200 



FUNDAMENTALS OF CYTOLOGY 



The principal cytological element in the new explanation is the fact 

 that 12 of the 14 chromosomes occupy constant positions in a ring at 

 meiosis and show a regularly alternate mode of disjunction in a high 

 percentage of the sporocytes (Fig. 148). The main genetical element 

 in the explanation is the evidence that there are two groups of genes, 

 known as Renner complexes, that segregate in sporogenesis and that each 

 complex carries a gene which prevents development when in the homo- 

 zygous state (Fig. 149). If the two Renner complexes (designated as 

 gaudens and velans in this species) are located, respectively, in the two 

 groups of seven chromosomes regularly passing to opposite poles in meiosis 

 (six from the ring, plus one member of the free pair carrying similar 

 Renner complex factors), it is possible to account for the production of 



Fig. 149. — Diagram of cytogenetic constitution of an evening primrose, Oenothera 

 lamarckiana. A, ring-of-12 and one free pair at diakinesis. Factors of Renner complex 

 gaudens (G) represented by large dots, those of velans complex (F) by circles, those common 

 to both complexes by shaded circles. Factors ordinarily recombining by crossing over 

 represented by small dots. B, segregation of two Renner complexes by alternate disjunc- 

 tion of chromosomes in ring at anaphase /. C, effect of lethal factors in Renner complexes: 

 of three possible combinations only the heterozygotes (GF) survive. 



spores and gametes of two main classes : those carrying the gaudens com- 

 plex and those carrying the velans complex. Random combinations of 

 male and female gametes of these two kinds produce zygotes of three 

 kinds, but since the development of two of these is prevented by homo- 

 zygosity of lethal genes, the only plants appearing are of the original 

 heterozygous 0. lamarckiana type. Thus the type breeds true not 

 because it is a pure species, but because it is a hybrid whose homozygous 

 offspring do not survive. 



The mutations which occasionally appear in 0. lamarckiana have 

 been found to have several causes: (1) gene mutation of the ordinary 

 kind; (2) the removal of a lethal gene from a Renner complex by crossing 

 over; (3) the occasional nonlethal action of a lethal gene; (4) nondis- 

 junction, giving trisomic and other types; (5) chromosome doubling. 



Other species of Oenothera also exhibit the results of reciprocal translo- 

 cation, and relationships are strongly suggested by the ways in which the 



