APOMIXIS 



Stood by all mcndelians; a distinction to which wc shall have to 

 return. 



It was the mutations, of course, which first attracted De Vries' 

 attention to Oenothera lamarckiana in 1886 and led him to propound 

 his Mutation Theory of Evolution. What part do the famous 

 mutations in fact play in this genetic system? They represent the 

 breakdown in the mechanism of the true-breeding hybrid. The ring 

 of chromosomes sometimes fails to arrange itself alternatingly on 

 the spindle, and germ cells are then produced with 8 and 6 instead 

 of only 7 chromosomes. Hence the series of trisomies with 15 

 chromosomes of which 68 types are to be expected in Oenothera 

 lamarckiana, whose chromosomes normally form a ring of 12 and 

 one pair. Again, segregation may fail altogether and unreduced germ 

 cells will then give triploids and tetraploids. And finally the ring 

 may break down by crossing-over, as we saw, to give new half- 

 heterozygous forms. The mutants o£ Oenothera are therefore nothing 

 more than symptoms of its peculiar hybridity and as such are of 

 little significance in evolution. Later we shall see the ways in which 

 the Oenothera system is significant. 



Apomixis 



The third way in which a high degree of hybridity may be main- 

 tained while the ancient system of crossing is allowed to lapse, is 

 by apomixis, the suppression of sexual reproduction. Apomixis 

 comes about in two ways. Some organisms achieve apomixis, others 

 have it thrust upon them by the circumstances of their birth. The 

 first are of less importance to us. Many organisms, in producing 

 their female spores or eggs under certain external conditions, are 

 compelled to forgo meiosis. A single mitotic division replaces 

 meiosis and gives a diploid egg which can develop without 

 fertilization by a male germ cell. 



So is it with many blackberries and other plants. So it is also with 

 the summer-brood females of aphides, which thereby continue to 

 multiply vegetatively like a plant propagating itself by suckers.When 

 the colder weather comes, a first symptom of meiosis begins to 

 appear in some females. One of the two sex chromosomes is lost 

 at the single maturation division of the egg, so that diploid XO eggs 



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