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The Maternal Effect Genes

Classic embryological experiments demonstrated that there are at least two "organizing centers" in the insect egg. One is the anterior organizing center, the other the posterior organizing center. Klaus Sander (1975) postulated that these two organizing areas form two gradients, one initiated at the anterior end and the other at the posterior end. Each of these gradients forms its own structures at the poles and interacts with the other gradient to form the central portion of the embryo. Sander based this model on experiments that involved ligating the embryo at various times during development and transplanting regions of polar cytoplasm from one region of the egg to another (Figure 1). First, if he moved cytoplasm from the posterior pole more anteriorly, he obtained a small embryo anterior to the posterior pole plasm, while extra segments, not organized into an embryo, formed behind it (see Figure 1D). Second, if he ligated the egg early in development, separating the anterior from the posterior region, one half developed into an anterior embryo and one half developed into a posterior embryo, but neither half contained the middle segments of the embryo. The later in development the ligature was made, the fewer middle segments were missing. Thus, it appeared that there were indeed gradients emanating from the two poles during cleavage and that these gradients interacted to produce the positional information determining the identity of each segment.

Figure 1
Figure 1   Sander's ligature experiments on the embryo of the leafhopper insect Euscelis. (A) Normal embryo seen in ventral view. The black ball at the bottom represents a cluster of symbiotic bacteria that marks the posterior pole. (B) After ligating the early embryo, partial embryos form, but the head and thoracic segments are missing from both embryos. (C) When ligated later (at the blastoderm stage), more of the missing segments are formed, but the embryos still lack the most central segments. (D) When the posterior pole cytoplasm is transplanted into an embryo ligated at the blastoderm stage, a small but complete embryo forms in the anterior half, while the posterior half forms an inverted partial embryo. These results can be explained in terms of gradients at the poles of the embryo that turn on one set of structures and repress the formation of others. (After Sander, 1960, and French, 1988.)

The possibility that mRNA is responsible for generating the anterior gradient was suggested in a series of experiments by Kalthoff and Sander (1968). They found that when the anterior portion of the Smittia (midge) egg was exposed to ultraviolet light at wavelengths capable of inactivating RNA (265 and 285 nm), the resulting embryo lacked its head and thorax. Instead, the embryos developed two abdomens and telsons (tails) with mirror-image symmetry: telson-abdomen-abdomen-telson. Further evidence that RNA is important in specifying the anterior portion of the fly embryo was obtained by Kandler-Singer and Kalthoff (1976), who submerged Smittia eggs in solutions containing various enzymes and then punctured the eggs in specific regions. Double abdomens resulted when RNase was permitted to enter the anterior end. Other enzymes did not cause this abnormality, nor did RNase effect this change when it entered other regions of the egg. Thus, Sander’s laboratory postulated the existence of a gradient at either end of the egg, and it seemed likely that the egg sequestered an RNA that generated a gradient of anterior-forming material.

From Developmental Biology, Fifth Edition by Scott F. Gilbert. © 1997 Sinauer Associates, Inc., Sunderland, MA.

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