Lack Of Genetic Variation Research Articles

Reproducing Bobcats to Cumberland Island, Georgia

AbstractMany felids are threatened by loss of habitat, lack of genetic diversity, and over‐exploitation. The reintroduction of bobcats (Felis rufus) to Cumberland Island, Georgia provided an opportunity to reintroduce a mid‐sized felid without the concern for species survival that is paramount with endangered species. We captured bobcats from the coastal plain region of Georgia, briefly held them in captivity, and released them on Cumberland Island. We describe and evaluate the protocols and techniques used to accomplish the reintroduction. Future reintroductions of felids should consider the problem of post‐release dispersal, although our island was relatively isolated and inhibited dispersal. Also, any reintroduction effort should invest effort and resources into post‐release monitoring of the population. Empirical knowledge about the effects of spatial distribution, genetics, population dynamics, especially mechanisms of population regulation, behavior, and environmental conditions on the viability of populations is critical to the conservation of endangered species. Future research of the bobcats on Cumberland Island will be able to address aspects of the population and genetic dynamics of a small, insular felid population.

Mate finding, dispersal, number released, and the success of biological control introductions

Abstract. An analysis of published data and a mathematical model of the population dynamics of introduced parasitoids were used to explore the possibility that biological control introductions fail because an Allee effect drives small, introduced populations extinct. Such an Allee effect would arise because low densities, resulting from dispersal into a new environment, lead to failure to mate, which leads to a male‐biased sex ratio, which, if extreme enough, could cause population extinction. For chalcidoids, ichneumonoids and tachinids, the proportion of parasitoid populations that established when introduced for control of lepidopteran pests increased with the number of parasitoids per release, the total number released, and the number collected when each variable was analysed separately. For chalcidoids alone, establishment increased with the number of releases for this variable analysed separately. However, stepwise logistic regression of establishment on these variables included only the total number released for chalcidoids and the number per release for ichneumonoids and tachinids. This suggests that an Allee effect may limit the establishment of introduced parasitoids more than stochastic environmental variation or lack of genetic variation. A reaction‐diffusion model of parasitoid introductions was developed, which included mate finding, dispersal, reproduction and survival. Sensitivity analysis showed that the critical number of females needed to establish a population decreased hyperbolically as mate detection distance and net reproductive rate were increased, but the critical number increased linearly as mean‐square displacement was increased. The critical number of females did not change when the gross distance traversed per generation was varied. This was because increased area searched by males compensated for increased displacement. Changing from virgin females producing all males (arrhenotoky) to virgin females producing no progeny increased the critical number of females by over 30%. The analysis of past introductions and the sensitivity analysis of the reaction‐diffusion model both suggested a threshold of about 1000 insects per release to ensure establishment of introduced parasitoids. The implications of our results for the design of biological control introductions are discussed. Limitations in retrospective analyses and current knowledge indicate the need for an experimental approach to introductions.

DNA fingerprint analysis of a free-range koala population.

Thirty-six koalas in a free-range Queensland population were fingerprinted using an M13 probe in combination with MspI digestion. The technique was found to be highly repeatable, with estimates of 0.1-1.6% within-gel error and 0.1-2.5% between-gel error. Of the 43 different-size fingerprint bands produced in the population, only 2 bands were common to all 36 koalas. Ten bands were quite rare, occurring at a frequency of 0.2 or less. All 36 koalas had unique DNA fingerprints (probability of 1.88 x 10(-7)), which enabled them each to be uniquely identified. Despite this, there was still a high level of band sharing in the population (mean number of shared bands = 0.749). This level is much higher than that reported for humans, birds, cats, dogs, and cattle but not as high as that reported previously for Victorian koalas. This lack of genetic variation may influence the ability of the population to respond to stress situations, such as lack of food, habitat destruction, and disease.

