The Control of the Crossover Localization in Allium

Abstract

Meiotic crossovers/chiasmata are not randomly distributed and strictly controlled. The mechanisms behind crossover (CO) patterning remain largely unknown. In Allium cepa, as in the vast majority of plants and animals, COs predominantly occur in the distal 2/3 of the chromosome arm, while in Allium fistulosum they are strictly localized in the proximal region. We investigated the factors that may contribute to the pattern of COs in A. cepa, A. fistulosum and their F1 diploid (2n = 2x = 8C + 8F) and F1 triploid (2n = 3x = 16F + 8C) hybrids. The genome structure of F1 hybrids was confirmed using genomic in situ hybridization (GISH). The analysis of bivalents in the pollen mother cells (PMCs) of the F1 triploid hybrid showed a significant shift in the localization of COs to the distal and interstitial regions. In F1 diploid hybrid, the COs localization was predominantly the same as that of the A. cepa parent. We found no differences in the assembly and disassembly of ASY1 and ZYP1 in PMCs between A. cepa and A. fistulosum, while F1 diploid hybrid showed a delay in chromosome pairing and a partial absence of synapsis in paired chromosomes. Immunolabeling of MLH1 (class I COs) and MUS81 (class II COs) proteins showed a significant difference in the class I/II CO ratio between A. fistulosum (50%:50%) and A. cepa (73%:27%). The MLH1:MUS81 ratio at the homeologous synapsis of F1 diploid hybrid (70%:30%) was the most similar to that of the A. cepa parent. F1 triploid hybrid at the A. fistulosum homologous synapsis showed a significant increase in MLH1:MUS81 ratio (60%:40%) compared to the A. fistulosum parent. The results suggest possible genetic control of CO localization. Other factors affecting the distribution of COs are discussed.

Keywords: ASY1; Allium; GISH; MLH1; MUS81; ZYP1; chiasmata localization.

Figure 1

GISH on mitotic metaphase chromosomes of F${}_1$ interspecific hybrids between A. cepa and A. fistulosum: (a) F${}_1$ diploid hybrid contains 8 chromosomes of A. cepa and 8 chromosomes of A. fistulosum; (b) F${}_1$ triploid hybrid contains 8 chromosomes of A. cepa and 16 chromosomes of A. fistulosum; (a${}^{\prime }$,b${}^{\prime }$) karyotypes of the F${}_{\mathbf{1}}$ diploid and F${}_{\mathbf{1}}$ triploid hybrids, respectively. Bars represent 10 $\mathsf{\mu }$m.

Figure 2

Acetocarmine-stained squash preparations of PMCs at metaphase I: (a)—A. cepa, (b)—A. fistulosum, (c)—F${}_1$ (A. cepa×A. fistulosum) diploid hybrid (2n = 2x = 8F + 8C), (d)—F${}_1$ (A. cepa×A. fistulosum) triploid hybrid (2n = 2x = 16F + 8C). Roman numbers mark different types of bivalents: I—ring bivalent with two distal and single interstitial chiasmata, II—ring bivalent with two distal chiasmata, III—heteromorphic open bivalent with interstitial chiasma, IV—univalent, V—cross-bivalent, VI—H-form of cross-bivalent, VII—ring bivalent with two interstitial chiasmata. Bars represent 10 $\mathsf{\mu }$m.

Figure 3

The behavior of ASY1 (red) and ZYP1 (green) during prophase I in A. cepa, A. fistulosum and their F${}_1$ diploid and triploid hybrids. Chromatin was stained with DAPI. (a–d) Arrows in zoomed regions are pointing at regions with absence of the ASY1 signal after loading ZYP1.

Figure 4

Immunolocalization of the MLH1/ZYP1 or MUS81/ZYP1 at pachytenes of A. cepa and A. fistulosum. MLH1 (on the left) marks class I COs, and MUS81 (on the right) marks class II COs. Bar = 10 $\mathsf{\mu }$m.

Figure 5

Immunolocalization of the MLH1/ZYP1 and MUS81/ZYP1 at pachytenes of F${}_1$ diploid hybrid (2n = 2x = 8F + 8C) and F${}_1$ triploid hybrid (2n = 3x = 16F + 8C). Bar = 10 $\mathsf{\mu }$m.