICK, M. W., 1992. Biometry of oospores and oogonia of Pythium (Oomycetes): the independent taxonomic value of calculated ratios. The oogonia and oospores of 80 isolates, representing 40 species of Pythium , were examined to determine the taxonomic values of direct measurements and derived indices. It was demonstrated that 20 individual oogonia from an isolate provided a suitable data base, with acceptable levels for standard errors of the means. All four measurable and all three calculable variables showed continuous variation between isolates and species, so that no single criterion was able to effect taxonomic separation. The inadequacy of the traditional plerotic/aplerotic concept was revealed. Derived indices were shown to have a significant taxonomic value in addition to the measured parameters from which they were derived. When taken together, using canonical variate analysis, the variables enabled isolates to be grouped into their respective species without recourse to other taxonomic features. The enhanced precision of the assessment of oogonial characteristics has enabled a new approach to Pythium classification to be developed.     

Biometry and medical statistics

The "biometric school" founded by K. Pearson, F. Galton, and W. F. R. Weldon was concerned especially with heredity and variation, and between the wars "biometry" was not widely used as a general term for quantitative biology. The foundation of the Biometric Society encouraged this wider usage, and medical and biological statistics were seen to share a common methodology. In recent years, medical statistics has developed more rapidly, and this growth has been reflected in the contents of Biometrics. The author stresses the essential unity of the subject, the central core of which is the application of statistics in the life sciences. The Society should encourage contact with those workers in quantitative biology who are outside this central tradition.     

Viability and maturation of Aphanomyces cochlioides oospores

Plasmolysis, tetrazolium bromide staining and microscopic appearance were tested for their usefulness in determining viability of oospores of Aphanomyces cochlioides. For comparison, three lethal treatments were employed to contrast the reaction of dead oospores and untreated, presumably viable oospores. Few oospores stained with tetrazolium bromide, even though plasmolysis and microscopic appearance indicated that 85% were viable. Cytoplasm of viable oospores was densely organized and uniformly granular (DOUG), whereas cytoplasm of oospores exposed to lethal treatments was loosely organized and non-uniformly granular (LONG). Dose-response bioassay experiments were conducted with untreated oospores of varying inoculum densities or with mixtures of untreated DOUG and heat-treated LONG oospores in varying proportions. The number of DOUG oospores was correlated (R(2) = 0.62, P < 0.001) with severity of damping-off of sugar beet seedlings caused by A. cochlioides. Thus, the granular appearance of cytoplasm offered a fast, easy and reliable indicator of viability of A. cochlioides oospores. Tests with newly formed oospores/oogonia showed that >80% harvested at 3-4 d after inoculation of hypocotyls stained with tetrazolium, but by 8-9 d <10% stained, apparently because of declining permeability of the spore wall to tetrazolium as oospores matured.     

Symbols and Terminology in Biometry

The many biologists whose work requires statistical science must be concerned for sound management and interpretation of quantitative data. Biometricians also need increased care for literacy and clarity in speech and writing, with precise phrasing for every numerical statement. This paper illustrates common confusions that arise from inexact terminology, or from words and symbols used without adequate definition. Careless statements on quantitative relations, or on probabilities, may be ambiguous; bad practices seriously pollute scientific journals and obstruct transmission of information. Such faults can affect daily life for a modern citizen. Pedantry is unwanted, and to be dogmatic about corrective measures would be stupid. This paper suggests that biologists and biometricians should examine the practicability of a system yet to be devised for standardizing use of symbols and the generally accepted terminology for the methods, techniques, and processes of statistical analysis. The outcome should influence all that we biometricians say and do — as authors, as consultants, and as referees for journals.     

An Electron Microscope Study of Cysts in the Saprolegniaceae

The structure of the zoospore cysts of various members of theSaprolegniaceae has been studied by electron microscopy. InSaprolegnia ferax, S. dioica, in Isoachlya eccentrica and I.unispora the primary cysts were smooth, whilst the secondarycysts bore stalked double-headed hooks. In S. parasitica thesecondary cysts bore tufts of longer hooks. In Protoachlya,Achlya, and Brevilegnia neither type of cyst bore hooks. InDictyuchus sterile the secondary cysts bore large spiny projections.The primary cysts in many species bore tufts of radiating hairs,and it is suggested that these are the remains of ciliated flagella.     

