ISOLATING MECHANISMS AND SPECIES FORMATION 



other. In other words, most species of Oenothera are highly heterozygous. 

 Hence two types of contrasting offspring are formed when any other spe- 

 cies of the genus is crossed to hookeri. Yet, in spite of their heterozygosity, 

 the plants breed true when selfed. Independent assortment is prevented 

 by the alternate distribution of the chromosomes of the rings, which re- 

 sults in all of the translocated chromosomes being segregated into one 

 class of gametes, while all of the untranslocated chromosomes are segre- 

 gated into another class of gametes. Only parental type zygotes form the 

 progeny, because the homozygous ones are eliminated by the balanced 

 lethals. 



The problem of the absence of cross-over effects in Oenothera crosses 

 is somewhat more complex. An inspection of the diagram of the pachytene 

 chromosomes of translocation complexes involving several pairs of chro- 

 mosomes will reveal that each chromosome consists of two ends which are 

 perfectly paired and a central segment which is mechanically held out of 

 contact with its homologue. It is evident that crossing over should occur 

 normally in the pairing arms of such chromosomes. But the pairing arms 

 appear to be largely homozygous throughout the genus, with the result 

 that crossing over does not have genetic effects. The central segments, 

 however, are protected from crossing over because homologous parts are 

 prevented from establishing synaptic contact. The genetic differences be- 

 tween the various chromosome complexes in the genus are concentrated 

 in these segments, and so the sets of genes characterizing the various 

 complexes are highly permanent. Rarely, crossing over does occur in the 

 central segments, and when it does, radically different character combina- 

 tions occur. These are the "half mutants" ( apparently because they affect 

 half of the progeny) of early genetic literature. Thus the distinctive gene- 

 tic features of the genus Oenothera are simple results of the fact that these 

 plants are permanent translocation heterozygotes. 



Chromosomal Mutants of Datura. In the genus Datura, the Jimson 

 weeds, Blakeslee and his collaborators have found translocation phenom- 

 ena which are, in some respects, even more complicated than those found 

 in Oenothera. The normal chromosome number is 24, so that there should 

 be 12 pairs of chromosomes on the metaphase plate of the first maturation 

 division. This is generally the case, but in interracial and interspecific 

 crosses, rings of four and of six commonly are found. This shows that the 

 various species, and the races within a single species, differ by a few 

 translocations for which the several races and species are themselves 

 homozygous. Each translocation complex appears to be associated with 

 a different phenotype. 



Trisomies are also common in this genus, that is, one chromosome of 

 the set may be present in triplicate, making a total of 25 chromosomes 



Figure 105. Trisomics in Datum. The center column shows seed capsule, meiotic 

 chromosomes, and plant for the primary trisomic 2n -|- 1'2, called "Rolled." The left 

 hand column shows one of its secondary trisomics, 2n + l-l, or "Polycarpic." The right 

 hand column shows its other secondary, 2n + 2*2, or "Sugarloaf." ( From Blakeslee, 

 A. F., /. Hered., V. 25, 1934. ) 



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