THE CYTOGENETICS OF AUTOHETEROPLOID PLANTS 



353 



involves a close union of only two of the three homologues, although one 

 of them may be synapsed with both of the others in different regions 

 (Figs. 159; 199, a to/). As chiasmata develop, the trivalent may assume 

 a rather complicated structure. If the three members remain associated 

 until the metaphase, the trivalent usually appears as a compact but 

 asymmetrical triple body in the equator of the spindle; but if only two 

 of the three chromosomes synapse or remain together, one of them may 

 appear at metaphase as a univalent while the two others constitute a 

 bivalent (Fig. 201, a). The chiasmata or points of contact between the 

 members of the trivalent at diakinesis and metaphase may be situated in 

 various positions along their length in some plants {Hyacinthus, Fig. 199), 

 while in others the association is practically always terminal {Datura, 



Fig. 201. — Chromosomes in heteroploid maize. <i. nataphase /, showing three triva- 

 lents, seven bivalents, and one univalent. (Spot at upper left is not a chromosome.) b, 

 three trivalents in metaphase. c, trivalent disjoining two from one. d, tardily split uni- 

 valents ready to separate at equator of spindle. (After McClintock, 1929a.) 



Fig. 200). The genetic data indicate that crossing-over may occur 

 between all three chromosomes (in different regions), that it takes place 

 when six chromatids are present, and that crossover values between 

 certain genes differ somewhat in diploids and triploids.'^ 



In the first meiotic mitosis the three members of a trivalent tend to 

 disjoin at random, except in those regions near the spindle-attachment 

 point where crossing-over may affect the type of disjunction. In the 

 second mitosis the halves of all of the chromosomes pass to opposite poles. 

 Often one chromosome of the group lags and is lost. Typically, as in 

 triploid Zea, the different trivalents are variously distributed in mitosis 

 / (Fig. 201), so that spores are formed with chromosome numbers ranging 

 from the monoploid to the diploid. Although many of the microspores 



' Bridges and Anderson (1925) and Redfield (1930, 1932) on Drosophila. 



