Terminal regions of wheat chromosomes select their pairing partners in meiosis

Abstract

Many plant species, including important crops like wheat, are polyploids that carry more than two sets of genetically related chromosomes capable of meiotic pairing. To safeguard a diploid-like behavior at meiosis, many polyploids evolved genetic loci that suppress incorrect pairing and recombination of homeologues. The Ph1 locus in wheat was proposed to ensure homologous pairing by controlling the specificity of centromere associations that precede chromosome pairing. Using wheat chromosomes that carry rye centromeres, we show that the centromere associations in early meiosis are not based on homology and that the Ph1 locus has no effect on such associations. Although centromeres indeed undergo a switch from nonhomologous to homologous associations in meiosis, this process is driven by the terminally initiated synapsis. The centromere has no effect on metaphase I chiasmate chromosome associations: homologs with identical or different centromeres, in the presence and absence of Ph1, pair the same. A FISH analysis of the behavior of centromeres and distal chromomeres in telocentric and bi-armed chromosomes demonstrates that it is not the centromeric, but rather the subtelomeric, regions that are involved in the correct partner recognition and selection.

Figure 1.—

Rye-specific DNA sequences in centromeres of wheat and rye chromosomes and their arrangement at early meiosis. (A) Bivalents 2Brc–2Brc and 2B–2Brc at MI with rye centromeres labeled with probe pAWRC.1. Signals of the pSc119.2 probe identify B-genome chromosomes. (B) Rye centromere in univalents 2Brc and 2R at MI; chromosome 2R shows the location of the rye-specific pSc74 DNA repeat. (C) Nuclei at early leptotene (EL), leptotene–zygotene transition (LLEZ), late zygotene (LZ), and pachytene (P) of homozygotes 2Brc–2Brc and double monosomic 2Brc–2R. Homologous centromeres of rye (arrows) are separated in EL and LZ and associated in LLEZ and P, respectively. Wheat centromeres and telomeres were labeled with the 6C6 and pAtT4 DNA probes, respectively. (D) Frequency of associations of homologous rye centromeres in pairs 1Brc–1Brc, 2Brc–2Brc, and 2Brc–2R at EL, LLEZ, mid-zygotene (MZ), LZ, and P stages in Ph1+ and Ph1− wheat lines. Mean number of PMCs, n = 43 ± 3. Bars, 10 μm.

Figure 2.—

The centromere and the terminal and subterminal knobs of chromosome 2RL and their arrangement in premeiotic interphase and in early meiosis. (A) The centromere and knobs of telocentric 2RL in a mitotic prometaphase after two rounds of FISH. (B) The arms 2RL and their rye centromeres in the ring bivalent formed by the 2BS.2RL translocation pair at MI after two-color FISH. (C) PMCs at the premeiotic interphase (PI), early leptotene (EL), and mid-zygotene (M) in the Dt2RL and 2BS.2RL lines showing the arrangement of the rye centromeres (red arrows), knobs (green), and telomeres (orange). Centromeres are separated in PI and ELs and associated in MZ. In the EL of Dt2RL, rye centromeres are incorporated at the telomere pole. Distal knobs are separated in PI and associated in the three other PMCs while subdistal knobs are associated only in the EL and MZ PMCs of Dt2RL. Bars, 10 μm.

Figure 3.—

The frequencies (%) of association of centromeres (Cen), and the terminal (Tm) and subterminal (Sm) knobs at the premeiotic interphase and in early meiosis in the Dt2RL and 2BS.2RL wheat lines. PI, premeiotic interphase. EL, early leptotene, LLEZ, late leptotene–early zygotene. MZ, mid-zygotene. LZ, late zygotene. Mean number of PMCs, n = 52 ± 6.