Polyploidy occurs when the zygote, or first cell, has more than two sets of haploid chromosomes. In insects, polyploidy is mainly restricted to parthenogenetic species and is largely limited to 3n and 4n. Chromosomal sex determination is regarded as a major barrier to the formation of polyploids among bisexual species of insects because duplicated sex chromosomes, such as XXYY, would lead to uniformly XY sperm and therefore no possibility of sex determination.
Endopolyploidy is the occurrence of a multiplicity of the ploidy in the zygote in the somatic tissues of an organism. The term "endomitosis" is used if the chromosomes appear during cycles of endoreduplication but with no formation of a mitotic spindle and no cell division. Endopolyploidy, including endomitosis, is commonly seen in the somatic tissues of most insects. In a special case of endoreduplication, involving only one round of replication under a variety of treatments, the chromosomes that subsequently appear may remain closely associated at the centromeres, forming diplochromosomes, which were first seen in a locust. The formation of polytene chromosomes (Fig. 3) is also a special form of endoredupli-cation. Endopolyploid cells are very common in the tissues of all insects, and the phenomenon seems to reflect a tendency for insects to increase the bulk of certain tissues by increasing cellular size rather than cell number.
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