SEX-RATIO AND REPRODUCTIVE VARIATION IN THE MISTLETOE PHORADENDRON JUNIPERINUM (VISCACEAE)

Despite the fact that many parasitic and hemiparasitic plant species such as mistletoes are dioecious and occur in both the new and the old world, few data exist on variation in the sex ratio and allocation to reproduction in these taxa. We investigated 1) the sex-ratio of the xylem-tapping mistletoe Phoradendron juniperinum in relation to its age and position within the canopy of its host tree Juniperus osteosperma, and 2) reproductive effort in relation to the gender and age of mistletoe plants. Our surveys showed that P. juniperinum has a male-biased sex ratio. Despite this predominance of male individuals, females lived longer and had a greater reproductive effort than did males. A statistical analysis of the age distribution data indicated that the peak in the frequency of reproductively mature individuals was later in females than in males. These gender-specific distributions may have resulted 1) from sequential hermaphroditism (age-specific sex switching), or 2) because the average age of peak reproduction is later in female individuals. Because sex is genetically determined in a closely related genus of mistletoe and because we have no data to indicate sex switching in this species, we feel that our data support the interpretation that female individuals, on average, show a peak in reproductive vigor at an older age relative to males. While delayed reproduction in females may be favored because reproductive effort and success appear to be age-dependent in females of this species, both sexes can become reproductively mature relatively early in life. Further, because 1) allocation to reproduction as a function of age increases more rapidly for females of this species relative to males, and 2) because there may be a higher resource cost associated with reproduction in females, we hypothesized that female individuals would be more abundant in the best quality locations within the host tree so as to maximize the opportunity to meet those costs. In spite of the association between gender and some host characteristics, there was no indication that female plants were located in sites most favorable to either their carbon or water balance. We discuss reasons why this may be the case.     

AGE STRUCTURE OF PHORADENDRON JUNIPERINUM (VISCACEAE), A XYLEM-TAPPING MISTLETOE: INFERENCES FROM A NON-DESTRUCTIVE MORPHOLOGICAL INDEX OF AGE

We investigated the age-structure of the xylem-tapping mistletoe Phoradendron juniperinum in relation to characteristics of its host tree, Juniperus osteosperma. We first correlated branch structure in the mistletoe with age of the mistletoe infection as determined anatomically; this correlation provided a nondestructive, field method of obtaining age structure information. We then surveyed the mistletoe plants, applying our aging index, within a population of their host trees in southwestern Utah; the majority of mistletoe plants were 2–12 years old. This peak in abundance of mistletoe infections showed no correlation to total annual precipitation within or 1 year previous to the peaks, minimum winter temperature, or to warmer than average winter temperatures. However, there was a positive correlation (r = 0.51, P < 0.06) with the amount of summer precipitation. A log-linear analysis indicated that a greater than expected number of mistletoe infections occurred at 5–7 years of age and at approximately 3 m in height among all host trees. We suggest that this pattern resulted because this canopy position had greater leaf and branch areas and was visited most frequently by seed-dispersing birds. The log-linear analysis also revealed that fewer than expected mistletoe infections occurred at ages older than 10 years, yet our data indicate that plants can reach 20 years of age. The lack of infections > 10 years of age was correlated to a period of below average precipitation, especially during the growing season, but not with cold winter temperatures, which in other studies had been suggested as a factor influencing mortality. We feel that drought may play an important role in influencing mistletoe mortality through its direct affect on host tree water status, but in addition we offer two alternative hypotheses to explain mistletoe longevity; the first is concerned with the relationship between carbon and nitrogen costs and maintaining large leaf areas in older plants, and the second addresses how increased hydraulic resistance in older and larger plants may be too costly for the plant, and stems are abscised.     

EXPERIMENTAL EVIDENCE FOR HOST RACES IN MISTLETOE (PHORADENDRON TOMENTOSUM)

In order to test the hypothesis that mistletoes (Phoradendron tomentosum) are differentially adapted for the host species that they occur on, mistletoe seeds from the three most common hosts in central Texas (hackberry, Celtis occidentalis, elm, Ulmus crassifolia, and mesquite, Prosopsis glandulosa) were planted onto different individuals of each of the three hosts. Germination of seeds and formation of haustorial disks by seedlings were followed in the subsequent 17 months. Germination of seeds was very high for all nine combinations of maternal (source) and seed (experimental) hosts (range 82%–98%). There were no significant differences in seed germination between the two groups when source and experimental hosts were the same species and when they were different species. In contrast, development of haustorial disks when source and experimental hosts were the same species was significantly greater than when experimental host and source host species were different. The data suggest that populations of mistletoes are genetically differentiated such that early seedling development is greatest when there is correspondence between maternal and seedling host species.     

