Hengchang Wang

Rosid radiation and the rapid rise of angiosperm-dominated forests

The rosid clade (70,000 species) contains more than one-fourth of all angiosperm species and includes most lineages of extant temperate and tropical forest trees. Despite progress in elucidating relationships within the angiosperms, rosids remain the largest poorly resolved major clade; deep relationships within the rosids are particularly enigmatic. Based on parsimony and maximum likelihood (ML) analyses of separate and combined 12-gene (10 plastid genes, 2 nuclear; >18,000 bp) and plastid inverted repeat (IR; 24 genes and intervening spacers; >25,000 bp) datasets for >100 rosid species, we provide a greatly improved understanding of rosid phylogeny. Vitaceae are sister to all other rosids, which in turn form 2 large clades, each with a ML bootstrap value of 100%: (i) eurosids I (Fabidae) include the nitrogen-fixing clade, Celastrales, Huaceae, Zygophyllales, Malpighiales, and Oxalidales; and (ii) eurosids II (Malvidae) include Tapisciaceae, Brassicales, Malvales, Sapindales, Geraniales, Myrtales, Crossosomatales, and Picramniaceae. The rosid clade diversified rapidly into these major lineages, possibly over a period of <15 million years, and perhaps in as little as 4 to 5 million years. The timing of the inferred rapid radiation of rosids [108 to 91 million years ago (Mya) and 107–83 Mya for Fabidae and Malvidae, respectively] corresponds with the rapid rise of angiosperm-dominated forests and the concomitant diversification of other clades that inhabit these forests, including amphibians, ants, placental mammals, and ferns.

Population genetic structure of two Medicago species shaped by distinct life form, mating system and seed dispersal.

BACKGROUND AND AIMS Life form, mating system and seed dispersal are important adaptive traits of plants. In the first effort to characterize in detail the population genetic structure and dynamics of wild Medicago species in China, a population genetic study of two closely related Medicago species, M. lupulina and M. ruthenica, that are distinct in these traits, are reported. These species are valuable germplasm resources for the improvement of Medicago forage crops but are under threat of habitat destruction. METHODS Three hundred and twenty-eight individuals from 16 populations of the annual species, M. lupulina, and 447 individuals from 15 populations of the perennial species, M. ruthenica, were studied using 15 and 17 microsatellite loci, respectively. Conventional and Bayesian-clustering analyses were utilized to estimate population genetic structure, mating system and gene flow. KEY RESULTS Genetic diversity of M. lupulina (mean H(E)=0.246) was lower than that of M. ruthenica (mean H(E)=0.677). Populations of M. lupulina were more highly differentiated (F(ST)=0.535) than those of M. ruthenica (F(ST)=0.130). For M. lupulina, 55.5% of the genetic variation was partitioned among populations, whereas 76.6% of the variation existed within populations of M. ruthenica. Based on the genetic data, the selfing rates of M. lupulina and M. ruthenica were estimated at 95.8% and 29.5%, respectively. The genetic differentiation among populations of both species was positively correlated with geographical distance. CONCLUSIONS The mating system differentiation estimated from the genetic data is consistent with floral morphology and observed pollinator visitation. There was a much higher historical gene flow in M. ruthenica than in M. lupulina, despite more effective seed dispersal mechanisms in M. lupulina. The population genetic structure and geographical distribution of the two Medicago species have been shaped by life form, mating systems and seed dispersal mechanisms.

Phylogenomic and structural analyses of 18 complete plastomes across nearly all families of early-diverging eudicots, including an angiosperm-wide analysis of IR gene content evolution

