Janis Antonovics

Heavy Metal Tolerance in Plants

Publisher Summary This chapter illustrates the literature on those plants and micro-organisms which can combat excessive quantities of heavy metal ions. Heavy metals include those metals which have density greater than five. Their common feature in regard to biological life is that in excessive quantities they are poisonous and can cause death of most living organisms. However, certain organisms possess an ability to survive under conditions of metal contamination which can prove toxic to other living things. Toxic levels of heavy metals can occur under several circumstances. The chapter focuses on the type of contamination in which the soil itself contains large quantities of these metals. The contamination results from the presence of undisturbed metal ore near the soil surface causing anomalies or from the actual mining of ore bodies. The vegetation in such areas is influenced by one overriding factor, namely, metal concentration. The habitats are usually spatially distinct and clear cut. It is observed that the sequence and pattern of genetic change responsible for colonization of metal-contaminated areas serves as a unique record of natural selection in action.

Host-Dependent Sporulation and Species Diversity of Arbuscular Mycorrhizal Fungi in a Mown Grassland

1 In laboratory microcosm experiments, co-occurring plant species were found to support very different rates of sporulation of arbuscular mycorrhizal (AM) fungi. These differences were not affected by the time of harvest, suggesting that they reflect host-dependent differences in fungal growth rates, rather than host-dependent timing of sporulation. 2 Spore counts in field soil and estimates from sorghum trap cultures showed that the association of AM fungi with particular host plants in the field was positively correlated with the sporulation rates observed on those hosts in the microcosm experiments. 3 The AM fungal species richness observed at the field site was high relative to estimates made in previous studies. 23 distinct species of AM fungi were found, seven of which have not been previously described. 4 The host-dependence of the relative growth rates of fungal populations may play an important role in the maintenance of fungal species diversity.

Evolution in closely adjacent plant populations

SummaryThe evolution of reproductive isolation along a cline was investigated using a computer simulation of 10 linearly arranged populations connected by gene flow and subjected to selection which varied either linearly or in a stepwise manner. For nearly all combinations of parameter values a monotonic cline was rapidly established in frequency of alleles at the selected locus. Only at high levels of selection (> 0.1) and high levels of assortative mating (> 0.4) was there divergence in frequency of the gene determining reproductive isolation. Under these conditions divergence was slow and the cline for the isolating gene was often inverse for many generations, although at equilibrium the cline was always monotonic. Linkage between the selected gene and the isolating gene promoted divergence. Both genetic divergence and reproductive isolation may therefore occur between populations connected by gene flow. Conditions leading to isolation are more stringent than those permitting genetic divergence suggesting that the “cohesion of the biological species” is neither maintained by gene flow nor by the uniformity of selection but by the weakness of forces leading to selection for reproductive isolation. The existence of inverse clines in isolating mechanisms strongly suggests the evolution of isolation in sympatry (following divergence either in sympatry or allopatry,) but it is invalid to conclude that a monotonic cline for reproductive isolation gives a priori evidence of its evolution in allopatry.

THE NATURE OF LIMITS TO NATURAL SELECTION

Insufficient genetic variability and the swamping effects of gene flow are inadequate explanations of limits to natural selection. Comparison of evolutionary responses in different ropulations subjected to similar selective forces, comparison of rare and widespread species, and comparison of marginal and central populations are all neglected research areas that bear on the nature of limits to natural selection. Plant populations provide us with well-defined, operationally viable systems for addressing these comparisons. Several possible constraints on range extension of ecologically marginal populations are considered in detail. Selection on fitness components that are themselves negatively correlated will be ineffective: such negative correlations are to be expected in natural populations. Small size of marginal populations will reduce severely the probability of obtaining appropriate character combinations; it will increase the swamping effects of gene flow; and it may lead to inbreeding depression effects. Gene flow will have different effects depending on whether the genes concerned are effectively neutral, advantageous, or deleterious in the population into which they migrate. Gene flow will spread beneficial genes rapidly, but may retard divergence if density of marginal populations is low and swamping effects are high. Finally a population entering a new habitat is likely to meet new competitors and predators: the coevolutionary responses of the latter may counteract adaptive responses by the species undergoing range extension. All these factors are likely to interact in important ways in marginal populations. The study of limits to natural selection is likely to be a fruitful future research area, and one in which the detailed documnentation of the systematist will provide invaluable baseline information.

Evolution in closely adjacent plant populations VIII. Clinal patterns at a mine boundary

SEVERAL previous papers in this series have been concerned with the evolution of differences between tolerant and non-tolerant populations of grasses growing over the boundaries of mine tip and pasture soils (Jam and Bradshaw, 1966, McNeilly and Antonovics, 1968, Antonovics, 1968a). Metal tolerant plants differ both physiologically and morphologically from normal plants in several features other than tolerance. Differences in calcium and phosphate response are recorded by Jowett (1959) and McNeilly (1966); Turner (1967) has shown differences in sulphur response; and Schwanitz and Hahn (1 954a, 1 954b) showed from a study of a range of plant species grown under standard conditions that in general tolerant plants had smaller flowers, smaller leaves and thinner stems. This was confirmed for Silene inflata by Broker (1963) who concluded from segregating progenies that dwarfness was not linked to tolerance and had probably been selected independently. Similarly, morphological differences have been recorded in the grass Agrostis tenuis on lead mines (Bradshaw, 1959; Jowett, 1964) and copper mines (McNeiIly, 1966). The morphological characters of plants of the grass Anthoxant hum odoratum L. taken from sites along a transect across a mine/pasture boundary were investigated with three main points in view.

