CYTOLOGICAL ASPECTS OF HYBRIDITY 223 



cottons, (26 + 13) X 2 = 78. In the mints and cottons the new types 

 were developed with the aid of colchicine. 



Ampliidiploids sometimes arise in ways other than by somatic chromo- 

 some doubling. Diploid spores and therefore gametes may appear after 

 haplosis has failed in sporogenesis in a diploid hybrid, two diploid gametes 

 then uniting. Although the chance of obtaining such plants in this 

 manner seems to be relativelj' small, they have evidently arisen thus from 

 interspecific diploid hybrids in Nicotiana, Triticum, and Digitalis, and in a 

 Raphanus-Brassica hybrid. A recentl}^ observed case is that in Madia, 

 a genus of western composites. Two rare and self-sterile species, M. 

 nutans {n = 9) and M. Rammii (n = 8) produced a nearly sterile diploid 

 hybrid (2/( = 17). Among several types in Fo there were two plants with 

 34 chromosomes which showed almost perfect synapsis into 17 bivalents. 

 After four generations the plants of this line were vigorous, fertile, true 

 lireeding, and different in several morphological characters from their 

 parents and all other species of the genus. 



A third important method is illustrated by amphidiploid snapdragons 

 (Antirrhinum) obtained by crossing two different autotetraploid strains. 



It should now be evident why polyploid hybrids confront the geneticist 

 with difficulties, as suggested earlier in the chapter. Even in the typical 

 amphidiploids with two genomes from each parent, much depends upon 

 the manner and degree in which synapsis is carried out. In the most 

 nearly true breeding of them the homologous chromosomes from the same 

 parent form bivalents, while those from the other parent do likewise 

 (autosynapsis). The result is that the genomes of the parent species 

 remain distinct in successive generations, the hybrid therefore breeding 

 true for characters due to interspecific hybridity. The genetical results in 

 such a case, where the two parents seem to have had relatively few genes 

 in common, are as though the plant were diploid with a large chromosome 

 number: the Mendelian characters of each original parent continue to 

 show disomic ratios. If the particular genes concerned are present in the 

 chromosomes from both parents, there appear certain tetrasomic ratios 

 like those in experimentally induced autotetraploids (page 208). When 

 some or all of the chromosomes from one parent synapse with members 

 from the other parent (allosynapsis) , the genetical data become even more 

 difficult to analyze. Since two crossable species may have genes in 

 common as well as unlike genes, the type of synapsis, degree of fertihty, 

 and genetical ratios tend to be variable in polyploid hybrids generally, 

 even in the most regular amphidiploids. 



Cytological Types of Hybridity. — The foregoing descriptions have 

 been based upon the essential feature of hybridity as it appears in plants 

 or animals with two or more genomes: the presence in the nucleus of 



