GENOTYPIC AND SEGREGATION AL STERILITY 



parts are differently distributed in the two species. So segregation 

 leads to unbalance, either numerical or genie, in the gametes of these 

 hybrids, just as it does in those of triploids and interchange hybrids. 

 The unbalanced gametes cannot survive, and sterility is the result — 

 until segregation is suppressed by tctraploidy. 



1.0 



o.9\ 

 o.s 



0-7- 



0.6\ 

 0.5- 



04- 



0.3- 

 0.2- 

 0.1 



/ 



X 



xv 



/ 





& 



ti/ 

 / 



Y .... 



/ f 



FL-L 



FL-F,-' 



4'/ 



& 



(67) 



LF-L.- 



C(S2) 



LF 



FL 



-BSIft- 



o.S 



CHIASMATA per BIVALENT 



Fig. 56. — Relationship of chiasma frequency, chromosome pairing and pollen 

 fertility in LoUum-Festuca hybrids. The graph shows that: (i) The upper limit 

 is set by chiasmata to pairing and by pairing to fertility, (ii) The F^ is male-sterile 

 probably owing to a defective reaction of the hybrid nucleus with both parental 

 cytoplasms, in respect Festiica is regularly worse than LoUum. (iii) The backcrosses 

 show segregation controlling chiasma frequency and hence fertility. Each point 

 represents an individual: circles for plants used as male and female parents; F, Festuca 

 pratcnsis; L, LoJiuin pcrcnne; squares for F^; triangles for backcrosses; numbers in 

 brackets for percentage of potential bivalents formed where first metaphase pairing 

 is incomplete (based on Peto, 1933). 



The distinction between segregational and genotypic sterility is 

 thus seen to lie in the time of taking effect. Genotypic sterility is due 

 to relative unbalance of the contributions of the two parents as 

 wholes, which gives absolute unbalance of the diploid Fj hybrid. 

 This is especially noticeable when sex chromosomes are concerned, 

 for in them the Fj has something of the nature of a backcross. 



23: 



