Reversal of preferential fertilization
--Carlson, WR

A surprising finding has emerged from studies of preferential fertilization. In two different crosses, the elimination of preferential fertilization has been accompanied by an apparent reversal of the fertilization pattern. The first study involved a tester stock which acted through the female to block preferential fertilization by B-containing sperm. Crosses of the type c c sh sh wx wx gl15 gl15 x hyperploid TB-9Sb lacked preferential fertilization. The rate of preferential fertilization ranged from 42 to 45% (Carlson, Genetics 62: 543, 1969, Table 5) rather than the random value of 50%. The values from 1969 are not statistically analyzable for a significant deviation from 50%, since they involve some data manipulation. However, a reversal of fertilization pattern was also found in the cross discussed above with the type 1 telocentric (40% preferential fertilization).

In a third case, elimination of preferential fertilization led to random fertilization, rather than a reversal of pattern. This was found when the B-9 chromosome was present in plants that also carried several standard B chromosomes. In this case, most pollen should contain at least one B-type chromosome in each sperm cell, due to random segregation of the chromosomes following nondisjunction. The rate of preferential fertilization by the B-9 was 52% (Carlson, Genetics 62: 543, 1969, Table 2), very close to a random value. The difference between the first two experiments and the last one can be explained on the basis of activity vs. non-activity of the system for preferential fertilization. The experiment with c sh wx gl15 tester involved inactivation of the system by the female parent. The experiment with the type 1 telo also produced inactivation of the system, this time by deletion of a proximal region of the B. However, the experiment which combined TB-9Sb with standard B chromosomes utilized an active system, which functioned in both sperm of the pollen. Therefore, competition between sperm occurred equally and the result was random fertilization.

In sum, inactivation of the preferential fertilization system appears to produce a reversal of preferential fertilization. The reversal could be due to 1) a non-specific effect of excess chromatin on fertilization, or 2) a specific effect of the B chromosome.

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