MOLECULAR DIVERGENCE BETWEEN ASIAN AND NORTH AMERICAN SPECIES OF LIRIODENDRON (MAGNOLIACEAE) WITH IMPLICATIONS FOR INTERPRETATION OF FOSSIL FLORAS

Botanists have long been aware of the floristic similarities between eastern Asia and eastern North America. Most who have considered this classic disjunction pattern have suggested that it arose through range disruption of a flora that was once more widely distributed in the Northern Hemisphere. There is less agreement on the timing of this process, with suggestions ranging from the Paleocene to the Neogene. In this study, molecular markers from two different plant genomes were used to assess the degree of genetic divergence between the two interfertile, morphologically similar species of the genus Liriodendron, i.e., L. tulipifera and L. chinense. Resulting molecular divergence estimates were translated into approximate dates of separation, independent of evidence from the fossil record. Allozyme data (Nei's genetic identity = 0.434) suggested a divergence time of 10–16 million years before present, whereas sequence divergence in the plastid genomes (1.24%) led to an estimate of approximately 11–14 million years before present. A review of the paleobotanical literature indicated that the fossil floras that included, or might have included Liriodendron could not have survived in Beringia after the late Miocene and the onset of southward-migrating Arctic air masses on the North American continent. This interpretation suggests a minimum time of separation of approximately 13 million years before present. Thus, both molecular data sets and the paleobotanical evidence concur in suggesting a divergence time of 10–16 million years before present. Interspecific compatibility and relative morphological stasis must have, therefore, persisted from at least the late Miocene. We emphasize the need for similar studies in other genera, especially those that have both a reasonable Tertiary fossil history and extant species in mesic temperate refugia in Asia, Europe, and western as well as eastern North America.     

ELECTROPHORETIC ENZYME ANALYSIS OF NORTH AMERICAN AND EASTERN ASIAN POPULATIONS OF AGASTACHE SECT. AGASTACHE (LABIATAE)

Agastache sect. Agastache consists of seven species in North America and one disjunct in eastern Asia. Starch-gel electrophoresis of enzymatic proteins was employed to assess genetic relationships among these species and to estimate the amount of genetic divergence between the North American and Asian populations. Species of the western United States appear to be better adapted for outcrossing than are the others and are much more genetically variable, with higher levels of heterozygosity per individual, more alleles per species, and higher percentages of polymorphic loci per population. Nonmetric multidimensional scaling of Nei's genetic distances among 32 populations partitioned the section into four discrete groups: 1) A. nepetoides (eastern North America), 2) A. scrophulariifolia and A. foeniculum (eastern and central North America), 3) the four species of the western United States (A. urticifolia, A. occidentalis, A. parvifolia and A. cusickii) and 4) A. rugosa (eastern Asia). Asian Agastache, separated from its American congeners for over twelve million years, differed from American populations at two of fifteen loci surveyed. Nei's genetic distances between Asian and North American populations ranged from 0.2877 to 0.6734.     

THE ONTOGENY OF THE INFLORESCENCE AND FLOWER OF LIQUIDAMBAR STYRACIFLUA L. (HAMAMELIDACEAE)

A developmental study of the inflorescence of Liquidambar styraciflua L. was conducted to clarify morphological discrepancies reported in the literature. Salient features of development are: 1) the inflorescence apex results from the conversion of a terminal, vegetative apex; 2) partial inflorescence apices arise as ellipsoid structures in axils of leaves, bracts, or transitional phyllomes; 3) development of male heads is acropetal whereas female heads differentiate basipetally; 4) the partial inflorescence apex becomes segmented into several distinct subunits indicating an axillary branch system of the third order; 5) distinct individual floral primordia are initiated on the subunits; 6) a complete absence of perianth development; 7) inception of carpel primordia in flowers of lower male heads as well as female heads, but a failure of the gynoecium to develop beyond an incipient stage in male heads; and 8) development of sterile structures around the base of the styles of only female flowers near the time of anthesis. Carpellary characteristics of the sterile structures are described, their morphological nature is discussed, and the phylogenetic position of Liquidambar is evaluated.     

