CSHL scientists discover how 'companion' cells to sperm protect them from gene...( Cold Spring Harbor N.Y. In plant po...)

Cold Spring Harbor, N.Y. In plant pollen grains, sperm cells, which carry the genetic material to be passed on to progeny, are cocooned within larger "companion" cells that are called pollen vegetative cells. These companions provide sperm with energy and nourishment, and push them towards their targets during fertilization.

A team of plant geneticists at Cold Spring Harbor Laboratory (CSHL) led by Professor Robert Martienssen, Ph.D., has discovered that the companion cells provide sperm with more than just succor. They also provide them with instructions that protect sperm DNA from damage and thus help pass on a stable genome to the next generation.

The instructions offered to sperm come in the form of small RNA molecules that companion cells pass on to sperm. These small RNAs, the CSHL team shows, have the power to inactivate or "silence" specific DNA sequences. Thus they help set up gene expression patterns in sperm, providing the next generation with instructions that specify which regions of the genome should be turned on and which should be switched off. These findings will be published in the February 6th issue of the journal Cell.

Transposons are "revealed" in a specific pollen cell type

The CSHL team made this discovery while studying how and when certain DNA sequences called transposons are activated in plant genomes. Transposons are the genomic equivalent of parasites. When activated, they become mobile, jump around the genome and insert themselves randomly in DNA, disrupting normal gene function, causing DNA damage and genome instability.

Transposons are kept in an inactive state in cells via a set of chemical modifications to histones, the proteins around which DNA is spooled. These modifications prevent the DNA from being expressed. Scientists call these epigenetic modifications; they alter patterns of gene expression without changing the actual sequence of DNA "letters."

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