POLLEN- AND SEED-STERILITY IN HYBRIDS 1 
Bradley Moore Davis 
Sterility of hybrids in various forms and degrees is a phenomenon so 
frequently presented to the plant breeder and geneticist that in some form 
and in some degree it is rather to be expected. The first problem in its 
study demands a critical examination to determine in the life history the 
place of those conditions that bring about the sterility in question. In 
earlier days the gametes were generally expected to carry the blame of 
failure to reproduce the line. More recent studies have shown that re¬ 
sponsibilities for sterility cannot be so easily placed. 
Sterility, as expressed by varying proportions of abortive pollen and 
abortive ovules, is very common, and since it is easily recognized this 
manifestation of sterility has received the greatest share of attention. It 
is expressed by failure of the pollen grain to attain full size, the structure 
shriveling and usually losing its protoplasmic contents. In a like manner, 
the megaspore or embryo sac does not reach a normal development in the 
ovule. Such behavior results in failure to produce gametes, and cytological 
studies, as far as they have gone, indicate that this form of sterility, at least 
frequently, has its causation in irregularities of the reduction divisions which 
immediately precede the differentiation of micro- and megaspores. 
During these mitoses spindles may not be normal in form, chromosomes 
may be distributed in varying and irregular numbers, and the preparations 
for the reduction divisions may show abnormalities. Such phenomena 
clearly indicate a breakdown in the mechanism of nuclear division at this 
critical stage in the life history. It seems reasonable to assume in these 
cases that the hybrid must carry a germ plasm the structural elements of 
which can not conduct themselves in the orderly manner so characteristic 
of meiosis. Speculation on the reasons for the obvious breakdown of the 
cell and nuclear mechanism at this point in the life history would lead us 
too far afield for the purposes of this paper. It seems clear, however, that 
the causes lie in the heterozygous nature of the germ plasm, since we do not 
find this form of sterility in pure material. Abnormalities of chromosome 
distribution are clearly invited when the two sets are of different genetical 
constitution, since irregularities of segregation are rendered much more 
likely. 
It should not be assumed, however, that the presence of abortive pollen 
and abortive ovules is proof positive that the parent plant is hybrid, although 
1 Read in the symposium on “Sterility in Plants,” at the joint meeting of Section G 
of the American Association for the Advancement of Science, the Botanical Society of 
America, and the American Phytopathological Society, at Cambridge, December 27, 1922. 
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