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chromosome distribution in triploid allium
59
Table 1. The distribution of S, chromosomes in the pollen grains. The upper part of the
table is Levan’s "Tabelle 24". Note the displacement to the right of the Fotind number of
S, chromosomes compared with the Expected number of S, chromosomes.
Ohromosomenzahl der 1’ollenkörner
_8_ 9 10 11 12 13 14 15 16_
1 S, 2 Sjjl S, 2 S, 1 S, 2 S,.l S, 2 S, 1 S, 2 S, 1 S, 2 S, 1 S, 2 S, 1 S, 2 S, 1 S.-2 S,
Anzahl Falle...... 10 0 117 i 2 48 [10 159 [24 49 42 1G [34 I 2 21 0 111 0 1 2
Gefundener °/o ... 100 0 89,5 10,5 82,8 17,271,1 28,9 53,8 46,2 32,0 68,()! 8,7 91,3 0 100 0 100
Krwartcter 11 i) ...100 0 87,5,12,5 75,0 25,062,5 37,5 50,0 50,0 37,:> 62,5,25,0 75,0 12,5 87,5 0 100
Found:
Number of
chromosomes ...... 80 171 580 913 1092 650 322 1 165 32
Number of St-
chromosomes .. 10 21 68 107 133 84 44 22 4
% S. of all
chromosomes ...... 12,5 12.3 11.7 i 11,7 12,2 12,9 13,7 13,4 12,5
Expected number
of
Sj-chromoso-rn.es with 12,5
°/o S[ ofall
chromosomes ...... 10 21,4 72,5 114,1 136,5 81,3 40,2 20,6 4
in Datum. lf sucli a selective orientation is acting, it must, however,
affect the binomial distribution of the chromosome numbers as in
Datura, hilt no tendencies of this kind are to be found in this
AUium case.
I find it most plausible lo think that the non-random distribution
of the Sj chromosomes is caused by the S, univalents being
non-randomly assorted to the tvvo poles at the first division. Wlien the
anaphase groups are of unequal size, the spindle will be asymmetrical,
and it is close al hand to suppose that the univalents will then be
non-randomly assorted, e.y. tliey will preferably go to the largest
anaphase group (when they are nol eliminated).
A condition for Ihis mechanism to have any result is that the
univalents remain undivided at the first division as are they in the
AUium plant in question. It may perhaps be objected that the cytological
consequences are rather slight in the case accounted for here, but it
reveals a mechanism al nieiosis, hitherto nnknown, which if ubiquitous
may be of great importance.
It may be supposed that in Allium this mechanism mainly affects
the relatively frequent Sj univalents and therefore its result will be too
slight to be noticeable in the distribution of the chromosome numbers
of the pollen grains. lf it were möre pronounced, it would certainly
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