Kim E. Zerba

Shaping intraspecifíc variation: development, ecology and the evolution of morphology and life history variation in tiger salamanders

The tiger salamander,Ambystoma tigrinum, is a geographically widespread, morphologically variable, polytipic species. It is among the most variable species of salamanders in morphology and life history with two larval morphs (typical and cannibal) and three adult morphs (metamorphosed, typical branchiate, cannibal branchiate) that vary in frequency between subspecies and between populations within subspecies. We report morphometric evidence suggesting that branchiate cannibals arose through intraspecific change in the onset or timing of development resulting in the wider head and hypertrophied tooth-bearing skull bones characteristic of this phenotype. We also quantified bilateral symmetry of gill raker counts and abnormalities, then evaluated fluctuating asymmetry as a measure of the developmental stability of each morph. There was a significant interaction between fluctuating asymmetry of developmental abnormalities in cannibals and typicals and the locality where they were collected, suggesting that relative stability of each phenotype could vary among populations. While altered timing of developmental events appears to have a role in the evolution and maintenance of morphs, novel phenotypes persist only under favorable ecological conditions. Predictability of the aquatic habitat, genetic variation, kinship, body size, intraspecific competition and predation all affect expression and survival of the morphs inA. tigrinum. This taxon provides an excellent model for understanding the diversity and complexity of developmental and ecological variables controlling the evolution and maintenance of novel phenotypes.

Biological Complexity and Strategies for Finding DNA Variations Responsible for Inter-individual Variation in Risk of a Common Chronic Disease, Coronary Artery Disease

Most common chronic diseases of humans aggregate, but do not segregate, in families. The segregation-linkage research paradigm has not provided great insights into their genetic etiology. In this paper, using coronary artery disease as an example, we discuss hierarchical organization, coherence, emergent properties and dynamism as features that characterize the complexity of genotype-phenotype relationships. We summarize a research strategy for evaluating the contribution of genetic and environmental factors to the prediction of inter-individual variation in risk of disease. We then review a statistical strategy that employs cladistic theory to identify individuals carrying mutant DNA sequences responsible for an observed association between marker variation in a gene and inter-individual variation in biological traits that determine risk of a common multifactorial disease. Finding these DNA sequences is a necessary step in our search for an understanding of the nature of the mapping of genetic variation into variation in risk of a disease like coronary artery disease.

Spatial Heterogeneity and Individual Variation In Diet of an Aquatic Top Predator

Since habitats are generally spatially heterogeneous, an important compo- nent of niche variation may be determined by the relationship between individual variation in feeding among and within habitats. We characterized variation in diet among and within individual tiger salamander larvae (Ambystoma tigrinum nebulosum) among and within sites in natural ponds. We also analyzed prey variation among sites. There was significant variation in diet among sites, controlling for variation in larval size among sites. Prey taxa varied significantly among sites. There were also significant size-associated differences in diet within sites. Controlling for larval size we found no further evidence of significant interindividual differences in diet within sites. Intrapopulation variation in diet among sites is likely a consequence of adaptive phenotypic plasticity in feeding behavior of in- dividuals on a spatially variable prey assemblage. This result supports Van Valens hy- pothesis that selection should favor multiple phenotypes within populations. Variation in diet among sites is directly analogous to the within- and between-phenotype components of niche variation defined by Roughgarden. Size-associated differences in diet represent a significant ontogenetic component to niche variation in feeding within populations of larval salamanders. Studies of food web structure that ignore predator diet and prey differences among sites are an incomplete summary of predator-prey linkages. In addition, phenotypic plasticity in feeding behavior as a source of niche variation has implications for traditional ecological theory of intra- and interspecific interactions, which has rarely included variation other than morphological size differences among animals.

The role of genome type-environment interaction and time in understanding the impact of genetic polymorphisms on lipid metabolism

Genome type-environment interaction is a poorly understood biological phenomenon, which plays a central role in the etiology of traits that are measures of lipid metabolism. The biological axioms that follow from the inherent complexity of genome type-environment interaction are usually ignored in interpretation of human studies of the associations between genetic variation and interindividual phenotypic variation. The consequence is the inflation of type I and type II errors of inference that may explain the failure to confirm most reported associations. Studies of the variation in patterns of lipid phenotypes over time, which are expressed as responses of genome types to experimentally defined changes in the environment, hold the greatest promise for understanding influences of important environmental interventions such as changes in diet and drug therapy.

