Cytogenetics and chromosomal structural diversity

Abstract

The cytogenetics of maize can be traced to the pioneering work of Barbara McClintock, who first defined the ten chromosomes of maize at the pachytene stage of meiosis (McClintock 1929). The chromosomes at this stage are not as condensed as in somatic cells and have many distinguishing characteristics such as knob heterochromatin, chromomere patterns and arm ratios that permit each member of the karyotype to be identified. This seminal contribution allowed the genetic linkage groups to be associated with the respective chromosome and initiated a series of findings ranging from the demonstration of the cytological basis of crossing over to the nature of chromosomal aberrations such as inversions, translocations, deficiencies and ring chromosomes (see Birchler et al. 2004). This ability to identify each chromosome and its structure allowed the analysis of the Breakage-Fusion-Bridge (BFB) cycle that led to the concept of the need for a special structure at the ends of natural chromosomes (McClintock 1939; 1941), now referred to as the telomere. Cytological analysis was also important in the early recognition of transposable elements as the means to detect the fact that Disssociation changed its site for induction of chromosomal breakage (McClintock, 1950). As alluded to above, the central fact of maize cytogenetics is that the gametic number of chromosomes is 10 and thus the sporophytic number is 20. The chromosomes were numbered from 1 to 10 in descending order of size, although variation exists in length due to the presence of extensive knob heterochromatin in different varieties. There is also variation for arm ratios due to small inversions surrounding the centromeres of some chromosomes (McClintock 1933; Lamb et al. 2007b). Monoploid as well as triploid to octoploid maize have been reported, although tetraploid varieties are the only other ploidy than diploidy that can be maintained from one generation to the next. This is due to the random assortment of chromosomes in monoploid and triploids or to sterility of the higher ploidies (Rhoades and Dempsey 1966).