Theodosius Dobzhansky’s Role in the Emergence and Institutionalization of Genetics in Mexico
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The significance of chromosomal inversions for investigating genetic and evolutionary questions can be traced to Sturtevant’s (1913) publication of the first chromosome map, which included six genes present on the X chromosome of D. melanogaster (Crow 1988). Sturtevant hypothesized that the recombination frequency between linked genes was directly related to their linear distance along the chromosomes, a proposal that initiated the fertile field of gene mapping. It would later be established that there is a linear correspondence between the sequence of genes in the genetic maps and the dark and light bands in the polytene chromosomes of the salivary glands. In 1926, Sturtevant (1926) showed that the so-called C factors, which had the property of suppressing crossing over, were associated with inversions in the gene sequence, which was later confirmed in giant salivary gland
chromosomes. Sturtevant and Dobzhansky(1936) later discovered that chromosome inversions were often present as polymorphisms in several natural populations of different species of Drosophila.
Chromosome inversions frequently overlap along a chromosome, an observation exploited by Dobzhanskyand Epling (1944) to infer the succession in which the inversions have arisen one from another, although not their temporal direction; i.e., whether a chromosome band arrangement was ancestral or descendant could not be determined (the method was first introduced and applied by Sturtevant and
Dobzhansky1936). Although the direction of a particular change could not be determined, this was often apparent from the entire set. Dobzhansky would for many years exploit this discovery for
reconstructing phylogenetic “trees” reflecting ancestor-descendant relationships between chromosome arrangements within and between species. This genetic method for reconstructing evolutionary biology would later be extended byFitch and Margoliash (1967) for determining the phylogeny of protein sequences from different species and, thereby, the phylogeny of the species. Protein and DNA sequences would eventually be used to reconstruct the evolutionary history of populations from the same species, a new subdiscipline called phylogeography. This has become a burgeoning field of evolutionary research (Avise 2000). The chromosome trees constructed by Dobzhansky and collaborators over many years may be seen as early examples of species phylogenies as well as of phylogeography, since chromosome polymorphisms were often associated with distinct geographic localities.
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