Martienssen's group has discovered that in plants, however, epigenetic marks are lost not in the sperm, but rather only within "companion" cells. The loss of these marks specifically occurs only in the regions of the genome that contain transposons, resulting in their activation. This specific activation of transposons only in the "companion" cells and not sperm cells achieves two crucial things, according to Martienssen.
"It forces transposons hidden among the DNA to 'reveal' themselves," says Martienssen, "thus alerting the companion cells to their exact locations in the genome". And because transposons are only activated within these companions -- which do not pass on their genetic material to the next generation -- the ensuing DNA damage is restricted to these cells alone while the genome of the sperm remains safe.
Companion cells generate transposon-silencing small RNAs
This activation of transposons in the "companion" cell, the CSHL team shows, has a more far-reaching consequence: it helps generate the epigenetic information that will prevent transposons in the genome of the neighboring sperm cells from ever getting activated.
The epigenetic instructions are small bits of RNA molecules called small interfering RNAs (siRNAs), which get transported from the companion cell and deposited into the sperm cells. There, they inactivate or "silence" transposon DNA via a phenomenon known as RNA interference.
"The companion cell is thus a completely selfless, altruistic cell," says R. Keith Slotkin Ph.D., a post-doctoral researcher in Martienssen's lab and lead author on the pap
|Contact: Hema Bashyam|
Cold Spring Harbor Laboratory