During the evolution of angiosperms AI has arisen several times, completely different lineages. More than 100 plant families, among which include the Solanaceae family, Poaceae, Asteraceae, Brassicaceae, Fabaceae and Rosaceae, are self-incompatible species. In fact, it is estimated that 39% of angiosperm species are AI. Such distributiontaxonomy range is compatible with the existence of several different genetic mechanisms that regulate the AI, according to the family concerned.
However, in all cases observed the same phenomenon: pollen grains the stigma achieve the same plant (or plants genetically related) are unable to perform fertilization and arrest their development in some stage of the process (seed germination pollen, pollen tube development in pistils and fertilization of the egg cell). Because of this barrier is not seed after self-pollination. 1 This cessation of growth or development of pollen tubes in the pistil consists, first, the recognition by the pistil of pollen tubes that are the same person and what other plants. On the other hand, occur after this recognition, the development of pollen tubes themselves (or genetically related) should be discontinued. These two phenomena, recognition and cessation of growth, have been widely studied both cytological, physiological, genetic and molecular.
As mentioned above, AI systems in higher plants have evolved several times in independent lineages. At the molecular level, there are three well-characterized systems: the system of self-incompatibility in Solanaceae and Rosaceae, the poppy (Papaver) and theBrassica. In two of these systems are currently known genes that encode components of pollen pistil recognition has clearly demonstrated that proteins of the pistil and pollen involved in the recognition are different from each other and the AI ??is a mechanism " lock "at the molecular level.
Types of mechanisms
When the flowers of the species are morphologically self-incompatibility that presents identical, it is said that self-incompatibility is homomorphic. However, when species have two or three different morphological types of flowers, it is said that self-incompatibility and heteromorphic heterostyly mechanism is called.
homomorphic self-incompatibility
The best studied mechanism of AI in plants act by inhibiting germination or growth of pollen tubes in the stigma or style. This mechanism is based on the interactions between proteins produced by a single locus called the S (English self-incompatibility or self-incompatibility). The S locus contains two genes actually base one of which is expressed in the pistil and the other in the anther or pollen grains. Reference is made to genes such as female and male determinants, respectively. These genes are physically close in the genomeare genetically related and are considered a single allele. The proteins produced by the male and female determinants of the same allele interact with each other at the time of pollen germination on the stigma and lead to the cessation of pollen tube growth. In other words, the identity of the proteins of pollen and stigma AI generates a response that stops the fertilization process since the beginning of it. When, however, an important determinant of women interacting with a male determining allele from another AI, no response from the IA and fertilization occurs. This brief description of the AI ??response in plants is usually more complex, often involving more than one locus of AI. Thus, in the grass, two loci that govern the response of AI called S and Z. There are two types of homomorphic incompatibility, AI and AI gametophyte sporophyte, which are described below.
