Sexual reproduction in protozoa

Sexual reproduction inprotozoans is by conjugation orsyngamy. Syngamy is the fusion of gametes. It may be isogamy. conjugation etc

1. Sexual Reproduction in Protozoa
Two processes-1. Syngamy
2. Conjugation
1. Syngamy:
It is the complete fusion of two cells or gametes,
resulting in the formation of zygote. The fusion
nucleus of zygote is called synkaryon.

2. a)Hologamy:Thetwoordinarymatureprotozoanindividualsdonotform
gametesbutthemselvesbehaveasgametesandfusetogethertoformzygote.
e.g.Copromonas
b)Isogamy:Whentwofusinggametesaresimilarinsizeandshapebutdiffer
inbehaviour,theyarecalledisogametesandtheirunion,isogamy.Isogametes
aregenerallyproducedbymultiplefission
e.g.Chlamydomonas
c)Anisogamy:Whentwofusinggametes,differmorphologicallyaswellasin
behaviour,theyarecalledanisogametesandthefusionofthesedissimilar
gametesiscalledanisogamy.Smallandmotilegametesarethemaleor
microgametesandlargenon-motileonesarethefemaleormacrogametes.
e.g.Plasmodium,Volvox
d)Autogamy:Itisthefusionofgametesderivedfromthesameparentcell.
Pseudopodiaarewithdrawnandacystisformed.Nowmeioticdivisiontakes
placeandtwodaughternucleiwithhalfnumberofchromosomesareformed.
Aftersometime,gameticnucleifusetoformazygotenucleus.
e.g.Actinophrys

3. 2. Conjugation (Amphimixis):
Itisthetemporaryunionoftwoindividualsusuallyatoralorbuccal
regionsoftheirbody.
ItischaracteristicofSuctoriaandholotrichciliates.
Thesetwoindividualsarecalledtheconjugants.
Theystopfeedingandtheirbuccalstructuresdisappear.
Thepellicleandectoplasmfuseatthepointofcontactanda
protoplasmicbridgeisformedbetweentheconjugants.
Macronucleibreakupanddisappear.
Thediploidmicronucleusofeachconjugantundergomeiotic
divisionand4haploiddaughtermicronucleiareproducedofwhich
3degenerateanddisappearineachconjugant.
Theremainingonemicronucleusdividesbymitosisforming2
unequalpronucleiorgameticnuclei.

4. Thesmalleroneistheactivemigratorygametenucleusor
malepronucleusandthebiggeroneisthepassive
stationarygametenucleusorfemalepronucleus.
Malepronucleusofoneconjugantpassesthroughthe
protoplasmicbridgeintotheotherconjugantandfuses
withitsfemalepronucleus,formingasinglediploidzygote
nucleusorsynkaryon.
Thecompletefusionoftwonucleifromtwodifferent
individualsformingazygotenucleusistermedamphimixis.
Afteraunionofabout12to48hours,twoindividuals
separateandarecalledexconjugants.
Eachconjugantsundergoesfurthernuclearandcytoplasmic
divisionsformingfourdaughterindividuals.

6. Factors and conditions of conjugants:
Doesnotoccurunderfavourablelivingconditions
Starvationandshortageoffoodandcertainchemicalsaresaidto
induceconjugation.
Acertainrangeoflightandtemperature
Usuallystartsearlyinmorningandiscontinuedtillafternoon.
Individualsaresmallerinsizethanthenormalone
Itnevertakesplaceamongthedescendantsofasingleindividual
Itoccursonlybetweenindividualsbelongingtodifferentmating
types.

7. Significance of conjugation
Rejuvenation:
Ifbinaryfissioncontinuesrepeatedlyforseveralgenerations,the
individuallosesitsvigour,ceasestomultiply,reducesinsizeand
eventuallydiesoff.
Toavoidthis,conjugationisresortedtoandtheprocessseemsto
rejuvenateandrevivethelostvigourforasexualreproduction.
Nuclearreorganization:
Duringconjugation,thenuclearapparatusisreorganizedanda
readjustmentoccursbetweenitandthecytoplasm.
Thereplacementofmacronucleusbyanewonebringsrenewed
vigourandvitalitytoacceleratethemetabolicactivities.
Hereditaryvariation:
Duringasexualreproductionbyfission,thehereditarymaterialofthe
parentpassesunchangedontotheprogeny,sothatallthe
descendantsofoneindividualhavethesameinheritance.
Theperiodicoccurrenceofconjugationensuresinheritedvariation.

8. AA aa
AA aa
AaAaAa
Aa
Aa Aa
Aa
Aa
AAAaAa aa
Genetic consequences of conjugation:

10. Regeneration
At fission or encystment

11. Economic importance
1.Useful protozoa
Helpful in sanitation
Planktonic protozoa as food
Symbiotic protozoa
Oceanic ooze and fossil protozoa
Protozoa in study

12. 2. Harmful protozoa:
Soil protozoa
Water pollution
Pathogenic protozoa