No genetic differentiation of giant clam (Tridacna gigas) populations in the Great Barrier Reef, Australia

Six Tridacna gigas populations were sampled in 1990 from locations throughout the central and northern Great Barrier Reef (GBR). Despite separations in excess of 1000 km, mean Nei's unbiased genetic distances among the populations was 0.0007. The complete lack of spatial variation observed among populations did not results from lack of genetic variability. Genetic variation within populations was high, with mean heterozygosities from 0.221 to 0.250. Gene frequencies were consistent with expectations under conditions of Hardy-Weinberg equilibrium. These data suggest panmixis, or random mating, throughout the highly connected reef system of the central and northern GBR. The large gene exchange among the giant clam populations has important implications for conservation management of one of the few large populations of T. gigas in the world. Small local effects are likely to be overcome in time by inputs from other sources. However, large genetic perturbations, particularly from up-current sources, may spread rapidly through the population.

Response of New Zealand honey bee colonies toNosema apis

SUMMARYSeventeen colonies of honey bees from 13 different sources were dosed with Nosema apis spores in sugar syrup. Spore loads carried by foraging bees were recorded for 11 weeks thereafter. Eleven further colonies, fed plain sugar syrup, were sampled as controls. Mean spore loads in all N. apis-dosed colonies increased to between 8 million and 27 million spores per bee two weeks after dosing. Five weeks after dosing, spore loads had decreased to levels comparable to those found in the control colonies (about 4 million spores per bee or less). There were no significant differences in the responses of colonies from different sources. Colonies from different sources showed a similar uniformity with regard to the rate of spread of infection among caged bees and chymosin levels in the guts of workers. These results show little variation in response to N. apis infection, and indicate a lack of genetic variability among New Zealand bees in respect of susceptibility to N. apis infection.

GENETIC VARIATION IN BROMUS TECTORUM (POACEAE): POPULATION DIFFERENTIATION IN ITS NORTH AMERICAN RANGE

Allelic variation in seedlings from 60 North American populations of the alien annual grass Bromus tectorum was determined at 25 loci using starch gel electrophoresis. Populations were collected from four regions; east of the Rocky Mountains, Nevada and California, the Intermountain West, and British Columbia. Compared to other diploid seed plants, genetic variation within these populations of B. tectorum is low: 4.60% of loci are polymorphic per population, with an average of 1.05 alleles per locus and a mean expected heterozygosity of 0.012. Although 2,141 individuals were analyzed, no heterozygous individuals were detected, and consequently, mean observed heterozygosity is 0.000. Extensive deviations from Hardy‐Weinberg expectations were observed at every polymorphic locus due to heterozygote deficiencies. The mean genetic identity (Nei's I) between population pairs was 0.980 and indicates a high level of overall genetic similarity among populations. The among‐population component of the total gene diversity is high (GST = 0.478), indicating substantial genetic differentiation among populations. These results are consistent with previous reports for highly self‐pollinating plants of low genetic variation and substantial genetic differentiation among populations. Despite the lack of genetic variation as measured by enzyme electrophoresis, this weedy grass has become exceedingly abundant in a diverse array of arid environments throughout much of western North America, perhaps due to phenotypic plasticity.

Phytotoxins as tools in breeding and selection of disease-resistant plants

Conventional plant breeding for resistance to pathogens, although successful, is in many cases still too slow to keep pace with pathogen adaptation, and suffers from the lack of genetic variability in cultivated varieties. Phytotoxins, because of their role in disease development, have been proposed as convenient markers for early screening of resistant genotypes and as selective agents for in vitro selection. The present review summarizes, firstly, the evidence for a genetic correlation between tolerance to toxins and resistance to pathogens, with particular reference to host-selective toxins (HST) and factors affecting early screening. There follows a discussion of results obtained from the use of phytotoxins for in vitro selection of resistant plants. The conclusion is drawn that this practice, while potentially useful in the case of HST, leads to contradictory results when ill-defined toxins or culture filtrates are used. Finally, prospects for future research are adumbrated.