SIEVE ELEMENTS IN THE MORPHOLOGICALLY REDUCED MISTLETOE,VISCUM MINIMUM HARVEY (VISCACEAE)

The absence of phloem in some Viscaceae may be a direct result of morphological reduction. Viscum minimum is a highly reduced, South African mistletoe that parasitizes Euphorbia. The aerial portions of this species are 2–3 mm long and consist of only 1–2 intemodes with a terminal triad of flowers. The collateral vascular bundles are composed of tracheary elements, parenchyma, and sieve tubes with associated companion cells. The sieve-tube members are similar to those described from other angiosperms, exhibiting simple sieve plates, P-protein, stacks of ER, plastids, and mitochondria. The morphological reduction of V. minimum thus is not associated with a loss of phloic elements in contrast to other reduced viscaceous mistletoes.     

亚热带季雨林植物罗伞气体交换对光质的反应

亚热带季雨林林下阴生植物罗伞(Ardisia quinquegona)叶片的气体交换速率(PN.μmol.m~(-2),s~(-1))随光强(PFD,μmol,m~(-2),s~(-1))增高而增大。在光强低于80μmol,m~(-2),s~(-1),PN=29.21PFD×10~(-3)+0.36。在光强150μmol,m~(-2),s~(-1)对出现气体交换的光饱和现象。在低光强下,气孔传导率(G,m mol,m~(-2),s~(-1)与光强(m mol,m~(-2),s~(-1)的关系为G=265.6 PFD+4.6。在低光强下。开阔地的阳生灌木桃金娘(Rhodmyrtus tomentosa)的气体交换速率和气孔传导率与光强关系曲线的直线部分斜率皆较罗伞的低,在红光上,罗伞叶片气体交换速率(μmol,m~(-2),s~(-1)与光强(μmol,m~(-2),s~(-1)的关系为PN=32.4 PFD×10~(-3)-0.04。气孔传导率(m mol,m~(-2),s~(-1)与光强(m mol,m~(-2),s~(-1)的关系为G=339.08 PFD+7.37。同时气体交换速率的饱和红光光强亦较白光的高。在蓝光光强低时,气体交换速率(μmol,m~(-2),s~(-1))与光强(μmol,m~(-2),s~(-1))的关系为PN=13.54 PFD×10~(-3)—0.17,而气孔传导率(m mol,m~(-2),s~(-1))与光强(mμmol,m~(-2),s~(-1))的关系为G=80.5 PFD+4.35。在低的蓝光下,体交换速率和气孔传导率与光强关系曲线的直线部分斜率显著较在白光和红光下的低。罗伞叶片气体交换对红光的反应敏感。     

LEAF SURFACE WETNESS AND GAS EXCHANGE IN THE POND LILY NUPHAR POLYSEPALUM (NYMPHAEACEAE)

As part of a continuing study of the effects of leaf surface wetness on gas exchange, the occurrence of leaf surface wetting by dewfall and associated effects on photosynthesis were evaluated for floating and aerial leaves of the pond lily Nuphar polysepalum Engelm. Because of nighttime radiation exchange with a cold sky, high humidity, and the presence of adaxial stomata, we predicted that pond lily leaves would be particularly susceptible to wetting events such as dewfall. A substantial reduction in net photosynthesis (up to 20%) occurred for leaves that were experimentally misted to simulate leaf wetting by dewfall. Aerial leaves remained below dewpoint temperatures for long periods on clear nights. However, floating leaves rarely approached dewpoint temperatures at night because minimum nighttime temperatures of leaves were up to 10 C warmer than air temperature. Thus, floating leaves of N. polysepalum did not experience dew formation primarily because of strong thermal coupling to a substrate (water) that was much warmer than air temperature at night. This coupling to a warmer substrate prevented a potentially strong inhibition of photosynthetic CO₂ exchange the following morning.     

