BIOCHEMICAL ADAPTATION AND LOSS OF GENETIC CAPACITY IN HELMINTH PARASITES

1. Adaptation and loss of genetic capacity differ chiefly in that adaptation is goal- directed whereas loss of genetic capacity is not. Given sufficient information about an individual organism and its environment, adaptations are recognizable without reference to historical events extending beyond a single generation. This is not true of loss of genetic capacity, which requires a preliminary judgement that genetic information now absent was present in ancestral organisms. Together, adaptation and loss of genetic capacity are the major contributors to overall reproductive fitness. Accidental selection is genetically associated with adaptation, but is not goal-directed. 2. Adaptations arevariant or invariant; invariant adaptations comprising biochemical unity, and variant adaptations contributing to biochemical diversity. Variant adaptations may be either exploitive or epigenetic. Exploitive adaptations are a measure of thegenetic capacity for phenotypic response to an altered environment, which the individual may not in fact encounter. Epigenetic adaptations are more rigidly programmed and are responsive to altered environments only insofar as these are a constant feature of the life cycle. 3. Selected observations in the biochemistry of helminth parasites are examined with respect to their interpretation in terms of adaptation, loss of genetic capacity and accidental selection. Secure judgements concerning adaptation are often possible at the most general level, i.e. when the physicochemical properties of the environment, such as temperature or oxygen supply, are clearly defined. I t is more difficult to make judgements concerning the specific mechanisms used in achieving these goals. Conclusions concerning loss of genetic capacity require knowledge of the specific function through-out the life cycle. In many cases loss of genetic capacity is only apparent, as the function appears in another part of the life cycle. Such apparent losses are in reality epigenetic adaptations. These concepts are helpful in interpreting past work and in devising new experiments. 4. Development in helminth parasites includes a pronounced capacity for the orderly release of information to be used in the next stage. As each stage may require a radically different environment, programming for it may lead to phenomena which are superficially puzzling, such as the existence of aerobic electron transport systems in a stage whose energy metabolism is fermentative. The concept of epigenetic adaptation is especially useful for interpreting such observations. 5. Although possible adaptations are most readily apparent in biochemically complex mechanisms, these mechanisms are an expression of the orderly effects of many different primary gene products which have not been much studied. There are indications that organisms possessing relatively complex life cycles may provide opportunities for relating primary gene products, such as isozymes, to their physiological functions.     

GENETIC CONSEQUENCES OF PASSIVE DISPERSAL IN POND-DWELLING COPEPODS

Pond-dwelling copepods have colonized habitats throughout North America after glaciers have receded. Most species are passively transported via resting eggs into new habitats. Colonists originating in a glacial refugium could lose some of the ancestral genetic diversity when they establish new populations and the attenuation may be substantial in populations far removed from the refugium due to multiple founder events. Genetic variation was measured in Heterocope septentrionalis from 27 populations at arctic sites near potential refugia and those more recently deglaciated to determine the effects of postglacial dispersal on patterns of genetic relatedness and diversity. Some populations were more distant, genetically, from others within the same site than those from other more distant sites. Eleven polymorphic enzyme loci were significantly more variable (F [1,294 df] = 5.94, P < 0.025) among individuals from populations near the Alaskan refuge than those at the eastern limit of their distribution. Because populations are typically extremely large and stable this loss of genetic diversity is attributed primarily to repeated founder events during colonization. This result suggests profound genetic changes may occur on a continental scale in passively dispersed copepods due to founder events alone. Their potential for divergence and speciation is greater than currently recognized.     

GENETIC CONSEQUENCES OF AUTOPOLYPLOIDY IN TOLMIEA (SAXIFRAGACEAE)

Although there is an extensive literature on the genetic attributes of allopolyploids, very little information is available regarding the genetic consequences of autopolyploidy in natural populations. We therefore addressed the major predicted genetic consequences of autopolyploidy using diploid and tetraploid populations of Tolmiea menziesii. Individual autotetraploid plants frequently maintain three or four alleles at single loci: 39% of the 678 tetraploid plants exhibited three or four alleles for at least one locus. Heterozygosity was also significantly higher in autotetraploid populations than in diploid populations: H_° = 0.070 and 0.237 in diploid and tetraploid Tolmiea, respectively. Most of the genetic diversity in T. menziesii is maintained within populations (ratio of gene diversity within populations to mean total genetic diversity = 0.636). The total genetic diversity due to differentiation between the two cytotypes is only 0.055. Such a low degree of differentiation between cytotypes would be expected between a diploid and its autotetraploid derivative. Most diploid and all tetraploid populations examined are in genetic equilibrium. Diploid and tetraploid Tolmiea share three or four alleles at six of eight polymorphic loci. This suggests that either autotetraploid Tolmiea was formed via crossing of genetically different diploids (perhaps via a triploid intermediate) or autopolyploidy occurred more than once in separate individual plants, followed by later crossing of autotetraploids.     

