Aneuploidy is the most common type of chromosomal aberration in the plant kingdom. At the same time, aneuploids represent valuable cytogenetic material for the study of chromosomal evolution (breakage, reunion, and rearrangement), phenotypic manifestation of chromosome gain or loss, and their effects on fitness and evolutionary success of a particular chromosome race as well as for genome mapping. Terminologically, aneuploidy (such as trisomy or monosomy) should be distinguished from dysploidy (Tischler 1937), which is a change in chromosome number due to karyotypic rearrangements (see also Rieger et al. 1991).

It is hardly surprising that most of our knowledge about aneuploids comes from investigation of economically important crops. The sensitivity of FCM was proven sufficient to detect individual rye chromosomes and/or, chromosome arms in wheat-rye addition lines (Pfosser et al. 1995), and monosomic individuals of Triticum aestivum (Poaceae; Lee et al. 1997). Similarly, deviations in nuclear DNA content signaled the presence of aneuploidy in Asparagus officinalis (Aspara-gaceae; Ozaki et al. 2004), Humulus (Cannabaceae; Sesek et al. 2000), Lolium (Poaceae; Barker et al. 2001), and Musa (Musaceae; Roux et al. 2003).

In contrast to a rather trivial task of estimating DNA ploidy levels, FCM detection of aneuploidy (or more precisely DNA aneuploidy) is a demanding task. An essential prerequisite is a high-resolution analysis because such differences in DNA content can only be discriminated in simultaneous FCM runs, which are at least twice the coefficient of variation of Go/Gj peaks (cf. Dolezel and Gohde 1995). It must be emphasized that conventional chromosome counting should follow any suspicion of chromosomal heterogeneity inferred from FCM data in order to elucidate its nature.

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