A test of pollen competition using TB-9Sb
Beckett (J. Hered. 73:29, 1982) tested male transmission of the B-translocations, TB-1La and TB-1Lc. The translocations were in the heterozygous condition: 1, 1-B, B-1. An approximate 1:1 ratio is expected for the normal vs. the translocation gamete, with other gametes being inviable or seldom transmitted. However, Beckett found that the translocation gamete was transmitted at a rate of 56-59%. This rate may be an underestimate of the translocation class. Beckett's approach was to cross the B-1 translocations into a high nondisjunction inbred background, so that most translocation gametes would show nondisjunction. Subsequently, the rate of nondisjunction was measured as an indicator of transmission of the translocations. This method will miss translocation gametes that lack nondisjunction, albeit a small number. The pollen competition effect of TB-1La and TB-1Lc was also minimized by missing the duplication gamete (1 B-1). It was counted in the normal (no nondisjunction) category. This group should be small, but may have been transmitted to some extent. Consequently, the true value for pollen competition is probably greater than that found by Beckett.
The findings for TB-1La and TB-1Lc are not consistent with published data on TB-9Sb heterozygotes. The Wx marker on TB-9Sb was used to detect transmission of the translocation gamete (Carlson, Maize Breeding and Genetics, p. 754, Table 4, 1978). The Wx marker is on the 9-B and is extremely close to the translocation breakpoint. It is unaffected by nondisjunction, since that phenomenon is confined to the B-9 chromosome. As a result, it marks all translocation gametes and does not need nondisjunction as an identifier. The data should, therefore, be very accurate in determining any advantage of the translocation gamete over normal. The table cited above contains data on male transmission of standard TB-9Sb (last entry in table). The TB-9Sb heterozygote gave 50% Wx transmission, indicating no competitve advantage to B-containing pollen.
One reason for the discrepancy in data between the B-1 translocations and the B-9 translocation could be survival of the duplicate gamete, A B-A. This gamete is ignored in both tests and becomes part of the "normal" gamete class. It is possible that transmission of the 9 B-9 duplicate gamete is much more frequent than that of either 1 B-1 gamete, due to a smaller amount of gene duplication. The result would be to reduce the apparent size of the 9-B B-9 gamete class.
A test was devised in which the duplicate 9 B-9 gamete could be identified and removed from the data. The heterozygote was marked as follows: 9 (C sh wx) 9-B (Wx) B-9 (c Sh). In a cross as a male parent to a c sh wx tester, the 9 B-9 gamete should give the phenotype, C Sh wx. While this phenotype could be produced by a crossover chromosome 9, it would require a double crossover on the B-9, including one crossover in the short C-Sh interval. Therefore, the C Sh wx class should consist almost entirely of the 9 B-9 class. Another consideration in identifying 9 B-9 gametes is the possibility that some crossover gametes are missed by identifying only the C Sh wx class. The crossover gametes, 9 (C sh wx) B-9 (C sh) and 9 (c Sh wx) B-9 (c Sh), would not be identified. These types occur by a crossover somewhere between C and the translocation breakpoint on the B-9. The rate of crossing over in this region can be estimated for normal (chromosome 9) gametes by measuring the frequency of c wx kernels per total wx. The total rate for all crosses is 1013 per 5788 or 17.5%. If the same rate occurs in 9 B-9 gametes, 17.5% of this gamete class would not be found. However, the noncrossover 9 B-9 (C Sh wx) class is small, and a change in it of 17.5% would not significantly affect the calculations.
Below are given data on the number of Wx and wx kernels produced in crosses of c c sh sh wx wx x9 (C sh wx) 9-B (Wx) B-9 (c Sh). Three ears were classified for each male parent.
Removal of the 9 B-9 class from the data increased the percent of Wx very little. The pollen competitive advantage of TB-9Sb is 53.3% (or 53.5% with an adjustment for crossing over). This value is below the 56 to 59% values found by Beckett for the B-1 translocations, despite the fact that every effort has been made to maximize the measurement of the TB-9Sb gamete class. The data do show an approximate 3.5% competitive advantage by TB-9Sb pollen over normal 9. However, one of the genes used to measure the phenomenon of pollen competition might account for the competitive advantage. The Wx allele transmits through the pollen at a slightly higher rate than the wx allele (Coe et al., Corn and Corn Improvement, p. 198, 1988). Consequently, the linkage of the 9-B B-9 gamete to Wx could, by itself, account for the advantage of TB-9Sb chromosomes over the normal 9. The data, therefore, show either a weak or a nonexistent pollen competitive effect of TB-9Sb.
The discrepancy between these results and those of Beckett could be
explained in a number of ways. Perhaps certain genetic backgrounds allow
pollen competition effects by the B chromosome and others do not. Alternately,
a gametophyte effect of genes on chromosome 1 may have produced excess
transmission of the B-1 translocation gametes (Beckett, 1982). Other ideas
can be invoked, such as a differential effect of meiotic loss on TB-9Sb
which cancels the pollen competition effect (Carlson and Roseman, Genetics
131: 211, 1992). However, all that can be concluded is that more work is
needed in different genetic backgrounds and with different translocations
to determine the significance of the pollen competition effect of B translocations.
|Male Parent||Classification (C Sh wx in parentheses)||% Wx||% Wx after removing C Sh wx|
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