DYNAMIC AND GENETIC CONSEQUENCES OF VARIATION IN HORIZONTAL TRANSMISSION FOR A MICROPARASITIC INFECTION

Transmission to a new host is a critical step in the life cycle of a parasite. Variation in the characteristics of the transmission process, for example, due to host demography, is assumed to select for different variants of the parasite. We have experimentally tested how variation in the time to transmission (early or late after infection) and exposure to adverse conditions outside the host (immediate or delayed contact with new host) interact to determine the success of the infection in the next host, using the trypanosome Crithidia bombi infecting its bumblebee host, Bombus terrestris. These two experimentally manageable steps mimic the processes of within- and among-host selection for the parasite. We found that early transmission led to higher infection success in the next host as did immediate contact with the new host. However, there was no interaction between the two parameters as would be expected if early-transmitted variants, resulting from rapid multiplication within the host, would be less adapted to the conditions encountered during the between-host transfer or infection of the next host. Furthermore, typing the genetic variability of the parasites with microsatellites showed that the four different transmission routes of our experiment selected for different degrees of allelic diversity of the infecting parasite populations. The results support the idea that variation in the transmission process selects for different genotypic variants of the parasite. At the same time, the relationship of allelic diversity with infection intensity suggested that the coinfection model of May and Nowak (1995) may be appropriate, where each parasite is able to infect and multiply independent of others within the same host.     

ECOLOGICAL AND GENETIC CONSEQUENCES OF POLLINATION BY SEXUAL DECEPTION IN THE ORCHID CALADENIA TENTACTULATA

Only orchids affect pollination by the deceptive sexual attraction of male insects, a syndrome particularly well developed in Australia. We examined the ecological and genetic consequences of exclusive pollination by sexually attracted male thynnine wasps in the orchid Caladenia tentaculata. Male wasps respond rapidly to flowers artificially presented in 1 × 1 m² experimental patches. Sixty of 287 wasps approached within centimeters of the flower, but did not land. Of the remaining 79% who made floral contact, only 7.5% attempted copulation, the step critical for pollination. Wasps only rarely moved among patches (19% of flights) and none attempted copulation a second time, resembling observations in natural populations. We confirmed outcrossing and long distance pollen flow by monitoring how colored pollen moved in natural populations. Pollen movements approximated a linear rather than a leptokurtic distribution (mean distance: 17 m; maximum: 58 m). Pollinator visits varied independently of flower density in three of four populations with most solitary flowers being visited. Allozyme analysis revealed within-population fixation indices (F) close to zero and low levels of differentiation (FST) among populations. Despite behavioral evidence for long distance pollen flow, significant local genetic structure exists, perhaps reflecting restricted seed dispersal. Long distance pollen flow in C. tentaculata may therefore promote outbreeding by minimizing pollen transfers among related neighbors. Although this species is self-compatible, outcrossed progeny develop significantly faster than selfed progeny. Effective pollination at low flower densities could accentuate this advantage. The data are consistent with the predictions that deceptive pollination will result in long distance pollen flow, which may be of selective advantage at low density. Comparative studies of how food reward, food deceptive, and sexual deceptive pollination systems vary within a phylogenetic framework could further illuminate the evolution of sexual deception.     

SOME POPULATION GENETIC CONSEQUENCES OF COLONY FORMATION AND EXTINCTION: GENETIC CORRELATIONS WITHIN FOUNDING GROUPS

Extinction and recolonization in an island model affects genetic differentiation among subpopulations through a combination of sampling and mixing. We investigate the balance of these forces in a general model of population founding that predicts first the genetic variance among new groups and then the effect of these new groups on the total genetic variance among all populations. We allow for a broad range of types of mixing at the time of colonization and demonstrate the significant effects on differentiation from the probability of common origin of gametes (φ). We further demonstrate that kin-structured founding and inbreeding within populations can have a significant effect on the genetic variance among groups and use these results to make predictions about lineal fission and fusion of populations. These results show that population structure is critically affected by non-equilibrium dynamics and that the properties of new populations, especially founding number, probability of common origin, and kin structure, are vital in our understanding of genetic variation.     

