THE INTERNAL ORIGINS OF ISOLATION 



inbreeding, besides combining the bases of variation from the 

 two parents. 



Polyploidy may also offer other advantages. An autopolyploid, 

 derived by the simple doubling of the chromosome number of an 

 existing species, can show physiological properties different from 

 those of its parent, while yet maintaining the balance, the capacity 

 for working as an integrated whole, of that parent. The drawback 

 of autopolyploidy, especially in aimuals, where seed production is 

 needed for maintenance, is the reduced fertility which follows 

 multivalent formation at meiosis. This may, however, be avoided 

 by restriction of pachytene pairing or of chiasma formation, so that 

 only bivalents occur, as is seen in the autotetraploids Lotus 

 corniculatHS and Tulipa chrysantha. 



Allopolyploidy avoids recombination within whole sets of 

 chromosomes. Interchange, in its turn, offers a means of avoiding 

 recombination of different individual chromosomes within the set, 

 so that two pairs have one linkage group. As we have already seen, 

 the recombinant gametes from interchange heterozygotes are lethal 

 in plants, because they show simultaneous duplication and deficiency. 

 In animals the zygotes to which such gametes give rise are lethal for 

 the same reason, except where, either by a rare chance or by 

 deliberate experimentation, fusion is with another gamete showing 

 complementary unbalance. Those gametes which, on the other 

 hand, carry a parental combination of chromosomes do not suffer 

 from this handicap, and consequently enjoy a greater chance of 

 success. Thus recombination of the chromosomes involved in the 

 interchange is rarely or never effected. 



In one sense the suppression of recombination in interchange 

 heterozygotes may be regarded as due to the exaggeration of the 

 iU-effects of all recombination. At the same time, in Oenothera 

 species and their like, we can see how, by the additions of differ- 

 entially balanced gametic lethals and of that competition between 

 potential embryo-sacs which we call the Renner effect, the loss of 

 fertiHty consequent on this suppression may be overcome. The 

 mechanism is then adjusted to give a situation resembling that in 

 polyploids, except that interchange abolishes recombination between 

 any number of chromosomes from two up to the whole set. Thus 

 interchange, like polyploidy, affords a means of attaining the 



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