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.

Polyploidy in Orchid Breeding, Advances and Perspectives

“…The frequency of multivalents has been used as a cytological factor to distinguish auto-and allopolyploids. For instance, a high frequency of multivalent pairing at metaphase I may point to homology between chromosome sets and thus autopolyploidy 19 . However, in contrast, a high formation of bivalents at diakinesis may result from pairing between non-homologous (homoeologous) parental chromosome sets, which may indicate allopolyploidy 19 , although this behavior is not absolute 19 .…”

“…For instance, a high frequency of multivalent pairing at metaphase I may point to homology between chromosome sets and thus autopolyploidy 19 . However, in contrast, a high formation of bivalents at diakinesis may result from pairing between non-homologous (homoeologous) parental chromosome sets, which may indicate allopolyploidy 19 , although this behavior is not absolute 19 . In the actual case of orchid breeding, the development of cultivars with multiple spikes involves crossing species such as Phalaenopsis micholitzii (with multiple short spikes) and Phalaenopsis tetraspis (long spikes) 20 .…”

“…In many cases, tissue culture induces spontaneous polyploidization of hybrids 20 , but selection is laborious, and crosses must be redone often to retain stability of traits in the progenies. Early generations of synthetic allopolyploids show quick and broad reorganization of the merged genomes, including chromosome rearrangements and changes in chromosome number as well as epigenetic modifications, such as transposon activation, chromatin modifications and altered methylation patterning 19 . Indeed, chromosome rearrangements are often observed in meiocytes of presumptive orchid allopolyploids 21 , along with micronuclei in tetrads.…”

Advances and Perspectives for Polyploidy Breeding in Orchids

“…The relative abundance of meiotic multivalents (more than three chromosome pairs) (see Table 1 ) has been used as a cytological factor to distinguish auto- and allopolyploids. For instance, the prevalence of multivalent pairing at metaphase I may point to homology between chromosome sets and thus autopolyploidy [ 32 ]. However, in contrast, a high formation of bivalents (see Table 1 ) at diakinesis may result from pairing between non-homologous (homoeologous) (see Table 1 ) parental chromosome sets, which may indicate allopolyploidy [ 32 ], although this behavior is not absolute [ 32 ].…”

“…For instance, the prevalence of multivalent pairing at metaphase I may point to homology between chromosome sets and thus autopolyploidy [ 32 ]. However, in contrast, a high formation of bivalents (see Table 1 ) at diakinesis may result from pairing between non-homologous (homoeologous) (see Table 1 ) parental chromosome sets, which may indicate allopolyploidy [ 32 ], although this behavior is not absolute [ 32 ]. In the actual case of orchid breeding, the development of cultivars with multiple spikes involves crossing species such as Phalaenopsis micholitzii (with multiple short spikes) and Phalaenopsis tetraspis (long spikes) [ 33 ].…”