From The Polyploidy Portal

Information

Basic: Advanced:

How are polyploids recognized?

To recognize polyploids, biologists have traditionally counted chromosomes and guess if taxa were diploid or polyploid with rules of thumb. For example, anything with over a certain number of chromosomes, especially if it were multiple of the number, was assumed to be polyploid (refs/examples). Often, an increase in organ size (e.g., stomates) was used as a substitute measure for comparing genome size (Masterson, 1994). Today, biologists use genomic tools (molecular cytogenetics, genetic maps) to observe a continuum of diploids and polyploids in various states of genome evolution. Many species, such as Arabidopsis, Zea mays, and yeast (TAGI, 2000; Blanc et al., 2003; Langkjaer et al., 2003) are believed to have undergone a doubling of their genome in the past but now behave as diploids (Wendel, 2000), . These ancient polyploids (paleopolyploids) have undergone a multitude of genomic changes, such as deletions of large fragments of chromosomes, silencing of duplicate genes, and recombining of homoeologous chromosome segments (Wolfe, 2001; Levy and Feldman, 2002) some of which may lead the organism to a more diploid-like state. In some polyploids, chromosomal reorganization is so extensive that the genome is no longer structured as an allopolyploid. Paradoxically, the more extensive this process of diploidization, the more difficult it is to discern. For example, “diploid” Brassica can be considered ancient polyploids when compared to Arabidopsis, and Arabidopsis itself shows ancient duplications. Since plant genomes show cycles of polyploidization and diploidization, it can be difficult to determine whether gene duplication arose by polyploidy or some other mechanism.

Slide3.gif

According to Grant (1971, 1981), the phenomena of polyploidy was discovered during the exploratory phase of plant cytogenetics in the early years of the twentieth century. Winkler (1916) introduced the term polyploidy, and Winge (1917) proposed that polyploidy occurred by somatic doubling in species hybrids. Early polyploid studies included those in in Nicotiana (Clausen and Goodspeed 1925), Raphanus-Brassica (Karpechenko 1927) and Galeopsis (Muntzing 1930, 1932). The distinctions between autopolyploidy and allopolyploidy were made by Kihara and Ono (1926) and later elaborated on by Clausen, Keck, and Hiesey (1945) and Stebbins (1950,1971). Stebbins also proposed the category of segmental allopolyploids , which are essentially intermediate forms between auto- and allo- polyploids (Stebbins 1950). Harlan and deWet (1975) suggested that most polyploids arose through the production of unreduced gametes as oppossed to somatic doubling. Ramsey and Schemske (2002) review the controversies surrounding the confusion over whether to classify polyploids by mode of origin criteria or by cytological criteria (Ramsey and Schemske, 2002). Using cytological criteria, allopolyploids display mostly bivalent chromosome pairing (aggregates containing two chromosomes) while autopolyploids can have higher frequencies of multivalent chromosome configurations. Here we follow Ramsey and Schemske (2002) and adopt mode of origin criteria: if the chromosomes of one genome within an organism or species are simply duplicated, the resulting polyploid is an autopolyploid. However, if genome duplication occurs during a cross of two different species, the resulting organism is referred to as an allopolyploid.

Personal tools