The grade of early-diverging eudicots includes five major lineages: Ranunculales, Trochodendrales, Buxales, Proteales and Sabiaceae. To examine the evolution of plastome structure in early-diverging eudicots, we determined the complete plastome sequences of eight previously unsequenced early-diverging eudicot taxa, Pachysandra terminalis (Buxaceae), Meliosma aff. cuneifolia (Sabiaceae), Sabia yunnanensis (Sabiaceae), Epimedium sagittatum (Berberidaceae), Euptelea pleiosperma (Eupteleaceae), Akebia trifoliata (Lardizabalaceae), Stephania japonica (Menispermaceae) and Papaver somniferum (Papaveraceae), and compared them to previously published plastomes of the early-diverging eudicots Buxus, Tetracentron, Trochodendron, Nelumbo, Platanus, Nandina, Megaleranthis, Ranunculus, Mahonia and Macadamia. All of the newly sequenced plastomes share the same 79 protein-coding genes, 4 rRNA genes, and 30 tRNA genes, except for that of Epimedium, in which infA is pseudogenized and clpP is highly divergent and possibly a pseudogene. The boundaries of the plastid Inverted Repeat (IR) were found to vary significantly across early-diverging eudicots; IRs ranged from 24.3 to 36.4kb in length and contained from 18 to 33 genes. Based on gene content, the IR was classified into six types, with shifts among types characterized by high levels of homoplasy. Reconstruction of ancestral IR gene content suggested that 18 genes were likely present in the IR region of the ancestor of eudicots. Maximum likelihood phylogenetic analysis of a 79-gene, 97-taxon data set that included all available early-diverging eudicots and representative sampling of remaining angiosperm diversity largely agreed with previous estimates of early-diverging eudicot relationships, but resolved Trochodendrales rather than Buxales as sister to Gunneridae, albeit with relatively weak bootstrap support, conflicting with what has been found for these three clades in most previous analyses. In addition, Proteales was resolved as sister to Sabiaceae with the highest support (bootstrap >90%) yet observed in plastome-scale phylogenetic analyses.

Complete Plastid Genome Sequencing of Trochodendraceae Reveals a Significant Expansion of the Inverted Repeat and Suggests a Paleogene Divergence between the Two Extant Species

The early-diverging eudicot order Trochodendrales contains only two monospecific genera, Tetracentron and Trochodendron. Although an extensive fossil record indicates that the clade is perhaps 100 million years old and was widespread throughout the Northern Hemisphere during the Paleogene and Neogene, the two extant genera are both narrowly distributed in eastern Asia. Recent phylogenetic analyses strongly support a clade of Trochodendrales, Buxales, and Gunneridae (core eudicots), but complete plastome analyses do not resolve the relationships among these groups with strong support. However, plastid phylogenomic analyses have not included data for Tetracentron. To better resolve basal eudicot relationships and to clarify when the two extant genera of Trochodendrales diverged, we sequenced the complete plastid genome of Tetracentron sinense using Illumina technology. The Tetracentron and Trochodendron plastomes possess the typical gene content and arrangement that characterize most angiosperm plastid genomes, but both genomes have the same unusual ∼4 kb expansion of the inverted repeat region to include five genes (rpl22, rps3, rpl16, rpl14, and rps8) that are normally found in the large single-copy region. Maximum likelihood analyses of an 83-gene, 88 taxon angiosperm data set yield an identical tree topology as previous plastid-based trees, and moderately support the sister relationship between Buxaceae and Gunneridae. Molecular dating analyses suggest that Tetracentron and Trochodendron diverged between 44-30 million years ago, which is congruent with the fossil record of Trochodendrales and with previous estimates of the divergence time of these two taxa. We also characterize 154 simple sequence repeat loci from the Tetracentron sinense and Trochodendron aralioides plastomes that will be useful in future studies of population genetic structure for these relict species, both of which are of conservation concern.

Floral organogenesis of Cocculus Orbiculatus and Stephania Dielsiana (Menispermaceae)

Floral ontogeny of two dioecious species in the Menispermaceae, Cocculus orbiculatus and Stephania dielsiana, was compared using scanning electron microscopy (SEM). In C. orbiculatus, flowers exhibit rudiments of a nonfunctional organ of the opposite sex. Female flowers have six stamens that remain primordial; however, male flowers usually have three carpellodes rather than six, and hence have a different number of carpels than female flowers. Development of carpellodes in the family was described under SEM for the first time. The male and female of C. orbiculatus are almost homomorphic in early developmental stages, with eight alternating whorls of four types of organs (sepals, petals, stamens/staminodes, and carpels/carpellodes) arising centripetally. In S. dielsiana, no vestigial sexual organs were found, and the symmetry of male and female flowers differs, with males being actinomorphic and females zygomorphic. The six stamens of C. orbiculatus are free, whereas the three or four stamens of S. dielsiana are united congenitally. There are six carpels in females of C. orbiculatus but a single one in S. dielsiana. In overview, all flowers are actinomorphic and trimerous, with the female flower of S. dielsiana being the exception. This may be the result of reduction and is yet to be determined.

