The volatility of the maize genome
--Peter A. Peterson

When appropriate searches are made for new instabilities, one is confronted with many surprises. Though a specific transposon is relegated to the search and the target, other transposons appear in the final analyses. This is the case for Ac-Ds. This has occurred in several instances and some of these are related here.

The first instance is that of the activation of quiescent Uqs, some of which have turned out to be Ac. This became apparent with the use of reporter alleles. In this case the reporter allele was c-ruq. Thus, in ordinary breeding lines, these events may occur with no cognizance of the activation of these elements because appropriate reporter alleles are not present. Thus, these particular transposons in this particular instance were quiescent due possibly to methylation, and subsequently, and in periods of normal growth and development, are periodically activated. There has been no stress induced activation such as a bridge-breakage-fusion cycle that has been particularly targeted for these events, but during normal processes of development. Note again that these would not appear except for the presence of the appropriate reporter allele.

Other cases are when specific genes are targeted. In a number of instances they appear as chromosomal breakers. Chromosomal breakers, because a number of markers are lost simultaneously. This is the case with C-I-b836024. In this case, it turns out to be a fully active Ac. It is not related to any two-element arrangement but certainly all the evidence suggests that it is a fully functional Ac. Its activity includes changes of state and transposition of the element to nearby sites.

Other elements are also known and these are elements that induce the same kind of event that has also been identified. This includes C-m897140 (1992 Newsletter). This also arose as an independent event in an isolation plot where the C locus was targeted. And, in this instance, a number of elements were present, however, only the Ac was identified with the breaker event.

It would appear that the corn genome is highly volatile. That is, these transposons are now known to be quiescent in the genome. They appear also to be spontaneously activated. This is evident in almost any line one looks at with appropriate reporter alleles as indicated in the paper by Pan and Peterson (1988), where sectors were pervasive in these lines. It would appear, therefore, that the corn genome has a built-in system of activation and redeployment of genome segments and in a final result, leads to a great deal of variability. 

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