General characters of Xanthophyceae & Life cycle of Vaucheria SMG

A General account of Xanthophyceae and Vaucheria - Thallus, Reproduction and Life cycle

1. General Characters of Xanthophyceae & Life Cycle of Vaucheria Dr. Saji Mariam George Associate Professor Assumption College Autonomous Changanacherry

2. CLASS XANTHOPHYCEAE (YELLOW GREEN ALGAE ) 75 genera & 450 sps . GENERAL CHARACTERS 1. Habitat Aquatic – mostly freshwater, as epiphytes or free floating forms Sub aerial Terrestrial Marine (e.g. Halosphaera ) Image:https ://megasun.bch.umontreal.ca/

3. 2. Thallus Organization i ) Unicellular motile forms e.g. Chloramoeba Phacomonas Heterochloris Chloramoeba Image: https://sr.wikipedia.org/

4. ii ) Palmelloid forms e.g. Chlorogloea Chlorosaccus © CCALA 2013

5. iii )Colonial forms e.g. Botryococcus Ophiocytium   Image: Protist Information Server http://protist.i.hosei.ac.jp/ Image: © Science Photo Library Limited 2020 https://www.sciencephoto.com/

6. iv ) Dendroid forms e.g. Mischococcus . Image:Protist Information Server http://protist.i.hosei.ac.jp/ Image:Y . Tsukii , Protist Information Server,  http://cfb.unh.edu/

7. v ) Rhizopodial form e.g. Stipitococcus Image: http://www.fishbiosystem.ru/

8. vi ) Coccoid forms e.g. Chlorobotrys Image : http://cfb.unh.edu/

9. vii) Filamentous forms e.g. Tribonema Heterococcus Image: http://protist.i.hosei.ac.jp/ Protist Information Server ©  Pavel Škaloud https://botany.natur.cuni.cz/

10. vii) Filamentous forms…… e.g. Heterothrix Copyright    Protist Information Server http://protist.i.hosei.ac.jp/

11. viii ) Siphonous forms e.g.Vaucheria Botrydium © Dwight Kuhn https://dkphoto.photoshelter.com/ © Copyright Malcolm Storey 2011-2118 https://www.discoverlife.org/mp

12. 3. Cell wall Often absent If present , made up of high content of pectic compounds. Some cellulose may be present. Silicified cell wall in a few species – has 2 unequal overlapping halves.

13. 4. Flagella Two, unequal in length, at the anterior end, Heterokont ( an organism with a hairy and a smooth flagellum) Longer flagellum – Tinsel or Pantonematic Type - Bears numerous fine flimmer hairs in two rows . Shorter Flagellum- Whiplash or Acronematic type.

14. 5. Pigments Pigments are located in discoid, yellowish green chromatophores . Many chromatophores are present in each cell. Pigments include Chlorophylls a, c and e ; Beta Carotene and one Xanthophyll . Pyrenoids are absent.

15. 6. Reserve food Fats , Oils, Glucose polymers like Leucosin (which occur as whitish lump) or Chrysolaminarin . 7. Reproduction i ) Vegetative – cell division, fragmentation ii) Asexual – Zoospores, Aplanospores , Statospores . ii ) Sexual – Rare. If present , isogamous , oogamous in Vaucheria .

16. VAUCHERIA SYSTEMATIC POSITION Division : Xanthophyta Class : Xanthophyceae Order : Siphonales ( Vaucheriales ) Family : Vaucheriaceae

17. Mostly found in fresh water. Some are terrestrial – in mud ( Vaucheria thuretti ); in damp soil ( V. sessilis , V. hamata ) A few are marine ( V. piloboloides )

18. Examples: Vaucheria sessilis – terrestrial or aquatic V. uncinata - Aquatic V. amphibia - Amphibious V. terrestris - Terrestrial V. mayyanadensis – Terrestrial (from Kerala) V. germinata - Aquatic V. polyperma - Aquatic.

19. Vegetative Structure of Vaucheria (Structure of Thallus ) Mostly a fresh water alga. Yellowish green thallus is the Gametophyte. Thallus form a tangled mass called ‘Green felt’.

20. Vaucheria thallus is filamentous, branched – branching may be monopodial , lateral or dichotomous. Image: https://fmp.conncoll.edu https://www.landcareresearch.co.nz

21. The filaments are siphonaceous . Aseptate – septa or cross walls are not present to separate the cells of the filament (Septa arise at the time of reproduction). Coenocytic – the protoplasm is continuous along its entire length embedding many nuclei.

