BREEDING SYSTEMS 



is coiifirnied when we look at a closely related cereal. Rye pushes 

 out its anthers and stigma in just the same way; but instead of 

 vitiating the crossing mechanisms by premature bursting, it rein- 

 forces it by incompatibility. Wheat has developed one way and 

 rye the other, after their ancestors diverged. Of peas the same story 

 can be told. To be sure, the cross-pollination was to have been by 

 insects, but its vitiation is still by premature bursting of the anthers. 

 And when we return to the mite Pediculopsis, premature mating 

 is again the means by which outbreeding is suppressed while the 

 sex system adapted to outbreeding is retained. 



Outbreeding, so far as we know, never supervenes on inbreeding. 

 Always it is the reverse. Complete inbreeding is evidently a dead 

 end. The reasons for this we shall touch on later. But there are 

 a variety of ways in which the effect of cross-breeding, the high 

 degree of hybridity, may be maintained while the ancient system 

 of crossing is allowed to lapse. One of these we have already seen 

 in the allopolyploid. A second method is that of the true- 

 breeding hybrid or complex heterozy^^ote. 



The Inbreeding Hybrid 



The true-breeding hybrid is known by the exact breeding analysis 

 of Renner, as well as by chromosome study, in a hundred or so 

 species of the American Evening Primrose, Oenothera. These species 

 are characterized by having 6, 8, lo, 12, or all 14 of their chromo- 

 somes linked in a ring at meiosis. They are interchange hybrids. 

 They are hybrid not for one only, but for two, three, four, five or, 

 in the extreme case of the complete ring, six, interchanges. Take 

 a ring of 6 resulting from two interchanges. We can represent its 

 chromosomes as follows: — 



a AB CD EF 



j8 BC DE FA 



The ring, in fact, usually arranges itself on the spindle in this 

 alternating way, so that two kinds of workable germ cell are formed, 

 both in eggs and pollen. They bear the two complexes a, or 

 AB 4- CD -h EF, and /3, or BC + DE + FA. When a complex 



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