Jeffry B. Mitton

Genetic variation in space and time in a population of ponderosa pine

SummaryThe genetic structure of a population of Pinus ponderosa was studied using seven electrophoretically-detectable protein loci. This population is composed of six groups of trees, which differ significantly from each other in their genetic constitutions. The results imply that the population is composed of groups of individuals assembled into genetically-related family units. Whenever such family clusters have been documented in other studies, marked inbreeding has also been noted as a consequence. In this population, no inbreeding is detectable. The groups also differ from one another in several characteristics which have genetic consequences within the population as a whole. These characteristics include age structures, seed output, level of infestation by woolly aphids and extent of damage by deer browsing. The population was also divided into four age classes which do not differ markedly from one another in their genetic characteristics. This suggests that genetic differentiation in time is much less marked than differentiation in space.

Genetic variation and the natural history of quaking aspen

In the fall, sightseers take to the highways of North America to enjoy the brilliant colors that are revealed as deciduous trees recycle the dominant greens of chlorophyll. In the western United States and Canada, the most colorful tree in the fall is the quaking aspen, Populus tremuloides. Brilliant yellows, rich golds, and shimmering shades of red shine, contrasting with the various green shades of the conifers. Quaking aspen earns its name for the distinctive fluttering of its leaves, even in the most gentle breezes. Early French-Canadian trappers called the tree an aspen because of its similarity to Populus tremula, a closely related species in Europe and Asia. One of the legends attached to the aspen reflects Judeo-Christian influences. The aspen quake in fear today, according to folklore, because Jesus Christ was crucified on a cross of aspen. In addition to the esthetics of quaking leaves and brilliant fall colors, extraordinary features of the natural history and genetics of aspen lend it special appeal for naturalists and laboratory scientists. Quaking aspen merits a variety of superlatives: It is North Americas most widely distributed native tree species and the second most widely distributed in the world (Barnes and

ELEVATIONAL GRADIENTS IN ADULT SEX RATIOS AND SEXUAL DIFFERENTIATION IN VEGETATIVE GROWTH RATES OF POPULUS TREMULOIDES MICHX.

Since Fishers (1930) early work on the effects of selection on the primary sex ratio which showed that mixtures should tend toward equal numbers of males and females, there have been several theoretical studies which have pointed out conditions in which deviations from 1:1 could be expected (e.g., Bodmer and Edwards, 1960; Emlen, 1970; Eshel, 1975). Empirical studies, usually of adult ratios, have shown strong imbalances in several cases (e.g., Putwain and Harper, 1972; Freeman et al., 1976; Bawa and Opler, 1977). The extent to which sex dependent mortality among adults influences changes in the primary ratio is still under theoretical investigation (Trivers and Willard, 1973; Werren and Charnov, 1978). The number of empirical studies which may be used to test these theories is still small, especially in plants (Godley, 1964; Putwain and Harper, 1972; Lloyd, 1973; Freeman et al., 1976; Bawa and Opler, 1977). We here report the results of field studies on adult sex ratios and concomitant patterns of vegetative growth rates for quaking aspen (Populus tremuloides Michx.) in the Colorado Front Range. Quaking aspen is the most widespread tree in North America, extending from Alaska to Mexico and from Maine to the state of Washington. Members of the genus are of substantial commercial, recreational and ecological significance (Little, 1971), as well as the subject of a large literature of physiological, silvicultural and general ecological attributes (Barnes, 1966, 1969, 1975; Reed, 1971; Anon., 1976). The extensive, beautiful stands or groves of quaking aspen familiar to travelers in the northern or western parts of Revised February 2, 1979

Population density, outcrossing rate, and heterozygote superiority in ponderosa pine

viously been shown to be negatively correlated with self-fertilization, clumped pollen transfer, pollen size, and the ratio of stigma area to the pollen-bearing area on the pollinator. Here, the four correlations are shown to be consistent with evolutionary models of optimal allocation of resources to male and female functions, providing evidence that selection on sex allocation in hermaphrodites is governed by the same principles as selection on the sex ratio.

Enzyme heterozygosity, metabolism, and developmental stability

Developmental homeostasis, measured as either fluctuating asymmetry or variance of morphological characters, increases with enzyme heterozygosity in many, but not all, natural populations. These results have been reported forDrosophila, monarch butterflies, honeybees, blue mussels, side-blotched lizards, killifish, salmonid fishes, guppies, Sonoran topminnows, herring, rufous-collared sparrows, house sparrows, brown hares, white-tailed deer, and humans. Because heterozygosity at a few loci can not predict heterozygosity of the entiry genome, these loci must be detecting localized zones that influence the developmental environment.Studies of malate dehydrogenase in honeybees,Apis mellifera, and lactate dehydrogenase in killifish,Fundulus heteroclitus, revealed that developmental homeostasis varied with heterozygosity of individual loci. Heterozygotes differed from homozygotes in fluctuating asymmetry, morphological variance, and in correlations between morphological characters.The protein loci in these studies code for enzymes, and therefore do not directly influence morphological characters. However, some enzymatic loci substantially influence metabolism, and contribute to variation in the amount of energy available for development and growth. This argument can be made most convincingly for the LDH polymorphism in killifish. LDH genotypes differ in enzyme kinetic properties that measure differences in physiological efficiency, and these differences produce measurable and predictable differences in physiology and development. Under environmental conditions which impose a stress upon development, genotypes at these loci may have different amounts of energy available for development, and consequently exhibit different levels of developmental homeostasis.

