From The Polyploidy Portal

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How are polyploids formed?

Two main modes of origin of the polyploid condition are recognized somatic doubling in mitosis, and nonreduction in meiosis (Heilborn, 1934; Grant, 1971). The mechanism of somatic doubling is exemplified by polyploid Primula kewensis, and nonreduction was the mode of origin seen in polyploid Rhaphobrassica. It used to be thought most that polyploids formed by hybridization followed by chromosome doubling. However, Harlan and deWet (1975) argued that unreduced gametes played an important role. While agronomy researchers took notice of this (e.g. Peloquin, 19XX), textbooks did not change. Recently, a lot of theoretical modeling (Rodriguez, 1996; Ramsey and Schemske, 1998, 2002) and fieldwork (Husband 1999, 2000) has contributed to the view that unreduced gametes and triploid bridges are a major source of polyploid formation. This is also a mechanism for how diploid and polyploid genomes can interact (thus, the new polyploid species are not strictly sealed off from its diploid progenitors).

During meiosis, homologous chromosomes pair and undergo crossing over resulting in the exchange of parts of their chromosomes. In diploid hybrids derived from crosses of two species, chromosomes from the two species may differ or one of the chromosomes may be absent. This can cause irregularities during meiosis and may result in cell cycle arrest and subsequent embryo abortion (Fig. 1a). However, if the chromosome number is doubled in the hybrid, allotetraploids are formed, which have four sets of chromosomes. This can occur by crossing autotetraploids of the two species, or more likely in nature, by the fussion of unreduced gametes. Allotetraploids generally will have pairing and crossing over only within the two chromosomes of each original parent (the homologous chromosomes AA) and only rarely between chromosomes from the two original parents (the homeologous chromosomes AA’). This meiotic behavior assures proper pairing of the chromosomes and the correct assortment into gametes (Fig. 1b).

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