Hello all deliberators on epigenetics.
This topic is a special interest of mine. I read quite a lot about epigenetics since I teach developmental biology. People who still think genes are controlling everything would say there is only ONE kind of epigenetics, and it is the following :
Genes wrap around giant proteins called histones. It is histones that are methylated, and gene expression is either repressed or promoted, depending on the site(s) of histone methylation. DNA can also be directly methylated at cystine residues, and this can in turn affect histone methylation.
Step 1: During the steps required to form sperm and egg, and around the time of fertilization, DNA "demethylates", meaning that silenced genes are un-silenced.
Step 2: Following fertilization, the DNA "remethylates" according to a new pattern that some believe may be in part determined by environmental signals. The most compelling example of this that I know of is called the "thrifty phenotype", you can google this by typing "thrifty phenotype epigenetics" (it's really interesting!). A different and important consequence of this de- and re-methylation is that each offspring is truly unique, not a mere "50-50 copy" of the two parents....because the selection of genes that are expressed is what determines its phenotype, and since each offspring generates its own unique methylation pattern it may express (or repress) genes that neither parent is expressing. (This is usually seen in polygenetic characteristics and not so much in straightforward single-gene traits that exhibit simple dominance).
But I think the definition of epigenetics is/should be broader, and can be extended to a second type:
Step 1: Environment sends some signal (temperature, gravity, chemical, etc) to the developing organism.
Step 2: Organism responds. If it is a crocodile embryo, temperature will determine its sex. If it is an amphibian, gravity will determine where it puts its backbone. If it is a bee, special molecules around the cells that cover the egg in the ovary (called "nurse cells") help the egg "decide" which end is its head, and which end is its tail.
I think that the above are examples of "epigenetics" because they illustrate the regulation of gene expression by *outside* signals....signals NOT generated by the organism and NOT in its own DNA.
Bees are sensitive to all kinds of signals in their environment that alters the way they express genes and probably alters behaviors, too. This is because chemical signals from the environment that diffuse into their cells can act as transcription factors....helping to determine whether a gene is active or not. I say that is "epigenetics".
That's my hypothesis based on what the textbooks say. But I don't think any of the effects I describe above are "transgenerational". I read what Wikipedia has to say about "transgenerational epigenetics" and it appears to be a section written by a wishful thinking geneticist, who says:
"The relative importance of genetic and epigenetic inheritance is subject to debate.[31]<http://en.wikipedia.org/wiki/Transgenerational_epigenetics#cite_note-31>[32]<http://en.wikipedia.org/wiki/Transgenerational_epigenetics#cite_note-32> Though hundreds of examples of epigenetic modification of phenotypes have been published,[33]<http://en.wikipedia.org/wiki/Transgenerational_epigenetics#cite_note-Jablonka-33> few studies have been conducted outside of the laboratory setting.[34]<http://en.wikipedia.org/wiki/Transgenerational_epigenetics#cite_note-34> Therefore, the interactions of genes and epigenes with the environment cannot be inferred despite the central role of environment in natural selection.?"
I gave several simple examples above which I believe refute this statement.
Christina
***********************************************
The BEE-L mailing list is powered by L-Soft's renowned
LISTSERV(R) list management software. For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html
|
