Moss - What genes cant do - 2003, страница 11

Alternatively,the chromosomes–Mendelian factors may contain only certain, andmaybe even only superficial, features of the organism, depending on thelarger context provided by the cytoplasm, the totality of the organism,and the environment for successful deployment.Jacques Loeb, a leading expositor of mechanistic materialism inbiology, suggested in 1916 that “the unity of the organism is due to thefact that the egg (or rather its cytoplasm) is the future embryo uponwhich the Mendelian factors in the chromosomes can impress onlyindividual characteristics, probably by giving rise to special hormones orenzymes” (Sapp 1987). Edwin Conklin, who followed Whitman andGenesis of the Gene35Lillie in cell lineage studies and was a lifelong advocate of the importance of the cytoplasm of the egg, argued in 1908 that “the characteristics of the phylum are present in the cytoplasm of the egg cell” (Sapp1987).

Even Boveri in 1903 had attributed to the cytoplasm the genericfeatures of the organism that provided the developmental context inwhich the individuating effects of the nucleus could and would becomeexpressed (Sapp 1987). Morgan, in 1910, rephrased the traditionalopposition between preformationism and epigenesis in terms of the “particulate theory of development” versus the theory of “physico-chemicalreaction.” He suggested of these that:The particulate theory may appear more tangible, definite and concrete becauseit seems to make a more direct appeal to a material basis of development andheredity.

The theory of physico-chemical reaction may seem more vague andelusive, since the responses and reactions to which it must appeal are as yet littleknown. But this distinction is not one of much importance. For the particulatetheory requires as elaborate a series of processes or changes to account for thedistribution of the postulated particles and their development into characters asdoes the reaction theory itself, and on the other hand the reaction theory mayrest its claims on as definite a physical or material basis as does the other view.One theory lays emphasis on the material particles of development, the other onthe changes or activities in the same material .

. . Whichever view we adopt willdepend first upon which conception seems more likely to open up further linesof profitable investigation, and second which conception seems better in accordwith the body of evidence at hand concerning the processes of development . . .it may be said in general that the particulate theory is the more picturesque orartistic conception of the developmental process. As a theory it has in the pastdealt largely in symbolism and is inclined to make hard and fast distinctions. Itseems to better satisfy a class of type of mind that asks for a finalistic solution,even though the solution be purely formal.

But the very intellectual security thatfollows in the train of such theories seems to me less stimulating for furtherresearch than does the restlessness of spirit that is associated with the alternative conception (Morgan 1910).Morgan’s sympathies had been with the physicochemical, or epigenetic,side of the coin. He contributed over many years to the evidence supporting the importance of the cytoplasm in the developmental acquisition of heritable characteristics.

It was, after all, the cytoplasm, not thechromosomes, that appear to undergo differentiation over developmental time. Morgan’s movement to the particulate camp, while reflective ofhis recognition that sex is chromosomally and not, as he had previouslysuspected, environmentally determined in Drosophila (Gilbert 1978), by36Chapter 1no means presupposed a rejection of all the evidence for the role of cytoplasm and physiochemical epigenesis in development.

The success ofMorgan’s “Fly Room” research mapping Drosophilia chromosomesmarked the partitioning of embryology into what would count as workin heredity and take center stage and what would not count as heredityand, hence, be marginalized.At the opposite conceptual pole from that of a particulate theoryof hereditary factors lies the continuing tradition of holistic theoriesattempting to explain orderly patterns of reaction in terms of biologicalorganization and/or developmental fields. An early attempt to articulatea field theory was made in 1924 by C.

M. Child, who sought to find biological organization in gradients of metabolic intensities. While not considered adequate by most contemporaries, the field concept was adoptedby such investigtors as Julian Huxley, Gavin de Beer, and Paul Weiss inyears to come (Sapp 1987). When Morgan turned to the chromosome,he did not attempt to subsume the questions of development and organization under a particulate model.

Rather, he followed the guidance ofJohannsen in rejecting the preformationist legacy of interpretingMendelian factors as Anlagen. By bracketing the questions of development and thus the role of the cytoplasmic-organizational context inrealizing the acquisition of hereditary characteristics, Morgan could, following his first desideratum stated above of “opening up profitable linesof investigation,” establish an independent science of the genotype.Morgan did not claim that a science of the genotype was tantamount toa science of the phenotype, and yet, in 1926 he stated: “Except for therare cases of plastid inheritance all known characters can be sufficientlyaccounted for by the presence of genes in the chromosomes.

