Philosophy of Biology

(Tuis.) #1

268 Raphael Falk


the model, the frequency of such marker-homozygotyzation was proportional to
their distance from the centromere, it allowed the mapping of the centromere —
the region of the chromosome with which the spindle fibers become associated,
i.e., a cytological entity — on the virtual linkage maps [L. V. Morgan, 1922]. No
information on the mechanism was available except that recombination could oc-
cur between any pair of the four chromatids, and that the closer the markers were
the more one exchange interfered with another one nearby, which indicated rigor
of the pairing chromosomes as expected in a mechanical model. These conclusions
were upheld and extended when recombination in fungi, primarily inNeurospora
crassa, were performed. In ascomycete fungi each meiotic division is confined to
a discrete sac, orascus. Micro-dissection of asci allows full “tetrad analysis,” fol-
lowing all the products of individual meiotic events (contrary to the situation in
most other organisms where the mass of all products of many meiotic events are
observed, except for special cases, like attached-X, where ”half tetrad-analysis” is
possible). Furthermore, in some species, like Neurospora, the linear arrangement
of the ascospores within the ascus allows discerning the products of the first meiotic
division from those of the second meiotic division (see Finchamet al.[1991]).


The observation that both chromatids of the paired chromosomes could be in-
volved in crossing-over events along the chromosome appeared to refute alterna-
tive to the mechanistic explanation, such as that exchange occurred between the
newly produced chromatids, due to “copy-choice” at replication [Belling, 1928].
Additional notions, like that of mutual induction of sites on the chromosomes, or
“gene conversion” [Winkler, 1930] were excluded by Barbara McClintock in maize
[Creighton and McClintock, 1931] and Curt Stern in Drosophila [Stern, 1931],
when they constructed stocks that allowed to follow both cytological physical ex-
change and genetic recombination of markers in the same individuals (see Falk,
[1995a; 2003]).


Linear linkage maps of distinct genes entities, among which recombination may
take place, became the dogma of genetic analysis. However, the resolution power of
recombination analysis was limited by the shear size of the experiments that could
be carried out: as a rule, few scores of progeny in mouse recombination experi-
ments, and not more than some hundreds in Drosophila or maize. This changed,
however, in the 1940s when fungi — primarilyNeurospora crassa— and later bac-
teria and their viruses became available for genetic analysis. Screening methods
were designed in which only (or almost only) the expected class of progeny, such
as recombinants between given markers, survived.


For many years it was doubted whether bacteria, who do not have a distinct
nucleus (they are prokaryotes), who multiply by fission, rather than by ordered
mitosis-like chromosome segregation, and in which adaptive (apparently “Lamar-
ckian”) heritable changes may be induced by changing culture conditions, obey the
rules of genetics as do nucleated organisms (eukaryotes). The evidence brought
by Luria and Delbr ̈uck [1943] that heritable changes in bacteria populations were
due to preadaptive mutations (i.e., “Darwinian” selection of preexisting muta-
tions, rather than “Lamarckian” adaptive responses to environmental challenges),

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