In vitro mitogen responses and lymphocyte subpopulations in cheetahs

Lack of genetic variability and apparent susceptibility of cheetahs ( Acinonyx jubatus) to coronavirus infection has lead to speculation that this species may have immune system deficits. To establish a foundation for evaluation of the immune function, cheetah peripheral blood mononuclear cells (PBM) were stimulated by a panel of six mitogens, and responses compared with those of domestic cat PBM. Individual responses in both species were variable, but evenly distributed throughout the range of stimulation for each mitogen. Proliferation by PBM from domestic cats occured within the same range as that of the cheetahs. However, a significantly lower response to peanut agglutinin (PNA) was observed with domestic cat PBM. Although responses varied between animals, certain individual cheetahs were consistent responders. The decreased values could not be explained by lack of IL-2 responsiveness since exogenous IL-2 significantly enhanced mitogen-stimulated proliferation in 11 of 12 cheetahs tested. The phenotypic distribution of domestic cat and cheetah lymphocyte subpopulations was similar as assessed by immunofluorescence staining for surface immunoglobulin (sIg) and cytotoxic T (Tc) cells (using a specific monoclonal antibody, FT2). Values for B cells (31.2% sIg+) and Tc (28.7% FT2 +) were slightly higher in domestic cats as compared with cheetah PBM (13.3% sIg+; 19.0% FT2+). Even though no species-specific deficits were detected, a significant negative correlation between PHA-stimulated proliferation and percent FT2+ (Tc) cheetah cells was observed. This indicates that proliferation can be used indirectly to assess relative numbers of functional T helper cells in cheetahs. Our studies suggest that these aspects of the cheetah's immune system are comparable with the domestic cat, and establish a basis for in vitro assays evaluating antigen-specific responses.

The evolution of Spartina anglica C.E. Hubbard (Gramineae): origin and genetic variability

An extensive survey of isozyme phenotypes in British populations of the amphidiploid salt marsh grass Spartina anglica and its putative parents has confirmed that the species arose by chromosome doubling in S. × townsendii, a sterile hybrid between S. maritima and S. alterniflora. Isozyme phenotypes and seed protein profiles indicate that S. anglica is almost totally lacking in genetic variation. Isozyme evidence also indicates that the parental species are characterized by low levels of genetic variation. The lack of variation in S. anglica is proposed as being due to a narrow genetic base resulting from a single origin, or a multiple origin from uniform parents; the fact that many populations are derived from very small founder populations; and because preferential pairing between identical homologous chromosomes prevents recombination between the divergent component genomes of the species. The low levels of isozyme variation that occur appear to be due to chromosome loss. The consequences for the future evolution of S. anglica, given its lack of genetic variation, are discussed.

QUANTITATIVE GENETICS OF FITNESS TRAITS IN A WILD POPULATION OF PHLOX.

To determine if the evolution of fitness traits in the annual plant, Phlox drummondii, is constrained by lack of genetic variation, we calculated the heritability and genetic correlation of 16 traits in a field population. Full- and half-sib families of seeds were generated in the greenhouse and planted back into the study population. Of 855 seeds that germinated, 609 survived to produce fruit. For each plant we measured several aspects of plant size and three components of female fecundity: total number of fruits produced, number of seeds per fruit, and mass of individual seeds. Heritability of traits ranged from 0.00 to 0.15. Several traits showed significant levels of additive genetic variance, but we found no evidence of additive genetic variance in components of female fecundity and no evidence of negative genetic correlation between fitness traits. These results suggest that evolution in this population would be constrained by lack of heritable variation in fitness traits.

Ecological genetics of Bromus tectorum : I. A hierarchical analysis of phenotypic variation.

