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Inheritance of coiling in snails.
Author: S. Gilbert
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"ter macromere, creating the characteristic spiral pattern. Looking down on the embryo from the animal pole, the upper ends of the mitotic spindles appear to alternate clockwise and counterclockwise (see Figure 8.24). This arrangement causes alternate micromeres to form oblique- ly to the left and to the right of their macromeres. At the third cleavage, the A macromere gives rise to two daughter cells, macromere 1A and micromere 1a. The B, C, and D cells behave similarly, producing the first quartet of micromeres. In most species, these micromeres are to the right of their macromeres (looking down on the animal pole). At the fourth cleavage, macromere 1A divides to form macromere 2A and micromere 2a, and micromere 1a divides to form two more micromeres, 1a 1 and 1a 2 (see Fig- ure 8.23). The micromeres of this second quartet are to the left of the macromeres. Further cleavage yields blastomeres 3A and 3a from macromere 2A, and micromere 1a 2 divides to produce cells 1a 21 and 1a 22 . In normal development, the first-quartet micromeres form the head structures, while the second-quartet micromeres form the statocyst (balance organ) and shell. These fates are specified both by cyto- plasmic localization and by induction (Clement 1967; Cather 1967; Render 1991; Sweet 1998). The orientation of the cleavage plane to the left or to the right is controlled by cytoplasmic factors within the oocyte. This was discovered by analyzing mutations of snail coil- ing. Some snails have their coils opening to the right of their shells (dextral coiling), whereas the coils of other snails open to the left (sinistral coiling). Usually the direc- tion of coiling is the same for all members of a given species, but occasional mutants are found (i.e., in a popu- lation of right-coiling snails, a few individuals will be found with coils that open on the left). Crampton (1894) analyzed the embryos of such aberrant snails and found that their early cleavage differed from the norm. The ori- entation of the cells after the second cleavage was differ- ent in the sinistrally coiling snails as a result of a different orientation of the mitotic apparatus (Figure 8.25). In some species (such as the pond snail Physa, an entirely sinistral species), the sinistrally coiling cleavage patterns are mir- ror-images of the dextrally coiling pattern of the right- handed species. In other instances (such as Lymnaea, where about 2 percent of the snails are lefties), sinistrality is the result of a two-step process: at each division, the initial cleavage is radial; however, as the cleavage furrow forms, the blastomeres shift to the left-hand spiral position (Shibazaki et al. 2004). In Figure 8.25, one can see that the position of the 4d blastomere (which is extremely impor- tant, as its progeny will form the mesodermal organs) is different in the two types of spiraling embryos. In snails such as Lymnaea, the direction of snail shell coil- ing is controlled by a single pair of genes (Sturtevant 1923; Boycott et al. 1930). In Lymnaea peregra, rare mutants exhibiting sinistral coiling were found and mated with wild-type, dextrally coiling snails. These matings showed that the right-coiling allele, D, is dominant to the left-coil- ing allele, d. However, the direction of cleavage is deter- mined not by the genotype of the developing snail, but by the genotype of the snail?s mother. A dd female snail can produce only sinistrally coiling offspring, even if the off- spring?s genotype is Dd. A Dd individual will coil either left or right, depending on the genotype of its mother. Such matings produce a chart like this: EARLY DEVELOPMENT IN SELECTED INVERTEBRATES 231 (B) Right-handed coiling AB CD AB CD B D AC B D AC 1A 1C 1B 1D 1c 1b 1a 1d 1A 1C 1B 1D 1c 1b 1a 1d 4A 4B 4C 4D 4d 4A4B 4C 4D 4d (A) Left-handed coiling FIGURE 8.25 Looking down on the animal pole of (A) left-coil- ing and (B) right-coiling snails. The origin of sinistral and dextral coiling can be traced to the orientation of the mitotic spindle at the second cleavage. Left- and right-coiling snails develop as mirror images of each other. (After Morgan 1927.) Chapter 8 pages/alt. 8/2/06 11:59 AM Page 231 The genetic factors involved in snail coiling are brought to the embryo by the oocyte cytoplasm. It is the genotype of the ovary in which the oocyte develops that determines which orientation cleavage will take. When Freeman and Lundelius (1982) injected a small amount of cytoplasm from dextrally coiling snails into the eggs of dd mothers, the resulting embryos coiled to the right. Cytoplasm from sinistrally coiling snails did not affect right-coiling embryos. These findings confirmed that the wild-type mothers were placing a factor into their eggs that was absent or defective in the dd mothers. A fate map of Ilyanassa obsoleta Joanne Render (1997) constructed a detailed fate map of the snail Ilyanassa obsoleta by injecting specific micromeres with large polymers conjugated to the fluorescent dye Lucifer Yellow. The fluorescence is maintained over the period of embryogenesis and can be seen in the larval tis- sue derived from the injected cells. The results of Render?s map, given in Figure 8.26, showed that the second-quartet micromeres (2a?d) generally contribute to the shell-form- ing mantle, the velum, the mouth, and the heart. The third- quartet micromeres (3a?d) generate large regions of the foot, velum, esophagus, and heart. The 4d cell?the mesen- toblast?contributes to the larval kidney, heart, retractor muscles, and intestine. The polar lobe: Cell determination and axis formation Molluscs provide some of the most impressive examples of both mosaic development?in which the blastomeres are specified autonomously?and of cytoplasmic localiza- tion, wherein morphogenetic determinants are placed in a specific region of the oocyte (see Chapter 3). Mosaic devel- opment is widespread throughout the animal kingdom, especially among protostomes such as annelids, nema- todes, and molluscs, all of which initiate gastrulation at the future anterior end after only a few cell divisions. In molluscs, the mRNAs for some transcription factors and paracrine factors are placed in particular cells by asso- ciating with certain centrosomes (Figure 8.29; Lambert and Genotype Phenotype DD � � dd � ? Dd All right-coiling DD � � dd � ? Dd All left-coiling Dd � Dd ? 1DD:2Dd:1dd All right-coiling 232 CHAPTER 8 AB A 1a 1A 1b 1B 1c 1C 1D 1d 2d 2D 3D 3d 4d ME1 ME2 (ME) 4D 3C 3c 2C 2c 3B 3b 2B 2b 2A 2a 3a 3AB B CD Left velum; left stomodeum; upper half left statocyst; mantle edge; upper left foot Left eye; left velum; apical plate Velar retractor; digestive glands Velar retractor; digestive glands Velum; apical plate Velum; dorsal stomodeum; mantle edge; foot retractor muscle Right velum; right esophagus Right velum; right stomodeum; upper half right statocyst dorsal mantle edge; upper right foot; heart Right velum; right statocyst; right half of foot; mantle edge Left velum; left statocyst; heart; left half of foot Velar retractor; digestive glands; style sac Left velar retractor; part of intestine Right velar retractor; heart; kidney; part of intestine Lumen of digestive glands; yolk Mantle edge; tip of foot Left velum near eye; apical plate Right eye; right velum; right tentacle; apical plate Left velum; left esophogus C D Zygote FIGURE 8.26 Fate map of Ilyanassa obsoleta. Beads containing Lucifer Yellow were injected into individual blastomeres at the 32-cell stage. When the embryos developed into larvae, their descendants could be identified by their fluorescence. (After Render 1997.) 8.3 Alfred Sturtevant and the genetics of snail coiling. By a masterful thought experiment, Sturtevant demonstrated the power of applying genetics to embryology. To do this, he brought Mendelian genetics into the study of snail coiling. WEBSITE Chapter 8 pages/alt. 3/6/06 3:10 PM Page 232 "
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