GAS EXCHANGE OF IMPATIENS PALLIDA NUTT. (BALSAMINACEAE) IN RELATION TO WILTING UNDER HIGH LIGHT

Impatiens pallida, a succulent annual herb of moist temperate forests, typically wilts on summer days after several minutes of direct sunlight. Time courses of gas exchange and leaf water potential were measured to determine if wilting resulted in substantially reduced photosynthesis, stomatal conductance, or leaf internal CO₂ concentrations. Leaves quickly wilted with the onset of high-light, but photosynthesis and stomatal conductance increased markedly. Photosynthetic rates and stomatal conductance declined slightly after several hours of high-light, and from morning to late afternoon shade conditions. Leaf internal CO₂ declined with increased photosynthesis, but there was no evidence that stomatal conductance limited photosynthesis through the day. We propose that rapid wilting is an adaptation that facultatively limits heat loading and extreme water loss under high-light. Further whole plant studies in natural settings are needed to fully evaluate the quantitative significance of wilting in relation to water use and photosynthesis.     

MITOSIS IN DUNALIELLA BIOCULATA (CHLOROPHYTA): CENTRIN BUT NOT BASAL BODIES ARE AT THE SPINDLE POLES

Centrin, a 20 kDa calmodulin-like protein, is located in various basal body-associated fibers in protists. We used indirect immunofluorescence of isolated cytoskeletons or methanol-fixed cells to analyze the distribution of centrin during mitosis of the biflagellate green alga Dunaliella bioculata (Butcher). The distance between the nucleus and the basal apparatus decreased in late interphase, presumably caused by the contraction of the two centrin-containing nucleus–basal body connectors (NBBCs). During prophase, centrin accumulated on the new basal bodies as shown by postembedding immunogold labeling of serial thin sections. The new basal bodies were in close contact with plaque-like structures on the nuclear envelope. In mitotic cells, basal body pairs were separated and positioned at a considerable distance from the poles of the mitotic spindle. At this stage, we observed four separated centrin dots, two associated with the pairs of basal bodies and two located at the spindle poles as shown by double immunofluorescence, including anti-tubulin staining. The latter signals corresponded to an accumulation of centrin between the plasma membrane and the nuclei, indicating that centrin could be involved in mitotic movements of the nuclei. In telophase, centrin was observed along the nuclear surface and one new NBBC developed in each cell half. Our results demonstrate that centrin is present at the acentriolar spindle poles of Dunaliella independently from its localization in the basal apparatus.     

SEEDLING MORPHOLOGY IN MARAH (CUCURBITACEAE) RELATED TO THE CALIFORNIAN MEDITERRANEAN CLIMATE

Studies in reproductive ecology were made in indigenous, western American plants in the genus Marah (Cucurbitaceae), with particular attention given plants of M. oreganus occurring in the Berkeley Hills near San Francisco Bay in California. These tuberous perennials produce capsular fruits on their annual aboveground shoots; the fruits dehisce in early summer, each one exposing about three large seeds with an average seed weight of 1.05 g. The embryo of a M. oreganus seed has two thick and fleshy cotyledons packed with protein granules. The embryonic axis, with shoot and root apices, is ca. 0.5-1.0 mm long, roughly ½0 or less the length of the seed. In the Berkeley Hills dispersal of the seeds is accomplished by nocturnal rodents, after which germination begins with the fall rains and cooler temperatures of November and December. Instead of a radicle emerging first from the seed at germination, the minute radicle and epicotyl are pushed or carried far out of the seed, down into the soil, by the elongating bases of the cotyledons. These cotyledon bases, or petioles, are fused, and as they elongate they form a hollow tube that bears the embryonic axis at its extreme tip. The cotyledonary petiole tube ceases elongation by January, when it may be 5-25 or more cm long in a seedling of M. oreganus. Then, from its tip, the radicle grows downward and the epicotyl upward—up the hollow petiole tube. The green shoot (epicotyl) reaches the soil surface by early March in this area, completes the first season's growth, and dries up by late May, when the arid summer season is beginning. But even before the epicotyl grows out of the petiole tube and above ground, the seedling's hypocotyl begins to enlarge, forming a tuber. The fleshy cotyledon blades remain in the seed coat below ground, and some food from the blades is transferred to the tuber that produces shoots in the following growing seasons. This pattern of germination and seedling establishment is now known for species of Marah and for a very few other dicotyledonous plants, all of them growing mainly in areas of hot and dry habitat that are generally referred to as having Mediterranean climate. This elongation of the fused hypogeal cotyledons is considered a complex adaptation in dicotyledons that helps ensure fast and successful seedling establishment in seasonally arid areas such as “Mediterranean” California.     