22. Terrestrial species have Holdfast or Haptera (Singular – Hapteron ), a rhizoid-like structure at the base of the thallus which help for the attachment to the substratum). Image:http ://www.biologydiscussion.com/

23. Vaucheria thallus Image:https :// www.naturepl.com

24. In some species ( V. debryana ), the surface of the filament may be impregnated with calcium carbonate. The wall of the filament is thin, composed of inner layer of cellulose and outer layer of pectic substances. Inner to the cell wall is the cytoplasm enclosing many nuclei , organelles, many oval or circular or elliptical chromatophores containing Chlorophyll a, e,carotenoids and xanthophyll pigments, reserve food materials like oil drops etc. Pyrenoids are absent. Cytoplasmic streaming ( Cyclosis ) is present which helps in the uniform distribution of nutrients, enzymes, metabolites etc. A large central vacuole filled with cell sap is present in the centre.

25. REPRODUCTION IN VAUCHERIA Vegetative – Fragmentation Asexual - Common method Zoospores Aplanospores Hypnospores Akinetes Cysts. 3. Sexual - Oogamy .

26. Vegetative – Fragmentation The thallus breaks into small fragments . Each fragment by repeated divisions will develop into a new thallus . 2. Asexual reproduction i ) By Zoospores (Compound zoospores or Synzoospores or Coenozoospores ). Most common method that occurs in aquatic species and also in terrestrial species in flooded areas.

27. Environmental factors which promote zoospore formation include low intensity light, change of medium from running to still waters, low nutrient level, complete darkness etc. During asexual reproduction, Vaucheria produce a motile, multiflagellate compound zoospore known as ‘ Synzoospores ’ or Coenozoospores inside a zoosporangium. A single compound zoospore develops inside the zoosporangium.

28. The tip of the lateral branch elongates and swells up and forms a long, club shaped zoosporangium. The cytoplasm containing many nuclei, chromatophores , organelles, reserve food etc. flows into it. Very soon, a septum develops at the base of the zoosporangium and separates it from the rest of the thallus .

29. The central vacuole of the zoosporangium becomes much narrow and the zoosporangium appears deep green. The protoplast contracts to form an oval, multinucleate, young zoospore. Within the zoospore, chromatophores and nuclei reverse their orientation in such a way that the chromatophores come to occupy the inner zone and nuclei come to occupy the outer zone. A pair of flagella develop opposite to each nucleus – Thus a multinucleate and multiflagellate zoospore is formed.

30. Asexual reproduction by Zoospores in Vaucheria Image:http ://www.biologydiscussion.com

31. The mature zoospore is large, yellow - green , oval or pyriform , multinucleate and multiflagellate . Its protoplasm is distributed in the periphery because of the presence of a large central vacuole. According to Fritsch, this is a compound zoospore or synzoospore or coenozoospore which represents a group of zoospores which failed to separate from one another.

32. The apical wall of the zoosporangium undergoes gelatinization and forms a terminal opening – through this opening the zoospore slowly moves out and escapes from the zoosporangium. Germination of zoospore – The released zoospore swims about slowly for 15 – 20 minutes. Then it comes to rest, withdraws its flagella, rounds up and secretes a thin cellulosic wall around it.

33. At this stage, the nuclei move inward and the chromatophores move outward. Then the zoospore gives out two tubular outgrowths – one of these undergoes branching to form the colourless , lobed holdfast while the other continues to grow and form the yellowish green tubular filament.

34. ii ) Asexual reproduction by Aplanospores Aplanospores are non - motile asexual spores. They are usually produced by terrestrial species under unfavourable conditions. Aquatic species produce aplanospores when they are transferred from light to darkness or from running waters to still waters. Aplanospore producing sps . – Vaucheria uncinata , V. hamata , V. piloboloides .

35. Aplanospores are produced inside aplanosporangia which are located apically on short branches. Aplanosporangia may be round ( Vaucheria uncinata ), ovoid ( V. geminata ), clavate ( V. hamata ), elongated ( V. piloboloides ) etc. The aplanosporangium get separated from the branch by the development of a septum.

36. The protoplast of the aplanosporangium is converted into a single, rounded, thin walled aplanospore which is liberated by the irregular rupture of the sporangial wall. The aplanospore germinates within the aplanosporangium or after liberation and produce tubular outgrowths and form a new thallus .

