XII.2 Diploid genome status can be restored not only by DNA replication but also by the fusion of two haploid cells

The simplest and quite natural way of renewing the original ploidy of a cell consists in simple replication of its DNA.The cell cycle occurs in this normal way in all prokaryotic organisms and in most cells of eukaryotic organisms (Kondrashov 1994).However, in addition to this simple way, other ways also exist, probably the most important of which is renewal of diploidy by the fusion of two haploid cells to form one diploid cell (in some cases, a relatively long stage of a cell carrying two haploid nuclei is part of the life cycle, see below).The process of fusion of two haploid cells in a diploid cell is of fundamental importance for modern organisms in sexual reproduction and, in this context, it is termed syngamy.In cells with a rigid cell wall, it need not have the character of fusion of two cells but, for example, only temporary interconnection by a mechanical bridge and passage of the nucleus containing the haploid genome of one cell into the other cell.Two nuclei present in the cell can combine to form one or can remain separated for a very long time, as in present-day fungi and some groups of protozoa.It is apparent that, in renewal of diploidy by the fusion of two cells and subsequently their nuclei, DNA is not multiplied but, rather than to an increase, this phenomenon frequently leads to a reduction in the number of individuals in the population.Nonetheless, it is possible that this process can be important from the standpoint of further multiplication of cells.Cell division is a relatively complicated process with participation by the products of a great many genes, many of which need not be important for the lives of nondividing cells.Consequently, it is possible that some of these genes can be damaged during the life of the cell and their products can be absent in the cell or can be dysfunctional.In this case, the cell would not be capable of commencing division even if it were able to replicate its DNA and pass to the diploid state.However, if fusion occurs between two haploid cells with damaged genomes, their functioning can be renewed.  It is not very probable that both cells would be damaged in the same gene.Renewal of the ability to divide through mutual complementation of two damaged genomes is, of course, not a permanent solution.Over time, such a genome would collect new mutations and the process of renewal of its functioning through fusion with another cell would have to be repeated.This would lead to a gradual increase in the ploidy of the cells, i.e. to a one-way process that cannot, in principle, become a normal part of the life cycle.Complementation of the genome must necessarily be followed by the formation of one completely functional genome from two damaged genomes, for example through removal of the dysfunctional and duplicate copies of the individual genes or repair of the damaged sites and subsequent renewal of the haploid state through nuclear and then cell division.s

Was this information useful for you?
The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.