Fig. 3

Oxytricha as a model system to study genome biology, epigenetic inheritance, and somatic differentiation. a-c Single (a,c) and mating (b) Oxytricha cells. Blue indicates DNA; Yellow is tubulin, highlighting the cilia. i  =  micronucleus, a  =  macronucleus. Image in (a) courtesy of National Human Genome Research Institute, (b) courtesy of Robert Hammersmith, Ball State University and (c) courtesy of Wenwen Fang and reproduced from [325]. d The sexual life cycle of Oxytricha trifallax and rearrangement of scrambled genes, reproduced from [47]. All vegetative cells (stages 1 and 10) contain one (bi-lobed) macronucleus (MAC) and two micronuclei (MIC). The two MIC are genetically identical, but for simplicity we show only one here. (2) When starved, two cells of compatible mating types partially fuse to initiate conjugation. Cell fusion occurs soon after mixing of mating types. (3) Both vegetative micronuclei in each partner enter meiosis I. (4) One product from each meiosis I is promoted to meiosis II, and one of the four meiosis II products is promoted to a post-meiotic mitosis. (5) The sister products of one mitosis develop into gametic nuclei: a posterior stationary and an anterior migratory nucleus. This happens in both partners, such that both cells emerge with identical zygotic genotypes after the exchanged migratory nucleus fuses with the retained stationary nucleus (6), resulting in (7) two genetically identical exconjugant cells. (8) The newly formed zygotic nucleus divides twice: one daughter nucleus is destroyed, two become the new micronuclei, and (9) the parental macronucleus in each partner cell degrades, leaving telomere-to-telomere RNA transcripts behind to guide rearrangement [44, 45]. One zygotic nucleus differentiates into the new macronucleus. This cycle takes approximately 48–60 h. Shown inside the circle are representative MIC and MAC versions of a scrambled gene. Retained DNA segments in purple; deleted DNA regions, including flanking DNA, in yellow; numbered segments correspond to the order in the expressed MAC version; segment 2 is inverted; telomeres are shown as black bars at the ends of the MAC chromosome