Copyright zyxwvutsrqpon 8 1988 by the Genetics Society of America zyxwvutsrq Spontaneous Activation of Quiescent zyxw Uq Transposable Elements During Endosperm Development in zyx Zea mays zy Yong-Bao Pan and Peter A. Peterson Department of Genetics and Department zyxwvut of Agronomy, Iowa State University, Ames, Iowa 5001 I Manuscript received April 27, 1987 Revised copy accepted February 29, 1988 ABSTRACT This study addresses the question of the activation of quiescent transposable elements in maize breeding lines. The a-ruq reporter allele of the Uq transposable element system expresses Uq activity (spots or sectors of spots in otherwise colorless aleurone tissue) when exposed to various genotypes of assorted maize inbred lines lacking any active Uq element. This activation ofquiescent Uq elements occurs randomly during the growth of the endosperm. It is concluded that there are components in the genome that enhance the rare activation of quiescent Uq elements. Further, it seems that this activation occurs in the absence of any stress-inducing treatment, but that normal growth conditions provide sufficient stimulus for such activation. T RANSPOSABLE elements in maize were initially observed in corn lines of families that were undergoing a cycle of chromosome bridge, break, and fusion (BBF) (MCCLINTOCK 1951b). Though transposable elements wereobserved earlier inge- netic nurseries (EMERSON 1914; RHOADES 1936), they were subsequently found in naturally occurring pop- ulations (PETERSON 1987). Otherreports have em- phasized the role of a stress environment that triggers the activity of quiescent elements. This was most notable in the Barley Stripe Mosaic Virus(BSMV) treatments of corn plants by SPRAGUE and MCKINNEY (1966, 1971) that uncovered unstable a-alleles later identified as belonging to the Ubiquitous (Uq) trans- posable element system (FRIEDEMANN and PETERSON 1982; PETERSON and FRIEDEMANN 1983). More re- cently, active elements have been recovered from lines originating from genotypes that were under a tissue culture regime that is considered a stress en- vironment (BURR, ARCHER and BURR 1987; PESCHKE, PHILLIPS and GENGENBACH 1985). Of all the transposable elements currently under study in maize, the Uq transposable element is one oftwo that is most pervasive and persistent in nu- merous maize breeding populations and several ge- netic testers (Mrh is the other) (PETERSON and SAL- AMINI 1986; PETERSON and FRIEDEMANN 1983; PE- TERSON 1986). Although found in the BSSS (Iowa StiffStock Synthetic) breeding populations (PETER- SON 1986; PETERSON and SALAMINI 1986), active Uq elements are not detectable in the BSSS-derived inbred lines and other commercial inbreds tested (CORMACK and PETERSON 1987; PAN and PETERSON 1986, 1987; PETERSON 1986; PETERSON and FRIEDE- MANN 1983; PETERSON and SALAMINI 1986). Their Genetics zyxwvutsrqpon 119: 457-464 uune, zyxwvutsrqpo 1988). disappearance in the derived inbreds has been hy- pothesized to be due to strong selective pressure against variability. It was early observed that among kernels of the standard zyxwv a-rug allele, occasional spots appeared at a low frequency even in the absence of an active Uq element. This became more apparent in outcrosses of the a-ruq allele acting, in this case, as a “reporter allele,” for the presence of an active Uq element in assorted inbred lines. What was revealed in these outcrosses was an unusual type of mutability that consistently appears as a single spot ofvariedsize (1-24 color cells) in an otherwise colorless aleurone layer. The genetic basis of this one-spot mutable phe- notype was assumed not to be due to the presence of a veryweak Uq element that was activated, but more likely to be due to a nonheritable instability of the ruq receptor element at the A locus (PETERSON and FRIEDEMANN 1983). Changes in the state of the ruq element have been found in two other studies, giving rise to several derivatives such as a-ruq(lo), nonresponsive colorless, nonresponsive pale (PEREIRA and PETERSON 1985), and A(m), a stable self-colored allele (SPRAGUE 1986) in the presence of a standard Uq element. In this paper, we report on the activation of quiescent Uq elements in four maize inbred lines, B70, C103, C123, and 187-2, that have not received any unusual treatment. It is our contention that quiescent mobile elements are continuously being activated even in the absence of any unusual treat- ment such as BBF, BSMV, or tissue culture regimes. The genetic basis of this activation as revealed by sectoring in the maize aleurone tissue has been de- termined and its possible mechanism is discussed.