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Number of complete chromosome sets in a nucleus: haploid (N), diploid (2N), triploid (3N), tetraploid (4N), and so on.



the number that indicates how many times the set of chromosomes characteristic of the germ cells of a given species is replicated in the cell nucleus. The majority of organisms that reproduce sexually are diploid, that is, each of their somatic cells contains two sets of chromosomes (one for each of the haploid germ cells or gametes). In nature and under artificial conditions, ploidy may change in multiples higher that two (eupioidy); there may also be a loss or gain of individual chromosomes (aneu-ploidy).

Euploid increase of the number of chromosome sets (poly-ploidization) occurs if the chromosomes do not separate during cell division. It may also result from the merging of nuclei containing more than one set of chromosomes. There are two types of polyploidy: autopolyploidy, in which the number of homologous sets of chromosomes is increased, and allopolyploidy, which arises when there is a joining, as a result of hybridization, of the genomes of various species. Nonseparation of genomes can occur only in part of the cells of some tissues (somatic polyploidy, which is characteristic of many multicellular animals and plants). Polyploidization can lead to the formation of new plant species and thus serves as an important factor in their evolution (as well as of unicellular organisms). Its significance for species formation in multicellular animals is apparently limited to par-thenogenetic forms. Aneuploidy leads to serious disruptions of development, which often lead to death of the organism and, therefore, does not play a significant role in evolution. Aneuploid cells are common in malignant tumors.

Methods have been elaborated for artificially altering ploidy. They are used to obtain economically valuable forms of plants and for research purposes (for example, to study the action of genes).

References in periodicals archive ?
Sex determination is haplodiploid, and thus females can control the sex of their offspring by whether or not they fertilize the egg (Cook 1993).
Inbreeding depression and haplodiploidy: experimental measures in a parasitoid and comparisons across diploid and haplodiploid insect taxa.
By controlling the access of sperm to the egg, females of haplodiploid groups such as the insect order Hymenoptera can determine the sex of their offspring.
Broad-sense heritability estimates for the performance traits, wet mass, and performance at each of the seven test temperatures were computed using standard haplodiploid models of inheritance (Collins et al.
Further, the haplodiploid sex determination leads to strong selection against deleterious alleles in the haploid males (Denholm et al.
Our results establish fertile ground for study by showing ample, detectable polymorphism and new paradigms regarding evolution of sociality in bees and the abundance of diploid males in haplodiploid populations.
In haplodiploid insects such as Hymenoptera, males develop from an unfertilized haploid egg, but females originate from a fertilized diploid egg.
Since Hamilton's (1964) development of inclusive fitness theory to explain the origin of eusociality in the haplodiploid Hymenoptera, much of the work addressing the evolution of insect societies has focused on the importance of genetic relatedness and kin selection.
Indeed, data are available for very few social insects that show both tight intrafamily relatedness and sexual allocation consistent with the haplodiploid theory of eusocial evolution (but see Packer and Owen 1994).
Haplodiploids (haploid males, diploid females) make up a large number of arthropod species, but little is known about the genetic basis of reproductive isolation in haplodiploid species.
These phytophagous mites have haplodiploid genetics and live gregariously in woven nests on the undersurfaces of leaves of perennial grasses.
Haplodiploid sibs, however, are related to their parents by different degrees; thus, the computations of causal components of variance differ from the diploid case (Margolies and Cox 1993).