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Abstract
Ciliates form a distinct group of single-celled eukaryotes that host two types of nuclei (micro and macronucleus) in the same cytoplasm and have a special sexual process known as conjugation, which involves mitosis, meiosis, fertilization, nuclear differentiation, and development. Due to their high species diversity, ciliates have evolved different patterns of nuclear events during conjugation. In the present study, we investigate these events in detail in the marine species Euplotes raikovi. Our results indicate that: (i) conjugation lasts for about 50 h, the longest stage being the development of the new macronucleus (ca. 36 h); (ii) there are three prezygotic micronuclear divisions (mitosis and meiosis I and II) and two postzygotic synkaryon divisions; and (iii) a fragment of the parental macronucleus fuses with the new developing macronucleus. In addition, we describe for the first time conjugation in amicronucleate E. raikovi cells. When two amicronucleate cells mate, they separate after about 4 h without evident nuclear changes; when one amicronucleate cell mates with a micronucleate cell, the micronucleus undergoes regular prezygotic divisions to form migratory and stationary pronuclei, but the two pronuclei fuse in the same cell. In the amicronucleate cell, the parental macronucleus breaks into fragments, which are then recovered to form a new functional macronucleus. These results add new information on the process of conjugation in both micronucleate and amicronucleate Euplotes cells.
Highlights
Ciliates are single-celled eukaryotic microorganisms that can be found in diverse habitats across the globe [1,2,3,4,5,6]
They are unique among eukaryotes for two distinctive features: the presence of two types of nuclei and the sexual process typically represented by conjugation
A germline micronucleus (MIC), which is inactive during the vegetative life coexists with a somatic polyploid macronucleus (MAC), which controls the general functions of the cell [7]
Summary
Ciliates are single-celled eukaryotic microorganisms that can be found in diverse habitats across the globe [1,2,3,4,5,6] They are unique among eukaryotes for two distinctive features: the presence of two types of nuclei and the sexual process typically represented by conjugation. A new MAC genome is generated through complex mechanisms of chromosome fragmentation, DNA elimination, and amplification [9,10,29,30] For these reasons, the functional advantage of sex in ciliates is related to nuclear reorganization, which allows cells to start a new life cycle, whereas reproduction is carried out by binary fission. We reanalyze the entire process in more detail, and report for the first time mating in amicronucleate cells that, surprisingly, are able to form pairs with both micronucleate (mic) and amicronucleate (amic) cells
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