LEAF ANATOMY OF TISSUE-CULTURED LIQUIDAMBAR STYRACIFLUA (HAMAMELIDACEAE) DURING ACCLIMATIZATION

Structural changes accompanying the acclimation process were observed in leaves of sweetgum, Liquidambar styraciflua, using light and transmission electron microscopy (TEM). Comparisons were made of leaves obtained from tissue culture, plantlets acclimated after transfer from the in vitro environment to soil, and field grown trees. Leaves of cultured plantlets lacked a differentiated palisade parenchyma and had spongy parenchyma interspersed with large air spaces. Field grown leaves showed distinct palisade and spongy tissues and a high cell density. New leaves from acclimated plantlets showed an elongation of the upper mesophyll with fewer intercellular spaces than cultured plants. Cells from leaves from in vitro plantlets had large vacuoles, limited cytoplasmic content and flattened chloroplast with an irregularly arranged internal membrane system. Acclimated and field leaf cells had a greater cytoplasmic content than cultured leaves, with the former having more dominate vacuoles. Chloroplasts had evident grana. Acclimated and field leaves had a well developed cuticle unlike leaves from culture.     

PATTERNS OF ISOZYME VARIATION IN WESTERN NORTH AMERICAN ANTENNARIA (ASTERACEAE: INULEAE) II. DIPLOID AND POLYPLOID SPECIES OF SECTION ALPINAE

Antennaria is a genus of dioecious, perennial, herbaceous Composites that are especially diverse in the cordillera of western North America. Section Alpinae consists of about nine taxa, among them A. aromatica, A. densifolia, A. media, A. pulchella, and A. umbrinella. Although diploids are morphologically distinct, the polyploid derivatives of the diploids obscure the morphological distinctness of the groups. A survey of 19 putative isozyme loci indicates that the diploids have diverged only moderately from one another with respect to biochemical genetics (I = 0.838 to 0.961). Additionally, only moderate amounts of genetic diversity were detected. Isozyme data are supportive of a hypothesis of a rapid mode of speciation in Antennaria, where morphological differentiation has been accompanied by small amounts of allozyme divergence. Polyploids have significantly higher amounts of heterozygosity than diploids and tetrasomic inheritance is inferred. Evidence from morphology and biochemical genetics suggests that the polyploids represent a continuum between interracial autopolyploids and segmental allopolyploids. In light of the relatively low degree of genetic and morphological divergence among many Antennaria species, taxonomic conservatism is advocated.     

KARYOTYPES OF EASTERN NORTH AMERICAN PHLOX

The karyotypes of 32 Phlox taxa were studied in many plants representative of populations found throughout eastern North America. Most plants were diploid with 2n = 14, but a few tetraploids and plants with B-chromosomes were also observed. Some species were virtually indistinguishable karyotypically, while others differed. Within and between the Subsections Speciosae, Divaricatae, and Subulatae, strikingly similar karyotypes were observed. Other Subsections differed significantly, with the Ovatae showing strong intrasubsectional variation. Increased karyotype asymmetry is correlated with morphological specialization. Where polyploidy was observed, discordant chromosome sets indicated allopolyploidy.     

CYTOGEOGRAPHY AND PHYLOGENY OF THE NORTH AMERICAN SPECIES OF VERBENA

Lewis , Walter H., and Royce L. Oliver . (Stephen F. Austin State Coll., Nacogdoches, Texas.) Cytogeography and phylogeny of the North American species of Verbena. Amer. Jour. Bot. 48(7): 638–643. Illus. 1961.—Chromosome numbers of 26 Verbena species in North America, separable into 2 series, are reported. The x = 5 series has no diploid representative, few tetraploids, and numerous widespread hexaploid species, while the x = 7 series has many pandemic diploid species and few tetraploid and hexaploid taxa. When compared with the data available for the South American verbenas, these frequencies and distributions were found to be almost opposite. The origin of the 2 groups from an x = 6 prototype followed by ascending and descending aneuploidy is suggested. Migration via tropical land bridges is proposed to explain the occurrence of both groups in the 2 continents. The following taxa are proposed: subgenus Verbenaca (Schauer) Lewis, and subgenus Glandularia (Gmelin) Lewis.     

