The maturation of pollen and its ability to function (germinate, produce a tube and penetrate the micropyle) are capable of being altered by environmental and genetical factors. For some purposes, in vitro germination studies may reveal aspects of this pollen plasticity. Studies proving, rather than just correlating, in vitro germination and the ability to complete fertilization are rare, although it has been demonstrated that an in vitro germinated male gametophyte may be recovered in an F1 zygote.
Earl Patterson and I have undertaken a study of the ability to function of adjacent segregants from a series of translocation heteromorphs. As part of that study, we have looked at the in vitro germination of certain classes of pollen grains.
Alternate segregants produce full, plump, functional pollen grains. Adjacent segregants are apparently less full, sometimes smaller, pollen grains. The adjacent segregants appear translocation-specific, indicating that their phenotype is a product of their genotype, in particular the duplication and/or deficient segment. Since many translocations in maize produce specific adjacent phenotypes in the pollen, there is the suggestion that loci for the 'biology' of the male gametophyte are distributed about the complement, at least in the interchange segments.
We have reported earlier that it is possible to utilize the size differences in the pollen from a translocation heteromorph by passing the pollen through a nest of sieves of decreasing pore size. Substantial populations of pollen may be obtained on some or all of the sieves.
Consistent features of pollen studied from field grown (1979, 1980, 1981, 1982) translocations (T5S.64-6S.89; T4-6 sat) include:
1. Pollen capable of germination was consistently recovered on sieves of pore size 125, 105, 90, 75 and 63 u. Pollen collected in the AM was more likely to 'clump,' rendering separation difficult, than pollen collected after 1200 h.
2. The percent germination was consistently highest for pollen collected on the 90 u sieve. The rank was 90, 105, 125, 75 and 63 with the differences usually significant (5%), except for the 90 and 105 u pollen.
3. The percent germination for pollen from the 90 u sieve averaged nearly 75% over the years; pollen from the 63 u sieve averaged 8%.
4. Very little additional germination occurred among the larger sized samples after 60 minutes. Additional germination of the smaller, including the 63 u pollen, continued for at least 150 minutes.
5. The rate of elongation of pollen tubes from the 90 u sieve decreased from over 5 u/min during the first hour to less than 1 u/min after 90 minutes. This latter rate was maintained for several hours. In general, the smaller pollen grains had slower rates of growth.
Most of these data were recorded from films, permitting the measurement of the diameter of the pollen grain. None of the populations (each sieve type) was entirely homogenous. Larger than expected pollen grains apparently were somewhat shrivelled when sieved and expanded with uptake of water from the medium. Smaller than expected grains did not proceed through the nest of sieves in accordance with their size, for a variety of reasons. However, none of these 'off-type' grains were entered into the data.
Thus, it would appear from in vitro studies that adjacent segregants fail to produce progeny most of the time because either they are actually sterile and do not become a functional gametophyte, or as a functional gametophyte they cannot compete well (i.e., a lag in germination and/or a slower rate of elongation) with alternate segregants.
Patterson and I have some data on the progeny produced by samples of pollen from the same translocations, sieved and used in sparse pollinations. We have examined several hundred dup/df F1's, the frequency of which is inversely proportional to the size of the sieve pore on which the pollen was collected. Some samples of pollen, passed through the 74 u sieve and collected on the 63 u sieve, have produced as high as 75% dup/df (estimated by uncovering a known allele) plants.
The combined use of sieving pollen and sparse pollination may permit the male transmission of cytogenetic and genic variants heretofore considered as 'non-transmissible.'
D. B. Walden
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