SHORTER ARTICLES AND DISCUSSION
POLYPLOIDY IN ZEA MAYS L.

Durina the course of our investigations on the chromosome
numbers in Zea Mays a triploid plant has appeared in a culture
obtained for cytological and genetical studies from Dr. A. C,
Fraser, Cornell University. As compared with the diploid plants
of the same culture with their ten pairs of chromosomes, the
triploid plant was notably more vigorous; it had a thicker stalk,
broader leaves, stouter tillers, larger anthers and distinctly larger
microsporoeytes. Genetically speaking, the plant appeared to be
a dilute sun red with a heterozygous tunicate tassel.

The aceto-carmine method was employed to fix and stain the
microsporocytes. Additional material was fixed in other fluids
for further study. The history of the chromosome complement
was followed from the late prophase of the first meiotic division
(I) through the second meiotic division (ZZ).

On the basis of other evidence not presented here it seems
apparent that the basic chromosome number in Zea is ten. The
plant under discussion possessed thirty chromosomes. At diaki-
nesis these were usually arranged in ten groups of three each, 1.e.,
ten trivalents (Fig. 1). However, in the same preparations a few
figures were observed in which the third member of one or more
groups appeared unassociated with its homologues. Thus in the
same nucleus at diakinesis univalents, bivalents and trivalents
were frequently observed.

Fig. 2 illustrates the appearance of the trivalent chromosomes
in the metaphase of J. (Compare with normal bivalents in Fig.
7.) In early metaphase there are usually ten groups. As in
some -cases in diakinesis, however, univalents occasionally ap-
peared not clearly associated with any other members of the com-
plement. This suggests a continuous non-association of certain
homologues through the late prophase and metaphase. The extra
member in each group most frequently appears as if it were
united by one end to some point on a normal bivalent, i.e., there
is a loose attachment of the third member (see first and last tri-
valent in Fig. 2). In such a case the third chromosome disjoins
from the other two; the latter then pass to opposite poles as usual.
In other trivalents of the metaphase complement the three

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chromosomes appear to be attached end-to-end with equal close-
ness. Other modes of association were observed, but whether or
not these are characteristic for certain trivalents is not as
yet known.

Very early in the anaphase the members of each trivalent
appear as three distinct units, and the assortment is apparently
a random one. Most frequently, as would be expected, the two
anaphase groups show fifteen chromosomes each. However, fig-
ures were observed in which the distribution was 14-16, 13-17,
and 12-18. In one case the thirty chromosomes were scattered
very irregularly between the two poles. It is not known whether
this results in the reorganization of one nucleus with the double
number of chromosomes, as seems possible from evidence dis-
cussed later, or merely indicates a greater delay in the passage of
the chromosomes to the two poles.

In the second meiotie division the chromosomes of the prophase

are characteristically long and slender, and distinetly X- or
No. 666] SHORTER ARTICLES AND DISCUSSION 101

H-shaped (Fig. 4). Here, also, the individual chromosomes are
easily distinguishable, sister cells showing most frequently fifteen
chromosomes each. Often both mitotic figures were so favorably
situated that the number of chromosomes in each anaphase group
were countable, so that the number of chromosomes in each miero-
spore of the quartet could be estimated. Occasionally we have ob-
served in one cell, whose size indicates that it is a microsporocyte,
a single mitotic figure showing all the characteristies of the
anaphase of IZ, but with thirty chromatids in each group (Fig.
5). In case two microspores develop from such a cell, each would
contain thirty chromosomes, instead of the usual ten to twenty.

Nothing is known concerning the manner in which triploidy
arose in the culture referred to above, but in other cultures the
following suggestive phenomena have been observed. In one
plant in which many miecrosporocytes with ten pairs of chromo-
somes were seen, there were found two binucleate microsporo-
eytes, with ten bivalent chromosomes in each nucleus (Fig. 6),
together with a few other microsporocytes showing twenty bi-
valents in each metaphase of I (Fig. 7). In another plant
normally showing twelve bivalents in meiosis, cells were observed
with twenty-four dyads in the prophase of IZ (Fig. 8), in addi-
tion to cells with twenty-four bivalents in the metaphase of J.
Should viable diploid gametes thus arise, their union with normal
haploid gametes would result in a triploid plant.

Since comparatively few cells with the chromosome number
thus doubled appear in the anther among normal diploid cells, it

 
102 THE AMERICAN NATURALIST [Vou. LX

would seem that the former must be due to aberrations occurring
in late premeiotic divisions. The occurrence of these various con-
ditions in cells so closely related points strongly toward the con-
clusion that the stages illustrated in Figs. 6-8 belong to one
process responsible for the origin of triploidy, and conceivably
of other forms of polyploidy in Zea Mays.

L. F. Ranpotpyu

B. McCuintocx
CoRNELL UNIVERSITY AGRICULTURAL
EXPERIMENT STATION AND THE
U. 8S. DEPARTMENT OF AGRICULTURE