SELF-PAIRING 201 



" haploid " has the constitution ABC, and is, relative to its basic 

 number, a triploid (jx). Pairing takes place between chromosomes 

 of the different sets. From 5 to 12 bivalents in the pollen mother- 

 cells and in the embryo-sac mother-cell even some trivalents are 

 formed. Haploids derived from other polyploids have shown less 

 autosyndesis. 



Table 28a. 

 Chromosome Pairing in Polyhaploids 



Triticum compactum (6x = 42), o — 3". Gaines and Aase, 



1926. 

 Nicotiana Tahacum (4X = 48), o — 3". Chipman and 



Goodspeed, 1927 ; Lammerts, 1934. 

 Digitalis mertonensis (i6x = 112), 5 — 12". Buxton and D. 



1932. 

 Mgilotricum (Sx = 56), o — 3". Katayama, 1935. 

 Solanum nigrum {6x = 72) 5 — 12". Jorgensen, 1928 



In these species the chromosomes are so far different that they pair 

 very little even in the absence of competition. The species are 

 approaching the condition of simple diploids. The variation found 

 in pairing in all these " poly-haploids " as they are described by 

 Katayama, is characteristic of that of the diploid, triploid and other 

 hybrids with which they correspond. In Digitalis and Mgilotricum 

 the process of doubling by which the new species arose from a 

 diploid hybrid has been reversed in the origin of the haploid form 

 from it by parthenogenesis. 



3. In " triploids" that have arisen through the functioning of an 

 unreduced gamete. These have each of the dissimilar sets of 

 chromosomes reproduced identically three times. Thus a tetraploid 

 AABB gives a " triploid " AAABBB. Since only two chromosomes 

 can pair at any one point at pachytene, the third set of A is left out 

 of combination, and is therefore (at any one point) in the same 

 relationship to the corresponding but dissimilar set of B as it would 

 be in a polyhaploid AB. Chromosomes of different sets will 

 therefore sometimes pair. But at other points such third-chromo- 