An understanding of how genetic differentiation and phenotypic plasticity may interact to promote the spread of an introduced species requires information on the hierarchical distribution of genetic variation within the species in its new range. For example, a lack of genetic variation within marginal populations of an introduced species may slow its rate of spread into new habitats. In a glasshouse study, we examined the phenotypic variation among populations, among families, within families, and the homogeneity of variances within families for morphological and life history characters of an alien, self-pollinating annual grass, Bromus tectorum. The populations of B. tectorum studied were collected from both large, central populations in steppe habitats and small, peripheral populations in forest sites along a broad temperature-moisture gradient in western North America. Most variation in average flowering time was due to differences among seed source populations; among family variation contributed less than 20% to the total variance for flowering time. Populations from arid steppe habitats were the earliest flowering while the population from the most mesic forest habitat was last to flower. In contrast, the within-family variance was a major contributor to the total variance for plant dry weight, seed number per plant, total seed weight, and individual seed weight. The amount of total variation explained by among-family differences ranged between 18% for average seed weight to 30% for total plant dry weight. There was no consistent difference in within-population genetic variability between large, central populations in steppe habitats and smaller, potentially more isolated populations in forest habitats. Significant heterogeneity in within-family variance in some of the source populations suggest that families differ in the capacity for phenotypic response to environmental variation. Considered independently from source population, there was no consistent trend linking a particular trait to increased heterogeneity of within-family variances.

Beta vulgaris NC-7 Collection as a Source of High Sucrose Germplasm

Among the major problems facing sugarbeet breed­ ers are lack of genetic diversity and the negative association between root yield and sucrose concen­ tration. The USDA-ARS Beta collection contains a wide array of material that has not been utilized fully. In this study 167 accessions of the NC-7 collec­ tion were evaluated for sucrose concentration on an individual root basis. A breeding population was formed by interpollinating 30 individuals with rela­ tively high sucrose concentration. Seed from each plant were harvested separately and progeny were evaluated the following year in replicated field plots. Subsequent selection was based upon both individual and family performance. Selected plants were interpollinated, seed were harvested again from individual plants and the progenies were evaluated as a family. This procedure was repeated in each of five selection cycles. After the second cycle it was apparent that root weight was decreasing drastically, so individual root weight was added to the selection criteria. The average sucrose concentra­ tion of fifth-cycle families was increased to 113 % of Ultramono. Five families combined high sucrose concentration with average root yield in the fourth and fifth selection cycles. This research indicates that germplasm collections of the USDA can be used for improving yield and quality of sugarbeet and for widening the genetic base of the sugarbeet crop.

Absence of nitrogen fixation in clover grown on soil subject to long-term contamination with heavy metals is due to survival of only ineffective Rhizobium

All strains of Rhizobium isolated from ineffective nodules on white clover (> 50 separate isolations) formed in metal-contaminated soils from a field experiment were demonstrated to be wholly ineffective in nitrogen fixation in plant infection tests on N-free nutrient agar. The plasmid profiles of these isolates were all very similar indicating a lack of genetic diversity in the population surviving in high concentrations of heavy metals. Isolates from comparable field plots of uncontaminated soil had a wide diversity of plasmid profiles. Inoculation of white clover at sowing with a large inoculum of effective Rhizobium leguminosarum bv. trifolii on metal contaminated soil resulted in effective N 2-fixation. However, if the inoculated soils were held for 2 months in a moist condition before sowing with white clover, N 2-fixation was not detected with inoculum doses of 10 7 cells pot −1 or less but was obvious where a very large inoculum of 10 10 cells pot −1 was added. This indicates that white clover rhizobia are unable to survive (or at least unable to remain effective) in the presence of concentrations of heavy metals close to the current Commission of the European Communities guidelines for environmental protection. A rapid method of assessment of the toxic effects of pollutants on populations of Rhizobium spp is described.

