How Paramoecium reproduces sexually?
In Paramoecium a peculiar type of sexual reproduction known as conjugation takes place. In conjugation there is an exchange of micronuclear material without the fusion of the cytoplasm.
After passing several generations of binary fission paramoecia take a nuclear reorganization by conjugation. Sonneborn (1937) first noticed that paramoecia with different genetic constitution only take part in conjugation. He named them as mating types. Mating type I will never conjugate with mating type I, but may conjugate with mating types II or III or IV etc. Jennings identified five mating types in P. caudatum. They are I-V mating types.
During conjugation two individuals called conjugants or gametocytes come close together and pair by the ventral surface. The interlocking between them is made stronger by the gullets which degenerate to form a protoplasmic bridge between them. The paired conjugants are however capable of movement. Soon a series of changes occur in the nuclei of both the conjugants which are to some extent comparable to gametogenesis of higher animals. These nuclear changes are:
(i) The macronucleus undergoes gradual disintegration and ultimately disappears.
(ii) The micronucleus of each conjugant divides twice successively giving rise to four micronuelei one of the division is perhaps meiotic and micronuclei become haploid.
(iii) Three of these four micronuclei in each conjugant degenerate and the remaining one undergoes an unequal division to form two gamete nuclei. One of the gamete nuclei is large and is called the stationary nucleus while the small one is called the migratory nucleus. Some authorities designate the stationary nucleus as female pronucleus and the migratory nucleus as the male pronucleus.
(iv) The migratory nucleus of one conjugant goes to the stationary nucleus of the other and vice versa through the protoplasmic bridge.
(v) The migratory nucleus of ore conjugant ultimately unites with the stationary nucleus of the other and forms the zygote nucleus or Synkaryon.
(vi) The conjugants with the zygote nucleus now separate and are called exconjugants.
(vii) In each exconjugant the zygote nucleus undergoes three successive divisions forming eight nuclei.
(viii) Of these eight nuclei four become macronuclei and four become micro-nuclei. Later on three of the four micronuclei degenerate leaving behind one active micronucleus.
(ix) The micronucleus divides and cytoplasmic division follows resulting into two paramoecia from each exconjugant and each of the two paramoecia is provided with two macronuclei and one micronucleus.
(x) The micronucleus again divides, followed by the cytoplasmic division, resulting in four paramoecia each with one micro and one macro-nucleus.
(xi) Thus from each exconjugant four paramoecia are formed and eight paramoecia are formed from one the conjugating pair.
Why conjugation is required in the life history of Paramoecium?
(i) Due to amitotic division of the macronucleus, the nuclear materials are not properly distributed to the daughter animals. As a result of that the paramoecia face ageing which may lead to the destruction of the community. Conjugation helps in rejuvenation of the vitality.
(ii) As conjugation occurs among different mating types, genetic variation occurs. This genetic variation is very much important for the existence of the species in long terms of evolution.
(iii) Conjugation gives rise to a rapid multiplication of the species, as a pair of conjugants gives rise to eight daughter individuals at a time.
Some peculiar reproduction in Paramoecium
Some species of Paramoecia show peculiar nuclear behaviour during fission and conjugation, as follow:
- 1. Endomixis: In endomixis in P. Aurelia macro nucleus first disintegrates and two macronuclei divide twice to form eight micronuclei. Six of the eight micronuclei disintegrate and two micronuclei remain, the Paramoecium divides into two cells by binary fission each having one micronucleus. This micronucleus of each daughter cell divides twice to form four micronuclei. Two micronuclei reorganized to form two macronuclei. The animal and its micronuclei divide so that two daughter individuals are formed. Thus from a single cell two Paramoecia develop and ultimately from a single specimen four daughter Pramoecia develop each with one macro and two micro nuclei. Significance of endomixis is claimed to be the same as that of conjugation.
- 2. Autogamy: In P. aurelia 2 micronuclei are present, which divide by meiosis to form 8 haploid daughter nuclei. Seven of them disintegrate and the rest undergoes a mitotic division forming 2 gamete nuclei. Macronucleus gradually get disintegrated and absorbed in cytoplasm. The two gamete nuclei fuse together to form a completely homozygous diploid zygote nucleus or synkaryon. Due to its division 4 nuclei are formed, 2 of which become macronuclei and 2 micronuclei and ultimately two daughter paramecia are formed. Autogamy leads to rejuvenation of the species.