As is the way of scientific progress, once one issue is resolved, other, more intriguing, unexpected problems are revealed.
In 1928, F. Griffiths discovered an ingredient in heat-killed cells of the pathogenic bacterium Streptococcus pneumoniae that could transform a live, non-pathogenic mutant strain of the bacterium into a pathogenic strain. In 1944, O.T. Avery and his colleagues identified the transforming principle in the cell-free extracts as DNA. The introduction of a piece of a homologous or foreign DNA molecule into an organism such that it is stably integrated into the host genome and stably alters its character is one of the most powerful tools in biology. It allows genes to be cloned through their ability to complement mutant pheno-types and to study their functions by selective knock out, the replacement of an endogenous gene with an engineered derivative.
From both basic reasons and practical benefits, it is of interest to determine the effects of introducing and integrating extra copies of a gene into the genome of a cell. The genes controlling pigmentation in Neurospora (Figure 9.1) or flower color in plants are especially useful as visual reporter systems to study the expression of transgenes, e.g., will the color be intensified? The unexpected finding, however, was that often the color is suppressed or extinguished. Such observations in fungi and plants led to the recognition of the phenomena of gene-silencing. This is considered to be a defense mechanism that evolved early for the preservation of the integrity of the genome from the onslaughts of viruses and transposons.
Because fungi reproduce rapidly and do so both sexually and asexually, they are particularly attractive for the study of gene-silencing phenomena in the vegetative and sexual phases. Additionally, the ability to fuse their vegetative cells allows the study of the dominance or recessiveness of the transgene in heterokaryons (Section 9.4.4). These experimental possibilities brought fungi to the forefront of research in this area.
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