Tetrahymena thermophila macronucleus chromosomes originate from chromatin breakage sequence defined TAD-like structures in micronucleus chromosomes

The somatic macronucleus (MAC) and germline micronucleus (MIC) of Tetrahymena thermophila differ in chromosome numbers, sizes, functions, transcriptional activities, and cohesin complex location. However, the higher-order chromatin organization in T. thermophila which contains both cohesin free MAC and cohesin located MIC are largely unknown. Here, we examined the higher-order chromatin organization in the two distinct nuclei of T. thermophila using the Hi-C and HiChIP. Interestingly, we found that the crescent MIC possess specific chromosome interaction pattern. All the telomeres or centromeres on the five MIC chromosomes clustering together, respectively, which could help to identify the midpoints of centromeres in the MIC. We found the transcriptionally active MAC chromosomes lack A/B compartments, topologically associating domains (TADs) and chromatin loops. The transcriptionally inert MIC chromosomes also without A/B compartments and chromatin loops, but have TAD-like structures. The boundaries of the TAD-like structures in the MIC were highly consistent with the chromatin breakage sequence (CBS) sites, suggesting that each TAD-like structure of the MIC chromosomes develops into one MAC chromosome during MAC development during conjugation. Overall, we revealed the higher-order chromatin organization in the T. thermophila and found the chromatin structures might play important roles during the development of the MAC chromosomes. Hi-C seq are generated from 1.5, 3, 24 hour post-mixing (hpm) Tetrahymena thermophila cells (B2086 and Cu428). HiChIP-seq are generated from 1.5, 3, 24 hpm Tetrahymena thermophila cells (B2086 with Smc1-HA and Cu428 with Rec8-GFP)