Incompatibility System in Theobroma cacao

Abstract

The genetics of self-incompatibility in T. cacao is considered unique among the identified systems in the flowering plants. In this species there is no inhibition of the pollen tube growth in incompatible crosses and the contents of the pollen tube are liberated in the ovules. Furthermore, hybrids obtained between 2 different isolated self-compatible populations turned out to be self-incompatible. The sequence of events, regarding S gene action, in T. cacao is suggested to be as follows: (1) premeiotic production of specific growth substances leading to S allele interaction, (2) post-meiotic production of specific incompatibility precursors, (3) conversion of specific precursors into specific incompatibility substances in pollen tubes in the male and in embryo-sacs in the female, (4) growth of all pollen tubes down the style and into the embryo-sacs because of lack of incompatibility substances in the sporophytic, stylar and ovarian, tissues and (5) incompatible reaction between the pollen tubes and embryo-sacs possessing the same dominant S allele, for which specific incompatibility substances are present and compatible reaction between the pollen tubes and embryo-sacs either of which have the recessive S allele and therefore lack the specific incompatibility substances. The self-compatibility of some populations of T. cacao is due to competition interaction between certain [image] alleles. A cross between such self-compatible populations may produce hybrids which have non-competing S allelic combination and may thus be self-incompatible. Unlike Cope''s hypothesis, which involves two loci, the present interpretation, requiring only one S locus, is simpler and is in line with observations made in other self-incompatible species, notably Oenothera.