CHROMOSOME NUMBERS AND EVOLUTION IN NORTH AMERICAN SPECIES OF LINUM

Among the North American species of Linum there are three basic chromosome numbers representing invasions from the Old World of three distinct evolutionary lines. N = 9 is found only in the blue-flowered group represented in North America by two species, L. lewisii and L. pratense which are closely related to and may be conspecific with the Old World L. perenne. The basic number for the yellow-flowered species is n = 18 which is characteristic of the Scabrella and Virginiana subgroups. The loss of chromosomes in the Neo-mexicana (n = 13) and Sulcata-Rigida (n = 15) subgroups suggests that the basic haploid number of 18 might be a polyploid derivative of an Old World ancestor with n = 9. The incidence of n = 9 among Old World species of Linum may indicate that this represents an ancestral condition. Linum catharticum has n = 8; this number and features of morphology and distribution suggest that it is not directly related to either the blue-flowered or yellow-flowered groups in North America but represents a separate introduction on this continent.     

GENETIC DIVERGENCE AND ISOZYME NUMBER VARIATION AMONG FOUR VARIETIES OF ALLIUM DOUGLASII (ALLIACEAE)

Allium douglasii comprises four, allopatric varieties confined to the northwestern United States: var. columbianum, var. constrictum, var. douglasii, and var. nevii. Although no apparent karyotypic differences distinguish the four taxa, evidence from starch gel electrophoresis indicates that they are very distinct at genes coding for enzymes. The mean genetic identity for pairwise comparisons of populations of all four varieties is 0.84. This value is lower than identity values generally observed for subspecific taxa, and may be indicative of a long period of geographic separation and subsequent lack of gene flow among the varieties. However, a higher genetic identity of 0.93 was observed between var. constrictum and var. columbianum relative to other varietal comparisons. In addition, var. constrictum exhibited two biochemical autapomorphies: 1) loss of expression for the gene coding for Tpi-3, and 2) duplication or reactivation of an ADH gene. These data indicate that var. constrictum may be a recent derivative of var. columbianum. The distribution of genetic variation within and among populations of the putative progenitor/derivative pair also supports this hypothesis.     

KARYOTYPIC VARIATION OF EASTERN NORTH AMERICAN CLAYTONIA CHEMICAL RACES

Considerable karyotypic differentiation has occurred within the group of taxa comprising the eastern North American members of the genus Claytonia. Patterns of karyotypic differentiation are congruent with evolutionary groupings based on flavonoid chemistry, particularly at the diploid level. The 2n = 16 diploid chemotype found in both C. caroliniana and C. virginica possesses a karyotype composed entirely of metacentric chromosomes, while acrocentric chromosomes predominate in the karyotypes of the 2n = 12 and 2n = 14 diploid chemotypes of C. virginica. The 2n = 16 diploid also has a karyotype significantly larger than those of the other diploid cytotypes. Polyploid karyotypes of both species show varying degrees of divergence from their presumed diploid progenitors.     

POLLEN MORPHOLOGY IN NORTH AMERICAN SPECIES OF TRILLIUM

The pollen morphology and tapetal membranes of 17 species of Trillium from North America have been examined by scanning and transmission electron microscopy. The pollen grains of the genus are consistently spherical and inaperturate. The exine is unstratified and ultra-thin. Five types of exine structure are recognized, namely, echinate-, corrugate-, granulate-, verrucate- and spinulate-type, based on the variations in the components of the exine. The granulate-type is the most common and is divided into six subtypes. The features of tapetal membranes also vary in relation with the variation of the exine structures. This study demonstrates the potential of palynological evidence in assessment of taxonomic relationships within the genus.     

