Understanding the cytological diploidization mechanism of polyploid wild wheats.
The allohexaploid Aegilops species (2n = 6x = 42), Ae. neglecta 6x (UUXtXtNN), Ae. juvenalis (DcDcXcXcUU), and Ae. vavilovii (DcDcXcXcSsSs) regularly form bivalents at metaphase I. However, in Ae. crassa 6x (DcDcXcXcDD) 0.27 quadrivalents per cell were observed probably as a consequence of the partial homology displayed by the D and Dc genomes. Likewise, the synthetic amphiploid Ae. ventricosa-Secale cereale (DDNNRR) is fertile and displays a diploid-like behavior at metaphase I, despite its recent origin. The pattern of synapsis at late zygotene and pachytene in the natural and artificial allohexaploids was analyzed by whole-mount surface-spreading of synaptonemal complexes under an electron microscope. It revealed that chromosomes were mostly associated as bivalents in all cases, the mean of multivalents per nucleus ranging from 0.17 (Ae. neglecta 6x) to 1.03 (Ae. crassa 6x) in the natural species and 1.05 in the Ae. ventricosa-S. cereale amphiploid. It can be concluded that the mechanism controlling bivalent formation in these species and also in the synthetic amphiploid acts mainly at zygotene by restricting synapsis to homologous chromosomes, but also acts at pachytene by preventing chiasma formation in the homoeologous associations. These observations are discussed in relation to the origin and evolution of the mechanism of diploidization in the allopolyploid species of the Poaceae family.
Cuñado
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N
Departamento de Genética, Facultad de Biología, Universidad Complutense, Madrid, Spain. nicuna@bio.ucm.es
Blazquez
S
S
Melchor
L
L
Pradillo
M
M
Santos
J L
JL
D023902:Chromosome Pairing / Q000235:genetics; D032461:Chromosomes, Plant / Q000235:genetics; D004171:Diploidy; D008677:Metaphase / Q000235:genetics; D011123:Polyploidy; D014908:Triticum / Q000166:cytology / Q000235:genetics* / Q000648:ultrastructure