Water stress and sporangial emptying in Achlya (Saprolegniaceae)

Time-course experimenls demonstrate that sporangial emptying in Achlya inlricata is inhibited by experimental depression of the external osmotic potential. The sporangial wall is a macromolecular sieve, impermeable to polyethylene glycols (PEGs) with molecular weight 3350. The process of emptying can be precisely manipulated using PEG-8000 which suggests that spore discharge in an aqueous environment is effected by a reduction in the osmotic potential of the sporangial lumen relative to the external solution.     

Phytochrome-mediated germination of very sensitive oospores

The light receptor and its mode of operation were studied in photosensitive oospores of Nitella furcata subsp. megacarpa (Allen emend. Wood). Brief pulses of light activated maximal germination of post-secondary dormant oospores removed from lake sediments. Fluence response data at 12 wavelengths were used to construct an action spectrum for germination. The shape of the action spectrum with its maximum at 669 nm provides evidence for the involvement of phytochrome. Germination was induced with photon fluences that established as little as 0.01% of the phytochrome in the far red-absorbing form, which suggests that phytochrome was operating in the very low-fluence response mode. The functioning of phytochrome in the very low-fluence response mode in Nitella is similar to that in higher plants.     

Nonhomologous pairing in oogonia and ganglia of Drosophila melanogaster

A high frequency of chromosome pairing between an X-duplication (Dp) and a compound 4 (–44), each of which lacks its homologue in the genome, has been observed in somatic ganglia cells in female and male larvae as well as in oogonia of adult Drosophila melanogaster. Three Dp's of different sizes were used. Pairing was determined both by inspection and by precise measurement of the distances between chromosomes; the two methods agreed in virtually every case. Nonhomologous pairing frequencies ranged from 23.0 to 62.6%, showing a strong dependency on size. The Dp that was closest in size to –44 produced the greatest frequency of pairing in each of the three tissues. Homologues in the same cells paired with a frequency of 89% or greater. The X and Y in the male ganglia showed a pairing frequency of only 45.7%, thus resembling nonhomologues in their pairing behavior. Genetic studies were also carried out on the three genotypes. Segregation data for the Dp's and –44 established that the same pattern of pairing observed in the mitotic cells was present in the oocytes — the Dp which was closest in size to the –44 again producing the greatest frequency of pairing. In the oocytes the pairing frequencies were much higher than those observed in the mitotic cells.From a dissertation submitted to the University of Tennessee in partial fulfillment of the requirements for the degree of Doctor of Philosophy. This work was jointly sponsored by National Science Foundation Grant No. GZ-1323, and by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation.     

Homothallic sexual reproduction of Pustula helianthicola and germination of oospores

Sunflower white blister rust has become an important disease in many countries with intensive cultivation of the important oil crop. The biology of the pathogen is still partly unclear, particular with respect to its sexual reproduction and primary mode of infection. Zoospores released from sporangia of Pustula helianthicola were isolated individually and used for the inoculation of sunflower in order to generate unithallic, genetically homogenous infections. Single zoospore inoculation of young seedlings resulted in mitotic sporulation within subepidermal blisters on cotyledons and true leaves after approximately 2 weeks. Three weeks postinoculation, the infected plants started forming oospores, hence indicating homothallic sexual reproduction of the pathogen. The development of oogonia and antheridia was studied using light and fluorescence microscopy. Oospores were isolated from infected plant tissue and used for infection and germination studies. Microscopic observation of isolated oospores showed germination that formed sessile vesicle-like structures, germ sporangia or only germ tubes. The rate of germination reached approximately 40 %. Germination was not dependant on a resting phase after oospore formation. Oospores applied to the above ground parts of sunflower seedlings lead to infections within a similar time frame as was achieved with mitotic sporangia. The results underline the importance of oospores for primary infection at the beginning of the season and for long-distance dispersal of the pathogen with sunflower seeds contaminated by oospores.