SPECIAL PROBLEMS OF MEIOSIS 73 



cytologically ; they are grouped together as corn- 

 pensating relationships. Direct observations on the 

 relative frequencies of the compensating and non- 

 compensating (diagonal) kinds of relationship have 

 only been made in a few cases, but in the grasshopper 

 Melanoplus femur -rithrum the compensating appear 

 to be about twice as frequent as the non-compensating 

 relationships.^* Probably it is the complementary 

 t;y^e which is unusually abundant ; there would 

 appear to be a kind of chromatid interference which 

 usually prevents the same chromatids from partici- 

 pating in adjacent chiasmata. 



Meiosis in Hybrids 



From the cytological point of view there are three 

 main types of hybrids, ordinary diploid hybrids, 

 complex keterozygotes (such as the Evening Primrose, 

 Oenothera spp.) and polyploid hybrids (allo-polyploids). 

 A diploid hybrid is an organism with its two haploid 

 sets of chromosomes derived from different parent 

 species. However similar they may be taxonomically 

 these parent species will nearly always differ in respect 

 of a number of genes ; moreover, the extent of the 

 taxonomic differences between two species is probably 

 a very unreliable guide to the number of gene 

 differences involved. 



In addition to simple gene-differences the parents 

 of a hybrid may also differ in the way in which the 

 genes are arranged in the chromosomes. Thus in 

 the case of Drosophila melanogaster and D. simulans 

 (which are taxonomically so similar that they were 

 originally regarded as one species), the number of 

 chromosomes is the same and their relative lengths 

 are almost identical, but a large portion of the Ilird 

 chromosome (about one- quarter of its total length) 

 is inverted in simulans in comparison with melano- 

 gaster ; in addition there are a number of differences 

 in the sex-chromosomes and in Chromosome IV 

 which are visible in the salivary gland nuclei and 



