CYTOLOGIGAL ASPECTS OF HYBRIDITY 217 



The genomic composition of a hybrid usually has well-known conse- 

 quences. Among these are the appearance of certain parental characters 

 to the exclusion of others, the appearance of a condition intermediate 

 between the parents as in "blending" or quantitative inheritance, the 

 production of gametes unlike in genetical constitution, and often a reduc- 

 tion in the degree of fertility. Hybrids resulting from narrow crosses, 

 such as those cited as illustrations of Mendelian heredity in Chap. XII, 

 commonly show regular chromosome behavior and good fertility. It is 

 mainly in hybrids resulting from wider crosses, i.e., in hybrids in the 

 traditional sense, that more extensive genie and structural differences in 

 the chromosomes lead to the cytological and genetical abnormalities to 

 be reviewed in this chapter. 



Chromosome Behavior in Diploid Hybrids. — The fundamental reason 

 why it is often difficult or impossible to obtain hybrids between members 

 of different species or genera lies in the genie dissimilarity of the parents. 

 The effects of this dissimilarity are various. The pollen of one species 

 may not grow successfully in the style of the other; the gametes, if they 

 meet, may not actually fuse; if fusion does occur, the zygote may fail to 

 develop because of disharmony within its chromosome complement or 

 between it and the surrounding tissues. In certain cases, on the other 

 hand, the cross results in a hybrid that develops well, even with greater 

 vigor than was shown by the parents. The two parental genomes, in 

 spite of their differences, may thus constitute a single harmonious system 

 during ontogenetic development. 



In most such hybrids, abnormalities appear at some stage in the 

 development of reproductive cells that render them partially or com- 

 pletely sterile. Although degenerative changes may set in at an early 

 stage of flower development, the most characteristic cytological aberra- 

 tions appear during the meiotic prophase and affect especially the course 

 of synapsis. Synapsis may be normal, indicating a close genie similarity 

 in the genomes. More often synapsis fails to occur between some or all 

 of the chromosomes (asynapsis), or after synapsing the chromosomes 

 may separate prematurelj'' (desynapsis) . Although asynapsis usualh'^ 

 indicates a considerable degree of genetical dissimilarity in the genomes, 

 it does not always do so, for even in pure lines and narrow crosses synapsis 

 sometimes fails because of certain mutant genes influencing the course of 

 meiosis or because of temporary physiological conditions induced by the 

 environment. Obviously, caution must be used in depending upon 

 synapsis as a criterion of relationship. 



The usual consequence of normal synapsis is regularity in the ana- 

 phasic distribution of the chromosomes, each spore and gamete bearing a 

 complete genome. This is distinctly favorable to fertility, yet it does not 

 guarantee it, for some of the genomes produced may haA'e chromosomes 



