Understanding the cytological diploidization mechanism of polyploid wild wheats

Abstract

The allohexaploid Aegilops species (2n = 6x = 42), Ae. neglecta 6x (UUX^tX^tNN), Ae. juvenalis (D^cD^cX^cX^cUU), and Ae. vavilovii (D^cD^cX^cX^cS^sS^s) regularly form bivalents at metaphase I. However, in Ae. crassa 6x (D^cD^cX^cX^cDD) 0.27 quadrivalents per cell were observed probably as a consequence of the partial homology displayed by the D and D^c 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.

Genomic and Meiotic Changes Accompanying Polyploidization

“…Among allopolyploid Aegilops species, regardless the degree of divergence between homoeologous genomes, the control of the diploid-like meiosis operates by means of restriction of synapsis to homologous chromosomes and suppression of chiasma formation in the infrequent homoeologous associations [ 199 ]. A similar system controlling restriction of chromosome synapsis initiation to homologous chromosomes has also been reported in allopolyploid species of Avena [ 200 ], Festuca [ 201 ], and wild forms of T. turgidum and T. timopheevii [ 202 ].…”

“…ventricosa , since it also operates in the synthetic amphiploid Ae. ventricosa-S. cereale despite its recent origin and the presence of rye genomes [ 199 ].…”