BRrF.DING SYSTEMS 



action. Moreover the properties of the pollen itself are determined 

 by the single allelomorph carried in its own nucleus after segregation : 

 tliere is no delayed effect from the other allelomorph present in its 

 diploid parent (and in its sister pollen). This rapid and specific action 

 puts one in mind of the relation between gene and antigen in the 

 determination ot blood groups. The analogy is still more evident 

 from the effect of a rise of temperature which, so Lewis found, 

 speeds up the growth of compatible pollen, yet slows down the 

 growth of the incompatible. Incompatibility is thus due to a positive 

 blocking reaction. 



In the haploid fungi, heterothaUy seems to work in the same way 

 as incompatibility in the flowering plants. In Coprinus rostntpianus, 

 for example, multiple allelomorphs exist within the species which 

 prevent the fusion of likes. In Coprinus lagopus and elsewhere there 

 are two series of allelomorphs, similarity in cither of which is 

 sufficient to prevent fusion (Fig. 60). This double scries in the haploid 

 plants, as opposed to the universal single series of allelomorphs in 

 the diploid plants, is significant. In the diploid plants the action of 

 the S gene in the style prevents any fusion of gametes from the 

 same diploid mother. In the haploid plants such fusions can never 

 be wholly prevented (Fig. 61). Indeed, with a single series of alle- 

 lomorphs, sister gametes can fuse in half the cases. A double series 

 is needed to reduce this chance to a quarter. A third series, of which 

 no case is known, would reduce it only to an eighth. Thus the 

 different systems of incompatibility genes are clearly related, we may 

 say adapted, to their effect in reducing inbreeding, or enforcing 

 outbreeding, in organisms with different systems of reproduction. 



Hctcrostyly 



Not all systems of incompatibility depend on the genetically 

 autonomous behaviour of the haploid pollen grains. There is another 

 type in which the properties of the pollen are controlled by the 

 diploid plant bearing it. The control is exercised by way of the 

 differentiation of the flower. The morphological aspects of this 

 system have long been recognized under the name of hctcrostyly 

 proposed by Hildebrand in 1864, Its physiology was first described 

 in detail by Darwin in his "Forms of Flowers" in 1877. 



248 