BIOCHEMICAL COMPOSITION OF HELIX SNAILS: INFLUENCE OF GENETIC AND PHYSIOLOGICAL FACTORS

This paper describes the biochemical composition of differentspecies (Helix lucorum, Helix pomatia) and sub-species of snails(Helix aspersa aspersa, Helix aspersa maxima) reared in thesame conditions with a feed (‘Helixal’) speciallydesigned for edible snails. In addition, the composition ofwild H. pomatia and H. lucorum is presented to allow comparisonbetween snails of different origins. Analyses determined thepercentages of proteins, lipids and minerals. They reveal bothsimilarities and differences in composition according to thespecies and the part analysed (whole body, pedal mass, and visceralmass). H. pomatia contains the highest percentage of mineralmatter and the lowest percentage of lipids. Surprisingly, proteincontents are slightly different between artificially rearedH. aspersa maxima of 3 months old and wild H. pomatia. The resultsmake it possible to evaluate nutritional quality of snails withthe composition of the body of four edible snail species. (Received 16 September 1996; accepted 24 May 1997)     

CHROMATOGRAPHIC COMPARISON OF TRAGOPOGON SPECIES AND HYBRIDS

Approximately 1,700 plants representing five species of Tragopogon (Compositae) and their F₁ and F₂ hybrids were analyzed by two-dimensional descending paper chromatography. Each species, or population within a species, was chromatographically distinct. Often, however, the differences were more quantitative than qualitative. The chromatographic data generally supported the species relationships which had been determined from previous morphologic, hybridization, and fertility studies. Inheritance of the flavonoid compounds was usually additive in the F₁'s. Segregation and recombination of the genes controlling the synthesis of these compounds sometimes approximated 3:1 or 9:7 ratios in the F₂'s. Occasionally parental compounds were missing from some of the hybrids. “Hybrid” compounds which had not been found in either parent were absent from the F₁ but did occur in several F₂ populations. Two linkage groups were present. The first contains genes controlling the synthesis of three compounds and the second, four compounds.     

POPULATION GENETIC CONSEQUENCES OF DEVELOPMENTAL EVOLUTION IN SEA URCHINS (GENUS HELIOCIDARIS)

Within the sea urchin genus Heliocidaris, changes in early embryonic and larval development have resulted in dramatic differences in the length of time larvae spend in the plankton before settling. The larvae of one species, H. tuberculata, spend several weeks feeding in the plankton before settling and metamorphosing into juveniles. The other species, H. erythrogramma, has modified this extended planktonic larval stage and develops into a juvenile within 3–4 days after fertilization. We used restriction site polymorphisms in mitochondrial DNA to examine the population genetic consequences of these developmental changes. Ten restriction enzymes were used to assay the mitochondrial genome of 29 individuals from 2 localities for H. tuberculata and 62 individuals from 5 localities for H. erythrogramma. Within H. tuberculata, 11 mitochondrial genotypes were identified. A G_ST analysis showed high levels of genetic exchange between populations separated by 1,000 kilometers of open ocean. In contrast, in H. erythrogramma, 13 mitochondrial genotypes differing by up to 2.33% were geographically partitioned over spatial scales ranging from 800 to 3,400 kilometers. Between distant localities, there was complete mitochondrial lineage sorting and large sequence divergence between resulting clades. Over much smaller spatial scales (< 1,000 km), genetic differentiation was due to the differential sorting of very similar genotypes. This pattern of mitochondrial variation suggests that these population differences have arisen recently and may reflect the historical interplay between the restricted dispersal capabilities of H. erythrogramma and the climatic and geological changes associated with Pleistocene Ice Ages.     

