218 FUNDAMENTALS OF CYTOLOGY 



that do not constitute a harmonious system. Some of the foregoing 

 points are iUust rated in the following cases. 



Maize (Zea mays) and Alexican teosinte {Euchlaena mexicana) each 

 have 10 chromosomes in the gamete. AVhen the two are crossed, the 

 resulting hybrids vary in fertility depending in part upon what race of 

 teosinte is used. Synapsis tends to be regular (Fig. 162), and crossing 

 over takes place. In subsequent generations some of the plants show 

 various parental character combinations, while others are precisel}^ like 

 maize or teosinte. This indicates a degree of cytological and genetical 

 similarity unusually high for plants assigned to different genera. On the 

 basis of a vaiiety of e^ddences it has recently been proposed that the two 



species of Euchlaena be transferred to 

 the genus Zea. 



Chromosomal compatibility and 

 fertility are also shown in various 

 degrees in crosses of European and 

 American species of grape, sycamore, 

 and larch, as well as in certain inter- 

 generic moth hybrids. After crossing 

 the garden pea {Pisum sativum) with. 

 P. humile, each with seven chromo- 

 FiG. 162 —Pachytene stage ill micro- ^^^^^ ^^ ^^le gamete, the hybrids in 



sporocyte of a teosmte-maize /< i hybrid, . 



showing the nearly perfect synapsis Subsequent generations show regular 

 of the chromosomes. {After A. E. nieiosis, but the Occurrence of abnor- 



Longley.) 



mal types and much sterility indicates 

 disharmonies in many of the complements produced by the random 

 assortment of the parental chromosomes. 



When the foxglove (Digitalis purpurea) is crossed with D. ambigua, 

 each having 14 chromosomes in the gamete, the number of bivalents 

 formed at meiosis in the hybrid varies from 5 to 12, the rest of the chromo- 

 somes remaining univalent. Such behavior is exhibited by many other 

 interspecific hybrids, and the irregularity may vary in amount with the 

 cultural conditions. One result of this partial asynapsis is irregular ana- 

 phasic distribution: the bivalents disjoin normally, but the univalents 

 either pass in various numbers to the poles or undergo equational division. 

 Further irregularity follows in the second division, so that numerous 

 abnormal complements and much sterility result. Irregularity in chro- 

 mosome distribution often leads to the formation of microspore groups 

 comprising spores varying in size and number (polyspory) instead of 

 normal quartets. Such a condition can, however, arise from temporary 

 environmental causes such as extreme fluctuations in temperature; 

 consequently, nonuniform pollen is not a sure sign of hybridity. Along 

 mth the in viable chromosomal combinations are some that are successsful. 



