Comparative cytogenomics

TC Hsu (17 April 1917 – 9 July 2003) has been called “the father of mammalian cytogenetics”. In 1952, he developed the hypotonic solution spreading method for mammalian chromosomes following a fortunate error in making up a solution, paving the way for Tjio and Levan to report the human chromosome number as 2n = 46 in 1956. TC’s career started in China, as an insect geneticist, before settling in the US, mostly working in Texas, and moving to the exciting world of mammalian cytogenetics. He established one of the first “Frozen Zoos” with cultures of animals from A to Z (aardvark to zebra), and used numerous species to study chromosome biology and comparative evolution. At the end of his lengthy career, he was using in situ hybridization to understand the nature of non-telomeric heterochromatin and organization of chromosomes. Now, with the new methods available including high-resolution in situ hybridization, chromosome sorting, high volume sequencing and bioinformatics, we can learn about the evolution of chromosomes, comparing and contrasting diverse genotypes, species, families and even the kingdoms of plants, fungi and animals, to build a picture of key events in evolution. Many of the same changes may be seen in abnormal karyotypes and disease, normally deleterious. However, the occasional chromosomal or whole-genome changes, beyond those from mutation and recombination, can provide the novel variation leading to evolutionary success, arguably over evolutionary time giving rise to all modern lineages. The field of comparative cytogenomics is developing rapidly (see www.cytogenomics.org and www.molcyt.com) and able to show how species have evolved in the past and letting us consider paths for their evolutionary future.