*** Welcome to piglix ***

Diploidization


Diploidization is the process of converting a polyploid genome back into a diploid one. Polyploidy is a product of whole genome duplication (WGD) and is followed by diploidization as a result of genome shock (1, 2, 4, 5). The plant kingdom has undergone multiple events of polyploidization followed by diploidization in both ancient and recent lineages (1). It has also been hypothesized that vertebrate genomes have gone through two rounds of paleopolyploidy (5). The mechanisms of diploidization are poorly understood but patterns of chromosomal loss and evolution of novel genes are observed in the process.

Elimination of Duplicated Genes

Upon the formation of new polyploids, large sections of DNA are rapidly lost from one genome (1, 2, 4). The loss of DNA effectively achieves two purposes. First, the eliminated copy restores the normal gene dosage in the diploid organism (1). Second, the changes in chromosomal genetic structure increase the divergence of the homoeologous chromosomes (similar chromosomes from inter-species hybrid) and promotes homologous chromosome pairing (2). Both are important in terms of adjusting to the induced genome shock.

Evolution of Genes to Ensure Correct Chromosome Pairing

There have been rare events in which genes that ensure proper chromosome pairing have evolved shortly after polyploidization. One such gene, Ph1, exists in hexaploid wheat (6). These genes keep the two sets of genomes separately by either spatially separating them or giving them a unique chromatin identity to facilitate recognition from its homologous pair. This prevents the need of rapid gene loss to speed up homeologous chromosome diversification.

1. Coordinate inter-genomic gene expression:

Duplicated genes often result in increased dosage of gene products. Doubled dosages are sometimes lethal to the organism thus the two genome copies must coordinate in a structured fashion to maintain normal nuclear activity (1). Many mechanisms of diploidization promote this coordination.

2. Maintain intra-genomic chromosome pairing at meiosis:

Chromosome pairing during meiosis is a significant challenge for polyploids. Homoeologous chromosomes with similar genetic content may pair with each other resulting in trivalent or tetravalent interactions (4). The resolution of these structures results in chromosome breakage, rearrangement, and gamete infertility. Diploidization is often required to restore the cell’s ability to stably go through meiosis (2).

3. Reduce costs of maintaining large, duplicated genomes:

Large genomes are costly to synthesize during replication and hard to maintain (2). The loss of duplicated genes during diploidization effectively reduces the overall size of the genome.


...
Wikipedia

...