Interspecific cross-incompatibility due to disturbed activation of the polar nuclei by the male nucleus

Abstract

A total of 74 interspecific reciprocal crosses was made among ten polyploid species of Avena. The development of hybrid kernels was very variable, and grouped into four types, Rd -, Rd +, D + and E - which indicate small or very small, small, mormal and empty, shrivelled kernels, respectively, as vvell as germination (+) and non-germination (-) of hybrid kernels. In a small number of crosses, however, some intermediate types of kernels were found. The results are almost completely explained in terms of a hypothesls of polar-nuclei activation by the male nucleus in double fertilization. The strength of activating stimulus of the male nucleus and reaction of the female nucleus is expressed by terms "activating value (AV)" and "response value (RV)", respectively. Activating values assigned to Avena species show a range from 0.4 to 3 where 1 to A. strigosa is standard. The degree of seed failure is closely related to the differenec between activatlng and response value which usually is represented by an activation index (AI) of the polar nuclei. The AI is c, omputed from a formula, AV/2 RV (or X100%), In a plant selfed the AI is 1/2=0.5 (or 50%), being a normal index, and resultlng in production of normal seeds. If AI deviates from 50% the development of endosperm is often arrested, and it causes the seed abortlon in the extreme case. In the present hybridizations the activation index varies greatly from 7 to 375%Expressing in round numbers less than 20, 90-30, 30-80 and more than 80% activation indexes show Rd -, Rd +, D + and E - type kernels, res-pectively. Those of intermediate-type kernels are always found on boundary zones between the four groups. In other ¥vords, it is shortly stated that interspecific crosses of Avena are only compatible when their activation indexes fall within a range from 20 to 80 % .Thus the triple fusion of the two pdlar nuclei with the male nucleus In angiosperms seems to be a sexual isolation mec.hanism and the most efffective barrier to interspecific hybridizations.