Reviews

of books by Eva Jablonka and Marion J. Lamb

EVA JABLONKA is a professor at the Cohn Institute for the History of Philosophy of Science and Ideas at Tel Aviv University. Jablonka started out in biology with a focus on non-random chromatid segregation and chromosomal silencing (e.g. X-chromosome inactivation) developing a theoretical framework for mechanisms of non-genetic inheritance.

MARION LAMB was Senior Lecturer at Birkbeck College, University of London, before her retirement. She studied the effect of environmental conditions such as heat, radiation and pollution on metabolic activity and genetic mutability in the fruit fly Drosophila before collaborating with Eva Jablonka.



Evolution in Four Dimensions
Genetic, Epigenetic, Behavioral and Symbolic Variation in the History of Life
by by Eva Jablonka and Marion J. Lamb
MIT Press (2005)

Jablonka and Lamb have been studying the inheritance of epigenetic variations since the late 1980s (e.g. in the Journal of Theoretical Biology, see J.Theor. Biol 1988; 1989; 2006). The current book is a lucid and very interestingly written account of the result of their work - mostly theoretical - and how the cell takes control of what happens to chromosomal DNA, particularly its effect on gene expression during development of multicellular organisms and what we could learn from this with respect to the evolution of cellular diversity. All multi-cellular organisms start with a totipotent cell that is the source of the formation of hundreds of different cell types in adult animals and plants. This cellular diversity is not dependent on changes in DNA sequences of genes (mutations), but which genes, in which combination and order are being used (see also: What is a gene?). The cell applies markers on the DNA that guides this differential use of genes. It is not only important what DNA sequences you have (genes and non-genes), but also if they can be used by the cell.

The structure of the cell is the overlooked partner of DNA in evolution. So, what is the cell and how does it control DNA? The authors discuss instructions of how and when to read DNA information at various levels, as indicated by the subtitle of the book. The central story is that instructions of how genes are accessed in parents can be inherited by their children. Some of these instructions are not found in the DNA sequence alone, but are the result of the cell's structural state, which can be influenced by environmental factors. These environmental influences are 'imprinted' on the DNA (epigenetic markers) and can be passed on to the next generation. These epigenetic changes - chemical modifications and markers that change the way enzymes and regulatory proteins have access to DNA - are currently being studied to explain many non-Mendelian patterns of inheritance. The best understood mechanism is nucleotide methylation that silences a gene. Methylation can be inherited during cell division, both asexually (mitotic) during development and wound healing, but in some instances also sexually (meiotic). Methylation is linked in some instances to RNA interference, the new and emerging science of RNA regulation of gene expression.

In addition, certain cellular structures - cell membranes and cytoskeletal networks are two examples - may not be directly encoded in the genome (the synthesis of their components is, of course). They are considered self-templating (to make a new membrane, you need an existing membrane, not just its components) and thus these cellular structures can be inherited independently of DNA, yet affect how genes are used or retained.

At an even higher level - that of populations - the inheritance of behavioral forms and social interactions can be inherited from generation to generation. This strategy is particularly effective in metazoan (animals), with humans having cultural and symbolic forms of inheritance, in addition to behavioral ones. The difficulty is to show that these forms of inheritance are truly independent of our DNA. Behavioral and social norms may simply reinforce successful genetic systems, being a self-governing selective process that rewards cooperation to benefit close-knit societies (kinship; shared genetics). Take for instance modern medicine, It affects the human gene pool by treating previously lethal diseases by simple means (antibiotics, nutrition, drugs, surgery). Other factors affecting gene pools are ethnic preferences, mating behavior, or simple geographic separation. In other words, human culture is affecting human genetics. The only question remains, if any of those are truly acquired characteristics inherited by the next generations (Lamarckism) or if they are special modes of selection (geographic separation is recognized as a form of selection different from natural selection, thought to be the result of competition (survival of the fittest)). Jablonka and Lamb argue, however, that cellular structures, behavior, cultural and symbolic instructions are phenotypic forms of inheritance independent of, but parallel to genetic inheritance.

No matter what one thinks about cultural influences on genetic fate - the inheritance of certain cellular structures independent of genes remains an interesting open question.
 

May 30, 2006 /  © 2006 Lukas K. Buehler / go back to Book Review Home