Interspecific Hybrids and the Cytogenetics and Cytotaxonomy of Euallomyces

Abstract

This paper presents a report of genetic, cytogenetic, and cytotaxonomic studies made with the aquatic fungus Allomyces over the past seventeen years. Because there have not, hitherto, been any extensive genetic analyses of interspecific hybrids in the Phycomycetes, the isolates and procedures used in these investigations are described in some detail. The life cycle and structural characters of the two species of Euallomyces, A. arbuscula with hypogynous male gametangia, and A. javanicus with epigynous male gametangia, are reviewed, and emphasis is placed on those features that are of special significance in genetic work. Methods for segregating the gametes, making the crosses, isolating the hybrid zygotes, determining viability of the products of meiosis (meiospores), and analyzing the progeny are explained. Reciprocal matings made between four strains of A. arbuscula and three of A. javanicus in various combinations show that the reaction of the gametes and the percentage of “takes” vary according to the particular parents used. Counts of epigyny (E) and hypogyny (H) in the gametophytes derived from over a hundred F₁ sporophytes have demonstrated that, in addition to both the parental types (E and H), a whole series of intermediate strains (I) segregate out. From these results it is concluded that the arrangement of gametangia is controlled either by more than one pair of independently segregating alleles or by a single pair of duplicated alleles that segregate independently. The viability of meiospores from F₁ sporophytes is shown to be reduced to less than one tenth that of the parents. In about one-fifth of the crosses all of the progeny exhibited pure hypogyny (H). For the following reasons we believe that hybridization failed to occur in these instances, the ‘F₁’ sporophyte having arisen by parthenogenetic development of the female gamete. (1) The female parent was always A. arbuscula. (2) Apomictic formation of sporophytes, accompanied by chromosome doubling, characterizes certain strains of A. arbuscula, and it was from “crosses” of just such strains that the purely H-type 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 (E, H, or I) of the F₁ 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 just 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. 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 F₁ 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. In 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 F₁ 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 a doubling of the arbuscula complement of chromosomes during or just prior to syngamy, i.e., 2 × 16 + 28 = 60 (55 detected). The irregular distribution of univalents in the F₁ explains the variable haploid chromosome numbers (20–44) which are found to occur in the F₂ and F₃ 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 F₂ and F₃ 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 var. 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 the laboratory; and the chromosome numbers of natural isolates range from 13 to 21, just as those of the F₂ and F₃ from artificial crosses vary, though over a somewhat higher and wider range. We are therefore raising var. macrogynus to a species, A. macrogynus n. comb., and designating A. X javanicus Kniep (arbuscula X macrogynus) as a hybrid species including var. javanicus and var. perandrus.