Moss - What genes cant do - 2003 (522929), страница 25
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Under the heading of “organizational structure” I have begun tomarshal evidence on behalf of the idea that cellular context as a wholeis basic to the nature and continuity of living beings and is irreducibleto any of its constituent parts. The membranous system of the cell, thebackbone of cellular compartmentalization, is the necessary presupposition of its own renewal and replication. Cellular organization in generaland membrane-mediated compartmentalization in particular are constitutitive of the biological “meaning” of any newly synthesized protein(and thus gene), which is either properly targeted within the context ofcellular compartmentalization or quickly condemned to rapid destruction (or cellular “mischief”).
At the level of the empirical materiality ofreal cells, genes “show up” as indeterminate resources, that is, as kindsof Gene-D.While even an uncontroversial depiction of the complexity andlongevity of cellular structural organization is in itself enough to defeatSchrödinger’s argument for why the aperiodic crystal must be the selfexecuting code-script, it is not yet enough to undermine a more discreteattribution of informational primacy to the genome. The structuralorganization of the cell, the basic membrane system, and the compartmentalization which it embodies is passed on from one generation to thenext by way of the maternal egg cell. If cellular membrane organizationis ever lost, neither “all the king’s horses and all the king’s men” nor anyamount of DNA could put it back together again.
But if the nature ofcellular structural information is basically the same throughout the livingworld and cannot be used to distinguish between an amoeba and ahuman, then something like a modified story about genetic code-scriptsdictating life-forms may still be defensible.
On the other hand, if organismal genomes consist of a compilation of sequence motifs and exons,which are common throughout the living world with no species-specific96Chapter 3stamp on them, then the onus of explaining where evolutionary innovation is to be found weighs even more heavily on the “gene-speakers”. Ifindeed genes are basically interchangeable across kingdoms and phyla,as a surfeit of empirical findings attests to, then surely the specificity oforganisms must be determined at a higher level of organization. If weare leveling our gaze at only that which is materially transmitted fromone generation to the next through the one-celled bottleneck of sexualreproduction, then higher-level organization begins with chromosomalstructure and ascends only to the level of the largely membranemediated topography and compartmentalization of the cell.The potential for heritable structural alterations of cellular organization to have evolutionary significance is most accessible to investigationin the case of single-celled organisms in which there is no distinctionbetween somatic and germ-line cells.
Classic studies of this sort have beencarried out on ciliates such as Paramecium and Tetrahymena (Jablonka& Lamb 1995). These cellular organisms are covered with rows of cilia,each of which is associated with a basal body and possesses a certainorientation. These units are asymmetrical and are the templates for theirown replication. When the orientation pattern of the cilia–basal body isexperimentally altered by environmental manipulation or microsurgery,the new pattern is transmitted to progeny.
This pattern is preservedthrough both repeated generations of asexual reproduction as well asthrough sexual conjugation. Larger-scale patterns of heritable variationhave also been witnessed, as in the case of the formation of “doublet”cells in Paramecium tetraurelia, experimentally induced by interferingwith cell division. The doublet phenotype is then transmitted to subsequent progeny.
Evidence for the natural emergence of a new species onthe basis of structural mutation is cited by Frankel (1983) with regardto the ciliate Teutophrys trisulca, which possesses a single trunk but threeanterior probosces, each of which is similar to the single proboscis of therelated species Deleptus. Teutophrys came about, it appears, through astructural mutation that produced the triplet organization and that wasthen perpetuated by the epigenetic inheritance of structural organizationand eventually stabilized by genetic changes.The basic character of metazoan development speaks to the plausibility of the idea that structural changes in the cell can be of evolutionaryA Critique of Pure (Genetic) Information97significance. Metazoan ontogeny consists of the progressive differentiation of cell lineages which, once differentiated, reproduce true to theidentity of the lineage.
If one were to look at ontogeny as a model ofphylogeny (not exactly novel), one would see the same exact genes situated in different cellular contexts of different cell lineages, giving riseonly to progeny determined by the cellular contexts. Once the membranesystem and cellular organization of a cell are differentiated along certainlines, they become a stable basis for maintaining themselves and reproducing subsequent generations of the same cell type.Helen Blau at Stanford carried out numerous experimental examplesof the ability of the cellular context to condition the differentiation stateof even foreign nuclei (Blau et al. 1983, Blau et al.
