Richard G. Harrison

Animal mitochondrial DNA as a genetic marker in population and evolutionary biology

Animal mitochondrial DNA (mtDNA) is playing an increasingly important role as a genetic marker in population and evolutionary biology. The popularity of this molecule derives, in part, from the relative ease with which clearly homologous sequences can be isolated and compared. Simple sequence organization, maternal inheritance and absence of recombination make mtDNA an ideal marker for tracing maternal genealogies. Rapid rate of sequence divergence (at least in vertebrates) allows discrimination of recently diverged lineages. Studies of mtDNAs from a diversity of animal groups have revealed significant variation among taxa in mtDNA sequence dynamics, gene order and genome size. They have also provided important insights into population structure, geographic variation, zoogeography and phylogeny.

Pattern and process in a narrow hybrid zone

This paper examines variation in morphology and allozymes in a hybrid zone between two closely related eastern North American species of field cricket (genus Gryllus). I show that patterns of variation across the zone do not conform to a simple model of monotonie clinal variation. In fact, the hybrid zone is a mosaic of populations. Pockets of “pure” parental forms are found within the hybrid zone, and striking reversals in mean character index score occur along transects across the zone. Treating hybrid zones as mosaics has important consequences for thinking about the dynamics of such zones.Patterns of variation in morphology and allozymes are not concordant across the hybrid zone. Rather, there is strong evidence for differential and asymmetric introgression, with morphological integrity maintained despite considerable introgression of alleles at allozyme loci. Species boundaries must be thought of as semipermeable, the permeability varying with the genetic marker used.I also show that there is strong positive assortative mating at one site within the hybrid zone and that assortative mating persists despite introgression at allozyme loci. Habitat isolation and behavioural differences may both affect the extent of assortative mating.

Ecological genetics of a mosaic hybrid zone: mitochondrial, nuclear, and reproductive differentiation of crickets by soil type

We investigated the effects that habitat variation has on the structure and dynamics of a hybrid zone between two closely related crickets in Connecticut. A collecting protocol was developed in which crickets were sampled from characteristic habitats on either side of the hybrid zone and from two distinct habitat types within the zone. Presumptive pure Gryllus pennsylvanicus were sampled from fields in northwestern Connecticut and represent “inland” populations. “Pure” Gryllus firmus were sampled from beaches along the coast and represent the “coastal” populations. Crickets from within the hybrid zone were sampled from two different soil types: the “loam” populations from loamy soils and the “sand” populations from sandy soils. Moreover, an attempt was made to identify closely adjacent sand and loam localities to determine the scale of habitat variation and its possible effects on hybrid‐zone structure. In general, there was little variation in morphological traits or in allozyme and mtDNA genotype frequencies among localities from within each of the four habitat types. Between each of the closely situated sand and loam localities within the hybrid zone, however, there were very significant differences in each of these sets of markers. In addition, crickets from hybrid‐zone populations were tested for reproductive isolation. The asymmetric outcome of hybrid crosses that exists across the zone (Harrison, 1983) also exists on a finer ecological scale within the zone. Thus, this hybrid zone is a mosaic of strikingly differentiated populations. The dynamics of hybrid zones with mosaic structures are discussed in contrast to the traditional clinal models. The data are also discussed in light of the semipermeable nature of species boundaries. The extent to which a species boundary is permeable varies not only from one genetic marker to the next, but also with the ecological and geographic context of species interaction.

EVOLUTION OF POLLINATION AND MUTUALISM IN THE YUCCA MOTH LINEAGE

The obligate pollination mutualisms between yucca moths and yuccas are some of the most obvious cases of coevolution, but the phylogenetic origins and extent of coevolution in these interactions are little understood. Ecological and phylogenetic information from the yucca moth family, Prodoxidae, shows that pollination has evolved at least three times from separate moth behaviors. Passive pollination occurs in Greya during nectaring by one species and during oviposition by two other species. Active pollination among prodoxids has evolved only once, in the yucca moths. Several life-history traits necessary for the evolution of obligate mutualism are common traits within the Prodoxidae, and only active pollination and modified mouthparts appear to have been novel traits in the yucca moths. We use moth and host biology in a phylogenetic framework to establish hypotheses for the evolution of active pollination and reciprocal specialization in the form of functional nectarlessness in yuccas.

Hybridization, Introgression, and the Nature of Species Boundaries

Species can be defined as populations that are diagnosably distinct, reproductively isolated, cohesive, or exclusive groups of organisms. Boundaries between species in sympatry are maintained by intrinsic barriers to gene exchange; these boundaries may not be uniform in space, in time, or across the genome. Here, we explore the nature of the species boundary, defined as the phenotypes/genes/genome regions that remain differentiated in the face of potential hybridization and introgression. We emphasize that species boundaries are semipermeable, with permeability (gene exchange) being a function of genome region. The early evidence for semipermeable species boundaries came from data on differential introgression in hybrid zones. This “genic view” of species was common in the hybrid zone literature even when few molecular markers were available to characterize genome-wide patterns of variation. Now, molecular tools allow detailed characterization of differentiation between diverging lineages and patterns of variation across natural hybrid zones, but the questions being asked by evolutionary biologists have remained much the same. Recent data (from DNA sequences and genotypes) reinforce earlier conclusions about the semipermeable nature of most species boundaries. However, debate persists over the nature and extent of genome divergence that accompanies speciation.

