THE CYTOGENETICS OF HYBRIDS 



361 



Here meiosis is regular in Fi, F2, and F3, but the high sterility and the 

 appearance of abnormal types in F2 and F3 indicate that many of the 

 combinations of humile and sativum chromosomes formed by random 

 assortment function poorly or fail completely in development (Lutkov, 

 1930). Certain Digitalis hybrids are also sterile in spite of good synapsis 

 (Haase-Bessell, 1932a). 



Partial Asjoiapsis and Its Results. — Some varietal crosses in Viola 

 arvensis show regular Mendelian segregation, but others exhibit faulty 

 synapsis coupled with genetic irregularities. When V. arvensis (n = 17) 

 is crossed with V. tricolor {n = 13), the Fi hybrid shows various numbers 

 of bivalents and univalents, and some sterility. Later generations include 



'A 



A 



4 ^^-^5 ■ ' 6 7 ^-^ 8 



Fig. 206. — Chromosome behavior in F\ hybrid, Crepis capillaris X Crepis tectorum. 

 1, 2, 3, somatic complements of C. capillaris, C. tectorum, and C. capillaris X tectorum. 

 4, 5, first meiotic mitosis in hybrid. 6. 7, second meiotic mitosis. 8, group of microspores, 

 including an extra one formed after chromosome lagging. The satellite on the tectorum 

 D-chromosome does not show in the hybrid. (After H ollingshead , 1930a.) 



the parental types and a number of new ones which are constant and 

 fertile (J. Clausen, 1924, 1926, 1931c). 



Crepis tectorum (n = 4) X C. capillaris (n = 3) gives an Fi hybrid 

 in which all of the seven parental chromosomes can be distinguished on 

 the basis of their morphology (Fig. 206). For some reason, however, the 

 tectorum satellite is not evident in the hybrid. When one strain of 

 tectorum is used in this cross, three bivalents and one univalent appear 

 most often in meiosis, but when another strain is used this configuration is 

 least common. The univalents divide in either meiotic mitosis, and the 

 hybrid is almost completely sterile (Hollingshead, 1930a; M. Nawaschin, 

 19276). 



Synapsis in Relation to Polyploidy in Hybrids. — Highly characteristic 

 of many hybrids is the type of meiosis first observed in Drosera by 

 Rosenberg (1904a, 19096). When Drosera rotundifolia (10 gametic 

 chromosomes) was crossed with D. longifolia (20 gametic chromosomes). 



