T.H. Morgan,neither an epistemological empiricist nor a “Methodological” empiricist

T. H. Morgan (1866–1945), the founder of the Drosophila research group in genetics that established the chromosome theory of Mendelian inheritance, has been described as a radical empiricist in the historical literature. His empiricism, furthermore, is supposed to have prejudiced him against certain scientific conclusions. This paper aims to show two things: first, that the sense in which the term empiricism has been used by scholars is too weak to be illuminating. It is necessary to distinguish between empiricism as an epistemological position and the so-called methodological empiricism. I will argue that the way the latter has been presented cannot distinguish an empiricist methodology from a non-empiricist one. Second, I will show that T. H. Morgan was not an epistemological empiricist as this term is usually defined in philosophy. The reason is that he believed in the existence of genes as material entities when they were unobservable entities when they were unobservable entities introduced to account for the phenotypic ratios found in breeding experiments. These two points, of course, are interrelated. If we were to water down the meaning of empiricis, perhaps we could call Morgan an empiricist. But then we would also fail to distinguish empiricism from realism.     

Heterogeneity of the glutathione transferase genes encoding enzymes responsible for insecticide degradation in the housefly

One of the four glutathione-S-transferases (GST) that is overproduced in the insecticide-resistant Cornell-R strain of the housefly (Musca domestica) produces an activity that degrades the insecticide dimethyl parathion and conjugates glutathione to lindane. In earlier work, it was shown that the resistant Cornell-R carries an amplification, probably a duplication, of one or more of its GST loci and that this amplification is directly related to resistance. Using polymerase chain reaction (PCR) amplification with genomic DNA, multiple copies of the gene encoding the parathion-degrading activity (called MdGst-3) were subcloned from both the ancestral, insecticide-susceptible strain BPM and from the insecticide-resistant Cornell-R. In BPM, three different MdGst-3 genes were identified while in Cornell-R, 12 different MdGst-3 sequences were found that, though closely related to ancestral genes, had diverged by a few nucleotides. This diversity in MdGst-3 genomic sequences in Cornell-R is reflected in the expressed sequences, as sampled through a cDNA bank. Population heterozygosity cannot account for these multiple GST genes. We suggest that selection for resistance to insecticides has resulted in not only amplification of the MdGst-3 genes but also in the divergence of sequence between the amplified copies. Received: 22 November 1995 / Accepted: 23 February 1996     

Transformation of the extremely thermoacidophilic archaeon Sulfolobus solfataricus via a self-spreading vector

Abstract The comparative chromosomal locations of polymeric β-fructosidase SUC genes have been determined by Southern blot hybridization with the SUC2 probe in 91 different strains of Saccharomyces cerevisiae . Most of the strains exhibited a single SUC2 gene, but in some strains two or three SUC genes were found. All Suc⁻ strains carried a silent suc2⁰ sequence. The accumulation of SUC genes was observed in populations derived from sources containing sucrose and seems to be absent in strains from sources promoting the MEL gene.     

Structural characterization of genes corresponding to cotton fiber mRNA, E6: reduced E6 protein in transgenic plants by antisense gene

Two genes, each corresponding to fiber mRNA E6, were isolated from cotton cultivars Coker 312 (Gossypium hirsutum L.) and Sea Island (G. barbadense L.). E6 is one of the predominant fiber-specific mRNAs present during early fiber development. The distinguishing feature of the nucleotide-derived E6 protein is the presence of a motif where a dimer, Ser-Gly, is repeated several times. Two of the Sea Island genes contained a pentameric motif, Ser-Gly, while one of the Coker genes had one and the other had four motifs each. cDNA clones containing one or five Ser-Gly motifs were also identified. Thus, it appears that the strict conservation of this motif may not be critical to E6 protein function. Sequence characterizations of the genes and cDNAs showed that multiple members of the E6 family are transcribed in fiber and may result in proteins 238 to 246 amino acids long. The 3 ends of the genes and cDNAs showed considerable heterology among them. Transgenic plants containing antisense genes were generated to decipher E6 function. Transgenic cotton with reduced E6 protein levels in the range of 60 to 98% were identified. However, no discernible phenotypic changes in fiber development or properties were apparent. This result leads to the conclusion that E6 is not critical to the normal development or structural integrity of cotton fibers.     

