EVIDENCE FOR GENETIC HETEROZYGOSITY IN A HOMOSPOROUS FERN

Homosporous fern sporophytes from natural populations exhibited heterozygous electrophoretic patterns for several enzyme systems. Genetic tests utilizing individual gametophytes demonstrate that the observed heterozygosity is coded by alleles at single loci. This simple procedure makes it possible to distinguish segregating from fixed or phenotypic heterozygosity, previously a problem in homosporous vascular plants. THIS REPORT presents the first evidence from allozymes for the segregation of alleles at single gene loci in homosporous vascular plants. The data result from electrophoretic examination of enzymes in individual gametophytes from single sporophytes collected in two natural populations of Pellaea andromedaefolia, a homosporous fern native to California (Tryon, 1957). Homosporous plants are genetically interesting because they produce a single kind of spore which generally gives rise to bisexual gametophytes capable of self-fertilization. Each gametophyte produces its sperm and eggs through mitotic cell divisions, thereby generating genetically identical gametes. When selffertilization occurs, a completely homozygous sporophyte is produced. Extreme inbreeding of this type is expected to lead to sharply curtailed genetic variability.2 Consequently there has been considerable speculation regarding the mechanisms of evolution in this group and the processes that might produce genetic variability in spite of homospory. The most widely discussed model addressing this problem (Klekowski, 1973) proposes: 1) that genetic variability is stored in the form of ' Received for publication 6 November 1981; revision accepted 13 December 1981. Research supported by travel and fellowship funds to G.J.G. from the College of Arts and Sciences and the Office of Research and Graduate Development of Indiana University and by NSF Grant DEB 79-22196 to L.D.G.; G.J.G. thanks Drs. T. R. Pray and S. Whitmore for help with the Point Mugu population. 2 Certain morphological and hormonal characteristics of the gametophytes of some homosporous vascular plants are thought to promote outcrossing (Klekowski, 1969; Haufler and Gastony, 1978), but no survey of the genetic structure of any natural fern population has been published to date. Levin and Crepet (1972) did find more heterozygosity than expected in their survey of the non-fern homosporous plant Lycopodium lucidulum, but the genetic basis of that heterozygosity is unknown. duplicated unlinked loci on homoeologous chromosomes originating as a result of polyploidy; and 2) that the variability is expressed in different combinations following occasional recombination between the homoeologous chromosomes. Evidence cited in favor of this model has been proposed (Klekowski and Hickok, 1974; Bierhorst, 1975; Klekowski, 1976a, b; Hickok, 1978a, b3; Chapman et al., 1979) and critiqued (Lovis, 1977). However, there has not been a direct genetic test of this model utilizing single gene products, in part because of the typically long generation times in homosporous plants. We were able to make such a test because we examined single gametophytes, which are the genetically unaltered derivatives of the meiotic products (spores). Our analysis follows from the observation that many enzymes detectable by gel electrophoresis in the sporophytic tissue of ferns are also expressed in their gametophytes (DeMaggio and Lambrukos, 19744). Two alleles are present at each gene locus in the sporophyte, but at meiosis only one or the other allele is distributed to each spore. Therefore comparison of the electrophoretic patterns of isozymes in individual gametophytes can determine whether the phenotypic heterozygosity of their parental sporophyte is coded by 3 Hickok's data are based on the segregation of chloroplast mutants in gametophytes and sporophytes. Studies of segregation of chloroplastid and other morphological markers in ferns date from early in this century (review by Andersson-Kott6, 1938). They reveal simple Mendelian patterns of inheritance for some markers (e.g., Lang, 1923) but require more complex explanation for others, as in Andersson's (1923) and Hickok's work with chloroplasts. 4 This paper, the only previous electrophoretic comparison of gametophytic and sporophytic tissues of homosporous vascular plants, was a developmentally oriented study of peroxidase expression.