Embryological studies on seed incompatibility in primula

Abstract

Valentine termed the phenomenon of post-fertilization failure in interspecific crosses “seed incompatibility.” On the basis of the results obtained from crossing them, he allotted different “genetic values” to the genomes of three species of Primula. The genetic ratio “R” (ratio of genomes of endosparm and maternal tissue) was calculated for each cross. When “R” is greater than the normal 1.5, so-called “type B” seeds, large and often empty, are produced. When R is less than 1.5, “type A” seeds, small and often well-filled, are formed. The present study is a detailed histological investigation of seed development of some crosses in the genus Primula. “type A” seeds have been shown to have overdeveloped inner integuments but relatively normal endosperms; “type B” seeds have thin integuments and poor endosperms. In every case the condition of the embryo apparently depends on that of the endosperm. In the cross between diploid P.Veris and its autotetraploid, the 4n x 2n mating (R = 1,25) produces extreme “type A” seeds, the reciprocal (R = 2.0) gives extreme “type B”. The results of these and other crosses involving various values of “R” all give support to the “genetic value” concept. The nature of genetic value is discusses, and the hypothesis advanced that a diploid species may evolve toward the polyploidy state by the accumulation of small duplications, giving it an increased amount of chromatic material, and thus increasing the “genetic value” without changing the chromosome number. Important previous work on seed incompatibility is reviewed and critically discussed. Finally the hypothesis is reviewed and critically discussed. Finally the hypothesis is advanced that in Primila crosses, the incoming pollen imparts an abnormal stimulus to the embryo sac, and, depending on the value of “R”, there is more or less serious physiological unbalance between endosperm and maternal tissues.