THE ORIGm OF SPECIES 



( 2n + 1 ) rather than the usual 24 in the zygote. Any one of the twelve 

 chromosome pairs may be so affected, and the phenotype of the plant 

 depends upon the particular chromosome which is present in triplicate. 

 Each is called a "prime type." Although any trisomic causes phenotypic 

 changes throughout the plant, they have been named on the basis of 

 changes in morphology of the seed capsules. At meiosis, the extra chromo- 

 some should be distributed to half of the gametes, but it tends to lag 

 behind the other chromosomes and be destroyed in the cytoplasm, and so 

 less than half of the gametes formed by a trisomic plant will transmit the 

 trisomic condition. As the n + 1 condition is a pollen lethal, it can be trans- 

 mitted only by the ovule parent. Such trisomies, in which a completely 

 normal chromosome is present in triplicate, are called primary trisomies. 

 All of the twelve possible primary trisomies in Datura are known. 



But secondary trisomies are also known, in which the extra chromosome 

 represents only one half of a normal chromosome, the half, however, being 

 duplicated (Figure 105). These again are associated each with a distinc- 

 tive phenotype. Twenty-four secondary trisomies are possible, but not all 

 of these have been found. Finally, there are tertiary trisomies, in which 

 the extra chromosome is a translocation product, being made up of halves 

 of two different chromosomes. These again are characterized each by a 

 distinctive phenotype. At meiosis, the extra chromosome binds together 

 the two tetrads with which it shares homology, thus making a ring of five 

 on the metaphase plate. 



Evaluation of the Data. It is thus clear that in some well demonstrated 

 instances homozygous inversions and translocations are among the char- 

 acters which differentiate races and species. Less detailed evidence is 

 available with respect to duplications and deficiencies ( Chapter 14 ) , yet 

 these rearrangements are known to be involved in the differentiation of 

 Sciara species, and differences in the metaphase chromosomes of many 

 insects for which the salivary gland chromosome technique is not avail- 

 able are most easily understood in terms of duplications and deficiencies. 



This brings us back to the question with which this discussion began: 

 to what extent do chromosomal rearrangements function as genetic iso- 

 lating mechanisms? And are they also responsible for the phenotype dif- 

 ferentiation of related species, or is this a matter of independently 

 accumulated gene mutations? 



With respect to the latter question, all geneticists agree that position 

 effects are a common result of chromosomal rearrangements. But many 

 rearrangements have been investigated without any evidence of corre- 

 sponding position effects coming to light, particularly in maize, perhaps 

 the most thoroughly known of all plants genetically and cytologically. 

 Hence the majority of geneticists doubt that chromosomal rearrangements 

 play a major role in the phenotypic differentiation of species. Singleton 

 has found it useful to disregard the distinction between chromosomal and 

 gene mutations, while Goldschmidt regarded the former as the basis for 

 systemic mutations of fundamental importance for speciation. 



With regard to the first question, there is less difference of opinion. 

 There seems to be no room for doubt that chromosomal rearrangements 



288 