GENETIC VARIATION IN TRAGOPOGON SPECIES: ADDITIONAL ORIGINS OF THE ALLOTETRAPLOIDS T. MIRUS AND T. MISCELLUS (COMPOSITAE)

Genetic diversity in the introduced diploids Tragopogon dubius, T. porrifolius, and T. pratensis and their neoallotetraploid derivatives T. mirus and T. miscellus was estimated to assess the numbers of recurrent, independent origins of the two tetraploid species in the Palouse region of eastern Washington and adjacent Idaho. These tetraploid species arose in this region, probably within the past 50–60 yr, and provide one of the best models for the study of polyploidy in plants. The parental species of both T. mirus and T. miscellus have been well documented, and each tetraploid species has apparently formed multiple times. However, a recent survey of the distributions of these allotetraploids revealed that both tetraploid species have expanded their ranges considerably during the past 50 yr, and several new populations of each species were discovered. Therefore, to evaluate the possibility that these recently discovered populations are of recent independent origin, a broad analysis of genetic diversity in T. mirus, T. miscellus, and their diploid progenitors was conducted. Analyses of allozymic and DNA restriction site variation in all known populations of T. mirus and T. miscellus in the Palouse and several populations of each parental diploid species revealed several distinct genotypes in each tetraploid species. Four isozymic multilocus genotypes were observed in T. mirus, and seven were detected in T. miscellus. Tragopogon mirus possesses a single chloroplast genome, that of T. porrifolius, and two distinct repeat types of the 18S-26S ribosomal RNA genes. Populations of T. miscellus from Pullman, Washington, have the chloroplast genome of T. dubius; all other populations of T. miscellus have the chloroplast DNA of T. pratensis. All populations of T. miscellus combine the ribosomal RNA repeat types of T. dubius and T. pratensis, as demonstrated previously. When all current and previously published data are considered, both T. mirus and T. miscellus appear to have formed numerous times even within the small geographic confines of the Palouse, with estimates of five to nine and two to 21 independent origins, respectively. Such recurrent polyploidization appears to characterize most polyploid plant species investigated to date (although this number is small) and may contribute to the genetic diversity and ultimate success of polyploid species.     

THE GENETIC CONSEQUENCES OF WORKER ANT POLLINATION IN A SELF-COMPATIBLE,CLONAL ORCHID

The self-compatible orchid Microtis parviflora is pollinated by the flightless worker caste of the ant Iridomyrmex gracilis. The orchid is clonal and forms small patches, usually less than 1 m², of disconnected individual ramets. Ant pollinators visited and revisited a limited proportion of available inflorescences, and 40% of all flower visits occurred within plants promoting self-pollination. Pollen labels indicated that self-pollination accounted for 51% of the pollen transfers, although pollen carryover extended beyond 16 flowers on 2 or 3 inflorescences. The distribution of ant movements between plants was leptokurtic with a mean of 12.4 ± 14.9 cm and a maximum of 89 cm, but a high proportion of movements were within clones accentuating the level of self-pollination. However, some pollen transfers between inflorescences of unlike genotypes contributed to a low incidence (max = 8%) of outcrossing. In 12 patches examined by electrophoresis, the density varied from 11 to 61 inflorescences per m² and a maximum of only 4 genotypes were detected. Electrophoretic analysis revealed populations were highly inbred: only 23% (N = 17) of the loci were polymorphic and the mean gene diversity h, was 2.7%. Heterozygotes were observed in only one population given a mean fixation index F, of 0.982. These results reflect the combined effects of restricted ant foraging and clonality. Nevertheless, while ant foraging was restricted, some outcrossing occurred and in the absence of clonality it is likely that ant foraging would have yielded a mixed mating system similar to those reported for a wide array of insect pollinators. Given the ability of ants to generate pollen flow, the reasons for the rarity of ant pollination appear to lie elsewhere.     

COMPLEX COLONY STRUCTURE IN SOCIAL INSECTS: I. ECOLOGICAL DETERMINANTS AND GENETIC CONSEQUENCES

For social insect species, intraspecific variation in colony social structure provides an opportunity to relate the evolution of social behavior to ecological factors. The species Myrmica punctiventris is a cavity-dwelling forest ant that exhibits very different colony structures in two populations in the northeastern United States. Combined data from seasonal censuses, allozyme electrophoresis, and worker hostility tests showed that a population of M. punctiventris in Vermont was strictly monogynous and seasonally polydomous. The same procedures showed that a population of M. punctiventris in New York was facultatively polygynous and predominantly monodomous. Genetic relatedness among colony-mates was not different from Hamilton's expected values in the Vermont population and was consistent with little exchange of ants between colonies and single-mating of queens. In contrast, relatedness was lower in New York, and examination of nest-mate genotypes revealed exchange of ants between colonies, high rates of colony loss and replacement of queens, or multiple-mating of queens. The genetic structure of the Vermont population was consistent with no inbreeding, but in New York, the population genetic structure reflected microgeographic subdivision and inbreeding. Previous study of the ant communities at these sites implicates nest-site limitation in New York as a primary constraint on social structure.