Why reproduce sexually

Abstract There is reason to believe that intense selection, such that only a small minority at the top of the fitness distribution has any appreciable chance of survival, can sometimes give sexual reproduction an immediate (one-life-cycle) advantage over asexual. The advantage must be great enough to balance the 50% loss of genetic material in meiosis.One model shows the advantage to be frequency-dependent in life cycles in which there are several asexual generations and one sexual. The observed frequency of sexual reproduction in such a life cycle is explained as an evolutionary equilibrium by this model. In another model the optimum frequency of asexual reproduction drops to zero as fecundity and competition increase. This explains the exclusively sexual reproduction of such fecund organism as elms and oysters. Once lost, asexual reproduction may be difficult to evolve secondarily. This explains the presence of such exclusively sexual, low-fecundity organisms as the higher vertebrates.

Mountain pine beetle develops an unprecedented summer generation in response to climate warming.

The mountain pine beetle (MPB; Dendroctonus ponderosae) is native to western North America, attacks most trees of the genus Pinus, and periodically erupts in epidemics. The current epidemic of the MPB is an order of magnitude larger than any previously recorded, reaching trees at higher elevation and latitude than ever before. Here we show that after 2 decades of air-temperature increases in the Colorado Front Range, the MPB flight season begins more than 1 month earlier than and is approximately twice as long as the historically reported season. We also report, for the first time, that the life cycle in some broods has increased from one to two generations per year. Because MPBs do not diapause and their development is controlled by temperature, they are responding to climate change through faster development. The expansion of the MPB into previously inhospitable environments, combined with the measured ability to increase reproductive output in such locations, indicates that the MPB is tracking climate change, exacerbating the current epidemic.

Life-history patterns and sociality in canids: Body size, reproduction, and behavior

SummaryEmpirical associations among co-adapted traits such as body size and patterns of reproduction, development, and behavior are unknown for most animal species, despite numerous theories suggesting otherwise. One way to study these complex relationships is first to consider closely related species and then to generalize findings to other groups. In the present study, relationships among body size, reproductive patterns, development, and sociality were examined in 17 members of the family Canidae (canids). Large canids are more social than smaller species, and offspring of large species achieve independence and tend to breed first at a later age. Large females give birth to absolutely larger young, but relative to their own body weight they allocate fewer resources to bringing a large pup to term. Overall, sexual dimorphism in size is small to moderate, and this is associated with monogamous mating habits and paternal care of young.

Population Genetics of Marine Pelecypods. I. Ecological Heterogeneity and Evolutionary Strategy at an Enzyme Locus

The hypothesis is tested that some isoenzyme-synthesizing loci are responding to natural selection by physical factors of the environment and that consequently correlation in strategy as well as concomitant changes in genetic composition with a changing environment should be detectable among homologous loci of organisms living in very similar habitats. Polymorphisms at the leucine aminopeptidase (Lap) locus of the intertidal mussels Mytilus edulis and Modiolus demissus were studied and may be characterized as follows: (1) both species are polymorphic for three common and two very rare alleles, but the alleles in the two species are not electrophoretically alike; (2) the rank order of relative abundance of alleles of particular electrophoretic mobility is nearly identical for both species at all locations; (3) there is significant heterogeneity in zygotic distributions among localities in both species, but the zygotic distributions compared for the two at any one locality do not differ; and (4) deviations from expected zygotic proportions are observed in all samples, and both species deviate in the same way for all genotypes at any one locality Three additional marine pelecypods were examined, and all were polymorphic for at least three Lap alleles. Both Mytilus and Modiolus are polymorphic for two alleles at one Mdh locus and the same allelic frequencies are observed at all localities. The existence of correlated strategies among such different organisms living under nearly identical ecological conditions suggests: (1) natural selection is effecting a particular evolutionary strategy at the Lap locus. (2) organisms adapt to certain patterns of environmental heterogeneity in the same way, and (3) such loci cannot provide meaningful information about subpopulational structuring but can be used to estimate patterns of environmental heterogeneity.

Paternity and gene dispersal in limber pine (Pinus flexilis James)

This study provides empirical information on intrapopulation gene dispersal via pollen, the size of genetic ‘neighbourhoods’, and interpopulation gene flow in a long-lived conifer, limber pine (Pinus flexilis). We used allozyme loci for a paternity analysis of 518 seeds produced in an isolated population of limber pine located in north-eastern Colorado, U.S.A., separated by 2 km from the nearest conspecific trees and nearly 100 km from populations in the Rocky Mountains. We also used indirect techniques (FST analyses) to estimate gene flow rates among subdivisions of the study population and among five widely separated populations. Within the main study population limber pine exhibited a polymorphism level of 50%, observed heterozygosity of 0.159, and 2.36 alleles per polymorphic locus. Mountain populations were slightly more variable. The main study population showed significant differentiation in allozymes among neighbouring subpopulations. The mean FST was 0.031 and the gene flow rate among subpopulations was estimated as 7.8 migrants per generation. Among widely separated populations the mean FST was 0.035 and the gene flow rate was estimated as 6.9 migrants per generation. The paternity analysis indicated a best estimate of 6.5% pollen immigration (minimum 1.1%) from populations 2 km to 100+ km away. For 4% of the seeds examined, paternity could be ascribed to a single tree in the study population. Fractional paternity and likelihood methods were used to estimate pollen dispersal distances for the remainder of the seeds. Mean pollen dispersal distance was estimated at 140 m using the fractional method, similar to results from the other techniques. This compares with a mean distance of 172 m between potential mates. These results suggest near-panmictic pollen dispersal over this population, which covers about 15 ha. The observed allozyme differences and surprisingly low estimates of among-subpopulation gene flow are ascribed to a probable restriction of gene dispersal by seeds.