. The Biological bulletin. Biology; Zoology; Marine biology. HOMEOTIC TRANSFORMATION OF CRAB LIMBS 319. Figure 4. Regenerated limbs of the third or fourth walking leg in Cancer amhonyi after receiving the autotransplant of claw tissue. (A) Chimeric claw regenerated from the stump of the fourth left walking leg. (B) Regenerated walking leg with two pollexes and three dactyls. (C) Fork-shaped walking leg. (D) Regenerated walking leg with a clawlike dactyl. transformation in nature is caused by a genetic defect of homeotic genes, such as Drosophila HOM or vertebrate Hox (reviewed by Krumlauf, 199
. The Biological bulletin. Biology; Zoology; Marine biology. HOMEOTIC TRANSFORMATION OF CRAB LIMBS 319. Figure 4. Regenerated limbs of the third or fourth walking leg in Cancer amhonyi after receiving the autotransplant of claw tissue. (A) Chimeric claw regenerated from the stump of the fourth left walking leg. (B) Regenerated walking leg with two pollexes and three dactyls. (C) Fork-shaped walking leg. (D) Regenerated walking leg with a clawlike dactyl. transformation in nature is caused by a genetic defect of homeotic genes, such as Drosophila HOM or vertebrate Hox (reviewed by Krumlauf, 1994; Lawrence and Morata, 1994). Homeotic transformation can be experimentally induced with either morphogens or ectopic expression of Hox genes (Schneuwly et ai, 1987; Kessel and Gruss, 1990; Morgan et ai, 1992). The mechanism for the crab claw transformation is unknown. We propose three pos- sible models: intercalary regeneration, morphogen, or change of Hox code. Intercalary regeneration is a process that regenerates the missing part in some insects and amphibians (French et ai. 1976; Bryant ^/ a/., 1981). It is possible that claw cells in the stump of the walking leg may regenerate the missing part to form a complete claw or else claw cells interact with walking-leg cells to regenerate a chimeric claw. This model is supported by the observation that autotransplanted frozen tissue had no activity. However, the result of contralateral autotransplantation does not favor this model since the positional value of the right and left limb is oriented in opposite directions. When the distal part of the right limb is transplanted to the proximal part of the left limb, it should regenerate three limbs from the stump according to the polar coordinate model (French et ai, 1976; Bryant et ai, 1981). In our contra- lateral autotransplantation experiments, we have never observed three limbs regenerating from the stump of a walking leg. Instead, a normal claw with the normal handedness can be
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