SEXUALLY TRANSMITTED DISEASES IN ANIMALS: ECOLOGICAL AND EVOLUTIONARY IMPLICATIONS

Sexually transmitted diseases (STDs) have been generally thought of as a small subset of infectious diseases, rather than as an important group of diseases that occur in numerous species. In this paper, we have (1) briefly reviewed theoretical studies on the dynamics of STDs; (2) documented the distribution of STDs in the animal kingdom; and (3) investigated whether STDs have characteristics which distinguish them from other infectious diseases. The dynamics of STDs should differ from those of ordinary infectious diseases because their transmission depends on the frequency rather than density of infectives. With this type of transmission, there is no threshold density for disease spread, and the conditions for host-pathogen coexistence are more restrictive. Nevertheless, a wide variety of disease characteristics may allow a sexually transmitted pathogen to coexist with its host. We found over 200 diseases for which there was evidence of sexual transmission. They occurred in groups as diverse as mammals, reptiles, arachnids, insects, molluscs and nematodes. Sexually transmitted pathogens included protozoans, fungi, nematodes, helminths, and cancerous cell lines, as well as bacteria and viruses. Detailed comparison of the characteristics of sexually transmitted mammalian diseases with those that are transmitted by non-sexual means, showed that STDs cause less mortality, are longer-lived in their hosts, are less likely to invoke strong immune responses, have narrower host-ranges, and show less fluctuation in prevalence over time. These shared features are related to mode of transmission rather than either host or pathogen taxonomic affiliation. This suggests an evolutionary explanation based on shared ecologies rather than one based on phylogenetic history.

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. I. A TEST OF THE FREQUENCY‐DEPENDENT SELECTION HYPOTHESIS

This study tests the hypothesis that one evolutionary advantage of sexual reproduction is that it produces genetically variable progeny with a density-dependent advantage mediated by resource partitioning or pest pressure. Our experimental approach involved planting separate plots of sexually-derived and asexually-derived tillers of the grass Anthoxanthum odoratum in density gradients at the two natural sites from which the source material was taken. The sexual progeny displayed a significant fitness advantage compared to the asexual progeny. But, in contrast to the expectations of the density-dependent selection hypothesis, the advantage of the sexually produced progeny is most marked at lower densities. Thus, the results of this experiment and our previous report (Antonovics and Ellstrand, 1984) seem to best support the frequency-dependent selection hypothesis for the advantage of sexual reproduction.

Evolution in closely adjacent plant populations IV. Barriers to gene flow

THE importance of isolation in promoting population divergence and speciation has long been recognised (e.g. Mayr, 1942; Dobzhansky, 1941; Baker, 1959). Isolation was considered a prerequisite for population divergence until Thoday (1958) showed that disruptive selection could effect such divergence in the absence of isolation. Recently the occurrence of divergence in nature in the face of gene flow has been shown in Papilio dardanus (Clarke and Sheppard, 1962), Maniola jurtina (Creed et al., 1959) and various grasses (Jam and Bradshaw, 1966; Aston and Bradshaw, 1966; McNeilly, 1967). However, gene flow is not without effect. Generally it slows down population divergence (but see Millicent and Thoday, 1961, and Streams and Pimentel, 1961) and produces ill-adapted genotypes from the crossing of two adapted types. In such situations we might expect the evolution of mechanisms to restrict gene flow. Evidence for the development of breeding barriers between adjacent (parapatric), or sympatric populations, and their absence between allopatric populations of the same species or group of species, has been presented in Drosophila (Dobzhansky and Koller, 1938; King, 1947; Ehrman, 1965), cotton (Stephens, 1946), Streptanthus (Kruckeberg, 1957), Solanum (Grun and Radlow, 1961) and Gilia (Grant, 1966). The process has also been demonstrated experimentally (Knight et al., 1956) and theoretically (Crosby, 1964). In all these instances there is evidence that breeding barriers have been developed between populations that have undergone prior allopatric divergence and which have subsequently met. However, Thoday and Gibson (1962) have shown that in Drosophila divergence and the evolution of breeding barriers can occur without isolation under disruptive selection. Since no evidence has been presented for the origin of breeding barriers under disruptive selection in natural populations, the occurrence of mechanisms reducing gene flow were investigated in closely adjacent plant populations at metal mine boundaries. The evidence suggests that considerable gene flow occurs in such situations (McNeilly, 1967; McNeilly and Bradshaw, 1967) and that mine populations are the product of recent evolution by disruptive selection (McNeilly, 1967; Antonovics, 1966).

The Cost of Resistance and the Maintenance of Genetic Polymorphism in Host-Pathogen Systems

By using models which incorporate both numerical and gene-frequency dynamics, we investigate the conditions for a stable polymorphism in host disease resistance when there is a genetically uniform pathogen. We show that polymorphism is more likely when the difference in resistance conferred by alternative alleles is large rather than small. This conforms with the frequent observation of major gene effects on resistance. Moreover, when allelic differences in resistance are large, polymorphism is possible over a wide range of costs, including situations where costs approach values close to zero. The actual resistance cost that can be sustained in such polymorphic populations is dependent on the transmission mode and the intensity of disease-independent population regulation. Expectations regarding resistance costs in any particular host—pathogen system will be dependent on knowledge of the epidemiological and genetic characteristics of that system.

Silene as a model system in ecology and evolution.

The genus Silene, studied by Darwin, Mendel and other early scientists, is re-emerging as a system for studying interrelated questions in ecology, evolution and developmental biology. These questions include sex chromosome evolution, epigenetic control of sex expression, genomic conflict and speciation. Its well-studied interactions with the pathogen Microbotryum has made Silene a model for the evolution and dynamics of disease in natural systems, and its interactions with herbivores have increased our understanding of multi-trophic ecological processes and the evolution of invasiveness. Molecular tools are now providing new approaches to many of these classical yet unresolved problems, and new progress is being made through combining phylogenetic, genomic and molecular evolutionary studies with ecological and phenotypic data.