Section 13 — Plant Genetics and Breeding 



types of N. rustica with such of N. tabacum, the 

 author succeeded in finding genotypes which 

 permit relatively easy crossing. With the help 

 of acenaphten treatment of Fi subjects a great 

 number of amphidiploids with partial fertility 

 could be obtained, offering thus the possibility 

 of observing many amphihaploid and amphidi- 

 ploid plants under normal field conditions. On 

 this occasion the inheritance of morphological 

 and some most important physiological and 

 agronomical characters was studied. It was 

 interesting to note the following particularities: 

 disappearance of the "aurea" character in Fi, 

 increased resistance of Fi hybrids to some 

 diseases and unfavorable environmental con- 

 ditions, a clearly expressed heterosis with 

 amphihaploids and diminution of this phenome- 

 non with amphidiploids. 



Testing of smoking qualities of fermented 

 leaves of amphidiploids N. rustica N. tabacum 

 gave positive results, especially with regard to the 

 use of such raw material as cigar filler. 



13.7. Incompatibility Among Species of Darwinia. 



Barbara G. Briggs (Sydney, Australia). 



Interspecific crosses in Darwinia (Myrtaceae) 

 have shown several cases of non-reciprocal 

 incompatibility acting at about the time of 

 fertilization. The twelve eastern Australian spe- 

 cies and subspecies may be divided into two 

 groups, within which the species are interfertile. 

 In one group the flowers are small and regularly 

 self-pollinate. The members of the other group 

 are also self-compatible but show considerable 

 outbreeding. Crosses between the groups are 

 fertile when an inbreeder is used as the female 

 parent. In the reciprocal cross, best studied in 

 the example D. fascicularis $ x D. bifiora $, 

 pollen of an inbreeder germinates on the stigma 

 and the growth of pollen tubes to the ovary is 

 unimpaired, but failure apparently occurs just 

 before fertilization. The combination of incom- 

 patibility and breeding behaviour prevents or 

 greatly restricts the formation of natural hybrids 

 between some combinations of species. The 

 genetical and physiological basis of the incom- 

 patibility reaction is unknown. 



13.8. Incompatibility in Betula verrucosa Ehrh. and 

 Betula pubescens Ehrh. Max Hagman (Maisa- 

 la, Finland). 



Self-incompatibility is found in both Betula 

 verrucosa and B. pubescens. The incompatibility 



inhibition occurs in the style and is expressed 

 through retarded growth of the pollen tube. 

 Interspecific incompatibility of the same type 

 occurs when the two species are crossed. How- 

 ever, the incompatibility reaction is more pro- 

 nounced when B. verrucosa is used as father. In 

 the reciprocal cross many of the combinations 

 hitherto made show a certain percent of B. pu- 

 bescens pollen tubes with good growth in the 

 style of B. verrucosa. This agrees with earlier 

 results that more filled seed is obtained when the 

 species cross is made in the direction B. verrucosa 

 $ > B. pubescens $• 



In the 25 combinations made with 5 trees of 

 B. verrucosa from the same stand there are 7 

 combinations indicating cross-incompatibility. 

 In 20 individuals from the distribution limit of 

 B. verrucosa in Northern Lapland and from 

 isolated islands in the southwestern Finnish 

 archipelago there was no tendency toward 

 increased self-compatibility. 



Low temperature slows down the incom- 

 patibility reaction so that fertilization may 

 occur after selfing. The same effect may be 

 reached through pollination at a late stage of 

 development of the female flower. The knowledge 

 of these facts may help the forest tree breeder 

 carrying out inbreeding as a breeding programme 

 for birch. 



There are serologically detectable differences 

 between the pollen of the two species. Individual 

 differences which could be used as a guide for 

 selecting compatible combinations are under 

 investigation. 



13.9. Radiation-induced Chromosomal Aberrations 

 which Simulate Point Mutation of Incompatibil- 

 ity Alleles. James L. Brewbaker (Honolulu, 

 U.S.A.). 



Self-incompatible flowering plants of the 

 gametophytic or Nicotiana type mutate with 

 comparative ease to self-fertile (SF) forms. 

 Mutants were sought in petunia following pie- 

 meiotic X-irradiations among 40 x 10 6 pollen 

 grains. A 16-fold increase in mutation was ob- 

 tained at 1000 r. Over 300 mutants were studied 

 cytogenetically. Most mutants were self-fertile, 

 and most of these reacted in crosses as if the 

 pollen activity alone had been lost ("pollen- 

 part" mutations). With few exceptions, these 

 plants carried supernumerary centric fragments on 

 which the S gene was located. Fragments varied 

 greatly in size, and could be shown to pair 

 with trisomes bearing the S locus. Subsequent 

 fragment loss was held to account for self- 



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