360 INTRODUCTION TO CYTOLOGY 



of the zygote at some stage, or to unfavorable relations existing between 

 the embryo and the surrounding tissues. Even when a hybrid is obtained 

 it may prove to be sexually sterile, as pointed out above, and so fail to 

 maintain its type. 



In earlier chapters we have dealt with ordinary Mendelian hybrids 

 and with those special cases in which heterozygosity is automatically 

 maintained by virtue of peculiar meiotic mechanisms and lethal factors 

 (p. 336). The present chapter is mainly a series of examples of the more 

 prevalent modes of chromosome behavior which have been observed 

 in hybrid plants derived from fairly wide crosses. The chief interest 

 of these phenomena is in the light they throw upon the causes of normal 

 chromosome behavior and upon the problem of the origin of new stable 

 and fertile types through hybridization. Most of the researches so 

 far cari'ied out in this field have dealt with meiotic phenomena and their 

 genetic results.^ Somatic chromosome complements have also been 

 studied, but relatively little attention has been devoted to syngamy in 

 such plants. 



Diploid Hybrids with Good Synapsis and Fertility. — When Zea Mays 

 (n = 10) is crossed with the Durango or the Chalco race of Euchlcena 

 mexicana {n = 10), the fertile Fi hybrid shows 10 bivalents which behave 

 regularly in meiosis. In subsequent generations the parental types are 

 recovered among others, indicating a random distribution of the chromo- 

 somes and the occasional reconstitution of essentially normal Zea and 

 Euchlcena sets. Furthermore, some crossing-over occurs between the 

 Zea and Euchlama chromosomes. This high degree of chromosome com- 

 patibility, which is somewhat exceptional in interspecific or intergeneric 

 hybrids, doubtless indicates a close natural relationship between the two 

 parents. When a Florida race of Euchlcsna mexicana is crossed with 

 Zea, the hybrid shows some unequal chromosome pairs and considerable 

 sterility. Triploid hybrids formed by crossing Zea with the tetraploid 

 Euchlcena perennis show much meiotic irregularity and a rather high 

 degree of sterility.^ 



In the Lepidoptera fertile interspecific and intergeneric hybrids are 

 frequently obtained. For example, a hybrid between Charocampa 

 elpenor {n = 29) and Metopsilus porcellus (n = 29) shows regular synap- 

 sis and fertility, as well as the combined characters of the parents. In 

 moth hybrids the amount of synapsis varies with the closeness of relation- 

 ship of the parents (Federley, 1923, 1928). 



That good synapsis alone does not insure complete fertility is shown 

 by hybrids between Pisum humile (n = 7) and P. sativum (n = 7). 



^ For a more extensive account of chromosome behavior in polyploids and hybrids, 

 see Darlington (1932a). 



^ Data in this paragraph are from Kuwada (1919), Longley (1924c), Beadle (1932c), 

 and Randolph. 



