Mitotic crossing-over as an important mechanism of floral sectoring in Tradescantia

A purple flowered Tradescantia hirsuticaulis is described exhibiting three classes of somatic sectors: red-only, red/blue twin spots, and blue-only. The twin spots are shown to be the results of a single event since the association of red and blue cells in the same stamen is non-random and intervening purple cells occur far too infrequently to permit hairs containing both red and blue sectors to arise by sequential, facilitated, red-only and blue-only events. As in T. clone 02, deletion, as evidenced by the presence of micronuclei (chromosome fragments), is indicated to be one mechanism producing some kinds of sectors (red-only) in response to ionizing radiation. The predominant, if not exclusive, mechanism of spontaneous sectoring and important mechanism of even 60 R-induced sectoring in this T. hirsuticaulis (and by inference, in other Tradescantia) is argued to be mitotic crossing-over, since other mechanisms operating on its genotype do not allow for twin spots arising as the result of one event. This is supported by the prediction of all three types of sectors in the proper order of frequency as well as by the calculation of a 'coefficient of coincidence' and a radiation response that are similar to those found for mitotic crossing-over in Drosophila