Floral morphogenesis in Primula : inheritance of mutant phenotypes, heteromorphy, and linkage analysis

Abstract

Interest in Primula flowers from both a horticultural and scientific perspective dates back over 400 years. Floral mutations were first used for ornamental value in the latter part of the 16th Century but had attracted little scientific attention. The phenomenon of floral heteromorphy as a mechanism to promote out-breeding was immortalised by the work of Darwin in the mid 19th Century. Subsequent analysis of this breeding system has attracted much attention, including the genetic definition of the S locus as a cluster of tightly linked genes that control pin and thrum flower development and mediate self-incompatibility. Mutant phenotypes of British Primula have been collected by the author for over twenty years. Classical genetic analysis of some of these mutants is included and provides the first detailed analysis of existing and new mutant phenotypes. Genetic analysis of these mutants is presented in the context of the ABC model of flower development. Detailed analysis of the early ontogeny of wild type and mutant flowers by scanning electron microscopy provides new insights into the control of Primula flower development. As Primula flowers were found to be homomorphic during early ontogeny development of pin and thrum heteromorphic features of Primula were investigated to maturity. A new heteromorphic feature was discovered; thrum flowers have a wider corolla tube mouth than pin flowers due to the corolla tube cells above the anthers being wider in thrum flowers than in pin flowers. Three of the mutant phenotypes Hose in Hose, Staminoid Carpels and sepaloid are predicted to arise through misexpression of a B function gene. The first two are dominant mutant phenotypes and all are linked to the S locus. A fourth recently discovered dominant mutant phenotype, Oakleaf, affects both leaf and flower development, and is also linked to 1he S locus. As 1he dominant nature of the Hose in Hose mutation precludes complementation tests three point crosses were used both as segregation tests and for mapping genes linked to the Primula S locus. Gene order was found to be Oak Leaf, S locus, Hose in Hose, with sepaloid either allelic to Hose in Hose or very tightly linked. In combination, these analyses have enabled 1he assembly of the first genetic map of genes around the S locus including flanking markers on either side