The Uq transposable element is one of two transposable elements consistently found in maize (Zea mays L.) populations. Populations developed from two independent recurrent selection programs initiated in the Iowa Stiff Stalk Synthetic (BSSS) maize population were tested for the frequency of Uq transposable elements to determine how Uq frequency has changed with cycles of recurrent selection. In the first programme, 13 cycles of half-sib and S2 progeny recurrent selection [BSSS(S)C13] have been completed and 10 of the 13 cycles were assayed for active Uq elements. In the second programme, 11 cycles of reciprocal recurrent selection [BSSS(R)C11] have been completed and five of the 11 cycles were assayed for active Uq elements. The frequency of Uq was different for the two recurrent-selection programmes. The percentage of plants containing active Uq elements increased from 19% (BSSS) to 91% [BSSS(S)C13] at a linear rate after 13 cycles of half-sib and S2 progeny recurrent selection, whereas the percentage of plants containing active Uq elements decreased from 19% (BSSS) to 0% [BSSS(R)C11] after 11 cycles of reciprocal recurrent selection, with extinction of the Uq element occurring between the fifth and sixth cycles of selection. Our data suggest that the increase in frequency of Uq with half-sib and S2 progeny recurrent selection was predominantly due to random genetic drift coupled with a selective advantage possibly associated with a region of the genome linked to Uq. Neither replicative transposition or chromosome assortment and segregation can be invoked to explain the change in frequency of Uq in these populations. The extinction of Uq after reciprocal recurrent selection was best explained by random genetic drift.
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