56 AN INTRODUCTION TO MODERN GENETICS 



action between the pollen tube and the tissues of the style in which it 

 is growing. Factors of this kind give rise to the phenomenon of self- 

 sterility, in which a plant cannot be fertilized by its own pollen, a state 

 of affairs which is found in many different genera (Nicotiana, Vero- 

 nica, Verbascum, Prunus, etc.).^ Usually the incompatibihty extends to 

 a group of plants, so that the species is divided into a set of inter-fertile 

 groups within each of which fertilization is impossible. The genetic 



Fig. 20. Pollen Lethals in Stocks (Matthiola incana). — ^The "eversporting" stock, 

 when selfed, gives about equal numbers of single flowered stocks and plants with 

 sterile double flowers. It must, therefore, be heterozygous for singleness; but it 

 transmits only doubleness through the pollen, since the F^ from a cross of pure 

 single by eversporting male consists entirely of heterozygotes. The factorial 

 Interpretation is that in the eversporting race a recessive pollen lethal factor / 

 is closely linked to the dominant factor S for singleness. The crosses are, then: 



Eversporting SI sL selfed gives SI sL (eversporting singles) and sLsL (doubles), 



the SI pollen not functioning. 

 Pure single SL SL by eversporting SI sL gives only SL sL (heterozygous singles), 



which give a normal 3 : 1 ratio in the F2. 



The lethal factor / can be identified as the absence of a small fragment (trabant) 

 which is normally attached to the chromosome bearing the singleness factor. 

 The appearance of this chromosome in the three types is shown below. 



(After Philp and Huskins.) 



a If a 



PURE EVERSPORTING DOUBLE 



SINGLE SINGLE 



basis of the infertility is a group of allelomorphic factors S^, 5*2, S^ . . . 

 5n, such that a pollen grain containing a gene S^ cannot grow satis- 

 factorily on a style which contains ^S^. Thus self-fertilization is always 

 impossible but cross-fertilization may be possible giving either two 

 classes of progeny in equal numbers (^^^g X Sj^S^ gives S^S^ and ^Sg^g) 

 or four classes in equal numbers (S1S2 X S^S^^ gives S^S^, ^i^^y '^2*^33 

 and S^S^). Analysis of self-infertility has considerable economic 

 importance in those plants where the crop depends on the setting of 

 fruit and thus on fertilization as in apples. Unfortunately in many of 

 the important cases the plants are polyploid, so that very complicated 

 possibilities of interaction between stylar tissue and pollen tubes must 

 be envisaged. 



^ Revs. Brieger 1930, East 1929. 



