Embryo development in reciprocal crosses of Phaseolus vulgaris L. and P. coccineus Lam

Embryo development was examined in reciprocal crosses of Phaseolus vulgaris cv. 'Great Northern' and P. coccineus cv. 'Scarlet Runner'. The formation of abnormal (shrunken and underdeveloped) embryos constituted the primary crossing barrier between the two species when P. coccineus was the female parent. Plants of P. coccineus X P. vulgaris were obtained by embryo culture. Although the P. vulgaris X P. coccineus cross resulted in normal seed development, the fertility of the resulting hybrids was much lower (27%) than that of the reciprocal hybrids (81%). Three classes of F2 embryos, normal, shrunken, and underdeveloped were formed on reciprocal F1s and the frequencies did not differ between reciprocal populations. Thus, the interactions between embryo and endosperm and/or maternal parent rather than cytoplasmic-nuclear effects seem to be important in the determination of the extent of embryo growth. The examination of pollen fertility of F2 plants and the development of F2 and F3 embryos suggests that the formation of abnormal embryos and reduced male fertility are independent events. The P. vulgaris - P. coccineus crosses may be useful in studying the possible involvement of interspecific differences in hormonal metabolism in the development of hybrid embryos.