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The Arabidopsis ATK1 gene is required for spindle morphogenesis in male meiosis

Changbin Chen1,2,*, Adam Marcus1, Wuxing Li1,2,3, Yi Hu1,2, Jean-Philippe Vielle Calzada4,5, Ueli Grossniklaus4,6, Richard J. Cyr1,3 and Hong Ma1,2,3,{dagger}

1 Department of Biology, The Pennsylvania State University, University Park, PA 16802 USA
2 The Life Sciences Consortium, The Pennsylvania State University, University Park, PA 16802 USA
3 The Intercollege Graduate Program in Plant Physiology, The Pennsylvania State University, University Park, PA 16802 USA
4 Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724 USA
5 CINVESTAV-Irapuato, Plant Biotechnology Unit, CP 36500, Irapuato, GTO, Mexico
6 Institute of Plant Biology, University of Zürich, CH-8008, Zürich, Switzerland
* Present address: University of Pennsylvania, Plant Science Institute, Biology Department, Philadelphia, PA 19104, USA



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  		Fig. 1. Wild-type, atk1-1 and revertant phenotypes. (A) A region of an atk1-1 plant with both mutant branch (arrow) and Ac-induced revertant sectors (arrowhead) that had normal siliques. (B) A wild-type flower. (C) An atk1-1 flower. (D) A wild-type tetrad with four microspores. (E) A product of atk1-1 meiosis, showing six microspores of unequal size. (F) Wild-type pollen grains of uniform size are brightly stained, indicating that they are functional. (G) atk1-1 pollen grains of variable sizes; most were faintly stained, indicating that they are dead. (H) Polar view of a wild-type pollen grain, with three apertures (arrows) that partition the pollen surface equally. (I) A side view of a wild-type pollen grain, showing the long apertures. (J) An atk1-1 pollen grain, showing that three apertures (arrows) were located unevenly on the pollen surface. (K) An atk1-1 pollen grain, showing that the apertures (arrows) were shorter than those of the wild-type pollen. Scale bars: 0.5 cm in A; 0.5 mm in B,C; 25 µm in D-G; 10 µm in H-K.

 


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  		Fig. 2. Analysis of the ATK1 locus. (A) A diagram of the ATK1 gene with exons shown as boxes and introns as lines. The Ds element is inserted in the first intron, with the 8 bp duplication 53 bp downstream of the 3' end of the first exon and 246 bp upstream of the 5' end of the second exon. (B) Sequence near the Ds insertion site in the wild type (WT), atk1-1 and three revertants (rev1, rev5 and rev6); the inserted nucleotides in atk1-1 and the revertants are underlined.

 


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  		Fig. 3. In situ RNA hybridization. For each row of three panels, the left panel is a bright-field image, the center one is a dark-field image and the right one a composite using Photoshop. (A,B) Wild-type floral sections hybridized with the ATK1 antisense RNA. (A) A stage 8 floral bud with ATK1 transcripts in young anthers before meiosis and the tip of a petal primordium (arrow). (B) A stage 9 floral bud with male meiocytes (arrows) in the anther and the developing pistil, both showing ATK1 expression. (C) An atk1-1 section of a stage 9 flower, showing background levels of silver grains.

 


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  		Fig. 4. Wild-type (A-D,I,J,L,Q-T) and atk1-1 (E-H,K,M-P,U-X) male meiosis from chromosome spreads. (A,E) Zygotene, with condensed chromosomes seen as thin threads. (B,F) Pachytene, showing synapsis of homologous chromosomes, forming thick threads. (C,G) Diplotene with partial separation of homologs. (D,H) Diakinesis with condensed bivalents of attached homologs. (I) Metaphase I, with five bivalents at the equator. (J) Anaphase I, showing separated homologs. (L) Telophase I, with two groups of chromosomes. (M) Metaphase I of atk1-1, with one bivalent (arrow) located slightly away from the equator; three others (arrowheads) are not aligned in parallel with each other. (N) Metaphase I-anaphase I transition in the atk1-1 mutant, showing one pair of separated homologs and four bivalents. (K,O) Anaphase I in atk1-1 meiocytes, showing four pairs separated homologs and one remaining bivalent (arrows); at least one pair of homologs are misaligned (arrowheads). (P) Telophase I in atk1-1. (Q,U) Prophase II. (R,V) Metaphase II. (S,W) Anaphase II. (T,X) Telophase II. In the wild type, a narrow zone of organelles is present between the separated homologs (Q-S); however, the organelles were more diffuse in atk1-1 cells (U-W). Arrow in V indicates two superimposed chromosomes.

 


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  		Fig. 5. Spindle structures in wild-type and atk1-1 male meiocytes. The microtubules are in green and chromosomes are in red. (A-D) Wild-type meiosis I. (E-H) atk1-1 meiosis I. (I-L) Wild-type meiosis II. (M-P) atk1-1 meiosis II.

 




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