Transposition of the En/Spm transposable element system in maize (Zea maysL.): reciprocal crosses of a1-m(Au) and a1-m(r) alleles uncover developmental patterns

Transposition studies of the transposon, En/Spm, have dealt with general as pects of the timing of the excision event with regard to DNA replication an d plant development, but without describing details of the process. By foll owing the excision events of an En transposon inserted at the a1 locus [a1- m(Au)], several features of this process can be elucidated. In progenies fr om reciprocal crosses between the nl-m(Au) allele containing an En insert, and a nonautonomous En allele, [a1-m(r) is a deficiency derivative of En], several features of the En at the a1-m(Au) allele can be observed taking pl ace during ear development and during microsporogenesis. First, it has long been known that the distribution of mutant kernel phenotypes on an ear ind icates that En transposes late in most of the events during ear development . Second, the phase change of En (presence and absence of activity) is obse rved during cob development. Third, discordant kernel phenotypes of two ear s, reported herein, resulting from a reciprocal cross with the parental phe notype can be deduced to arise from the transposition of En during microspo rogenesis and subsequent fertilization, leading to a discordant genotype be tween endosperm and embryo. The phase change and discordance lead us to con clude that these events can arise from transposition after host DNA replica tion. It can also be concluded that the activity of the En inserted in this al-m(All) allele is not limited to a specific stage or timing during plant development. Further, this study illustrates the power of genetic analysis in the determination of cellular events.