Bouquet Formation Failure in Meiosis of F1 Wheat-Rye Hybrids with Mitotic-Like Division

Abstract

Bouquet formation is believed to be involved in initiating homologous chromosome pairings in meiosis. A bouquet is also formed in the absence of chromosome pairing, such as in F₁ wheat-rye hybrids. In some hybrids, meiosis is characterized by a single, mitotic-like division that leads to the formation of unreduced gametes. In this study, FISH with the telomere and centromere-specific probe, and immunoFISH with ASY1, CENH3 and rye subtelomere repeat pSc200 were employed to perform a comparative analysis of early meiotic prophase nuclei in four combinations of wheat-rye hybrids. One of these, with disomic rye chromosome 2R, is known to undergo normal meiosis, and here, 78.9% of the meiocytes formed a normal-appearing telomere bouquet and rye subtelomeres clustered in 83.2% of the meiocytes. In three combinations with disomic rye chromosomes 1R, 5R and 6R, known to undergo a single division of meiosis, telomeres clustered in 11.4%, 44.8% and 27.6% of the meiocytes, respectively. In hybrids with chromosome 1R, rye subtelomeres clustered in 12.19% of the meiocytes. In the remaining meiocytes, telomeres and subtelomeres were scattered along the nucleus circumference, forming large and small groups. We conclude that in wheat-rye hybrids with mitotic-like meiosis, chromosome behavior is altered already in the early prophase.

Keywords: bouquet; centromeres; mitotic-like meiosis; subtelomeres; telomeres; wheat–rye amphihaploids.

Figure 1

Telomere–centromere dynamics through meiotic prophase in wheat (a,i) and wheat–rye disomic substitution lines 1R(1A) (c,e,g,k,m), 2R(2D) (b,d,f,h,j,l,n). (a) Mid–late interphase with 3 nucleoli. (b–h) Leptotene with two nucleoli. Early–mid zygotene (i,j) with one peripherally located nucleolus. (k) Late zygotene, (l) early pachytene. (m) Pachytene. (n) Early diplotene. (ai–ni) The same as (a–n), DAPI. Telomeres are stained red and centromeres, green. Scale bar = 10 μm.

Figure 2

Telomere and centromere distribution in 2R(2D) × R hybrids. (a,b) Leptotene. (c,d) Early zygotene. (e–h) Zygotene. (i) Meiocytes at late zygotene (1, 2) and pachytene (3). Telomere bouquet is preserved (1) and resolved (2). Dispersed point telomeres and elongated centromeres are scattered within the nucleus (3). (ai–ii) the same as (a–i), DAPI counterstaining. Telomeres are stained red, centromeres, green. Scale bar = 10 μm.×.

Figure 3

Telomere associations in 5R(5D) × R. Zygotene. (a) A tight telomere cluster. (b) Telomeres formed a part of circular arc. (c) Telomere clumps. (d) Diffuse telomere cluster. (ai–di) the same as (a–d), DAPI counterstaining. Telomeres are stained red, centromeres, green. Scale bar = 10 μm.

Figure 4

Telomere and centromere dynamics in 1R(1A) × R (a,c,e–h,j,l) and 6R(6A) × R (b,d,i,k) hybrids. (a,b) Leptotene. (c–h) Zygotene. (i,j) Pachytene, (k,l) Diplotene. (ai–li)—the same as (a–l) DAPI counterstaining. Telomeres are stained red, centromeres, green. Scale bar = 10 μm.

Figure 5

CENH3 and pSc200 behaviour at leptotene-to-pachytene stages, marked ASY1 loading, in 2R(2D) × R hybrids. (a) Leptotene. CENH3 indicates centromere clustering, continuous ASY1 signal at restrict area, subtelomeres began to associate in a small area. (b–d) Zygotene. ASY1 signals are visible along chromatin axes. Centromeres distributed over nucleus. Subtelomeres are associated in single cluster (b), in two clusters (c). (d) Pachytene. Initiation of ASY1 disassembly, subtelomere distribution. ImmunoFISH. Confocal image, maximum intensity projection. Scale bar = 5 μm.

Figure 6

CENH3 and pSc200 behaviour at leptotene-to-pachytene stages, marked by ASY1 loading in 1R(1A) × R hybrids. (a) Leptotene. CENH3 indicates centromere clustering, ASY1 signals form part of circular arc, subtelomeres form clumps and occupy a circular arc. (b–f) Zygotene. ASY1 signals are visible along chromatin axes. (b) Centromeres and subtelomeres are polarized and occupy circular arcs. (c) Centromeres distributed over nucleus, subtelomeres formed part of circular arc. (d,e) Large and small subtelomere clumps as well as centromeres distributed over nucleus. (f) Centromeres distributed over nucleus, subtelomeres are associated in cluster. (g) Late zygotene–early pachytene. Initiation of ASY1 disassembly, subtelomeres distributed over the nucleus. (h) Pachytene. ASY1 disassembly, subtelomeres distributed over the nucleus. ImmunoFISH. Confocal image, maximum intensity projection. Scale bar = 5 μm.