Section 13 — Plant Genetics unci Breeding 



incompatible offspring (called "revertibles" by 

 some authors) among the mutants. Exceptional 

 SF mutants included several tetraploids, and 

 rare deletions. Colchicine-induced tetraploids 

 were selfincompatible only when homozygous for 

 S alleles (5 tested); all heterozygous tetraploids, 

 triploids, and S-gene trisomes were SF. The 

 following conclusions are drawn: Incompatibili- 

 ty mutations, in petunia at least, rarely if ever 

 involve intragenic change; aberrations which 

 lead to loss of pollen activity include duplication 

 (largely through centric fragments) of the locus 

 and deletion. The data do not support theories 

 of bipartite S gene structure or of revertible 

 mutations; they do encourage consideration of 

 the possibility that point mutations are not 

 expressed due to the probable multiple-stranded 

 structure of the chromosomal gene. 



13.10. Pollen-Style Incompatibility in Oenothera. 



Adolph Hecht (Pullman, U.S.A.). 



Self-incompatible races of Oe nothera organensis, 

 O. rhombipetala and O. caespitosa are being 

 studied by the author and his students in an 

 attempt to discovei the mechanisms of the 

 incompatibility reaction. Unlike most other 

 plants that have a gametophytically determined 

 incompatibility, the pollen of these species is 

 inhibited at the time of its germination on the 

 stigma, and few if any pollen tubes in an in- 

 compatible mating even reach the style. When the 

 cut ends of styles were reunited in a moist cham- 

 ber and their stigmas pollinated with compatible 

 pollen, the cut surfaces resulted in only a slight 

 inhibition to the growth of pollen tubes from 

 the upper (scion) portion into the lower (stock) 

 portion of the style. When stylar tissues incom- 

 patible with the pollen were used as the stock 

 portion of the graft, there was an appreciable 

 inhibition of the pollen tubes, but some of them 

 did grow several centimeters into this otherwise 

 incompatible tissue. Compatible pollen tubes 

 passed readily from the cut end of a style into a 

 stigma lobe which was also compatible with the 

 pollen, but when a stigma incompatible with the 

 pollen was used in this position below a cut 

 style, the pollen tubes were completely stopped 

 at the stigmatic surface. These experiments 

 show that the incompatibility reaction in these 

 species is considerably stronger in the stigma 

 than in the style. 



Work supported by National Science Foun- 

 dation Grant G-14056. 



13.11. A Theory of the Control of Complex Unilateral 

 Incompatibility in Lycopersicon hirsutum. 



Franklin W. Martin (Mayaguez, Puerto 

 Rico). 



Unilateral incompatibility is the ability of 

 two hermaphroditic plants to cross in only one 

 of the reciprocal combinations. The type consid- 

 ered here is similar to self-incompatibility in 

 that both are due to inhibition of pollen-tube 

 growth in the style. In Lycopersicon hirsutum and 

 L. esculentum this phenomenon has 4 levels of 

 expression. The various compatibilities occur in 

 a step-wise sequence suggesting a single form of 

 behavior with various levels of manifestation. 

 Measurements of inhibited pollen tubes indicat- 

 ed two kinds of variability among 5 accessions of 

 plants tested: variability in strength of an in- 

 hibiting reaction in the style and of a stimulating 

 reaction in the pollen. The presence of different 

 balances of stimulating and inhibiting substances 

 could account for this hierarchy of incompatibi- 

 lity reactions. This theory is consistent with 

 knowledge of the occurrence of such substances. 

 Evidence for the occurrence of different balances 

 of such substances has come from 4 other sources. 

 First, measurements of pollen tube growth 

 within styles showed variances associated with 

 degree of heterozygosity of a single, partially 

 imbred parent. Secondly, segregation for degree 

 of inhibition of pollen-tubes occurred among 

 plants of a single family. Thirdly, crossing among 

 selfincompatible plants from different geogra- 

 phical sources sometimes destroyed the balance 

 resulting in pseudo-compatibility. Finally, the 

 inheritance of unilateral compatibility was 

 conditioned by many minor genes in hybrids. 

 The body of evidence suggests polygenic control 

 of the quantity of growth substances, modified 

 by environmental factors. 



13.12. Changes in Pollen Proteins during Pollen 

 Tube Growth from the Incompatibility Point 

 of View. Jaroslav Tupy (Prague, Czechoslo- 

 vakia). 



Proteins of the pollen with different S alleles 

 are serologically distinguishable (Lewis, 1952; 

 Linskens, 1960; Makinen, Lewis, 1962). 

 This fact supports the antigen-antibody hypo- 

 thesis of the incompatibility reaction. The ger- 

 mination and the growth of pollen tubes in in- 

 compatible styles is normal during first hours 

 with the gametophytic system of incompatibility. 

 In Nicotiana alata the incompatible tubes often 

 attain the length of as much as 4 cm without any 

 inhibition. That is why we can expect changes in 



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