An unusually large histone H3 protein ("protein X"; molecular weight 21,000) previously observed in Stylonychia micronuclear histone extracts (Schlegel et al. 1990) has recently been identified as a divergent histone H3 variant, H3.8, and appears to be replaced in the developing new MAC during macronuclear development, including by the H3.7 variant (Forcob et al. 2014).
These linear molecules form during macronuclear development by removing both inter- and intragenic spacer sequences from germline micronuclear DNA and then stitching gene fragments together.
Furthermore, we observed that methylated cytosines at both nanochromosomes mdp1 and mdp2 remained present during macronuclear development, whereas after incubation with TSA the DNA remained unmethylated.
The hypothesis in Figure 5 suggests that one or more errors during macronuclear development (perhaps epigenetically driven) result both in transfer of essential somatic micronuclear function(s) [ 1, 17] to the macronucleus and in the non-mating phenotype.
We surveyed the population of partially processed DNA molecules from two scrambled genes at different time intervals during macronuclear development in two ciliates, Stylonychia lemnae and Oxytricha trifallax.
The most elaborate known form of DNA rearrangement occurs during macronuclear development in ciliated protists, which provide model systems to study programmed DNA elimination, fragmentation, and reorganization during development [13] [18].
To understand the process of DNA deletion and reorganization during macronuclear development, we examined the population of DNA molecules during assembly of different scrambled genes in two related organisms in a developmental time-course by PCR.
During conjugation, the 5 chromosomes, each present in two copies in the diploid micronucleus, are fragmented into ∼225 chromosomes during macronuclear development, and then endoreplicated to ∼45 ploid in the vegetative macronucleus.
This would lead to their deletion from somatic chromosomes during macronuclear development, long before transcription.
Unlike acetylation, H3K4me1 and H3K4me3 are introduced de novo at an advanced stage of polytenization during macronuclear development.
