Causal relationships between the polar nuclei in double fertilization and interspecific cross-incompatibility in Avena.

Publisher Website
Google Scholar
Abstract
A diallel cross was made using 10 polyploid species, 2x, 4x and 6x, x = 7, in the genus Avena [A. strigosa, A. hirtula, A. longiglumis, A. pilosa, A. ventricosa, A. barbata, A. magna, A. fatua, A. sativa cv. Aurora and A. sterilis var. maxima]. In most 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 4 types: normal kernels, well developed and germinable (designated as D+); shrivelled-empty kernels which do not germinate (E-); small viable kernels (Rd+); and small inviable kernels (Rd-). Some intermediate types were occasionally found. 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 was discussed. 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 .times. 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 unually being the normal index results in the production of normal seeds. If AI deviates from 50% the development of endosperm is often arrested or aborted. Based on the degree of abnormality of hybrid kernels, activating values of the 10 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 7-375%. The distribution of these indices agrees closely with the grouping of the 4 kernel types. Thus, AI values of less than 20, 20-30, 30-80 and more than 80% show the Rd-, Rd+, D+ and E- kernel types, respectively. If the AV of the pollen parent is > 160 or < 40% that of the maternal parent they are cross-incompatible. Thus in double fertilization of angiosperms the triple fusion of 2 polar nuclei with a secondary male nucleus is apparently a biological isolation mechanism and may be the most effective barrier to hybridization on occasion.