GENETIC ORGANIZATION AND EVOLUTIONARY HISTORY IN TWO NORTH AMERICAN SPECIES OF CIRSIUM

Relationships involving progenitor and derivative species pairs have been documented for a number of plant taxa. In this study, we used starch-gel electrophoresis to examine the probable relationship between Cirsium pitcheri, an endemic thistle restricted to the shorelines of the Great Lakes, and Cirsium canescens, a closely related species of the central Great Plains (U.S.A.). The species shared the same enzyme loci but differed greatly in levels of genetic diversity. C. pitcheri was genetically much less variable than its congener, with fewer polymorphic loci and significantly lower heterozygosity. The genetic variation in C. pitcheri represents a depauperate subset of the alleles present in C. canescens, consistent with the hypothesis that the two species represent a progenitor-derivative species pair. We suggest that C. pitcheri migrated into newly created dune habitats in the Great Lakes region near the end of the Wisconsin glaciation and that its low genetic variability results from repeated and probably prolonged population bottlenecks which accompanied its movement.     

PATTERNS OF VARIATION IN FOLIAR TRICHOMES OF EASTERN NORTH AMERICAN QUERCUS

Scanning electron microscopy of leaf trichomes of the forty two native species of oaks in eastern North America indicates five patterns of variability: 1) Eight trichome types are evident among the species and each species possesses a definite complement of trichome types. Certain trichomes are restricted to particular subgenera and series. 2) An obvious seasonal loss of trichomes occurs during leaf maturation. This loss may be both quantitative in terms of trichome density and qualitative in terms of trichome type. 3) There is an obvious difference between the adaxial and abaxial surfaces. The adaxial side of most oak leaves is dark green, lustrous, and glabrous or glabrate. The abaxial surface either remains pubescent, becomes glabrate or glabrous, or maintains trichomes along the midrib or in the axils of major secondary veins. There are also initial quantitative and qualitative trichome differences between the two sides. 4) Geographical and ecological variations are due in part to non-genetic ecophenic modifications, ecotypic differentiation, and random genetic differences not necessarily correlated with environmental conditions. Trichome types are considered to be less affected by environment than is trichome density. 5) Hybridization and introgression within a subgenus leads to localized variability. Trichomes of hybrids are usually a combination of the parental types. These five patterns of variation are predictable and appear to be held within rather narrow limits. The complement of foliar trichomes, therefore, is a reliable character in the taxonomy of the oaks.     

GIEMSA C-BANDED KARYOTYPES OF SEVEN NORTH AMERICAN SPECIES OF ALLIUM

The karyotypes of seven North American Allium species were studied by Giemsa C-banding technique. Two species (A. shoenoprasum and A. tricoccum) were diploids with 2n = 16 chromosomes. Three species (A. cernuum, A. douglasii and A. geyeri) were also diploids but with 2n = 14 chromsomes. Diploid and tetraploid populations of A. textile (2n = 14, 28) were studied. The population of A. canadense studied here was a tetraploid (2n = 28). All these North American species, except A. geyeri, possessed centromeric bands in all their chromosomes and nucleolar constriction bands in their satellited chromosomes. Allium shoenoprasum contained telomeric bands in most of its chromosomes but the other species had them only in a small number of chromsomes. Only three species (A. shoenoprasum, A. textile and A. tricoccum) were found to have intercalary bands in some chromosomes. The heterochromatin distribution in B chromosomes of three species was also observed. In A. cernuum, the heterochromatin occupied most of the length of all its Bs, but in A. shoenoprasum, heterochromatin was concentrated in the centromeric region. One population of A. textile (CC1179) was found to have a B chromosome in which very little heterochromatin existed.