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However, some organisms are polyploid. Polyploidy is especially common in plants. Most eukaryotes have diploid somatic cells, but produce haploid gametes (eggs and sperm) by meiosis. A monoploid has only one set of chromosomes, and the term is usually only applied to cells or organisms that are normally diploid.
Zygoidy is the state in which the chromosomes are paired and can undergo meiosis. The zygoid state of a species may be diploid or polyploid. [51] [52] In the azygoid state the chromosomes are unpaired. It may be the natural state of some asexual species or may occur after meiosis. In diploid organisms the azygoid state is monoploid.
The main goals of diploidization are: (1) To ensure proper gene dosage; and (2) to maintain stable cellular division processes. This process does not need to occur rapidly for all chromosomes in one or few steps. In recent polyploid events, segments of the genome may still remain in a tetraploid status.
Alternation between a multicellular diploid and a multicellular haploid generation is never encountered in animals. [33] In some animals, there is an alternation between parthenogenic and sexually reproductive phases , for instance in salps and doliolids (class Thaliacea). Both phases are diploid.
Speciation via polyploidy: A diploid cell undergoes failed meiosis, producing diploid gametes, which self-fertilize to produce a tetraploid zygote.. Polyploidy is pervasive in plants and some estimates suggest that 30–80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes.
This is the situation found in most animal hybrids. For a hybrid to be viable, the chromosomes of the two organisms will have to be very similar, i.e., the parent species must be closely related, or else the difference in chromosome arrangement will make mitosis problematic. With polyploid hybridization, this constraint is less acute. [citation ...
It has been suggested that many polyploidization events created new species, via a gain of adaptive traits, or by sexual incompatibility with their diploid counterparts. An example would be the recent speciation of allopolyploid Spartina — S. anglica; the polyploid plant is so successful that it is listed as an invasive species in many ...
The list of organisms by chromosome count describes ploidy or numbers of chromosomes in the cells of various plants, animals, protists, and other living organisms.This number, along with the visual appearance of the chromosome, is known as the karyotype, [1] [2] [3] and can be found by looking at the chromosomes through a microscope.