Viçosa, Brazil

Universidade Federal de Viçosa

B chromosome derivatives in maize somatic cells --Carvalho, CR, Saraiva, LS and Almeida, PM Accessory or B chromosomes have been described in hundreds of plant and animal species. They are typically smaller than the members of the regular complement, mostly heterochromatic, transmitted in a non-Mendelian pattern, and not necessary for normal development and reproduction. The maize B chromosome is responsible for its own non-disjunction, due to the induction of late knob replication during mitotic anaphase, when the heterochromatin adjacent to the centromere remains functionally non-divided. The morphology of the B chromosome in the root tip was analyzed. The stock used was the Black Mexican Sweet Corn inbred line with B chromosomes from the Maize Stock Center, at the University of Illinois. Seeds were germinated in Petri dishes containing a film of distilled water, and incubated at 29 C in the dark. Seedlings (1.5 to 2 cm root length) were transferred to a plastic mesh adapted inside 6 cm diameter plastic vessels containing 3 ml of 5 µM amiprophos-methyl (APM) solution. After 2h 30 min, the roots were washed for 15 min in running tap water, fixed in a fresh ice-cold methanol:acetic acid solution (3:1), and kept in a freezer for at least 24 h. Subsequently, root tip meristems were excised, macerated with 200 µL of freshly prepared Flaxzyme (NOVO) enzymatic solution plus 1.6 ml distilled water, and incubated at 34 C for 2h 30 min. The macerated cells were dissociated on a clean slide with a fresh fixative solution, air-dried and stained with a 3% Giemsa solution. Metaphasic figures were digitized directly from a microscope-coupled CCD video camera to a computer. The karyogram was elaborated (Figure 1) and the average length of the homologue A pairs and B set was determined in micrometers (Table 1). Interestingly, the data show that B chromosomes were longer than those measurements reported by other authors. On top of that, figures of the somatic cell-derived B chromosomes have not been widely documented. This makes any kind of comparison with the present results difficult. Besides B chromosomes with standard morphology (Bs), B derivatives (Bd) were observed that were reduced in length (Figure 1). It is possible that the different morphologies observed were due to the highly heterochromatic nature of the maize B chromosome as affected by the arresting agent (APM) used.

Figure 1. Maize karyogram of the 2n =20A+4B chromosomes. Note the different B chromosome morphologies. Bar = 5 µm.

Table 1. Measurements of maize root tips metaphasic A and B chromosomes.
 
A and B Maize chromosomes
A Pairs  1 2 3 4 5 6 7 8 9 10
Mean Length (µm) 2.22 1.99 1.98 1.86 1.39 1.49 1.41 1.4 1.32 1.11
B Types  Bs Bd1 Bd2 Bd3            
Length (µm) 1.48 1.20 0.91 0.89            
Bs = Standard B chromosomes
Bd = Derivative B chromosomes
 
 


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