Genomic and Meiotic Changes Accompanying Polyploidization

Abstract

Hybridization and polyploidy have been considered as significant evolutionary forces in adaptation and speciation, especially among plants. Interspecific gene flow generates novel genetic variants adaptable to different environments, but it is also a gene introgression mechanism in crops to increase their agronomical yield. An estimate of 9% of interspecific hybridization has been reported although the frequency varies among taxa. Homoploid hybrid speciation is rare compared to allopolyploidy. Chromosome doubling after hybridization is the result of cellular defects produced mainly during meiosis. Unreduced gametes, which are formed at an average frequency of 2.52% across species, are the result of altered spindle organization or orientation, disturbed kinetochore functioning, abnormal cytokinesis, or loss of any meiotic division. Meiotic changes and their genetic basis, leading to the cytological diploidization of allopolyploids, are just beginning to be understood especially in wheat. However, the nature and mode of action of homoeologous recombination suppressor genes are poorly understood in other allopolyploids. The merger of two independent genomes causes a deep modification of their architecture, gene expression, and molecular interactions leading to the phenotype. We provide an overview of genomic changes and transcriptomic modifications that particularly occur at the early stages of allopolyploid formation.

Keywords: allopolyploidy; cytological diploidization; genomic changes; interspecific hybridization; unreduced gametes.

Figure 1

Diploid behavior during meiosis in allopolyploids and autopolyploids. (A,B) Chromosome associations at metaphase I in the presence and in the absence of the Ph1 locus in wheat (T. aestivum; AABBDD). (A) Representative image showing regular bivalent formation in the presence of Ph1. (B) Formation of multivalents (arrow) in the absence of Ph1. (C–F) Fluorescence in situ hybridization showing chromosome associations at metaphase I in wheat lines carrying chromosomes from wheat related species, both in the presence and in the absence of the Ph1 locus. Introgressed homologous chromosomes are visualized associated in disomic lines independently of the presence of the Ph1 locus, although aberrant chromosome associations can be observed in the absence of the Ph1 locus (F). (C) Wheat + pair 6H^v from Hordeum vulgare (red), Ph1Ph1. (D) Wheat + pair 6H^ch from Hordeum chilense (green), Ph1Ph1. (E) Wheat + pair 6P from Agropyron cristatum (in red), Ph1Ph1. (F) Wheat + pair 6P from Agropyron cristatum (in red), ph1ph1. (G,H) Chromosome associations at metaphase I in a natural autotetraploid line from A. thaliana. 45S rDNA and 5S rDNA regions are detected in green and red, respectively, to identify the chromosomes. (G) Metaphase I showing a pair of univalents (arrows). (H) Metaphase I exhibiting univalents (white arrows) and a quadrivalent (yellow arrow). Bars for (A–F): 10 µm. Bars for (G,H): 5 µm.