The Genetic Control of Unilateral Incompatibility between Two Tomato Species.

LTHOUGH self-compatible (SC) species frequently can hybridize as females A with related self-incompatible (SI) species, the reciprocal cross is often impeded by inhibition of pollen tube growth in the style. This unilateral incompatibility can also be demonstrated between species of widely separated genera, or even of different families, which suggests that it is of fundamental significance in understanding the evolution of self-fertility and self-incompatibility. Certain SC species that cross freely with SI types appear to be evolving from self-incompatibility to self-compatibility (LEWIS and CROWE 1958). The authors developed an explanatory theory of such change, based on a three-step mutation of the S allele as it loses, progressively, the property to inhibit its own pollen, the property to inhibit the SC allele (Sc), and finally the ability of its pollen to grow through the styles of SI species. The above hypothesis has been weakened by the finding of unilateral incompatibility in unexpected combinations, such as between two SI entities (STOUT 1952; PUSHKARNATH 1953; PANDEY 1962; MARTIN 1964), or between a SI male and SC female (MARTIN 1964). In addition, it now appears that other genes account for unilateral incompatibility in some crosses (MARTIN 1964; GRUN and AUBERTIN 1966). Finally, unilateral incompatibility may be due to other mechanisms in addition to pollen tube growth inhibition, and these might be controlled by different genes ( GRUN and AUBERTIN 1966). In the tomato, unilateral incompatibility occurs between the red-fruited selffertile garden tomato (section Eulycopersicon) and the wild, green-fruited species (section Eriopersicon) . One of the green-fruited species (Lycopersicon hirsutum Humb. and Bonpl.) consists of a number of geographical races exhibiting a stepwise series of unilateral relations ( MARTIN 1964). Hybridization among races tends to break down unilateral barriers. Genetic analysis of F, and BC generations showed the phenomenon to be controlled by a pair of genes of major effect (incompatibility genes), and also by a multigenic background that rendered the expression continuous in gradation (MARTIN 1964). In crosses to L. esculentum, both selfand unilateral incompatibility generally are dominant ( MCGUIRE and RICK 1954; MARTIN 1961). In two species crosses, F, hybrids were partially unilaterally incompatible with both parents, as female with L. esculentum and as male with the wild parent. Although the cross L. esculentum x L. hirsutum has been accomplished many times, the inheritance of unilateral incompatibility has