37. Asexual reproduction in Vaucheria by Aplanospores Image:http :// www.biologydiscussion.com

38. iii ) Asexual reproduction by Akinets (Cysts, Hypnospores ) Some terrestrial and aquatic species, when exposed to unfavourable conditions like desiccation or lower temperature, form Akinets (Cysts, Hypnospores ). The branched filaments divide into rows of short segments by thick, gelatinous cross walls.

39. The protoplasts contain the reserve food material, oil. These resting, multinucleate, thick walled segments are called akinets or cysts or hypnospores . The cysts in the chain may remain connected by the membrane of the filament . In this stage , it resembles another alga Gongrosira and hence this stage is known as “ Gongrosira stage”.

40. During favourable conditions, each cyst may directly grow to a new filament. Sometimes, the protoplast of the cyst divides into small protoplasmic bits which are liberated through a pore in the cyst wall. After some time , this amoeboid structure come to rest and assumes a spherical form. It then secretes a cell wall around it and develops into a new filament.

41. Asexual reproduction in Vaucheria by Akinets Image:http :// www.biologydiscussion.com

42. 3. Sexual Reproduction All species of Vaucheria reproduce sexually. Sexual reproduction is by Oogamy – Occurs in moist land and quiet waters. The male reproductive organ is Antheridium and the female reproductive organ is Oogonium . They may be stalked or sessile.

43. Most of the species are monoecious (Homothallic)- The antheridia and oogonia develop close to each other on the same thallus . Majority of monoecious species are protandrous - antheridia mature earlier than oogonium . e.g Vaucheria sessilis , V. terrestris , V. geminata . Some are dioecious (heterothallic) – The antheridia and oogonia develop on separate thalli . e.g. V. dichotoma , V. littorea etc.

44. Antheridium In monoecious species, the sex organs antheridium and oogonium develop on the same filament . Most species are protandrous . The antheridial branch arise as a lateral outgrowth and has abundant cytoplasm, many chromatophores and nuclei . The mature antheridium is a slender, tubular , hook - like structure with a terminal pore – it is separated from the main thallus by a septum.

45. The nuclei of the antheridium undergo repeated mitotic division. Each nucleus with a bit of cytoplasm transforms into a spindle shaped and biflagellate antherozoid –The flagella are lateral in position - one of the flagella is long, tinsel type ( Pantonematic )and forwardly directing . The other flagellum is short, whiplash - type ( acronematic ) and backwardly directing. At the anterior end of the antherozoid , there may be a small process called proboscis. Mature antherozoids are liberated through the apical pore.

46. Oogonium The female reproductive organ In homothallic species, oogonium develop in the same filament just near the antheridium after some time. Oogonium begins as a small protuberance at the base of the antheridium by accumulation of cytoplasm and a large number of nuclei. This dense , colourless , multinucleate mass of cytoplasm is called wanderplasm (Couch , 1932) – It gradually increase in size and forms an ovoid or rounded oogonium which is separated from the rest of the filament by a septum.

47. Vaucheria : Development of Antheridium and Oogonium Image: http://www.biologydiscussion.com/

48. Vaucheria : Antheridium and Oogonium Image:https ://classconnection.s3.amazonaws.com

49. Mature oogonium is spherical or sub – spherical , with an apical beak. All the nuclei except one disintegrate and a round uninucleate egg develops inside the oogonium . At the anterior end of the egg, there is a hyaline area which acts as a receptive spot. At maturity, the beak gelatinizes and an opening develops.

50. Fertilization The motile antherozoids enter the oogonium through the apical pore and come in contact with the ovum (egg) – one of them penetrates the egg . Plasmogamy and karyogamy follow , resulting in the formation of zygote. Soon, the zygote secrete a thick, multilayered wall and transforms into an oospore . Vaucheria with zygotes  Image © Tom Adams/Visuals Unlimited/Corbis. https://www.dukeupress.edu/emergent-ecologies

51. Germination of Oospore After a period of rest, the zygote undergo germination. The diploid nucleus undergoes one meiotic division and several mitotic divisions and form a tubular filament. The life cycle is haplontic since the haploid phase is the dominant phase in the life cycle. The diploid phase is very short represented only by the zygote.

52. VAUCHERIA- LIFE CYCLE https://brainly.in

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