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If there is no selective disadvantage operating against one of the two phenotypes, the progenies of cross-compatible plants Distyly 29 Tristylic (Lythrum) Distylic (Primula) short = mid Pin x Thrum mid mid = long - ~I r--- short = long Long _____ I I s s - m miss _ X ~I Mid S,·MM = mid = long "I m~ Ss - Mm - ~ Pin: Thrum short X~ Short ) -X ~ short = mid short = mid = long Fig. 3. Segregations for pin and thrum (distylic system) and long, mid and short (tristyly) after compatible matings will again break down into two phenotypic classes, pin and thrum, which are present in equal proportions.

In any given population, a large number of different alleles segregate at the incompatibility locus. Bateman (1954) established, as an underestimate, that 22 different alleles were present in a natural population of Iberis amara which was composed of only 47 self-incompatible plants. Ockendon (1974) found 19 different S-alleles in 488 plants representing 16 cultivars of Brussels sprouts. The number of S-alleles per cultivar varied from four to 13. Sampson (1967) identified nine different S-alleles in 45 plants of Raphanus raphanistrum representing five different wild populations.

Stebbins refers, in this connection, to the situation in Bromus (Harlan, 1945) and in Myosurus (Stone, 1957) where outbreeding and inbreeding alternate from one season or from one set of environmental conditions to the next, but several other cases may be found in the literature dealing with pseudocompatibility (see Chap. 4) and the tendency of many self-incompatible species to display, under extreme environmental changes, a distinct capacity to set seeds upon selfing. A third reason for the origin and maintenance of self-compatible populations has been underlined by Baker (1955) who rightly concludes that the accidental long distance dispersal of a single seed cannot be successful in the absence of an association with self-compatibility.

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