In a wordthe cytoplasm may be ignored genetically” (Morgan 1910). Unless newlytempted by particulate preformationism, Morgan’s intent was to establish the acquisition of genes and genotypes as the definition of whatcounts as heredity, that is, to separate by definitional fiat the inheritanceof genes from the developmental context and mechanism which allowheritable traits to appear. As Sapp (1987) has detailed, the fixation ofthe genetic meaning of heredity was hardly uncontroversial or apolitical.Conklin in 1919 protested: “Development is indeed a vastly greater andmore complicated problem than heredity, if by the latter is meant merelyGenesis of the Gene37the transmission of germinal units from one generation to the next”(Sapp 1987).

Speaking perhaps on behalf of the accumulated sentimentsof that sector of the life sciences whose concerns had become partitionedoutside of the realm of inheritance, Ross Harrison in 1937 suggested:The prestige of success enjoyed by the gene theory might easily become a hindrance to the understanding of development by directing our attention solelyto the genome, whereas cell movements, differentiation and in fact all developmental processes are actually effected by the cytoplasm. Already we have theories that refer the process of development to genic action and regard the wholeperformance as no more than the realization of the potencies of the genes. Suchtheories are altogether too onesided (Sapp 1987, p.

50).Morgan’s willingness to conceptualize his work on the genetics ofDrosophila in terms which marginalized developmental concerns wasbolstered by several factors. Morgan’s Fly Room laboratory atColumbia received enthusiastic support from zoology department chairE. B. Wilson, whose advocacy of the cellular autonomy-nuclear idioplasm perspective Morgan once took issue with. Morgan benefited financially from rising industrial interest in agricultural genetics.Ideologically, Morgan, owing in some measure to the influenceof Jacques Loeb, had adopted a staunchly mechanistic materialistoutlook and found in genetics a considerably more tangible model ofbiological causation than that found in the lexicon of epigenetic fieldtheories (Allen 1985).

The constitution of a science of heredity whichpresupposes the bracketing out of mechanisms involved in the realization of organismal characteristics is, as Falk (1995) has argued, a kindof instrumental reductionism. The fruit of Morgan’s instrumental reductionism, in its advancing a theory of the chromosomes and in providinga handle for grappling with certain kinds of inheritance in animalsand plants, can’t be denied. But it is also the case that the propensity forthe instrumental reductionism of Morgan’s genetics to spawn naivelyreductionist-preformationist progeny has also had its consequences, bothat the level of pernicious social ideologies and, I will argue, conceptualconfusions.Genetics arose as a discipline proper when Morgan found that inDrosophilia factors affecting eye color, body color, wing shape, and sexsegregated together with the X chromosome (Gilbert 1978).

At that point38Chapter 1he turned to Johannsen for the conceptual framework with which tounderstand this. Following Johannsen, Morgan chose to separate traitsfrom genes as well as phenotypes from genotypes and to use traits asmarkers for the hereditary entities. As Falk (1995) points out:Not all saw the instrumental, tactical aspect of Morgan’s mechanistic materialism.

Many future developments of genetics, and especially offshoots of it, maybe traced back to the conceptual rather than heuristic interpretation of the geneas determinants of characters. The term “genes for” . . . became, notwithstanding Johannsen’s reservations, a decisive factor in our genetic thinking.Did Johannsen Get It Right?Whatever the full configuration of motivations were for Morgan, it isevident that once he turned toward the path of genetic analysis he didnot look back. The same cannot be said for Johannsen, whose formulation of the genotype-phenotype distinction enabled Morgan to constitutea practice of genetics distinct from development. In his 1923 remarksabout “units in heredity,” Johannsen raised three lines of questioningwhich merit a place in this ongoing conversation.Johannsen’s first question pertains to the extent to which the fundamental features of the organism are segregable as Mendelian units.Certainly by far the most comprehensive and most decisive part of the wholegenotype does not seem to be able to segregate in units; and as yet we are mostlyoperating with “characters,” which are rather superficial in comparison with thefundamental Specific or Generic nature of the organism.