Edward J. Klekowski

Petroleum pollution and mutation in mangroves

Abstract Chlorophyll-deficiency has often been used as a sensitive genetic end-point in plant mutation research. The frequency of trees heterozygous for nuclear chlorophyll-deficient mutations was determined for mangrove populations growing along the southwest coast of Puerto Rico. The frequency of heterozygotes was strongly correlated with the concentration of polycyclic aromatic hydrocarbons in the underlying sediment and with both acute and chronic petroleum pollution. Although epidemiological studies can seldom prove causation, a strong correlation is certainly compatible with a cause-effect relationship. Our results suggest that the biota of oil-polluted habitats may be experiencing increased mutation.

EVIDENCE AGAINST GENETIC SELF-INCOMPATIBILITY IN THE HOMOSPOROUS FERN PTERIDIUM AQUILINUM

Pteridium aquilinum is the only pteridophyte which supposedly possesses a genetic self-incompatibility system (Wilkie, 1956). Wilkie studied spore collections originating from three different sporophytes growing in Scotland. Populations of isolated hermaphroditic gametophytes established from these spore collections failed to form sporophytes when only selfing was possible. When populations of pairs of sister gametophytes (having a common parental sporophyte) were grown and allowed to cross fertilize, 50% of such pairs failed to form sporophytes. These observations indicated the presence of a single locus, multiple allelic, self-incompatibility system in this fern. This paper presents genetic data for five varieties of Pteridium aquilinum which suggest that self-incompatibility is not true of the entire species.

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

Progressive cross- and self-sterility associated with aging in fern clones and perhaps other plants

Plant reproductive cells differentiate from cell lineages derived ultimately from apical meristem initials, cells that are mitotically active throughout the life span of the plant. Because of the chemostat-like properties of these initial-cell populations, the frequency of mutant initials (or apical meristems) will increase as the plant ages. These properties led to three predictions when comparing long-lived species in which the frequency of sexual reproduction varies. The most asexual species should have the highest mean frequency of mutant ramets per chimeric genet, the highest genetic load, and the highest level of genetic load with dominant effects in embryonic and juvenile growth phases. These predictions were confirmed in a study of two fern species.

GENETICAL FEATURES OF FERNS AS CONTRASTED TO SEED PLANTS

pects. As Figure 1 shows, an angiosperm seed is a propagule which develops after meiosis and syngamy have occurred. This propagule contains the results of double fertilization; endosperm tissue and an embryo. Upon seed germination this embryo will develop into a mature sporophyte. The genotype of this sporophyte does not reflect its current isolation but rather is related to the breeding system of the parental sporophyte population. Thus it may be homozygous or very heterozygous. In contrast, an homosporous fern spore is a propagule that develops after meiosis but prior to syngamy. In this case only a single haploid cell is dispersed and upon germination undergoes a series of mitotic cell divisions leading to the development of a haploid, generally autotrophic plant, the gametophyte. This organism is functionally hermaphroditic, simultaneously developing male and female gametangia at some point during its life. Self-fertilization (intragametophytic selfing) results in a single diploid cell, the zygote, which is completely homozygous. This homozygous cell undergoes mitotic cell divisions leading to the development of an embryo and eventually a mature sporopbyte. Thus the establishment of a sporophyte from a single propagule in an homosporous fern results in a completely homozygous individual, whereas in a seed plant the sporophyte genotype may be heterozygous. Because of the above genetic distinctions between a seed and a spore, the evolutionary future of disjunct populations of homosporous ferns and angiosperms might predictably be very different. The angiosperm sporophyte, if it is perennial and dioecious, may live long enough to encounter a sporophyte of the opposite sex, outcross, and form progeny. These resulting progeny in all likelihood would be

Mangrove Genetics. II. Outcrossing and Lower Spontaneous Mutation Rates in Puerto Rican Rhizophora

The frequency of red mangrove trees, Rhizophora mangle L., that were monohybrid heterozygotes for chlorophyll-deficient alleles was determined for populations along the southwest coast of Puerto Rico. Segregation ratios for the offspring of these trees were also calculated. Although nuclear, the different mutant homozygous genotypes had profound effects on chloroplast ultrastructure. In comparison to Florida and San Salvador Island R. mangle populations, Puerto Rican mangroves were more outcrossed (28.8% vs. 4.8%) and had lower per generation per genome mutation rates for chlorophyll-deficient mutations (2.1 x 10-3 vs. 5.8 x 10-3).

Genetic load and soft selection in ferns

SummaryGenetic load data for two homosporous ferns are summarized and the mean frequency of recessive sporophytic lethals per zygote calculated. Fifty-one spore collections of Onoclea sensibilis had a mean of 0·587 lethals per zygote, and 129 spore collections of Osmunda regalis had a mean of 2·39 lethals per zygote. Each spore collection represents a meiotic sample from a single individual (genet). Both species form hermaphroditic gametophytes with extensive capacities for simple polyembryony. Simple polyembryony results in a form of soft selection in panmictic populations and thus higher equilibrium frequencies for lethals are possible than in organisms without simple polyembryony. When ferns are inbred, the high load levels are expressed. Load levels in ferns may reflect a balance between soft selection in panmictic populations and hard selection under conditions of inbreeding. The sheltering effect of simple poly-embryony is demonstrated for both mutational and heterotic load.

Mutation rates in mangroves and other plants

Red mangrove (Rhizophora mangle) possesses two traits that make it an ideal species for the measurement of per generation mutation rates in natural populations: vivipary and self-pollination. Vivipary allows the scoring of offspring phenotypes when they are attached to the maternal parent and self-pollination results in the attainment of near mutation/selection equilibria in relatively few generations. Mutation rates for the multigenic mutant phenotype chlorophyll-deficiency are presented for mangroves and the rates are compared to other plant taxa. Patterns are evident for marginal vs., central populations and for variations in life-form.

The Rapid Colonization and Emerging Biology of Cordylophora caspia (Pallas, 1771) (Cnidaria: Clavidae) in the Connecticut River

ABSTRACT Five freshwater populations of the otherwise estuarine colonial hydrozoan Cordylophora caspia were observed in the Connecticut River, New England. Histological comparison of the body wall of the freshwater and estuarine populations revealed no differences in cell shape or size in the epidermis and gastrodermis, either of the undifferentiated body wall or in regions of tentacles. One freshwater population examined in 2000 had polyps that were shorter and wider than estuarine animals and contained much fewer and smaller sporosacs. All freshwater populations examined in 2001 had feeding polyp shape and size and sporosac size and number comparable to the estuarine population. We conclude that C. caspia has undergone physiological and ecological adjustments for existence in very soft water with low alkalinity. The species has in only a few years become abundant in certain areas of the Connecticut River and has acquired all the facies exhibited during experimentally determined optimal growth conditions. Ecologically, C. caspia is filling the niche of a benthic colonial predator; the hydroid preys on larval insects, predominately chironomids.

REVIEW: MUTATION RATES IN DIPLOID ANNUALS-ARE THEY IMMUTABLE?

Nuclear mutations resulting in the chlorophyll-deficient or albino phenotype are the most commonly used end point of mutation in higher plants. Mutation rates per generation for albinism are remarkably similar in a variety of species of annuals even though these species vary greatly in their DNA levels, chromosome numbers, and apical ontogenies. These data prompt the hypothesis that the nuclear genes for chlorophyll deficiency or albinism, on the average, degrade at relatively constant rates regardless of many biological, ecological, and evolutionary parameters.