274 



INTRODUCTION TO CYTOLOGY 



tion")- This point has been especially neglected in many investigations 



of meiosis in hybrids. 



Recent studies have shown clearly that normal synapsis is due 



primarily to an interaction of small homologous portions of the two 



chromosomes rather than of the chromosomes as wholes (Gelei, 1921). 



Evidence for this is not easy to obtain in normal diploid organisms, but 



it is inescapable in certain other types to be dealt with in later chapters. 



A striking case is afforded by a strain of maize in which a small portion 



of one of the homologues has been "deleted." 

 At the time of synapsis the normal chromosome 

 pairs closely with the deleted one along the 

 regions present in both of them, while the region 

 having no counterpart in the deleted chromo- 

 some extends laterally as an unpaired loop (Fig. 

 182, c). Moreover, it sometimes happens that 

 an exchange of segments between non-homolo- 

 gous chromosomes gives rise to a condition in 

 which two regions of one chromosome are 

 homologous respectively with two different 

 chromosomes. Synapsis occurs here between 

 homologous parts and results in a group of 

 three or more chromosomes. Again, a small 

 free fragment of a chromosome tends to synapse 

 at the proper time with the portion of the 

 chromosome with which it would have paired. 



It is now a workable 

 hypothesis that the synaptic reaction occurs 



synapsed with one of the primarily between "homologous" genes, or 



others. All three are closely i, ■■, -ii-- tj. •••j. 



associated at the spindle- between substauces m their immediate vicmity, 

 attachment region. {From a the Synaptic pairing of whole chromosomes thus 



preparation by McClintock.) -u • j.i. ix ^ r xi j. x- 



being the resultant oi the separate synaptic 

 reactions of their many constituent genes. 



There is reason to believe further that the synaptic reaction is one 

 which occurs characteristically and fully between two elements only at 

 one time at a given locus in the chromosomes concerned. This is indi- 

 cated in an interesting way by synaptic behavior in polyploid plants, 

 which have more than two chromosomes of a kind.^^ In triploid tulips or 

 maize, for example, all three of the homologues come near to one another, 

 but the intimate synaptic union appears characteristically between two 

 only in a given region (except often at the spindle-attachment point) 

 (Fig. 159). The third one may be synapsed with either of them at 



^^ M. Lesley (1926) on Lycopersicum, Newton and Darlington (19276, 1929) 

 on Tulipa, Darlington (1929c, 1930c, 1931a) on Hyacinthus and Primula, McClintock 

 (unpubl.) on Zea. 



Fig. 159. — Trivalent chro- 

 mosome in early meiotic pro- 

 phase in Zea. Note that any 



one of the three members had it never bccomc free. 



shows clearly its two chro 

 matids only where it is not 