Endemism in Rock Outcrop Plant Communities of Unglaciated Eastern United States: An Evaluation of the Roles of the Edaphic, Genetic and Light Factors

Within the unglaciated eastern United States, where the natural vegetation is mostly deciduous forest, edaphically-controlled herbaceous plant communities occur on unshaded rock outcrops in shallow soil developed from serpentine, shale, granite, limestone, dolomite, sandstone, chert, rhyolite and other rock types. The most distinctive features of the flora of these communities are their endemic taxa. Many of the endemics are not restricted to a single geological substratum, and they grow equally well, or better, in non-outcrop as in outcrop soil. Thus, it seems unlikely that they are restricted to the outcrops because of a require- ment for some chemical, physical or biological element found there but nowhere else. Several of the endemics exhibit intraspecific variability in one or more of the following characteristics: morphology, physiology, breeding system, flower colour and enzyme pattern. Thus, it is unlikely that lack of genetic variation per se is the cause of endemism. ' A requirement that the endemics have in common for growth and survival is a high photosynthetic photon flux density. The obligate nature of the high light requirement is supported by: (1) field observations that the endemics grow on well-lighted portions of the outcrops but not in adjacent forests, (2) studies which show that growth and photosynthetic rates are maximal in high light but are severely inhibited at low light levels, and (3) field and glasshouse experiments which show that the endemics are shade intolerant and that they compete poorly with taller plants. Thus, of the three hypotheses examined to account for the proximal causes of endemism in the rock outcrop endemics, the requirement for a high level of light is the most important.

Study of electrophoretic variability in Epipactis helleborine (L.) Crantz, E. palustris (L.) Crantz and E. microphylla (Ehrh.) Swartz (fam. Orchidaceae)

Allozymic variation in seven enzymes coded by eight loci was studied in seven populations of terrestrial Epipactis orchids, including four populations of E. helleborine, one population of E. palustris and two of E. microphylla, from Latium (Central Italy). Heterozygosity estimates reveal that E. helleborine (EH) is the most variable species (HET=0.233; P=0.59; A=1.8), followed by E. palustris (EP) (HET=0.085; P=0.29; A=1.3) while E. microphylla (EM) is monomorphic for all the loci examined. It is suggested that the two EM populations examined, with a total lack of genetic variability, could originate from the same genetically impoverished ancestral population. Also the low variability amount of EP can be ascribed to genetic drift, probably due to a founder event. Nei's identity coefficients were also calculated. These show a high similarity in EH (Ī=0.987) and EM (Ī=1.000) populations. Average identity value between EP and EM is 0.289, between EP and EH is 0.336 and between EM and EH is 0.784. These data reveal that E. palustris presents a remarkable genetic, besides geographical and ecological, differentiation.

THE GENETIC BASIS OF THE ECOLOGICAL AMPLITUDE OF SPARTINA PATENS. II. VARIANCE AND CORRELATION ANALYSIS.

Reciprocal transplantations of Spartina patens genotypes from adjacent salt marsh, swale, and dune habitats provided evidence for genetic differentiation among subpopulations, due at least in part to contrasting selection regimes. Genet survival in the different habitats was related to the amount of genetic divergence. In the dune habitat, marsh ramets showed the lowest survival, swale ramets showed intermediate survival, and dune ramets showed the highest survival. This relationship was not reciprocal, however. The marsh habitat afforded an environment where survival was maximal for all genotypes. Thus, by comparison, the dune environment appeared to impose a more intense selection pressure, and the swale an intermediate selection pressure on Spartina patens. In each site resident genotypes tended to show greater relative fitness than aliens. This evidence for genetic divergence corroborates that previously reported on morphometric (Silander and Antonovics, 1979) and allozymic traits (Silander, 1984). High levels of phenotypic plasticity may permit greater adaptation to the spatially and temporally heterogeneous environment occupied by S. patens than would genetic variation alone. Dune and swale genets were more phenotypically plastic across traits examined than were marsh genotypes. The higher plasticity in these peripheral subpopulations may confer increased fitness among residents and compensate for observed declines in genetic variation. A slight decrease in genetic variability was evident from marsh to dune subpopulations. However, since the differences in genetic variation among subpopulations were small, and disparities did occur, it is unlikely that evolutionary divergence is retarded primarily by a lack of genetic variability in the characters considered. Evidence is presented to indicate that evolutionary divergence among subpopulations may be retarded by negative or unfavorable correlations among characters being selected simultaneously. These negative correlations may increase extinction probabilities in small peripheral populations, such as those represented by the dune or swale, and are likely to lower fitness. Based on these observations, I hypothesize that further microevolution may be retarded in peripheral dune and swale subpopulations, primarily by unfavorable genetic correlation structures among fitness components or characters under simultaneous selection. Contributing factors may include lowered genetic variance and higher levels of phenotypic plasticity.

