SYMPATRIC SPECIATION IN SNAILS; A LARGELY NEGLECTED MODEL

Usually, shells of snails are clearly either dextral or sinistral. That means that, if one looks at the apex, the whorls increase in width clockwise or counterclockwise, respectively. In a dextral shell, the soft parts of the animal have the proximal part of the genitalia situated asymmetrically at the right side of the head. Most species are dextral, a clear minority are sinistral, and a few species are polymorphic ifor the direction of coiling. In many species aberrant specimens of opposite coil are known (for additional information, see Johnson [1982] and Gould et al. [1985]). Reproductive isolation can be a consequence of opposite direction of coiling. The various degrees of isolation in the few cases studied until now have depended upon differences in interspecific behavior and shell shapes. The three species in which observations show that animals of opposite coil cannot copulate (see below), are characterized by globular shells in which the width equals or exceeds the height. In Partula sutoralis, which has more slender, relatively higher shells, copulation is hindered but not prohibited by opposite coil (Lipton and Murray, 1979; Johnson, 1982). For Laciniaria biplicata, a species with very slender shells, we know that mirror-image specimens can copulate but not how readily they do so (Degner, 1952). A realistic model for sympatric speciation in snails has not received much attention in the English literature, in part perhaps because some underlying notes and observations have been published in Dutch, German, and Russian. Meisenheimer (19 12 p. 131) stated that a dextral and a sinistral specimen of the helicid gastropod Helix pomatia will try to copulate but in vain: ". for days and weeks the animals fatigue each other in courtship, without achieving a final copulation. " Hesse (1914) reported that, in both Helix pomatia and Helix aspersa, copulations between individuals of opposite coil proved to be impossible because the globular shells interfered. This author suggested that more slender shells might have less dramatic consequences for the animals possessing them. Janssen (1966) published detailed observations concerning a sinistral specimen of the helicid gastropod Arianta arbustorum (which has a broad globular shell), kept for some months with a dextral animal of the same species. Several times, he observed that the animals tried to copulate, but these attempts were always without success, because the genital pores could never come into contact. Both snails produced unfertilized eggs. These facts brought Gittenberger (1973 p. 1) to state that "the fact that snails are either sinistral or dextral may underlie a mechanism resulting in sympatric speciation. " Alexandrov and Sergievsky (1979) dealt with sympatric speciation in snails on the basis of chirality. They distinguished four stages in the process: 1) mutant recessive genes for opposite coil are only present in heterozygotes and do not manifest themselves; 2) the opposite phenotype appears; 3) a stable population is formed; and 4) a new species may enlarge its range. Stages 1) and 2) do not require allopatric populations. The main question is whether a population with snails of opposite coil can develop within the cruising range of normal specimens. Johnson (1982) also described several possible components of sympatric speciation in work on Partula, including chirality clines and assortative mating. In many species of snails the individuals move only short distances after hatching, and therefore, inbreeding is common. This increases the likelihood of stage 2 being reached. Because of the so-called maternal or delayed inheritance, found in the three genetically studied cases (all Pulmonata), the opposite phenotype will show up only in the next generation, in those individuals that develop from the eggs of the homozygote (Boycott et al., 1930; Degner, 1952; Murray and Clarke, 1966). The genotype of the maternal parent is decisive for the direction of coiling of its young. As a consequence, a dextral snail can itself be homozygous for sinistral coiling, when the maternal parent is heterozygous and dextral coiling is dominant. Its eggs will develop into sinistral individuals, however. A sinistral snail can be homozygous or heterozygous for sinistral coiling. As a result of this form of inheritance, some gene flow is still possible between the sinistral and the dextral part of the population, even in those cases in which the different phenotypes cannot copulate. This gene flow will stop as soon as only homozygotes occur. Selection favoring homozygotes is probable, because the offspring of heterozygotes will, on average, have a lower probability of finding optimal sexual partners nearby. Pulmonate snails, many of which are hermaphrodites, may be capable of self-fertilization, but its frequency in nature is still largely unknown. Johnson (1982) reported that fewer young than usual are produced by mixed pairs in Partula, where copulation between such pairs is relatively rare. He does not mention the allelic compositions of the phenotypes involved. Because of the consequences for natural selection, it would be interesting to know whether both heterozygotes and homozygotes have reduced fertility. Johnson ( 1982) has argued that there will be selection against the rare form with opposite coil, thus favoring monomorphism. Selection will be strongest if mating between mirror-image snails is impossible. Despite this, because of low dispersal, locally strong inbreeding, and/ or genetic drift, the aberrant morph may occasionally