HETEROSIS IN POPULATION GENETICS 153 



Among animals, the largest ])art of which are not sessile and therefore not 

 bound to the ground, the differentiation into two sexes offers the best solution 

 to the problem of insuring a wide range of crossing among different geno- 

 types. But even here we see that special behavior patterns have been de- 

 veloped for this purpose. These may be courtship relationships, sexual selec- 

 tion, dominance relationships among a group of animals, or protandry 

 mechanisms, where the presence of two sexes in hermaphrodites could reduce 

 the amount of outcrossing and therefore endanger the survival of the species. 

 Even among parthenogenetic animals, such as Cladoceran Crustacea, the ap- 

 pearance of sexual generations after a long succession of asexual ones seems 

 to depend upon extreme environmental conditions. For its survival, the 

 species must shift over to sexual reproduction in order to obtain a wider 

 range of genetic combinations, some of which might be able to survive under 

 the new set of conditions. 



At the level of the chromosome mechanisms, several examples of perma- 

 nent hybrids are known well enough to be sure that they play an im- 

 portant role for the survival of some flowering plants. In animals, too, some 

 similar mechanism may be present. In a European species of Drosophila 

 which we are studying now^, Drosophila subobscura, one finds that practically 

 every individual found in nature is heterozygous for one or more inversions. 

 It looks as if the species were a permanent hybrid. 



Rarely, though, one finds individuals giving progeny wath homozygous 

 gene arrangement. Such cases have been observed only three times: once 

 in Sweden, once in Switzerland, once in Italy; and they are very peculiar 

 in one respect. The three homozygous gene arrangements are the same, even 

 though the ecological and climatic conditions of the three original popula- 

 tions were as different as they could be. It looks as if the species could 

 originate only one gene arrangement viable in homozygous condition, and 

 that this may occur sporadically throughout its vast distribution range 

 (Buzzati-Traverso, unpublished). 



At this level too is the fine example of heterozygous inversions from the 

 classical studies of Dobzhansky (1943-1947). They have demonstrated that 

 wild populations of Drosophila pseudoobscura show different frequencies of 

 inversions at different altitudes or in the same locality at different times of 

 the year. Variation in the frequency of inversions could be reproduced ex- 

 perimentally in population cages by varying environmental factors such as 

 temperature. It is shown in such a case that natural selection controls the in- 

 crease or decrease of inversions determining an interesting tyj)e of balanced 

 polymorphism. Finally, according to the investigations of Mather (1942- 

 1943) on the mechanism of polygenic inheritance, it appears that linkage rela- 

 tionships within one chromosome, even in the absence of heterozygous inver- 

 sions, tend to maintain a balance of plus and minus loci controlling quantita- 

 tive characters. 



