Transposon Invasion of the Paramecium Germline Genome Countered by a Domesticated PiggyBac Transposase and the NHEJ Pathway

Figure 1

Nuclear dimorphism and the sexual cycle in Paramecium. The merged picture on top shows a vegetative Paramecium cell (Veg.), with its DAPI-stained nuclei (in black). The figure represents the major steps of the sexual cycle observed during autogamy, a self-fertilization process triggered by starvation. Upon starvation, the two germline diploid MICs (red) undergo meiosis to give rise to eight haploid nuclei (I), a single of which migrates to a specialized cell compartment, where it divides once to give two identical gametic nuclei (II). Meanwhile, the remaining seven meiotic products are degraded (grey dots in II) and the old MAC (black) gets fragmented into ~30 pieces. During karyogamy, two gametic nuclei fuse to form the diploid zygotic nucleus (III). The zygotic nucleus then undergoes two successive mitotic divisions (IV): after the second division, the nuclei which migrate to the anterior cellular pole become the new MICs of the sexual progeny (red), while those that localize to the posterior pole differentiate into new developing MACs (blue) and undergo programmed genome rearrangements. At the first cell division (or karyonidal division), the new MICs divide by mitosis and each of the two developing new MACs segregates into a daughter cell (V), where it continues to amplify the rearranged somatic genome to a final ploidy of ~800n. During conjugation (not shown), meiosis is triggered by the mating of two compatible sexual partners, which undergo reciprocal exchange of their haploid gametic nuclei. As a result, the zygotic nucleus in each partner is formed by the fusion of a resident and a migratory haploid nucleus. Exconjugants separate between the first and second divisions of the zygotic nucleus, and MAC development takes place as described for autogamous cells.