DIFFERENCES IN GROWTH AND PHYSIOLOGY OF MARINE SYNECHOCOCCUS (CYANOBACTERIA) ON NITRATE VERSUS AMMONIUM ARE NOT DETERMINED SOLELY BY NITROGEN SOURCE REDOX STATE1

The preference of phytoplankton for ammonium over nitrate has traditionally been explained by the greater metabolic cost of reducing oxidized forms of nitrogen. This “metabolic cost hypothesis” implies that there should be a growth disadvantage on nitrate compared to ammonium or other forms of reduced nitrogen such as urea, especially when light limits growth, but in a variety of phytoplankton taxa, this predicted difference has not been observed. Our experiments with three strains of marine Synechococcus (WH7803, WH7805, and WH8112) did not reveal consistently faster growth (cell division) on ammonium or urea as compared to nitrate. Urease and glutamine synthetase (GS) activities varied with nitrogen source in a manner consistent with regulation by cellular nitrogen status via NtcA (rather than by external availability of nitrogen) in all three strains and indicated that each strain experienced some degree of nitrogen insufficiency during growth on nitrate. At light intensities that strongly limited growth, the composition (carbon, nitrogen, and pigment quotas) of WH7805 cells using nitrate was indistinguishable from that of cells using ammonium, but at saturating light intensities, cellular carbon, nitrogen, and pigment quotas were significantly lower in cells using nitrate than ammonium. These and similar results from other phytoplankton taxa suggest that a limitation in some step of nitrate uptake or assimilation, rather than the extra cost of reducing nitrate per se, may be the cause of differences in growth and physiology between cells using nitrate and ammonium.     

LEAF GAS EXCHANGE IN QUERCUS MACROCARPA (FAGACEAE): RAPID STOMATAL RESPONSES TO VARIABILITY IN SUNLIGHT IN A TREE GROWTH FORM

Responses in net photosynthesis (A), stomatal conductance to water vapor (g), and leaf xylem pressure potential (ψ) were measured in the deciduous tree Quercus macrocarpa during alternating periods of sun (photosynthetic photon flux, PPF > 1,500 μmol m^-2 sec^-1) and shade (ca. 350 μmol m^-2 sec^-1 simulating cloud cover). Measurements were made on trees growing at the gallery forest-prairie edge on the Konza Prairie Research Natural Area in northeast Kansas. The region is near the westernmost extension of the range of Q. macrocarpa where this species experiences significant seasonal water stress (minimum ψ < -2.9 MPa). Quercus macrocarpa was chosen for study because it has relatively high A (15 μmol m^-2 sec^-1) and g (300 mmol m^-2 sec^-1) in contrast to the deciduous and evergreen subalpine trees previously studied. Both trees and large saplings of Q. macrocarpa responded to alternating several minute periods of sun and shade with relatively rapid changes in A and g. Reductions in g (110 mmol m^-2 sec^-1) during shade periods lowered transpirational water losses (E) by 13% (and reduced A by 5%) relative to estimates of A and E made assuming g remained constant. Partial stomatal closure during shade was correlated with moderate enhancement in ψ (0.31 MPa) in Q. macrocarpa. However, greater increases in ψ were measured in adjacent prairie grasses exposed to similar periods of shade (0.72 MPa in Andropogon gerardii, 0.61 MPa in Sorghastrum nutans). Reduced variability in ψ in tree growth forms may reflect greater buffering of water relations associated with the large size of trees, the amount of tissue devoted to water storage, and differences in hydraulic resistance relative to herbs. Nonetheless, the gas exchange and water relations responses in Q. macrocarpa were much more similar to those previously measured in herbaceous subalpine and grassland species than to those documented for subalpine trees. Thus, rapid gas exchange responses to variable PPF may also occur in tree growth forms.     