Enzyme Polymorphisms in the Phylogenetic Relics Cycas rumphii and Dracaena draco

Summary Palmferns and dragon trees have survived largely unchanged since the Upper Carboniferous and the Tertiary. To find out if this absence of any evolutionary potential is possibly due to a lack of genetic variation, the level of enzyme polymorphism in 48 adult specimens of the Malaysian species Cycas rumphii and in 59 seedlings originating from three Dracaena draco trees from the Canary Island Tenerife was studied using horizontal starch gel electrophoresis. In Cycas a great variety of esterase patterns was obtained with at least two clear cases of allelic polymorphism. In Dracaena one esterase and one phosphoglucomutase exhibit polymophism, too, but alcohol dehydrogenase was monomorphic. Whereas Cycas exhibits appreciable heterozygosity, single dragon trees are homozygous and since they are forced to self-pollinate on account of their isolation, progenies are homozygous, too. On the whole, it is assumed that genetic “relics” are genetically not less variable than those forms evolving at standard rates. Their lack of evolutionary flexibility therefore must be due to other causes, perhaps to a monotonous environment and/or to genetic homeostasis.

LIMITATIONS TO THE USE OF PHYSIOLOGICAL VARIABILITY IN PLANT BREEDING

The possibility that breeding programs may be reaching a point of limited progress because of the lack of genetic diversity has often been raised. However, there is also an often-expressed feeling that considerable genetic variability may exist in fundamental physiological processes. In this review, an attempt is made to examine whether there is evidence of genetic variability in quantitative physiological characters, and if so, whether such variability is likely to be useful to crop improvement programs. The results, presented from the literature and the author’s laboratory, indicate that physiological characters demonstrate considerable genotypic variability in expression. Moreover, in cases where genotype performance has been studied over a range of environments or where crossing studies have been carried out, physiological characters often have relatively high heritabilities. This suggests that, at least from a genetic standpoint, improvements in the expression of physiological processes may be possible. On the other hand, the problems of environmental sensitivity and its resultant error variability, combined with cumbersome methodology and complex physiological interactions, make it difficult to relate individual physiological characters to agronomic benefits, and make large genetic studies difficult. The exploitation of physiological diversity remains a major challenge to plant scientists.Key words: Genetic heritability, physiology of yield

ASEXUAL REPRODUCTION AND GENETIC POPULATION STRUCTURE IN THE COLONIZING SEA ANEMONEHALIPLANELLA LUCIAE

Populations of Haliplanella luciae on the Atlantic coast of North America typically are composed of one or a few strictly asexual clones. The lack of genetic variability in one local population, that at Blue Hill Falls, Maine, is reflected in the population's response to extremes of temperature and salinity. As the limits of tolerance are approached, there is an abrupt and epidemic incidence of mortality, rather than a gradual one. Genetic and the concomitant physiological uniformity explain the well-known tendency for local populations of Haliplanella to disappear suddenly and illustrate a common outcome of the founder effect. The success of Haliplanella as a colonizer is due to its extreme hardiness toward physical environmental factors and to its prolific asexual nature. Asexual reproduction by longitudinal fission and by pedal laceration not only provides a means of rapid colonization of a new habitat, but also a means of producing multiple copies of genotypes that have proved to be successful under local conditions. The resultant population structure recalls that in other animal and plant colonizers, in which there is heavy reliance on general purpose genotypes in isolated populations. The comparative lack of colonizing success in the ecologically similar anemone, Diadumene leucolena, relates to its reproductive biology. Not only is the asexual potential for the rapid spread of successful genotypes lacking, but sexual reproduction typically requires a more optimal set of environmental circumstances which will not necessarily be met in a new habitat.