Much of the progress made during the past two decades in understanding oogenesis is based on the identification of genes whose function is important for oocyte/egg development. Mutations that cause either a non-egg-laying pheno-type or a highly abnormal laid-egg morphology are special types of female ster ile mutations, because they visibly and directly demonstrate a requirement for the normal functions of the affected genes in oogenesis. About 30% of zygotic lethal mutations can have an ovarian phenotype as well, an observation based on mosaic analysis of such mutations (109). Furthermore, weaker hypomorphs and conditional alleles of genes identified as zygotic lethals can produce an ovarian phenotype. Any of these mutations may affect either the germ-line or the soma (usually assessed in follicle cells), which can be tested by inducing germ-line and/or follicle cell clones homozygous for the mutation of interest. This can be achieved either by pole cell transplantation (9) or by mitotic recombination.
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