BLASTOCLA D/A LES 63 I 



development of the female gamete. (I) The female parent was always A. 

 arbuscula. (2) Apomictic formation of sporophytes, accompanied by chromo- 

 some doubling, characterizes certain strains of A. arbuscula, and it was from 

 "crosses'" of just such strains that the purely H-typc progeny arose. (3) The 

 reaction of the gametes in these crosses was atypical and zygotes were not seen. 



Second and third generation progeny from the true crosses generally 

 exhibit high meiospore viability and perpetuation of the characters (F. H, 

 or I) of the ¥ v gametophytes from which they were derived. 



Studies of chromosome number and behavior in parental and hybrid 

 strains have provided a precise basis for understanding and interpreting the 

 genetic experiments jusr summarized. A polyploid series consisting of natural 

 strains with 8, 16, 24, and 32 haploid chromosomes exists in A. arbuscula, 

 and counts of 14, 28, and 50 (probably 56) have been established in A. 

 javanicus var. macrogynus [A. macrogynus]. The parental strains originally 

 selected for hybridization have 16 and 28 chromosomes respectively in their 

 gametophytic nuclei, and hence the sporophytes, with 32 and 56 chromosomes, 

 represent a tetraploid condition. 



Two patterns of chromosome number and behavior have been observed at 

 the first meiosis in F x hybrids obtained by crossing these tetraploid parents. 

 In Type A nuclei, there are 16 -+- 28 = = 44 chromosomes, but only very few 

 bivalents, and the univalents are distributed randomly to the poles. !n Type B 

 nuclei approximately 55 chromosomes have been detected; about half of 

 them form bivalents, and again many univalents go to the poles at random. 

 The irregular distribution of chromosomes in both types accounts for the 

 greatly reduced viability of h\ meiospores. More precise determinations 

 within the hybrid population show, furthermore, a direct correlation between 

 the degree of pairing and the percent meiospore viability: Type A, few pairs 

 and ca. 0.1% viability; Type B, many pairs and ca. 3% viability. From the 

 observed number of chromosomes, the amount of pairing, and the fact that 

 Type B hybrids always have A. arbuscula as the female parent, it is concluded 

 that Type B nuclei arise by doubling of the arbuscula complement of chromo- 

 somes during or just prior to syngamy, i. e., 2 > 16 - 28 = <>0 (55 detected). 



The irregular distribution of univalents in the F, explains the variable 

 haploid chromosome numbers (20-44) which are found to occur in the F 2 and 

 F 3 progeny and also gives insight into the mechanism whereby E and H types, 

 as well as a series of intermediates, I, segregate out. Chromosome pairing is 

 usually regular and complete again in F 2 and F, meioses, and meiospore 

 viability returns to the high level of the parents. 



All of the evidence, morphological, genetic, and cytological, points to the 

 conclusion that Allomyces javanicus wax. javanicus is a natural hybrid of A. 

 arbuscula x A. javanicus var. macrogynus. Gametophytes are characterized 

 by an intermediate condition in which epigyny and hypogyny occur on the 

 same plant; this intermediacy has been exactly duplicated by crosses made in 



