MUTATIONAL LOAD IN CLONAL PLANTS: A STUDY OF TWO FERN SPECIES.

The homosporous fern life cycle is a classic example of the alternation of generations in plants. Sporophytes produce spores via meiosis, the spores develop into hermaphroditic gametophytes which form gametes via mitosis. The self-fertilization of a gametophyte (intragametophytic selfing) results in a zygote which is completely homozygous (of course such self-fertilization is not obligatory in nature). If a gametophyte genotype contains recessive lethal or deleterious genes which are restricted in their expression to the sporophyte generation, the homozygous zygote will express these traits. Because of these two attributes (free-living gametophytes and intragametophytic selfing), the ferns are very useful organisms in genetic load studies (Klekowski, 1979). Many different fern species have been investigated for genetic load (Klekowski, 1970, 1973; Ganders, 1972; Verma and Kapur, 1972; Holbrook-Walker and Lloyd, 1973; Lloyd, 1974a, 1974b, 1974c; Lloyd and Gregg, 1975; Saus and Lloyd, 1976; Lloyd and Warne, 1978; Cousens, 1979; Khare and Kaur, 1979; Masuyama, 1979; Schneller, 1979; Warne and Lloyd, 1981). Load levels vary greatly between species, the highest load levels documented occur in Osmunda regalis with an average of 2.39 recessive sporophytic lethals per zygote (Klekowski, 1982b) to a low of .006 recessive sporophytic lethals per zygote in Ceratopteris pteridoides (calculated from Warne and Lloyd, 1981). Why such great interspecific variation exists is unknown. Although a number of causes of genetic load have been advanced (Crow and Kimura, 1970), only two are relevant in considering the origin of load in ferns: mutational load and heterotic load. The former was first discussed by Muller (1950) in reference to the consequences of human inbreeding. Mutational load results from mutant alleles which are present in very low frequencies because they are selected against, but which persist because of recurrent mutation. Heterotic load (Wright, 1977 or balanced load of Dobzhansky, 1955, 1970 or segregational load of Crow and Kimura, 1970) occurs when the load components are deleterious in double dose (homozygotes) but give rise to heterosis in heterozygotes. Lethal or deleterious alleles are maintained in the population at equilibrium frequencies by the selective advantage of the corresponding heterozygotes. The relative contributions of mutational and heterotic load in the overall genetic load to populations has spawned a large literature (Simmons and Crow, 1974; Wallace, 1981) with little consensus. Fern biologists have held generally that heterotic selection, and consequently heterotic load, is the cause of genetic load in fern populations (Lovis, 1977). This opinion was not based upon any real evidence other than the fact that heterozygosity for lethals existed in many fern populations. Levin and Crepit (1973) studied polymorphisms of 11 proteins encoded by 18 loci in 16 populations of the pteridophyte, Lycopodium lucidulum, and showed that the observed heterozygosity exceeded the level of heterozygosity predicted from allele frequencies with random mating. Nei (1975) analyzed these data and concluded that the frequency of heterozygotes had little to do with heterozygote advantage. The possibility that mutation was a source of genetic load in ferns was suggested by Verma and Kapur (1972), but the idea was not developed