The gonad of C. elegans has a mirror-symmetric structure. This is because two distal tip cells (DTCs) migrate to the opposite direction. However, the mechanism regulating initial DTC migration is not known. In addition to DTC migration, polarity of two somatic gonadal precursors (SGPs; Z1 and Z4) that produce each DTC is known to be mirror-symmetric in terms of asymmetric localization of Wnt signaling components regulating asymmetry of the SGP divisions. Although we previously showed that Wnts are not essential for SGP polarity, since quintuple Wnt mutant has normal polarity, we recently found that SGP polarity is controlled by redundant functions of Wnts and lin-17/Frizzled. in-17 cwn-1/Wnt egl-20/Wnt cwn-2/Wnt quadruple mutant shows strong loss of SGP polarity, resulting in the no-DTC phenotype. In lin-17 egl-20 cwn-2 triple mutant, polarity of Z1 is reversed, so that it has the same polarity orientation with Z4. In this background, migration of Z1-derived DTC is also reversed, resulting in posterior-directed migration of both DTCs. Furthermore, in this triple mutant, we found perfect correlation between polarity orientation of the DTC mother (a daughter of Z1) and the migratory direction of DTC produced from it. Therefore, mirror-symmetric polarity of SGPs and their daughters determines initial migratory directions of DTCs to produce the mirror-symmetric structure of the gonad. Our results show a novel mechanism of cell migration regulated by polarity of asymmetric division.
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