Section 13 — Plant Genetics and Breeding 



generations, 4 mutations have been produced: 

 vp YW (I); vp YW (II); vp YW (III); vp YW 

 (IV) (the three last ones were spontaneous), 

 each one conditioning the viviparous character 

 or premature germination of the embryo. The 

 former mutations, with the exception of the last 

 one, are not allelemorphs. The 4 mutations 

 behave as simple recessive genes in the various 

 genetic environments tested. 



In this pedigree apparent cases of homozygous 

 double crossing-over appeared, (theoretically 

 probable) without interference, once in a million 

 cases. Mosaics in aleurone color due to instabi- 

 lity of the A2 gene. Progeny with necrosed 

 grooves. 



To be able to interpret the conjunction of these 

 phenomena in closely related individuals, we 

 believe that there are interactions between cer- 

 tain loci with metabolic states or with molecules 

 of variable localization of specific mutagenic 

 action. 



The mutant genes vp which determined a 

 specific mutational tendency were saved, and 

 crossed with lines from other stocks; they be- 

 haved as simple genes of monogenic mendelian 

 segregation. The specific mutational tendency, 

 even only in a limited number of generations, 

 and in exceptional lines of a species, could clear 

 up some of the phenomena of evolution. On the 

 other hand, families with a certain advantageous 

 mutational tendency could probably be used in 

 breeding objectives. 



13.68. On a Spontaneous Mutation of the Cauli- 

 flower-head Type in Medicago sativa L. A. Pa- 

 nella, F. Lorenzetti, A. Mariani and G. M. 

 Briganti (Perugia, Italy). 



On a wide program carried out at the Plant 

 Breeding Department, University of Perugia, 

 Italy, regarding methods and techniques applied 

 in alfalfa breeding, in single plant selfed proge- 

 nies, the appearance of a spontaneous mutation 

 of the cauliflower-head type was observed. 



A morphological and cytological research was 

 made on the mutant material along with a 

 study of the hereditary behaviour in three subse- 

 quent selfed generations. 



Morphological and cytological description of 

 the mutants and data gathered on the segregat- 

 ing material are discussed. 



13.69. A New Inhibitor of Aleurone and Plant Colour 

 in Maize. N. K. Notani and Chandra Mouli 

 (Bombay, India). 



A new inhibitor (proposed symbol h) of aleur- 



one color has been discovered in maize. Its mode 

 of action is somewhat different from the only 

 other known inhibitor — C 1 . Whereas C 1 C 1 when 

 reciprocally crossed with homozygous colored 

 aleurone stocks (AA CC RR) gives only color- 

 less aleurone, h h does so only when used as 

 the female parent. In the reciprocal cross, hh 

 only partially inhibits pigment formation. Oc- 

 casionally a sector of lighter colored kernels 

 has also been noted on such cobs. 



The hh stock also has the property of inhibit- 

 ing plant color when crossed to intense — red 

 {A A BB PIP1) plants. The extent of inhibition 

 varies from partial to complete. Comparison 

 between the results from reciprocal crosses has 

 shown slight differences in inhibition. 



13.70. The Heterogametic Sex in Dioecious Flower- 

 ing Plants. J. N. Hartshorne (Manchester, 

 Great Britain). 



Dioecism in the angiosperms has probably 

 arisen independently on many occasions. Of the 

 13 species or groups of species where the sex- 

 determining mechanism is known, all except 

 one have heterogametic males, although the 

 animal kingdom shows that female heterogam- 

 ety is an effective alternative. There are three 

 possible explanations for this preponderance 

 of male heterogamety. The first is that by coin- 

 cidence an excess of species of the one type 

 happen to have been investigated. The second 

 is that dioecism based on male heterogamety 

 evolves more readily than the alternative. The 

 third is that although both systems are equally 

 likely to arise, male heterogamety confers an ad- 

 vantage which increases its chance of persisting. 



Evidence supporting the third possibility 

 comes from Correns's demonstration that com- 

 petition between two sorts of pollen leads to 

 an excess of females in the progeny. Since only 

 females produce seed, and a single male can 

 fertilize several females, this arrangement could 

 help to ensure survival of the species. Experi- 

 ments to test for Correns's certation effect, using 

 natural agencies of pollination, are being car- 

 ried out with dioecious species of Melandrium, 

 Mercurialis, Rumex and Silene. Results so far 

 do not confirm Correns's observations, but show 

 that: 



(a) the quantity of pollen reaching the stigma 

 has no effect on the sex-ratio of the prog- 

 eny; 



(b) sex-ratios either do not deviate signifi- 

 cantly from 1 : 1 or in some species show 

 a significant excess of males. 



232 



