Biology Department Seminar

Heterochromatin is composed of transposable elements (TE) and related repeats which silence genes located nearby, and play a major role in epigenetic regulation of the genome. Far from being inert, heterochromatin is transcribed and small interfering RNA corresponding to heterochromatic sequences can be detected in plants, animals and fission yeast. Recently, we have proposed a role for heterochromatin in reprogramming events that occur in plant reproductive cells, as well as in the embryo and endosperm after fertilization. 21nt epigenetically activated small interfering RNA (easiRNA) from transposable elements accumulate in cultured cells and in pollen, and are translocated from the surrounding pollen grain into the sperm, while in the maturing seed 24nt siRNA are primarily maternal in origin. Thus maternal and paternal genomes likely contribute differing small RNA to the zygote and to the endosperm. If transposable elements in the seed are not targeted by small RNA from the pollen, or vice versa, this could lead to hybrid seed failure, in a mechanism reminiscent of hybrid dysgenesis. Unexpectedly, mutants in the easiRNA pathway lead to specification of diploid functional megaspores from somatic cells in the ovule, reminiscent of apospory in other species. Thus heterochromatin reprogramming may play a role in apomixis, and may utilize a similar mechanism. In fission yeast and in Arabidopsis, centromeric repeats are transcribed, but the transcripts are rapidly turned over by RNA interference, through the combined action of DNA dependent RNA polymerase, Argonaute and RNA dependent RNA polymerase, each of which is associated with heterochromatin. Histone H3 lysine-9 dimethylation (H3K9me2) depends on RNAi, mediated by the Rik1-Clr4 complex. We have found that heterochromatin is lost transiently during chromosomal replication, allowing heterochromatic transcripts to accumulate. Rapid processing of these transcripts into small RNA during S phase promotes restoration of