Two additional characters of wild tomatoes, self-incompatibility (SI) and allogamy (i.e., mating system), have played an important role in the evolution of the clade. SI is an ancestral character in the tomato clade (Rick et al. 1977; Miller and Tanksley 1990), and loss of SI has occurred independently several times. Notably, SI hasbeenlostinsomepopulationsof S. pennellii (Rick and Tanksley 1981), some populations of S. habrochaites (Rick et al. 1979), and in the clade containing colored-fruited tomatoes and the species S. neorickii and S. chmielewskii (Ricket al. 1976; Taylor 1986). In this latter group there is support for loss of SI being of monophyletic origin (Miller and Tanksley 1990; Marshall et al. 2001).
Lower levels of genetic variation are expected in self-compatible (SC) species as compared to their SI
relatives, primarily because of the reduction in Ne due to the increase in inbreeding (Charlesworth 2003). Various molecular markers, including allozymes (Breto et al. 1993), SSRs (Alvarez et al. 2001), RFLPs (Miller and Tanksley 1990), random amplified polymorphic DNAs (RAPDs) (Egashira et al. 2000), and nuclear DNA sequences (Baudry et al. 2001; Roselius et al. 2005) have consistently documented lower levels of genetic variation in SC wild tomato species.
Among SC wild tomato taxa, variation exists in the degree of allogamy, ranging from the almost entirely selfing species, S. cheesmaniae, to the facultative outcrossers, S. chmielewskii and S. pimpinellifolium (Taylor 1986). S. pimpinellifolium is notable in that variation in the degree of allogamy occurs among populations, with levels of outcrossing varying between 0 to 40% (Rick et al. 1977). Also of interest are the species S. chmielewskii and S. neorickii,which, though sympatric, differ in that S. chmielewskii is a facultative outcrosser, while S. neorickii is autogamous (Rick et al. 1976). The two species often grow in contact with each other, leading to the speculation that S. ne-orickii may have evolved from S. chmielewskii by acquiring autogamous reproduction (Rick et al. 1976); predictably, greater levels of allozyme diversity are found in S. chmielewskii (Rick 1984). Both within and between SC species, levels of outcrossing have been found to be correlated with various morphological features, including larger corolla size and greater stigma exsertion, which aid in cross-pollination (Rick et al. 1976,1977; Rick 1984).
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