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fungi has, however, uncovered certain instances of exceptional chromo-
some behaviour that are not revealed by less penetrating methods,

In Neurospora, Lindegren and Lindegren (1942) have demonstrated
specific patterns of chromatid interference. Unfortunately, the
markers were morphological mutants which some workers have
found difficult to classify, at least in combinations. This fungus is
unquestionably very well suited to the analysis of crossing-over, and

' fortunately there are now available an unlimited number of bio-
chemical mutant markers which can be scored without ambiguity
in any combination. It is to be hoped that the current revival of
interest in crossing-over may motivate a reinvestigation of interference
in Neurospora.

The coprophilous species Bombardia lunata exhibits some a2utono-
mous characters of the ascospores. Segregation of these characters
can be determined by inspection, thus circumventing the necessity
of manual isolation and transfer of each spore. From data tabulated
by Zickler (1934), Catcheside (1944) inferred that the Segregation was
polarised, the mutant allelomorph tending to the proximal pole of
the ascus. Elucidation of the mechanism operating has been hindered
by our ignorance of Zickler’s whereabouts and of his unique Bombardia K
cultures. Attempts to recover comparable species from field collections
have been unsuccessful, Dr Catcheside tells me.

A different sort of bias, affecting the proportions of Symmetrical
(AaaA or aAAa) and asymmetrical modes of post-reduction, has been
studied by Whitehouse and Haldane (1946). They suggested that
asymmetrical segregation will result from the retention of crossover
chromatids medially, the non-crossover chromatids more usually
teaching the poles, and the preservation through the second division

: - — of this orientation, Their analysis of data from diverse sources
2aAu ance Suggested a doubtfully significant excess of asymmetric reductions.

Vy “Ns . . - Shay and Keitt (1945) tested segregating asci of the apple scab

gus Venturia inegualis, from a somewhat different viewpoint.

L. . Cytological study had been inconclusive as to the Possible rearrange-

af wrtio Bm ment of nuclei in the developing ascus as might prevent discrimination
(a BU ert ca be

tween first- and second-division Segregations. Breaking the order

 

 

 

_ pT in the four-nucleus Stage would result in Spurious, asymmetric post-

{ . Cc Feductions in asci in which there had actually been a first-division
TS Segregation. Other things being equal, this would result in an excess
_ of asymmetric asci, which was not found, however, for factors cone

Tolling pathogenicity on differential hosts.
. The mathematical problems of tetrad analysis have been discussed

ee Mather (1935) and Mather and Beale (1942). Complete analysis
\7- depends on the recovery of spores in intact linear order; but some

 

information can be obtained as to centromere relations of factors even
i tetrads, such as yeast asci, where the linear order is not preserved,

~ If two independent factors are both closely linked to a centromere,
fp De 4b the Majority of asci will contain either two parental combinations or
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