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OOMYCETES I INTRODUCTION & SAPROLEGNIALES. IB 371 – GENERAL MYCOLOGY LECTURE 13 THURSDAY, OCTOBER 9, 2003. OOMYCETES DEFINING CHARACTERISTICS. Vegetative thallus filamentous, coenocytic, in some groups unicellular Cell walls lacking chitin (except for a few taxa), made of glucans & cellulose

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oomycetes i introduction saprolegniales

OOMYCETES IINTRODUCTION &SAPROLEGNIALES

IB 371 – GENERAL MYCOLOGY

LECTURE 13

THURSDAY, OCTOBER 9, 2003

oomycetes defining characteristics
OOMYCETESDEFINING CHARACTERISTICS
  • Vegetative thallus filamentous, coenocytic, in some groups unicellular
  • Cell walls lacking chitin (except for a few taxa), made of glucans & cellulose
  • Asexual reproduction by biflagellate zoospores
  • Sexual reproduction by oogonia (female) & antheridia (male) - no motile gametes
  • Sexual spore is the oospore
  • Diploid life cycle
oomycetes defining characteristics1
OOMYCETESDEFINING CHARACTERISTICS
  • Sexual reproduction is oogamous
  • Female gamete (oosphere) produced by an oogonium
  • Depending on taxon, there may be one to many oospheres per oogonium
  • Male gamete is produced by antheridium and transferred to the oogonium by gametangial contact and migration of male nuclei
oomycetes defining characteristics2
OOMYCETESDEFINING CHARACTERISTICS
  • Asexual reproduction is by zoospores
  • Zoospores have flagella that are anisokont (unequal) and heterokont (different) (straminipilous)
  • The anteriorly directed flagellum bears two rows of tubular tripartite hairs
  • The posteriorly directed flagellum is a whiplash type
slide7

DIPLOID

LIFE

CYCLE

(From Alexopoulos et al.)

oomycetes
OOMYCETES
  • Phylogenetically unrelated to the fungi but similar in morphology & physiology
  • Now in Chromophyta with brown algae
  • 9 orders, 25 families, 95 genera
  • Between 500-800 species
  • Called Peronosporomycetes by Dick
orders of oomycetes to be covered
ORDERS OF OOMYCETES TO BE COVERED
  • Saprolegniales
  • Leptomitales
  • Lagenidiales (Salilagenidiales)
  • Peronosporales
saprolegniales
SAPROLEGNIALES
  • Found in water and soil.
  • Captured by baiting soil and water samples with hemp seeds or dead flies.
  • Saprobic on both dead plants and animals.
  • A few species are parasitic on fish.
saprolegniales1
SAPROLEGNIALES
  • Thallus is filamentous.
  • Hyphae is hyaline, broad & coenocytic.
  • Asexual reproduction is by biflagellatezoospores formed in large, cylindrical sporangia.
  • Sexual reproduction is by oogonia containing several oospheres that receive nuclei directly from antheridia.
  • A fertilized oosphere is called an oospore.
zoospores
ZOOSPORES
  • Primary – oval with two flagella at apex, one is tinsel and the other is whiplash.
  • Secondary – kidney shaped with two lateral flagella, one anteriorly directed tinsel and one posteriorly directed whiplash
slide15

From

Introductory

Mycology

by

J. Webster

zoospores1
ZOOSPORES
  • Monomorphic – having only one type of zoospore.
  • Dimorphic – having both types of zoospores.
zoospores2
ZOOSPORES
  • Monoplanetic – having only one swimming period followed by encystment.
  • Diplanetic – having one swimming period followed by encystment, emergence and a second swimming period followed by encystment.
  • Polyplanetic – Having multiple swimming and encystment periods.
saprolegniales2
SAPROLEGNIALES
  • Saprolegnia – Dimorphic, diplanetic with swimming periods of equal duration.
  • Achlya – Dimorphic, diplanetic but primary zoospores encyst just outside the sporangium.
  • Dictyuchus – Monomorphic, polyplanetic, no primary spores are liberated. Primary spore encysts in the sporangium, each cyst releases a secondary zoospore that can swim and encyst repeatedly.
saprolegniales3
SAPROLEGNIALES
  • Thraustotheca – Monomorphic, monoplanetic, primary zoospores encyst within sporangium, secondary zoospores swim only once.
  • Geolegnia – Aplanetic, both swimming periods have been repressed, aplanospores germinate directly from sporangia.
sexual reproduction
SEXUAL REPRODUCTION
  • Most species are monoecious and self-fertile.
  • A few species are self-sterile and heterothallic (Achlya ambisexualis, Achlya bisexualis).
sexual reproduction1
SEXUAL REPRODUCTION
  • Hormone A (antheridiol) - produced by the vegetative hyphae of female, causes the vegetative hyphae of the male strain to form antheridial branches and grow toward the oogonia along a concentration gradient (chemotropic).
  • Hormone B (oogonial) - produced by the antheridial branches, causes female vegetative hyphae to produce oogonial initials.
slide31

From

Lower Fungi in

the Laboratory

leptomitales
LEPTOMITALES
  • Small group of about 30 species.
  • Differs from Saprolegniales in having hyphae that is constricted at regular intervals.
  • Cytoplasm of coenocytic hyphae contains conspicuous granules.
  • Oogonia contain a single oosphere (except in Apodachlyella completa)
slide36

From

Lower Fungi in the Laboratory