PATTERNS OF VARIATION AND LINKAGE DISEQUILIBRIUM IN A FIELD CRICKET HYBRID ZONE

The distribution of multilocus genotypes found within a natural hybrid zone is determined by the sample of genotypes present when the hybrid zone first formed, by subsequent patterns of genetic exchange between the hybridizing taxa, and by drift and selection within each of the hybrid zone populations. We have used anonymous nuclear DNA restriction fragment polymorphisms (RFLPs) to characterize the array of multilocus genotypes present within a well‐studied hybrid zone between two eastern North American field crickets, Gryllus pennsylvanicus and Gryllus firmus. These crickets hybridize along a zone of contact that extends from New England to Virginia. Previous studies have shown that both premating and postmating barriers exist between the two cricket species, but the absence of diagnostic morphological and allozyme markers has made it difficult to assess the consequences of these barriers for genetic exchange. Analyses based on four diagnostic anonymous nuclear markers indicate that hybrid zone populations in Connecticut contain few F1 hybrids, and that nonrandom associations persist among nuclear gene markers, between nuclear and cytoplasmic markers, and between molecular markers and morphology. Field cricket populations within the hybrid zone are not “hybrid swarms” but consist primarily of crickets that are very much like one or the other of the parental species. Despite ample opportunity for genetic exchange and evidence for introgression at some loci, the two species remain quite distinct. Such a pattern appears to be characteristic of many natural hybrid zones.

Phylogeny and Evolutionary History of the Ground Squirrels (Rodentia: Marmotinae)

Although ground squirrels (Spermophilus) and prairie dogs (Cynomys) are among the most intensively studied groups of mammals with respect to their ecology and behavior, a well-resolved phylogeny has not been available to provide a framework for comparative and historical analyses. We used complete mitochondrial cytochrome b sequences to construct a phylogeny that includes all 43 currently recognized species in the two genera, as well as representatives of two closely related genera (Marmota and Ammospermophilus). In addition, divergence times for ground squirrel lineages were estimated using Bayesian techniques that do not assume a molecular clock. All methods of phylogenetic analysis recovered the same major clades, and showed the genus Spermophilus to be paraphyletic with respect to both Marmota and Cynomys. Not only is the phylogeny at odds with previous hypotheses of ground squirrel relationships, but it suggests that convergence in morphology has been a common theme in ground squirrel evolution. A well-supported basal clade, including Ammospermophilus and two species in the subgenus Otospermophilus, diverged from all other ground squirrels an estimated 17.5 million years ago. Between 10 and 14 million years ago, a relatively rapid diversification gave rise to lineages leading to marmots and to several distinct groups of ground squirrels. The Eurasian ground squirrels diverged from their North American relatives during this period, far earlier than previously hypothesized. This period of diversification corresponded to warming climate and spread of grasslands in western North America and Eurasia. Close geographic proximity of related forms suggests that most species evolved in or near their current ranges.

The population genetics of a biological control introduction: mitochondrial DNA and microsatellie variation in native and introduced populations ofAphidus ervi, a parisitoid wasp

Introductions of biological control agents may cause bottlenecks in population size despite efforts to avoid them. We examined the population genetics of Aphidius ervi (Hymenoptera: Braconidae), a parasitoid that was introduced to North America from Western Europe in 1959 to control pea aphids. To explore the phylogeographical relationships of A. ervi we sequenced 1249 bp of mitochondrial DNA (mtDNA) from 27 individuals from the native range and 51 individuals from the introduced range. Most individuals from Western Europe, the Middle East and North America shared one of two common haplotypes, consistent with the known history of the introduction. However, some A. ervi from the Pacific Northwest have a haplotype that is most similar to haplotypes found in Japan, raising the possibility of a second accidental introduction. To examine population structure and assess whether a bottleneck occurred upon introduction to North America, we assayed variation at 5 microsatellite loci in 62 individuals from 2 native populations and 230 individuals from 6 introduced populations. Introduced samples had fewer rare alleles than native samples (F1,34 = 13.5, P = 0.0008), but heterozygosity did not differ significantly. These results suggest that a mild bottleneck occurred in spite of the introduction of over 1000 individuals. Using a hierarchical Bayesian approach, the founding population size was estimated to be 245 individuals. amova showed significant genetic differentiation between the European and North American samples, and a Bayesian assignment approach clustered individuals into four groups, with most European individuals in one group and most North American individuals in the other three. These results highlight that genetic changes are associated with founder events in rapidly growing natural populations, even when the founding population size is relatively large.

Mitochondrial DNA size variation within individual crickets.

The mitochondrial DNAs of two closely related cricket species (genus Gryllus) share a size polymorphism as evidenced by analysis of restriction fragment patterns. Moreover, 12 of 100 field-collected crickets are heteroplasmic, that is these individuals have more than one size class of mitochondrial DNA. No heteroplasmy for restriction site variation is observed. Intraindividual variation in cricket mitochondrial DNA provides a useful marker for studying the transmission genetics of mitochondrial DNA. Available data on patterns of variation in mothers and offspring suggest that random segregation of mitochondrial DNA variants does not occur rapidly in cricket germ-cell lineages.

European Corn Borer: Pheromone Polymorphism or Sibling Species?

Electrophoretic analyses of the (Z) and (E) pheromone-attracted males of Ostrinia nubilalis (H�bner), the European corn borer, in an area of coexistence indicate that these strains are not freely interbreeding. Although the populations are morphologically indistinguishable, studies of allozyme, pheromone, and hybridization suggest that the (Z) and (E) entities are genetically differentiated, perhaps to the status of semi- or sibling species.