Causal Relationships between the Polar Nuclei in Double Fertilization and Interspecific Cross-incompatibility in Avena

Abstract

A diallel cross was made using ten polyploid species, 2x, 4x, and 6x, x=7, in the genus Avena. Inmost crosses hybrid seeds were successfully obtained though the frequency of seed set varied greatly. The development of hybrid seeds differed widely between cross combinations, especially in reciprocal crosses. Hybrid seeds were classified into four types, 1) normal kernels, well developed and germinable (designated as D+), 2) shrivelled-empty kernels which do not germinate (E-), 3) small viable kernels (Rd+) and 4) small inviable kernels (Rd-). Some intermediate types were occasionally found.
The results are fairly well explained in terms of a hypothesis of polar-nuclei activation in which the strength of the activating action of the male nucleus, and the reaction of the female nucleus is expressed by terms ‘activating value (AV)’ and ‘response value (RV)’, respectively. The degree of seed failure is closely related to the difference between the activating and response values, expressed by the activation index, AV/2RV (or×100) of the polar nuclei. In a selfed plant the activation index (AI) of the polar nuclei is 1/2=0.5 (or 50%), and this usually being the normal index results in the production of normal seeds. If AI deviates from 50 per cent the development of endosperm is often arrested or aborted.
Based on the degree of abnormality of hybrid kernels, activating values of the ten species are arbitrarily assigned from 0.4 in A. ventricosa(2x) to 3 in A. sterilis(6x) compared with 1 in A. strigosa(2x) as the standard. Among 74 reciprocal interspecific crosses the AI of the polar nuclei varied widely from 7 to 375 per cent. The distribution of these indices agrees closely with the grouping of the four kernel types. Thus, AI values of less than 20, 20-30, 30-80, and more than 80 per cent show the Rd-, Rd+, D+ and E- kernel types, respectively. In other words, if the AV of the pollen parent is >160 or <40 per cent that of the maternal parent they are cross-incompatible. Thus in double fertilization of angiosperms the triple fusion of two polar nuclei with a secondary male nucleus is apparently a biological isolation mechanism and may be the most effective barrier to hybridization on occasion.