GENETIC POLYMORPHISM IN THE MORPHOLOGICALLY REDUCED DWARF MISTLETOES (ARCEUTHOBIUM,VISCACEAE): AN ELECTROPHORETIC STUDY

Arceuthobium is a well defined genus of obligate parasites of conifers. Assessment of taxonomic and phylogenetic relationships in this group is difficult owing to morphological reductions and complex host relationships. In the present study, genetic relationships within and among 19 taxa were examined using starch gel electrophoresis of triploid seed endosperm tissue. Allelic frequency data for eleven polymorphic loci were derived from analysis of 40 natural populations collected from the U.S. and Mexico. The genus showed remarkably high levels of genetic diversity: averaged across the 19 taxa, 66.7% of the loci were polymorphic with an average of 2.23 alleles per locus. This level of polymorphism is approximately double the average value reported for many dicotyledons and stands in contrast to the overall uniform morphology of these parasites. Unweighted pair group cluster analysis (UPGMA) of genetic similarity measures was conducted for all sampled populations and the results compared to a phenetic system of classification for the genus. Similarities between the two studies include the recognition of two subgenera based upon the segregation of the verticillately branched Arceuthobium americanum from the remainder of the taxa examined. Analysis of isozyme data supported a grouping of six taxa: A. vaginatum ssp. cryptopodum, A. vaginatum ssp. durangense, A. gillii, A. rubrum, A. divaricatum, and A. douglasii. The placement of the latter three taxa in other groups by phenetic criteria provides evidence for molecular divergence not seen using morphological features. The Campylopodum group of taxa comprised eleven members that were linked at similarity values of 80% or greater. Populations were not unambiguously grouped according to species as defined by the phenetic study or at similarity levels comparable to other well defined species in this study. This group is either not reproductively isolated or molecular differentiation is cryptic due to rapid adaptive radiation onto numerous host tree species.     

DIFFERENCES IN LIFE-HISTORY CHARACTERISTICS ARE CONSISTENT WITH THE VERTICAL DISTRIBUTION PATTERN OF FUCUS SERRATUS AND FUCUS VESICULOSUS (FUCALES, PHAEOPHYCEAE) IN THE CENTRAL BALTIC SEA

On the rocky shores of southeast Sweden, in the central Baltic Sea, the brown seaweed species Fucus serratus L. and Fucus vesiculosus L. form dense constantly submerged stands. The relative proportion of the two species in the belt is shifting both vertically and geographically. Our hypothesis is that differences in life-history characteristics and the tolerance to physical factors such as wave force, freezing, and light may alter the competitive outcome between the two species with increasing wave exposure. This study included a combination of field studies and laboratory experiments. In sheltered locations, F. serratus and F. vesiculosus grow in mixed stands from 0.5 to 3 m deep, whereas at exposed sites F. vesiculosus grow from 0.5 to 2 m deep and F. serratus mainly from 1 to 5 m deep. The two Fucus species differed markedly in their tolerance to physical stress as well as in a number of reproductive and vegetative characteristics. Tolerance to freezing and wave action was higher in F. vesiculosus compared with F. serratus . Fucus vesiculosus also produced 10 times more eggs per receptacle, allocated significantly more tissue per frond for reproduction, and produced more reproductive fronds per individual than did F. serratus . In addition, the survival rate of F. vesiculosus germlings was slightly higher than that of F. serratus germlings during the first year. These differences in life history characteristics are suggested to partly explain the observed distribution patterns of the two Fucus species in the Baltic Sea.     

MORPHOLOGICAL ADAPTATION TO HOST PLANTS IN UROLEUCON (HOMOPTERA: APHIDIDAE)

The morphology of host-specific Old World Uroleucon (Homoptera: Aphididae) species is related to features of their host plant species (Asteraceae and Campanulaceae) by canonical correlation analysis. Aphid species on pubescent host species have longer rostra and shorter hind tarsi, adaptations for feeding and walking on hirsute surfaces. Also, aphid species on taller hosts are larger. The positive relationship between rostrum length and host trichome density is further supported by simple regression analyses on two species complexes. Regression parameters differ between groups, suggesting different responses to different trichome types. This morphological adaptation involves characters used extensively by taxonomists and could result in convergence obscuring phylogenetic relationships of herbivore taxa and, particularly, the relation of herbivore phylogeny to host phylogeny.     

