Applied Cytogenetics

Historically, most cytogenetics was carried out on (meiotic) pachytene cells where individual chromosomes can be readily identified (e.g. Anderson et al. 2004). A weakness of pachytene analysis is that whole plants must be grown to near maturity to collect samples. Root tip chromosomes offer a simpler way to collect chromosome information, but they have been viewed as too small to accurately identify chromosome variants and cytological features.

The power of mitotic chromosome analysis changed dramatically with the discovery of new FISH methods to label and identify root tip chromosomes. Birchler and colleagues showed that by mixing a collection of repetitive probes labeled with differently colored tags (fluorophores) it was possible to rapidly identify all ten maize chromosomes (Kato et al. 2004). They also introduced an important nitrous oxide method for increasing the number of condensed chromosomes from a single root tip. Subsequently, the same group went on to show that the sensitivity of FISH could be increased dramatically by increasing the amount of a key enzyme (DNA polymerase) in the labeling protocol (Kato et al. 2006).