Sexual Reproduction: Conjugation

Sexual Reproduction: Conjugation

Alongside asexual fission, many ciliate protozoans (such as Paramecium) are also capable of a form of sexual reproduction called conjugation. Conjugation is defined as the temporary union of two individuals of a compatible mating type, during which the two cells exchange micronuclear genetic material before separating again, each then continuing its own independent existence (rather than fusing permanently into one cell, or producing new individuals directly from the union itself).

During conjugation, two individuals come together and adhere at their oral surfaces. Within each conjugating individual, the micronucleus undergoes a specialised division process producing several haploid micronuclei; most of these degenerate, but in each partner, one surviving haploid micronucleus divides once more into two genetically identical haploid micronuclei — one of which stays within its own cell (referred to as the stationary pronucleus), while the other migrates across the point of contact into the partner cell (the migratory pronucleus). Within each partner, the stationary and the newly arrived migratory pronucleus then fuse, restoring the diploid condition but with genetic material now drawn from both original individuals. After this nuclear exchange and fusion is complete, the two individuals separate.

At the moment of separation, the two formerly joined individuals are given distinguishing names based on which was the more "active" partner during the initial approach and pairing: the proter is generally considered the more active, typically anterior partner of the pair, while the opisthe is the other, typically posterior or more passive partner. Following separation, each of the two individuals (proter and opisthe) subsequently undergoes its own further nuclear reorganisation and typically then proceeds through fission, but the union itself (conjugation) does not, by itself, immediately produce new individuals — it produces two genetically reorganised, still-separate individuals, which then continue to reproduce asexually as before. …