THE CIRCADIAN RHYTHM OF BIOLUMINESCENCE IN PYROCYSTIS IS NOT DUE TO DIFFERENCES IN THE AMOUNT OF LUCIFERASE: A COMPARATIVE STUDY OF THREE BIOLUMINESCENT MARINE DINOFLAGELLATES

The biochemistry and circadian regulation of luminescence in two Pyrocystis species, P. lunula Hulburt and P. noctiluca Murray et Haeckel, were compared with a well-studied species, Gonyaulax polyedra Stein. All exhibit circadian rhythms and all have similar luciferins and luciferases. However, the Pyrocystis species lack a second protein involved in the reaction in Gonyaulax , the luciferin (substrate) binding protein, which sequesters the luciferin at the cytoplasmic pH and releases it upon acidification, thus controlling the characteristic flashing, which is similar in the three species. More striking is the difference in the circadian regulation of luminescence, which in Gonyaulax involves the daily synthesis and destruction of the two proteins, along with the luminous organelles (scintillons) from which light is emitted, and which are present in all species. In the Pyrocystis species, the amount of luciferase is the same in extracts made during the day and night phases; its circadian regulation in vivo may be attributed to a change in its localization from day to night phase.     

CHROMOSOME NUMBERS IN NORTH AMERICAN LORANTHACEAE: (ARCEUTHOBIUM,PHORADENDRON, PSITTACANTHUS,STRUTHANTHUS)

Chromosome numbers are reported from 67 populations of 36 taxa, mostly in Phoradendron. The basic number is 14 in Phoradendron and probably also in Arceuthobium. The 4 species of Struthanthus for which chromosome numbers are recorded suggest that the basic number is 8. Numbers of n = 8 and n = 10 have been reported for Psittacanthus. In Phoradendron the 22 taxa examined are all diploid, although 1 instance of polyploidy was discovered. Objects interpreted as supernumerary chromosomes were discovered in a number of species of Phoradendron; no evidence of sex chromosomes previously reported in Phoradendron was observed. The Loranthaceae have chromosomes comparable in size to the largest in the plant kingdom.     

FACTORS RELATED TO THE DOMINANCE OF CYLINDROSPERMOPSIS RACIBORSKII (CYANOBACTERIA) IN A SHALLOW POND IN NORTHERN TAIWAN1

Seasonal succession of phytoplankton was investigated in a shallow pond in northern Taiwan from August 2009 to January 2011, with particular reference to the dynamics of Cylindrospermopsis raciborskii (Woloszynska) Seenayya et Subba Raju. The abundances of the representative species in the pond increased during high‐temperature seasons, whereas only C. raciborskii became dominant in the pond from summer to autumn in both 2009 and 2010. The high shade tolerance of C. raciborskii was likely one of the factors that enabled the cyanobacterium to grow during the summer when the transparency was low. Moreover, the heterocyst production of C. raciborskii was enhanced during summer when the concentration of dissolved inorganic nitrogen was low, implying that nitrogen fixation also played an important role in supporting the growth of C. raciborskii. Autumnal rainfall was a critical factor in the collapse of C. raciborskii blooms. C. raciborskii formed blooms with relatively small trichomes, whereas larger trichomes dominated during winter. The dependence of the trade‐off between growth rate and trichome size on temperature was assumed to be an adaptation strategy of C. raciborskii.     

GENETIC DIVERGENCE ALONG THE SPECIATION CONTINUUM: THE TRANSITION FROM HOST RACE TO SPECIES IN RHAGOLETIS (DIPTERA: TEPHRITIDAE)

Studies of related populations varying in their degrees of reproductive isolation can provide insights into speciation. Here, the transition from partially isolated host races to more fully separated sibling species is investigated by comparing patterns of genetic differentiation between recently evolved (～150 generations) apple and ancestral hawthorn‐infesting populations of Rhagoletis pomonella to their sister taxon, the undescribed flowering dogwood fly attacking Cornus florida. No fixed or diagnostic private alleles differentiating the three populations were found at any of 23 microsatellites and 10 allozymes scored. Nevertheless, allele frequency differences were sufficient across loci for flowering dogwood fly populations from multiple localities to form a diagnosable genotypic cluster distinct from apple and hawthorn flies, indicative of species status. Genome‐wide patterns of differentiation were correlated between the host races and species pair comparisons along the majority of chromosomes, suggesting that similar disruptive selection pressures affect most loci. However, differentiation was more pronounced, with some additional regions showing elevated divergence, for the species pair comparison. Our results imply that Rhagoletis sibling species such as the flowering dogwood fly represent host races writ large, with the transition to species status primarily resulting from increased divergence of the same regions separating apple and hawthorn flies.