Analyzing crosses involving misdivision
--Carlson, WR

There are peculiarities in crosses that utilize an unpaired type 1 telocentric for studies of preferential fertilization. For example, the rate of Yg bz + yg Bz kernel types is much lower (20% in the cross analyzed above) than the usual nondisjunctional rates (50-100%). This is not unexpected, since misdivision is a different process from nondisjunction. Another oddity is the fact that a considerable number of kernels in these crosses are variegated for endosperm phenotype (Bz/bz). In the cross above, using 9-B 9-8 telo B-9 plants, 65/941 Wx kernels were variegated ( 6.9%). When these were germinated, 54 green (Yg), 6 yellow (yg) and 3 variegated (Yg/yg) plants were found. Two did not germinate. The predominance of a stable (green) seedling phenotype suggests an explanation for the variegated kernels. Most likely, the source of endosperm variegation was usually an unstable telocentric, produced by misdivision at meiosis. The telocentric was unstable due to the absence of a telomere at its centric end. It divided equationally at the second pollen mitosis to send an unstable telocentric into each sperm cell. The daughter telocentrics were unstable in the endosperm but stable in the plant, because telomeric healing only occurs in the plant (McClintock, Proc. Natl. Acad. Sci. USA 25: 405, 1939). Two other classes in the cross, Yg bz and yg Bz, can also be explained by misdivision. The Yg bz kernels were analyzed by growing bz kernels in the field and testcrossing them, as female parents, to bz bz. Testcross data can be used to determine the number of B-9 chromosomes in a plant (Robertson, Genetics 55: 433, 1967). The testcrosses showed that 35 out of 39 plants tested were hyperploid. That is, they had two chromosomes carrying Bz. The remaining plants were heterozygotes, with one Bz-carrying chromosome. It is believed, although not yet tested, that the two chromosomes in the hyperploid plants were telocentrics. They probably resulted from two unstable telos migrating to one pole and undergoing "healing" to form two stable telos. This type of disjunction (0-2) is distinct from the 0-iso disjunction found earlier for the type 1 telocentric (Carlson, MNL 70: 218, 1996). The difference between crosses that produced 0-2 disjunction and ones that produced 0-iso disjunction may be attributable to genetic background.

It seems very likely that the yg Bz kernels from the 9-B 9-8 telo B-9 crosses also resulted from 0-2 disjunction. The yg Bz kernels should have received the reciprocal fertilization of the Yg bz kernels, with two unstable telos entering the endosperm. Why was the endosperm of yg Bz kernels not variegated? Unlike the embryo, the endosperm lacks a system of telomeric healing that could stabilize the telocentrics. The explanation may be that two unstable telocentrics in the same cell readily fuse with each other to produce a stable isochromosome. A stable endosperm phenotype may, therefore, be attributable to fusion at the centromere of two unstable chromosomes.

One final consideration is whether the Yg bz and yg Bz phenotypes could have been produced by chromosomal instability after fertilization, rather than through a dissimilarity of sperm before fertilization. If an unstable telocentric is produced at meiosis and is transmitted to both sperm of a pollen grain, could the chromosomes be unstable after fertilization? If so, calculations of preferential fertilization would be invalid, since the various recessive phenotypes (yg Bz, Yg bz and yg bz) would result from postfertilization loss rather than an effect on the fertilization process. This idea seems unlikely for two reasons. Considering the Yg bz kernels, two telos were found in the embryo in most cases. If B-9 loss accounted for this phenotype, there should be only one telo delivered to both the endosperm and embryo. Second, the variegated kernels that were found in these crosses gave primarily Yg seedlings, as noted above. If an unstable telocentric was delivered to both endosperm and embryo in these cases, there is little evidence of telo loss or instability in the seedling.
 


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