Taichung, Taiwan
National Chung Hsing University
The cytological B-breakpoints of seven B-10L translocations --Cheng, Y-M, Lin, B-Y The pachytene structure of the B chromosome comprises, from proximal to distal, the short arm, the centromeric knob, the proximal euchromatic region (including five chromomeres), the distal heterochromatic region (H1, H2, H3 and H4), and the euchromatic tip (Fig.1A). Of the 38 B-10L translocations previously isolated by Lin (MNL 46: 193-194, 1972), the cytological B-breakpoint of 31 had been determined based on the pachytene figure of hyperploids (Genetics 90:613-627, 1978; Genetics 92:931-945, 1979; Genetics 100:475-486, 1982 and MNL 60:54, 1986). This article reports the B-breakpoints of an additional five translocations. Also included are two of the previously analyzed breakpoints (TB-10L20 and TB-10L36).

Of the seven translocations, only one (TB-10L16) has a break in the proximal euchromatic region, at the fourth chromomere (Fig.1B). Six others (TB-10L13, TB-10L20, TB-10L32, TB-10L34, TB-10L36 and TB-10L38) have breakpoints in the distal heterochromatic region: TB-10L34 and TB-10L38 at the junction of H1 and H2 (Fig.1C and 1D), TB-10L32 in H3 close to the junction of H2 and H3 (Fig.1E) and TB-10L13 at the junction of H2 and H3 (Fig.1F). TB-10L20 and TB-10L36 break in H3 (Fig.1G and1H), consistent with Linís earlier observation (Genetics 90: 613-627, 1978). The breakpoint of TB-10L36 is distal but close to that of TB-10L20. The results are summarized in Figure 2.

Figure 1. A-H Pachytene figure of the B chromosome and seven B-10 chromosomes. (A) B chromosome. (B) TB-10L16. (C) TB-10L34. (D) TB-10L38. (E) TB-10L32. (F) TB-10L13. (G) TB-10L20. (H) TB-10L36. The breakpoints are marked with an arrow.

Figure 2. B-breakpoints of seven B-10L translocations.
 
 


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