Multivalentsed

Multivalentsed, or multivalents, are associations of more than two homologous chromosomes that align and pair during prophase I of meiosis. They arise in genomes with three or more copies of each chromosome (polyploids) or in cases where related chromosomes retain substantial homology due to structural rearrangements. When three homologs pair, a trivalent forms; with four, a quadrivalent; with five or more, pentavalents, hexavalents, and so on. The frequency and configuration of multivalents depend on the level of ploidy, the degree of homology among chromosomes, and the activity of genetic systems that control chromosome pairing.

Formation and types: In autopolyploids, all homologous chromosomes are similar enough to pair in various configurations, often forming quadrivalents in tetraploids or more complex multivalents in higher levels of polyploidy. In trisomies or other aneuploid conditions, trivalents may be observed. In allopolyploids, homeologous chromosomes—derived from different species—are more diverged, so pairing tends to occur between true homologs as bivalents, but multivalents can occur, especially during early meiosis or when homology across subgenomes is high.

Consequences: Multivalents influence recombination and segregation during meiosis. Recombination among three or more homologs can create novel allele combinations but can also produce unbalanced gametes if segregation is irregular, reducing fertility. They can contribute to segregation distortion and chromosomal rearrangements, affecting the evolutionary trajectories of polyploid lineages.

Examples: Bread wheat (Triticum aestivum) is an allopolyploid with three subgenomes; pairing is predominantly between true homologs, yielding mostly bivalents, though multivalents may occur under certain genetic or cytological conditions. Autopolyploids in various plant species show multivalent formation more readily, reflecting their higher chromosome similarity.