Fragmentation in Tradescantia

Abstract

life cycle, except for an occasional case of nondisjunction at meiosis.The meiotic behavior of the chromosomes was studied in 100 cells from plants having only one pair of fragments. These data are summarized below and in the following paragraphs. '.92 noo 8.00 Fragments none Normal bivalents 2.4 23.44 Average chiasma frequency . (Ic, of interstitial chiasmata . % unpaired .AMONG A group of Tradescantia 2 plants collected from Gentilly, Louisiana, by Small, Anderson and Woodson, there appeared a number of individuals having in addition to the usual diploid complement of 12 chromosomes, a varying number of fragments. When a preliminary examination indicated that the number of fragments in the somatic tissue seemed to che?k with the number found at meiosis, an opportumty was presented for studying the behavior of these fragments under rather stable conditions. The technique developed by Sax and Humphrey (1934) for demonstrating the internal structure of the chromosome offers a refined method for investigating the morphology of fragments. With the ordi-The normal bivalents show the usual chiasma frenary smear methods (either aceto-carmine or per-quency, and the percentage of interstitial chiasmata rnanent) and paraffin sectioned material, the frag-characteristic of the species. The fragment chromoments appear as small, darkly-stained blobs; with somes pair almost as regularly as the normal chromothe alcohol-alkaline pre-treatment method, structural somes. The chiasma frequency per unit of pairing features that are visible in the major chromosomes length in the paired fragments is more than twice are also evident in the fragments ( fig. 3, 4).that of the normal bivalents. The fragments form The len~th of the fragments found in this group either terminal or interstitial chiasmata, although of plants IS roughly one-sixth to one-seventh that of the latter are not nearly so numerous as they are in the major chromosomes. Thev seem to be of rather the ordinary bivalents. uniform length regardless of' the number found inThe types of configurations found in the paired a single cell. In respect to their size thev differ from fragments a.re illustrated in the diagrams of figure 2, he fragments studied by Lesley and L'esley (1929) together with a regular rod bivalent. With the m the tomato. In this case, the fragments were technique used, the chromatids are well differentiated. about one-half the size of the major chromosomes. In the regular bivalents the two chromatids of each The fragments studied by Darlington (1929) in homologue are closely associated in a coiled chromo-Tradescantia and (l9;30) in Fritillaria, by Belling nema at early metaphase, and separate to form two (1925) . in Uuularia, and by Hicks and Stebbins coiled chromatids at late metaphase and anaphase.

Structural Heterozygosity, Duplication of Telomeric (TTTAGGG)_n Clusters and B Chromosome Architecture in <i>Tradescantia</i> <i>virginiana</i> L.

“…Consequently, the terms 'B chromosome(s)' and 'Bs' are used in the present paper. Also in Tradescantia paludosa supernumerary chromosomes were not found to pair with standard chromosomes at MI [Whitaker, 1936]. In many studies it was shown that despite their univalency Bs have the capacity to pass though meiosis and to be still maintained in populations [Jones and Rees, 1982;Jones, 1995].…”

“…Mitotic/meiotic propagation of such an amalgamate of sequences seems possible by recruiting centromere DNA and/or generating epigenetic effects enabling spindle microtubule attachments, and/or by adding telomeric motifs stabilizing the ends of a newly arisen B chromosome [Dhar et al, 2002;Jones and Houben, 2003]. Previous models on the origin of supernumerary chromosomes in Tradescantia suggested fragmentation of a major portion of an A chromosome with the preservation of its original centromeric region, which then could act as a donor functional domain [Whitaker, 1936;Swanson, 1943]. However, the structural similarity of Bs and distal chromosome regions indicates quite a different route of B chromosome escape in T. virginiana .…”

“…To note, at least some supernumerary chromosomes in Tradescantia are likely to be of relatively recent origin, as can be inferred from their ability to pair with A chromosomes [Darlington, 1929]. The small supernumerary chromosomes of Tradescantia pass through the mitotic and meiotic division cycles, frequently possess a subterminal centromere, and can be accumulated in a single plant in numbers as high as 12 with no appreciable effect [Darlington, 1929;Anderson and Sax, 1936;Whitaker, 1936;Bhaduri, 1942;Swanson, 1943;Brown, 1960;Hauber, 1987]. Thus, even if sometimes not entirely identical with true B chromosomes [see Jones, 1995 for a definition], they are actually vastly alike.…”

EXTRA, DIMINUTIVE CHROMOSOMES INOENOTHERA

“…Perhaps the hypothesis suggested by Whitaker (1936) in an attempt to explain the pairing of extra diminutives in Tradescantia will apply also to Oenothera. Whitaker has evidence to show that the pairing extras in Tradescantia paludosa are fragments which represent median segments of a normal chromosome.…”

Implications of clonality for ageing research

“…According to the mutation accumulation theory of aging (Medawar 1952), organisms senesce because their machinery is unable to reverse deleterious mutations at a rate higher than the rate at which they occur. The plant ecophysiological literature is mined with examples of species that can compartmentalize risk such as cavitation, fungal attacks and other diseases (Orians et al 2005;Casper 2010, 2011;Schenk 1999;Schenk et al 2008;Whitaker 1946;Zanne et al 2006). Plant clonality, whereby modules (i.e.…”