Interspecific crosses among 27 species of Nicotiana, including all six S.I. species of the genus, revealed a complex, but highly systematic pattern of unilateral incompatibility, apparently encompassing the whole genus. There was a definite relationship between the type of incompatibility and the site of inhibition: in interspecific incompatibility the inhibition occurred in the stigma, in intraspecific incompatibility it occurred in the style.An hypothesis of the S-gene evolution explaining intra-and interspecific incompatibility, and having the following salient features, is proposed: (1) There are two kinds of specificity, each governed by a separate complex of genetic elements, involved in the incompatibility reaction -a primary speci/icily controlling interspecific incompatibility and a secondary speci/icity controlling intraspecific incompatibility. (2) The primary specificity developed early during the evolution of gymnosperms, as a means of avoiding indiscriminate fertilization by anemophilous pollen, thus combating, (i) chiefly, the sterilization of their "naked" ovules through pollination by relatively distant species ; and (ii) less significantly, the relatively easy introgression from closely related species. The primary specificity complex evolved, through duplication and redifferentiation, initially from a single specificity cistron. (3) The secondary specificity developed early during the evolution of angiosperms, as a means of excluding self-fertilization. It arose through a duplication of the entire set (or a major portion) of primary specificity cistrons. Redifferenfiation of the duplicate complex produced a large number of alleles controlling intraspecific incompatibility.Self-compatibility alleles and polymorphism of the S-gene are explained in terms of mutations and subsequent natural selection favouring different forms of the S-gene complex. The interspecific incompatibility patterns reflect the differences in the architecture of the S genes of the parents.The parallel, supposedly independent, evolution of the gene complexes controlling breeding behaviour in widely different phyla of the plant kingdom has been cited as an example of "channelling" in evolution. It is suggested that the rise of a primeval, specificity element in the nucleus, early in the evolution of plants, produced, through duplication and redifferentiation, an easy means for the evolution of diverse sexual mechanisms in plants.
Parthenogenetic development of seeds after pollination with irradiated pollen was studied in the dioecious species Actinidia deliciosa (kiwifruit). In total, 479 pollinations were made involving three recipient female cultivars, with five male and two hermaphrodite pollen donors. Pollen was irradiated with doses of 0.5, 0.7 and 0.9 kGy, all three doses produced parthenogenetic seeds. The 0.7 kGy dose yielded the highest number of germinating seeds (708 of a total of 723), of which 609 developed into seedlings, and 334 survived to grow into plants. Ploidy level was evaluated by cytological studies and stomatal guard cell size. In a total of 416 seedlings and plants evaluated, 332 plants were hexaploid ('diploid' 2n = 170) and 84 plants were triploids ('haploids' 2n = 85).
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