Meiotic loss of the B chromosomes of maize is influenced by the B univalent co-orientation and the TR-1 knob constitution of the A chromosomes

Abstract

The suppression of meiotic loss when the maize B chromosomes are unpaired is genetically determined. Two genotypes were selected in 1B × 0B crosses: the H line where the B transmission rate is Mendelian (50%) and the L line where the B is present in only about 40% of the progeny. Using the ZmBs probe located at the centromere and at the distal portion of the B chromosome in FISH, we found that the centromeric and telomeric ends of the B univalent co-orient at metaphase I. This feature seems to promote proper centromere orientation causing the lack of meiotic loss of the unpaired B. The co-orientation was observed in both lines, however in the L line the B univalents were not always properly oriented, showing amphitelic orientation in about 25% of the metaphase I cells. We also studied plants of the H and L lines with FISH to test the possible relation between the knob constitution and B loss. It has been found that the plants of both lines are similarly variable for the 180-bp knob repeat, but they differ in the TR-1 350-bp repeat, the L line having more TR-1 knobs. The use of a 45S rDNA probe which labels chromosome 6, allowed us to determine that this chromosome shows the main variability between the two lines: the L line has TR-1 in both arms, showing a large TR-1 knob on the long arm. The H line has only one, generally located on the short arm besides the NOR.

Chromosomes with a life of their own

“…As shown above, the B-specific sequence ZmBs seems to be involved in centromere function (Jin et al, 2005), nondisjunction , and efficient passage of the B through meiosis as a univalent (González-Sánchez et al, 2007). The ZmBs is therefore an excellent example of the functional plasticity that a repetitive DNA sequence can achieve.…”

“…Carlson and Roseman (1992) studied five different deletion types and showed that these deletions removed components of the B that serve to suppress meiotic loss. It has been recently suggested (González-Sánchez et al, 2007) that the subtelomeric region of the B-specific ZmBs sequence might be involved in the proper orientation of the B univalent, because in a number of metaphase I cells both the centromeric and the subtelomeric portion are stretched to the poles and co-oriented ( Fig. 1 f).…”

“…González- Sánchez et al (2007) suggested that the ZmBs region in the distal half of the fourth heterochromatic block may be involved in the passage of the B univalent through meiosis, which is one of the maize B accumulation mechanisms. They found that the centromeric and distal ZmBs signals are conspicuously co-oriented at metaphase I.…”

Sgo1 is required for co-segregation of sister chromatids during achiasmate meiosis I

“…32 Moreover, bi-orientation of achiasmate chromosomes has been observed during anaphase I in mammalian and plant cells. [36][37][38][39][40][41] However, sister kinetochores are mono-oriented during meiosis I in S. cerevisiae mutant cells defective in chiasma formation and protection of centromeric cohesion (spo11Δ rts1Δ). 15 Thus, it is possible that bi-orientation of achiasmate chromosomes during meiosis I is a general phenomenon, however, some organisms posses mechanisms that ensure mono-orientation of sister kinetochores during meiosis I in the absence of chiasmata.…”

Neocentrics and Holokinetics (Holocentrics): Chromosomes out of the Centromeric Rules

“…Two different tandem repetitive DNA sequences have been reported in maize knobs: the 180-bp repetition called Zm4-21 [Peacock et al, 1981], and the 350-bp TR-1 sequence [Ananiev et al, 1998b;González-Sánchez et al, 2007]. Both sequences may be simultaneously present or one may be predominant, interrupted by retrotransposons.…”