ENDOSPERM BALANCE NUMBER AND THE POLAR-NUCLEI ACTIVATION HYPOTHESES FOR ENDOSPERM DEVELOPMENT IN INTERSPECIFIC CROSSES OF SOLANACEAE AND GRAMINEAE, RESPECTIVELY

The Endosperm Balance Number (EBN) and the polar-nuclei activation (PN A) hypotheses have been developed to interpret, explain and predict in terspecific and interploidy crossabilities in the Solanums and the Gra mineae, respectively. Although these two hypotheses evolved independen tly, they share a number of common features. Assignment of EBNs and 'a ctivation/response values' (AVs/RVs) depend on plumpness, size, and ge rminability of hybrid seeds. Also, both hypotheses emphasize the impor tance of a balanced parental genic contribution for the normal develop ment of endosperm. However, in the EBN hypothesis a 2 maternal:1 pater nal EBN ratio is a prerequisite for successful interspecific crossabil ity, while the PNA hypothesis is based on the stimulative strength of the male nuclei to initiate mitotic divisions in the primary endosperm nucleus and is idependent on a 2:1 ratio between the RV of the polar nuclei and the AV of the male gamete. Differences and similarities bet ween the EBN and PNA hypotheses are summarized and contrasted. It is p roposed that EBN and PNA be considered as the same concept.