CHAPTER XXII 

 THE CYTOGENETICS OF HYBRIDS 



In modern genetics the principal criterion of hybridity is heterozygos- 

 ity rather than the relative taxonomic standing of the parents known or 

 supposed to have been crossed. The union of any two protoplasts 

 differing in hereditary potency, no matter how slight the difference, con- 

 stitutes an act of hybridization in the fundamental sense. Accordingly, 

 a hybrid is defined as an individual arising from the union of genetically 

 dissimilar gametes^ and maintaining the resulting heterozygosity in its 

 somatic nuclei. Moreover, its spores and gametes are commonly of more 

 than one kind. 



The degree of heterozygosity differs widely in different cases. It may 

 involve only one or relatively few genes, as in the ordinary intraspecific 

 hybrids so widely studied by geneticists; here there is usually no dis- 

 turbance of the normal pairing and distribution of the chromosomes in 

 meiosis. As will be shown below, the chromosomes in many interspecific 

 and even in some intergeneric hybrids may show a high degree of com- 

 patibility in both somatic development and meiosis, fertility not being 

 impaired in any considerable measure. In other instances, where the 

 chromosome sets brought together differ widely in genie composition, 

 the hybrid may undergo successful vegetative development and then 

 prove to be sexually sterile as a result of a partial or complete failure of 

 meiosis. This difficulty at the meiotic period commonly has its chief 

 visible expression in asynapsis or desynapsis (see p. 345), this being 

 followed by other irregularities which prevent the formation of func- 

 tional spores and gametes. Irregularities of this kind are even more 

 pronounced when the parental chromosome groups differ in number of 

 members as well as in genie composition. Deficient synapsis is not, how- 

 ever, wholly responsible for the sterility observed in hybrids, since 

 degenerative changes often begin well in advance of the synaptic period. 



As a general rule, one may expect the difficulty of obtaining a hybrid 

 to increase with numerical and qualitative dissimilarities in the chromo- 

 some complements of the parental types. Failure may be due to the 

 inability of the pollen tube to grow properly in a "foreign" style, to a 

 lack of gametic interaction, to a developmental incapacity on the part 



^ This holds for diploid and most other hybrids but not without qualification for 

 those of the amphidiploid type (see p. 369). It is understood that a hybrid individual 

 may also arise from a hybrid parent by vegetative means. 



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