1919] 



Frost: Mutation in Matthiola 



123 



Double 



3. Homozygous dominants not pro- 



duced by hybrids, because func- 

 tional pollen carries recessive 

 factor only. 



4. Eecessive (mutant) type the 

 more vigorous. 



5. Dominant factor or another fac- 



tor very closely linked with it 

 is incompatible with formation 

 of functional pollen. 



6. Eecessive type exceeds the ex- 



pected equality by about 3 per 

 cent among some 7000 indi- 

 viduals. 



Smooth-leaved 

 3. Homozygous dominants perhaps 

 not produced by hybrids. i* 



4. Recessive (normal) type the 

 more vigorous ; difference much 

 greater than with single and 

 double. 



5. Relation of dominant factor to 



viability of pollen not yet de- 

 termined. 



6. Recessive type exceeds equality 



by about 23 per cent among 

 234 individuals. 



The most probable hypothesis for smooth-leavedness, then, would so 

 far seem to be essentially the same as for doubleness — complete elimina- 

 tion of the weaker type in pollen formation, and partial elimination in 

 embryo-sac formation. Reciprocal crosses with Snowflake are obviously 

 necessary ; as w^e shall soon see, three of the other mutant types have 

 already proved to he carried hy l)oth eggs and sperms. 



The case of Oenothera lata (Gates, 1915) suggests the possibility 

 that the smooth-leaved form might arise by reduplication of a chromo- 

 some. AVith ordinary 0. lata the pollen is sterile, but pollination by 

 0. lamarchiana gives about 15-20 per cent of lata. This deficiency of 

 lata individuals is due, it seems, to a frequent loss of the extra 

 chromosome at meiosis in lata ovules, with a resulting formation of 

 more than 50 per cent of seven-chromosome {lamarckiana) eggs. 



If the smooth-leaved type originates through duplication of a 

 chromosome, we might suppose that other types of similar heredity 

 involve other pairs of chromosomes. The apparent parallel with 

 0. lata, which Bartlett (1917) has noted, was long ago suggested by 

 the data, but with at least two or three types to be described linkage 

 phenomena have seemed to conflict with this interpretation. Possibly 

 different processes have produced different mutant types as with 

 Oenothera; as we have considered types suggestive of 0. rubrinervis 

 (early) and of 0. lata (smooth-leaved), we may consider next a form 

 which in appearance is remarkably suggestive of 0. gigas. 



14 This possibility is only suggested by these cultures, but it becomes highly 

 probable when the data for other types are considered. 