Presence of unique repeated insertion sequences in nodulation genes of Rhizobium ‘hedysari’

Cloning and sequencing of DNA from a symbiotic large plasmid in Rhizobium hedysari strain IS 123 required for its nodulation of the mediterranean legume crop Hedysarum coronarium (sulla) and complementation studies of nod^- mutant derivatives led to the characterization of a 30-kb region containing common and host-specific nod genes. This DNA region also contained at least six copies of a novel insertion sequence-like structure, some of which appeared to have suffered deletions. This 0.8 kb novel element carries two 17-bp flanking inverted repeats and an open reading frame showing homology with a transposase from Staphylococcus aureus. Hybridization studies revealed that several strains of Rhizobium hedysari carry this element in various copy number. The six copies in strain IS 123 appear clustered specifically within the pSym nod region.The significance of this IS element in rhizobia and its possible use as a probe for taxonomic and phylogenetic studies of Rhizobiaceae is addressed.     

Association of RFLP markers with loci conferring broad-based resistance to the soybean cyst nematode (Heterodera glycines)

Soybean cyst nematode (SCN) is a major soybean yield-limiting pest. The present study was conducted to map broad-based SCN resistance loci from the cultivar Hartwig. Two-hundred F₂₃ lines derived from the cross Williams 82 x Hartwig were screened with a fourth-generation SCN inbred and 56 polymorphic molecular markers. Allele states and phenotypes were analyzed using stepwise regression and the model selection was made at P 0.01. Four unlinked RFLP markers (A006, A567, A487, A112) were associated with SCN resistance and the partial coefficient of determinations (R²) were 91%, 1%, 1%, and 1%. We have mapped a new, major SCN resistance locus (A006) and three minor loci (A567, A487, A112). This complete mapping will accelerate the transfer of broad-based resistance without linkage drag and aid in the determination of relationships among various SCN-resistant germplasm sources.     

THE DISTRIBUTION AND ORIGIN OF GENES FOR RACE-SPECIFIC RESISTANCE TO MELAMPSORA LINI IN LINUM MARGINALE

The genetic basis of resistance in wild flax (Linum marginale) to its host-specific pathogen Melampsora lini was investigated in seven lines collected from a single population growing at Kiandra, New South Wales and in an additional ten lines collected more widely across southeastern Australia. All lines showed different phenotypic patterns of resistance and susceptibility. Genetic analyses indicated the presence of single dominant genes for race-specific resistance in all but one of these lines. That particular line appeared to carry two linked dominant genes for resistance. Intercrosses between lines in each of these groups of L. marginale detected substantially more linkage between the resistance genes in the Kiandra population sample than between those in the broader geographic collection. This result is interpreted to indicate a possible mechanism whereby resistance genes are generated in natural populations.     

Contrasting rates of nucleotide substitution in the X-Linked and Y-Linked zinc finger genes

We have sequenced the entire exon (1,180 bp) encoding the zinc finger domain of the X-linked and Y-linked zinc finger genes (ZFX and ZFY, respectively) in the orangutan, the baboon, the squirrel monkey, and the rat; a total of 9,442 by were sequenced. The ratio of the rates of synonymous substitution in the ZFY and ZFX genes is estimated to be 2.1 in primates. This is close to the ratio of 2.3 estimated from primate ZFY and ZFX intron sequences and supports the view that the male-to-female ratio of mutation rate in humans is considerably higher than 1 but not extremely large. The ratio of synonymous substitution rates in ZFY and ZFX is estimated to be 1.3 in the rat lineage but 4.2 in the mouse lineage. The former is close to the estimate (1.4) from introns. The much higher ratio in the mouse lineage (not statistically significant) might have arisen from relaxation of selective constraints. The synonymous divergence between mouse and rat ZFX is considerably lower than that between mouse and rat autosomal genes, agreeing with previous observations and providing some evidence for stronger selective constraints on synonymous changes in X-linked genes than in autosomal genes. At the protein level ZFX has been highly conserved in all placental mammals studied while ZFY has been well conserved in primates and foxes but has evolved rapidly in mice and rats, possibly due to relaxation of functional constraints as a result of the development of X-inactivation of ZFX in rodents. The long persistence of the ZFY-ZFX gene pair in mammals provides some insight into the process of degeneration of Y-linked genes.Correspondence to: W.-H. Li