The role of transposons in the origin and morphogenesis of maize
--Walton C. Galinat
The first direct evidence of host regulation over the transposons that control gene expression during the origin and morphogenesis of maize comes from connecting-link stocks developed to analyze floral differences between teosinte and maize. Previous studies by others of host regulated transposon excision and incision involved color producing genes expressed as spots in the aleurone or stripes in the pericarp. By extrapolation to genes in general, McClintock predicted that this transposon process controlled evolution and morphogenesis.
In the connecting-link stocks the teosinte gene Pd-Ac (pd) on chromosome 3 that represses development of the pedicellate female spikelet had a variegated expression along the ear (Fig. 1). Normally in teosinte, the female spikelets are solitary in adapting for the protective function of grain enclosure within a fruitcase while the male spikelets on the same plant are paired showing host regulation by sex over Ac transposition and Pd expression. In maize, both male and female spikelets are paired. Paired female spikelets in maize are adaptive for the domestic function of increased productivity per ear and easier shelling. Stabilizing selection in certain of my stocks restored absolute sexual control in producing female spikes that were stable for either two-ranks of single spikelets as in teosinte or else two-ranks of paired spikelets as in maize.
Apparently during the origin of maize, the Ac transposon of Pd became banished to exile, perhaps into some distant intron of a different chromosome. The pairing of spikelets, by a reactivation of the second spikelet, then had a freedom of expression without sexual discrimination. Humankind's greatest plant breeding achievement was well underway.
Presumably the different transposons can be identified molecularly by different known probes, and in due course someone will do that for the Pd transposon that I have perhaps incorrectly indicated here as being the Ac transposable element.
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