1985, Miller et al.1988). Blau’s experiments consisted of using a “syncytial” muscle fiberthat comes about naturally through the fusion of muscle cells and isthereby multinucleate. Nuclei from other tissues types of the same orother species can be experimentally transferred to a cultured muscle fiberin order to see how the muscle cytoplasmic “context” will affect, forexample, the transcriptional activity of a nucleus derived from a livercell. Using a liver nuclei from a different species enables the investigatorto readily distinguish, by immunological means, which newly synthesizedproteins in the muscle fiber were derived from the genetic templates ofa muscle nucleus from the “host” cell versus that of the “donated” livernucleus.
When a nucleus from a liver cell of species A was transferredinto the muscle syncytium of species B, Blau found that cultured syncytium produces muscle proteins, but not liver proteins, with theimmunological identity of species A. The ability of a somatically heritable cytoplasmic context to regulate genomic expression, as demonstratedby Blau, certainly further suggests the possibility that epigenetic changeswhich are heritable across generations may be the source of evolutionary innovations.Of course the real question concerning metazoan ontogeny is just howa single cell gives rise to the requisite number of differentiated cell lineages with all the right inductive developmental interactions required toreproduce the form of the mature organism.
Understanding the dynamics between different components of the fertilized egg cell (and its surround) that become the developmental pathway of the nascent organism98Chapter 3is still a central challenge. It is well established that the compartmentalization of the cell in general and of its messenger RNA—the legacy ofthe maternal egg cell—is extremely influential in setting early development along a certain course. That this organization is not merely theproduct of nuclear inheritance but also significantly of structural inheritance is directly addressed by Grimes and Aufderheide (1991) in theirextensive review of the subject:The highly organized cytoplasm of a metazoan egg, therefore, cannot be solelythe consequence of direct nuclear gene activity.
Given the background of information from the Cliophora, one would predict that structurally heritable information systems must be present in addition to direct nuclear (genic) controlsystems in the metazoa. The ciliated protozoa are a group of organisms that havemade exceptional use of the posttranslational, ‘epigenetic’ systems that contribute to the localization of gene products . . .
Processes homologous, or at leastanalogous, to the directed assembly and directed patterning seen in ciliates arealso functional in metazoa, and are of fundamental developmental significance(p. 67).Order from the Inheritance of Self-Sustaining Dynamics and/or theEmergence of Self-Sustaining Order for “Free”The Global ViewIn his contribution to a fiftieth anniversary retrospective of Schrödinger’sWhat is Life?, Stuart Kauffman (1995) points out that the force ofSchrödinger’s argument was based on the assumptions of equilibriumthermodynamics held by “most physicists of his day.” Macroscopicorder, in this view, is attributable to “ averages over enormous ensembles of atoms or molecules .
. . not to the behavior of individual molecules.” At equilibrium the predictability of the location of thecomponents in a system in relation to one another is very weak exceptfor the case of those atoms that are held together in a crystalline array.If given the presuppositions of equilibrium statistical physics and askedto account for the stability of complex life-forms through and across generations, then it would follow that some form of solid-state structure,i.e., a crystal, and an aperiodic one to make it more interesting, must beinvolved. But life is not an equilibrium phenomenon, and so-calleddissipative far-from-equilibrium systems are not only capable of sus-A Critique of Pure (Genetic) Information99taining complex highly ordered structures and dynamics outside of thesolid state but also of self-organizing into other and even more highlyordered organizational regimes.2The idea that stable biological order can emerge from the dynamicinteractions of catalytic molecules goes back to the advocates of the“protein-first” model for the origins of life in the 1920s (Moss 1999).Following that tradition, Kauffman (1993, 1995) has argued that anaperiodic crystal, that is, the securing of genetic information in somesolid-state array, is neither necessary nor, for that matter, sufficient forthe existence of a stable system capable of evolving by the differentialselection of heritable variations.
Life emerges, in his view, on this basisof a phase transition in which a hitherto chaotic milieu of molecules ina thermodynamically open system self-organizes into an autocatalyticcycle. An “autocatalytic cycle” is one whose components cyclically catalyze their own production with their respective concentrations beingmaintained at a dynamic equilibrium over time. Where origins-of-lifetheorists had previously conceived of the emergence of life as a very lowprobability event, Kauffman (and collaborators) have used computersimulations to show that the state-space of a system of reactors and reactants will, given certain parameters, predictably converge on an autocatalytic cycle through a limited number of states which constitute anattractor cycle for the system.
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