Time and site of theS-gene action, breeding systems and relationships in incompatibility

Summary

In self-incompatible angiosperms correlations have been discovered between the genetic systems of compatibility control, floral morphology, pollen cytology, and site of pollen inhibition. The present paper critically reviews these relationships and their exceptions with particular reference to the time and site of theS-gene action. A hypothesis explaining these relationships and some of the exceptions has been proposed earlier. The major factor affecting these relationships is believed to be time of theS-gene action — late- or post-meiotic (not earlier than telophase I) in the gametophytic system and early (homomorphic) — or premeiotic (heteromorphic) in the sporophytic system. Recent work of Heslop-Harrison and his colleagues has provided additional evidence about the suggested timing of theS-gene action in the gametophytic species. In the sporophytic species, however, Heslop-Harrison suggests that theS-gene action occurs in the tapetum and not in the meiocytes. In the present discussion it is argued that tapetal action of certain elements of theS-gene complex is possible, but only in the heteromorphic species. It is suggested that major exceptions are by-products of secondary evolution of incompatibility after the primary incompatibility had been disturbed, or had broken down.