ABSTRACT. The freeze-cracking technique was applied to chop and divide cell randomly in root meristems of Drosera falconeri into two pieces and obtain ultrastructural information on chromosomes, nucleoli, mitotic spindles and their attached sites on chromosomes as well as the poles. Certain phenomenon on diffused-centromeric chromosomes were found to strongly support Kondo's hypothesis on "diffused-centromeres in Drosera chromosomes." The mitotic spindles were 60~90 nm diameter at many separate portions on a poleward side of prometaphase and metaphase chromosomes. During the course of investigations in a series of chromosome cytology in Drosera and its close relatives made by Kondo and his associates (Kondo 1966;1969, 1971, 1973, 1984, Kondo and Olivier 1979, Kondo and Lavarack 1984, Kondo and Segawa 1988, Kondo hypothesized that Drosera might have "Diffused centromeres" for the first time since their chromosomes showed commonly relatively wide nonstaining gap between the chromatids of chromosome and non-visible, localized centromeric-regions throughout mitotic prophase, prometaphase and metaphase. suggested that it might be a case of polycentric (or diffused centromeric) chromosomes due to lack of single, localized centromere. Observations of C-banding and silver staining on chromosomes of Drosera petiolaris confirmed that the chromosome in the genus could have C-banding pattern and numerous silver-staining position segments suggesting multi-centromeres (Kondo and Lavarack 1984). Among the dicot genera, only Drosera chromosomes were concluded to have diffused centromeres (Kondo and Lavarack 1984). Later, Sheikh and Kondo (1995), Hoshi and Kondo (1998a, b) and Nontachaiyapoom et al. (2000) supported Kondo's hypothesis on the "diffused-centromeres in Drosera chromosome".
KEYWORDS:The present study by using scanning electron microscopy on the cracking flozen specimen of Drosera falconeri chromosomes used displayed new evidence with respect to diffused centromeres.
MATERIALS AND METHODSPlant material Drosera falconeri Kondo et Tsang collected in a small population along the Fennis River in Northern Territory, Australia was chosen because it had big size chromosomes and not so great numbers of chromosomes as 2n=12. Respective chromosomes of the species from mitotic early prophase to late metaphase and even anaphase showed distinct characteristics of wide, non-staining gap between chromatids (Kondo 1984). Plants of the species were easily, vegetatively propagated and supplied enough research materials within a short time in vivo and in vitro.Cracking frozen specimens Root tips of D. falconeri were harvested and fixed in either 3:1 (v/v) methanolacetic acid or 3:1 (v/v) ethanol-acetic acid at 4°C for 24 h. Fixed root-tips of the species were washed in a series of ethanol from 90, 70, 50, 30, 20 and finally to 10%. They were excised under a stereo microscope to remove non-meristematic tissues. They were rinsed with phosphate buffer solution at pH 7.4 for 10 min each, three times and were, then, treated in 1% O S O 4...