A diamond anniversary: the first chromosomal map
A DIAMOND ANNIVERSARY: THE FIRST CHROMOSOME MAP S EVENTY-FIVE years ago this month A. H. STUR- TEVANT (1 9 13) published the first linkage map. It involved five X-chromosomal loci in Drosophila ampelophila, now called Drosophila melanogaster. This was early genetics at its most exquisite. From seemingly irrelevant counts of the number of different kinds of offspring from various matings, and with no idea of the nature of the genes, STURTEVANT could nevertheless infer their sequence and relative
... quence and relative distances apart on the chromosome. These quiet beginnings stand in abrupt contrast to the current hubbub over the human linkage map and the proper definition of a map (ROBERTS 1987). With its rival factions and the glare of publicity, the mapping race is almost a genetic Olympics. One other contrast: the 19 13 Drosophila paper had one author, the 1987 paper on the human map has 33 (DONIS- KELLER et al. 1987). STURTEVANT was still an undergraduate student at Columbia University when he had the key idea. In his words (1 965 ): In the latter part of 191 1 , in conversation with MORGAN . . ., I suddenly realized that the variations in strength of linkage, already attributed by MORGAN to differences in the spatial separation of the genes, offered the possibility of determining sequences in the linear dimension of a chromosome. I went home and spent most of the night (to the neglect of my undergraduate homework) in producing the first chromosome map. The first publication in I91 3 was a masterpiece of clarity. Here is a sample: By determining the distances . . . between A and B and between B and C, one should be able to predict AC. For, if proportion of cross-overs really represents distance, AC must be, approximately, either AB plus BC or AB minus BC. Figure 1 shows STURTEVANT'S original map, together with the current distances as given by LINDSLEY and GRELL (1 968). Considering the primitive laboratory conditions and large distances between markers, the agreement is remarkable. This pathbreaking paper and 32 more of STURTEVANT'S most important contributions have been reprinted (STURTEVANT 196 1). STURTEVANT and C. B . BRIDGES were both students in MORGAN'S course in elementary zoology at Columbia in 1909. They were both given places to work in the "fly room" and immediately became members of the research team. This room was only 16 by 23 feet and, somehow, eight desks were crowded into it. The room also included fly food preparation, with an always-present stalk of bananas. It soon became filled with additional geneticists, notably H. J. MULLER who joined the group in 1912. Included in this closepacked area was PHOEBE REED, who washed glassware, prepared media, and later became MRS. STURTEVANT. Each of the researchers made important contributions, of both data and ideas, to the rapid mapping of the Drosophila genome. MULLER introduced the ideas of coincidence and interference. BRIDGES concerned himself with the technology, working out standardized culture conditions and mating systems designed to minimize viability complications. Curiously, the MOR-GAN school made no use of mathematical mapping functions, which would have been very useful in the early days when distances between known genes were large. Such functions were developed in England (HALDANE 19 19) but did not make it across the Atlantic for many years. The free exchange of data, the continuous discussion of each other's results and the scientific excellence of the group created a situation in which new results came at an enormous rate. Within a few years the rules of transmission genetics and the mechanical basis of sex-linked inheritance, crossing over, nondisb c P r m 0.0 1.0 30.7 33.7 57.6 0.0 1.5 33.0 36.1 54.5 Y W v m r F~GURE 1 ."STURTEVANT'S original linkage map of the Drosophila X chromosome, with his placements and the symbols US^ at the time (upper) compared to the current locations and symbols (Imey). The loci are $low body, white eyes, vermilion eyes, minkture wings and rudimentary wings.