Botany online 1996-2004. No further update, only historical document of botanical science!

Chromosomes - The Chromosome Theory, Part II

The early studies of chromosomes, that have already been described, led to the discovery of mitosis and meiosis and the characterization of their different phases. The correspondence between the pattern of the chromosomal segregation and the genetic data indicated that chromosomes are the vehicles of the hereditary factors of genes.

And still - the chromosome theory was, at the beginning of the 20th century, accepted only by few and was generally regarded rather sceptically. Its real breakthrough came with the recognition that the chromosomes of a genome are individual and differ from each other. Suddenly, changes in the cytological appearance of the chromosomes and genetic changes could be brought into line.

The choice of a suitable model organism, the fruit fly Drosophila melanogaster, proved to be decisive. It was introduced into genetic research by T. H. MORGAN (Columbia University, New York) around 1910. Drosophila offers a number of advantages; among them are:

a short life cycle of just 14 days;

the need for only small spaces, where thousands of flies can be kept;

the high number of individuals that offers a relatively good chance to select and identify mutants;

the fact that Drosophila has just n = 4 chromosomes; the X-chromosome differs clearly from the Y-chromosome;

the fact that the larva of Drosophila contains very large chromosomes, the polytene or giant chromosomes, with precisely aligned side-to-side adhered individual chromatin strands in several organs like, for example, the salivary gland. They are ideal for the research into the structural changes of chromosomes.

There are some plants that meet most of these requirements. But the life cycle of higher plants takes, without exception, much longer. Only rarely can more than two generations be analyzed in one year. Plants need a lot of space so that experiments with them are not only time-consuming but also expensive.

Many plants, especially monocots, have bigger chromosomes than Drosophila but polytene chromosomes have only exceptionally been found in some highly specialized cells of a few plant species (like, for example, the suspensor cells of the bean Phaseolus coccineus). Up until today, no results with a profound impact on pure research have been obtained by the study of polytene chromosomes of plants.

Even though this is a textbook on botany, some basic results of Drosophila-research have to be mentioned here. Among them are the discovery of

sex chromosomes and sex-linked inheritance ;

the discovery that genes are organized linearly on chromosomes;

chromosomal mutations and

the correlation of gene maps and chromosomal structures.

The chief discoveries were later on also confirmed in plants. The model organism used was mostly maize (Zea mays). Due to its great economic importance, particularly in the United States, enough money and space was available to carry the experiments through on the necessary scale. The expenses were worthwhile, since hardly any other agricultural plant brought as huge increases in yield due to the consequent application of basic genetic knowledge.

	a short life cycle of just 14 days;
	the need for only small spaces, where thousands of flies can be kept;
	the high number of individuals that offers a relatively good chance to select and identify mutants;
	the fact that Drosophila has just n = 4 chromosomes; the X-chromosome differs clearly from the Y-chromosome;
	the fact that the larva of Drosophila contains very large chromosomes, the polytene or giant chromosomes, with precisely aligned side-to-side adhered individual chromatin strands in several organs like, for example, the salivary gland. They are ideal for the research into the structural changes of chromosomes.