Towards an understanding of the molecular mechanisms regulating gene expression during diploidization in phylogenetically polyploid lower vertebrates.

Polyploidization and regional gene duplication have occurred frequently during vertebrate evolution, providing the genetic material necessary for creating evolutionary novelties. Mammals, including man, can be regarded as diploid species with a polyploid history of evolution. Polyploidization steps during the phylogeny of mammals probably took place in the genomes of amphibian- or fish-like mammalian ancestors. The polyploid status has subsequently been shaped by the process of diploidization, leading to genomes that are polyploid with respect to the amount of genetic material and the number of gene copies, and diploid with respect to the level of gene expression and chromosomal characteristics. Phylogenetically tetraploid amphibian and teleost species together with their diploid close relatives can be used as a model system to study the effect of polyploidization and the mechanisms of diploidization of a parallel event during early mammalian evolution. Experimental evidence permits the assumption that the diploidization of gene expression in tetraploid cyprinid fish may be functionally correlated with structural modifications of the ribosomal components, RNA and protein. These findings are discussed in the light of reduced protein synthesis in diploidized tetraploid species and a mechanism to explain diploidization during mammalian evolution.