1. Chytridiomycota
Kingdom - Fungi

2. Chytridiomycota
Asexual reproduction by zoospores produced in zoosporangia
Zoospores have one posterior flagellum
Vegetative thallus variable – range from globose, multinucleate to hyphal forms
Growth may be determinate or indeterminate

3. Zoospores
Initially zoospores encyst – withdraw or lose flagellum, rapidly form a cell wall
Cyst then germinates to form rhizoids and enlarges

4. Chytridiomycota
Habitats – zoospores require free water in which to swim – many occur in aquatic habitats, also found in soil water
Many species are saprotrophic – grow on a variety of substrates, most are aerobic, some anaerobic
Some are parasitic on algae, other fungi, aquatic animals, some parasitic on higher plants (crops)

5. Chytridiomycota
Include 5 orders (will discuss 2), distinguished on basis of habitat, zoospore ultrastructure, other characterisitics
Chytridiales – mainly aquatic
Spizellomycetales – mainly in soils
Neocallimasticales – occur in rumen
Blastocladiales – mainly aquatic
Monoblepharidales – small no of species, all filamentous, unique sexual reproduction

6. Zoospore ultrastructure

7. Chytridiales Primarily aquatic
Saprotrophs grow on variety of substrates – “baiting”
Parasites of algae, fungi, animals, higher plants –
“black wart of potato” caused by Synchytrium endobioticum
Olpidium brassicae is a cabbage parasite that is a vector for a plant virus

8. Chytridiales
One species is parasitic on amphibians – mentioned in decline of frog populations - Batrachochytrium dendrobatidis

9. Sexual reproduction
Great deal of variation, but nuclear events, e.g. meiosis, not clearly determined
Fusions have been seen between zoospores, gametangia, rhizoids

10. Vegetative thallus Single multinucleate thallus with no appendages
If grows within host cell it is endobiotic
If entire thallus is converted to zoosporangium, it is holocarpic

11. Vegetative thallus
Many species form rhizoids – tapering structures that anchor thallus and increase surface area for absorption of nutrients
During differentiation, the entire thallus is not converted into a zoosporangium – eucarpic
May be within host cell – endobiotic or outside - epibiotic

12. Vegetative thallus
Some chytrids produce only one zoosporangia per thallus – monocentric
Others produce multiple zoosporangia – polycentric
Produce rhizomycelium

13. Vegetative thalli

14. Zoosporangia Thallus (or part) differentiates into zoosporangium
Triggered by environmental conditions, thallus size, nutrient concentration
Multinucleate cytoplasm is cleaved into a number of zoospores
Golgi produce vesicles that are deposited around nuclei – form plasma membrane, flagella
Once formed zoospores escape sporangium

15. Zoosporangium

16. Zoosporangium formation

17. Zoosporangium Zoospores are released from zoosporangia by
Breakdown of sporangium wall
Forming 1 or more discharge papillae
Opening in papilla may be
A lid = operculum
By becoming thin and dissolving - inoperculate

18. Resting spores
Chytrids may form resting spores – thick cell wall, may be ornamented with spines, knobs or may be smooth
Typically undergo a period of dormancy

19. Blastocladiales
Relatively small order – mainly saprotrophs, great variation in vegetative thallus
Characteristics
Produce brown, thick-walled pitted resting sporangia
Characteristic zoospore (nuclear cap containing cellular ribosomes)
Representative genera

20. Coelomomyces
Obligate parasite of aquatic animals – diploid phase on mosquito and midge larvae, haploid phase on copepods
Forms a holocarpic, endobiotic thallus
Forms isogametes that are motile for sexual reproduction
Possible biological control agents for mosquitoes (importance in understanding life cycles)

21. Coelomomyces life cycle

22. Blastocladiella Monocentric thallus, eucarpic
Asexual life cycle – can form two types of sporangia depending on environment
Thin walled zoosporangia
Thick walled resting sporangia when CO2 concentrations are high
Has been used to examine the biochemistry of differentiation along these two pathways

23. Blastocladia
Forms polycentric thallus but exhibits determinate growth

24. Allomyces Great deal of research on development and genetics
Some species reproduce both sexually and asexually, in others only asexual reproduction
Some species exhibit a haploid – diploid life cycle
Haploid vegetative mycelium
Diploid vegetative mycelium

25. Allomyces
Haploid and diploid mycelia are identical except for the reproductive structures they produce
Haploid mycelium produces gametangia
Diploid mycelium produces zoosporangia and resistant sporangia
Hyphae branch dichotomously, produce septa with many perforations

26. Allomyces life cycle Haploid zoospore germinates to form 1n thallus
Tips of hyphae produce male and female gametangia
Male gametangia orange
Female gametangia colorless

27. Allomyces life cycle
Cytoplasm in gametangia cleaves to produce gametes
Both gametes are motile, leave gametangia through discharge pores in papillae

28. Allomyces life cycle Gametes swim Male gametes smaller, orange
Female gametes larger, colorless
Female gametes produce substance, sirenin that attracts male gametes chemotactically
Male and female gametes fuse (plasmogamy and karyogamy) to form zygote

29. Allomyces life cycle Zygote swims and encysts
Germinates to produce diploid mycelium
Produces zoosporangia – 2n zoospores that encyst and germinate to produce 2n thallus

30. Allomyces life cycle
Zoosporangia

31. Allomyces life cycle
2n mycelium also produces resistant sporangia – thick walled, pitted, brown structures that can remain dormant
When resistant sporangia germinate, they undergo meiosis to form haploid zoospores that start the cycle over

32. Allomyces life cycle

33. Allomyces
Divided into several subgenera based on reductions in the haploid portion of the life cycle, some only reproduce asexually like Blastocladiella

34. Monoblepharidales
Small order of filamentous eucarpic thalli that produce zoosporangia on ends of hyphae
Produce small motile male gametes and large non motile female gametes
Thought that life cycle is haploid, i.e. germination of zygote includes meiosis