Application of cladistics to the analysis of genotype-phenotype relationships

We seek to understand the relative contribution of allelic variations of a particular gene to the determination of an individuals risk of atherosclerosis or hypertension. Work in progress is focusing on the identification and characterization of mutations in candidate genes that are known to be involved in determining the phenotypic expression of intermediate biochemical and physiological traits that are in the pathway of causation between genetic variation and variation in risk of disease. The statistical strategy described in this paper is designed to aid geneticists and molecular biologists in their search to find the DNA sequences responsible for the genetic component of variation in these traits. With this information we will have a more complete understanding of the nature of the organization of the genetic variation responsible for quantitative variation in risk of disease. It will then be possible to fully evaluate the utility of measured genetic information in predicting the risk of common diseases having a complex multifactorial etiology, such as atherosclerosis and hypertension.

Traversing the biological complexity in the hierarchy between genome and CAD endpoints in the population at large

An emerging challenge facing those who are concerned about the efficacy of public health programs is to understand how information from the DNA revolution might be used to improve our ability to predict the initiation, progression and severity of a common disease having a complex multifactorial etiology. In the course of research to evaluate the role of information about DNA, combinations of genome types and environmental exposures that predispose to disease will be identified. Such information is expected to be useful in efforts to identify individuals and families at higher risk of disease and to predict their responses to a proposed therapy. This paper begins with a discussion of the features of a realistic biological model for the study of a common multifactorial disease. We present evidence for the complexity in the relationship between genome type variation and variation in risk of coronary artery disease (CAD) and review the preliminary results of our studies to determine whether information about genome type variation can improve our ability to predict the distribution of CAD among individuals in the population at large. Such studies make it apparent that new analytical strategies are necessary to deal with the plethora of genome type information available for the evaluation of risk of a common disease like CAD. This shift in the research paradigm will build upon new strategies to understand the organization of natural systems that are coming from outside the mainstream of genetic research.

Genetic structure of five susceptibility gene regions for coronary artery disease: disequilibria within and among regions

Abstract We analyzed two-locus disequilibria for 16 polymorphic loci of seven susceptibility genes for coronary artery disease located in five chromosomal regions distributed across four chromosomes. Included were the genes coding for apolipoprotein B (ApoB, chromosome 2, four marker loci), lipoprotein lipase (LPL, chromosome 8, three marker loci), apolipoproteins AI, CIII, AIV (ApoAI–CIII–AIV, chromosome 11, three marker loci), apolipoprotein E (ApoE, chromosome 19, two marker loci), and the low density lipoprotein receptor (LDLR, chromosome 19, four marker loci). Our sample included 540 unrelated individuals from the Rochester, Minn. population. There were no statistically significant deviations of single-locus genotypes from Hardy-Weinberg equilibrium. The strongest associations within genes were for composite diallelic disequilibria; 17/19 were significant (13 at Pr <0.001, 1 at Pr <0.01, 3 at Pr <0.05). These observations suggest marker alleles within genes have a shared evolutionary history reflected by disequilibria that have not been dissipated by recombination. Disequilibrium was not generally concordant with the physical orderings of markers. Only two significant higher-order disequilibria were observed although 12 triallelic disequilibria were at maximum possible values. We observed 19 statistically significant disequilibria (Pr <0.05; 4 composite diallelic, 13 triallelic, and 2 quadriallelic) between 101 pairs of marker loci, where each locus in a pair was from a different unlinked region. These unexpected results are most likely explained by recent historical factors, including worldwide population expansion and amalgamation with continuous admixture, that influence the genetic structure (organization of alleles and non-alleles into genotypes) of a population. We conclude that disequilibria between loci from unlinked regions may be more extensive than is commonly assumed. Our findings also suggest that it is, on average, at least 15 times more likely to not detect significant disequilibrium among unlinked loci when it is really present than to make a false positive inference. Disequilibria between functional loci within or between regions will impact estimates of genetic variance associated with particular functional mutations within a susceptibility gene region.