Floral Development of Stephania (Menispermaceae): Impact of Organ Reduction on Symmetry

Stephania is the sole genus in the basal eudicot family Menispermaceae that possesses both actinomorphic and zygomorphic flowers. Variation in perianth merism can have an important impact on flower symmetry and thus eminent biological significance in evolution of Menispermaceae. Using SEM, we studied the floral development of four representative species, which present the two predominant floral patterns of the genus, namely, homomorphy of both male and female flowers (actinomorphy) or heteromorphy (actinomorphy/zygomorphy). The sepals of the male flowers are arranged mostly in two alternate whorls of three or four each, whereas in female flowers they are in a single whorl of three or four or there is only a single sepal. Petals of male flowers are in a whorl of three or four organs, whereas female flowers of some species have only two petals. Trimerous and tetramerous perianths can coexist in the same umbellets of some species. Variation in perianth merism and loss of perianth parts of the female flowers may result in flower symmetry switching from actinomorphy to zygomorphy. The two main floral patterns are consistent with a distinction of two subclades within Stephania. The unicarpellate genera of the Menispermaceae share a unique combination of characters, including a synandrium, unitegmic ovules, and absence of vestigial sexual organs. However, Stephania differs from its unicarpellate relatives by two features: a two-whorled arrangement of floral organs of the male flowers and a free perianth. The investigation provides new and valuable developmental information on flowers of the little-known Menispermaceae and provides a background for a discussion of the evolution of merism and unisexual flowers in the basal eudicots.

Chromosome Behavior During Meiosis and Development of Spore Mother Cells in the Chinese Quillwort Isoetes sinensis T. C. Palmer (Isoetaceae)

Abstract Chromosome behavior during meiosis of the tetraploid Isoetes sinensis was observed to be similar to that of basic diploid Isoetes species. This observation was consistent with the previous report that polyploid species of Isoetes are allopolyploids. Meiosis was generally similar in both megaspore and microspore mother cells, but differences were observed. Due to the smaller size of the microspore mother cells, during prophase I, chromosomes are not as dispersed as those in megaspore mother cells. Cytomixis was observed in all stages of meiosis in microspore mother cells, but it was not seen at any stage of meiosis in megaspore mother cells. Cytomixis, lagging chromosomes, chromosome bridges, chromosome fragments, and micronuclei, observed during meiosis in I. sinensis, can affect sexual reproduction, but it does not appear to be a major cause of population decline in this species. The major cause of decline is habitat degradation from human disturbance. The few remaining populations of the Chinese quillwort are fragmented and the numbers of individuals contained in these populations dwindle as a result of habitat degradation. Conservation efforts are needed to save I. sinensis from extinction.

Floral development of Phyllanthus chekiangensis (Phyllanthaceae), with special reference to androecium and gynoecium

The floral development of Phyllanthus chekiangensis has been studied by scanning electron microscopy. The perianth organs are initiated in two whorls, dimerous in male flowers and trimerous in female flowers, with a longer plastochron between whorls than between the organs within a whorl. Male flowers have two stamens. The prominent connective protrusions begin development simultaneously with the floral disk. The disk is two-lobed in male flowers but continuous in female flowers. In female flowers, the developing gynoecium remains open relatively long, so the developing ovules are visible from the outside for some time. The direction of the hemitropous ovules in the carpels is antitropous (epitropous). Two small obturators are formed per carpel, one above each ovule. The prominent nucellar beak extends far beyond the “micropyle”. A micropyle in the classical sense formed by integuments closing over the nucellus apex is not present at any stage of development. Thus, it is not correct to say that the nucellar beak “grows through the micropyle”. The exposed nucellar beak continues the curvature of the antitropous (epitropous) ovule and becomes contiguous with the obturator. The unusual length of the nucellar beak may be a potential synapomorphy of the enlarged Phyllanthus clade as inferred from molecular phylogenetics.

A new species of Argentina (Rosaceae, Potentilleae) from Southeast Tibet, with reference to the taxonomic status of the genus

Argentina songzhuensis, a new species of tribe Potentilleae (Rosaceae) from Southeast Tibet, is described and illustrated. Placement of this species within Argentina was assessed based on morphological comparisons with related species and on phylogenetic analyses of nuclear ITS and plastid trnL-F sequences. The new species is similar to A. glabriuscula, but can be easily distinguished by its densely hairy leaflets, number of stamens (ca.10–12) and decurrent bases of the uppermost pair of leaflets. Our results support the generic status of Argentina and suggest that ventral stipular auricles may be a good synapomorphy for the genus. The recent transfer to Argentina of several Sibbaldia species as well as the genera Piletophyllum and Tylosperma is also confirmed by our molecular phylogenetic analyses. A taxonomic key to A. songzhuensis and other Argentina species in Tibet is provided.

Cross‐species amplification of 92 microsatellites of Medicago truncatula

Medicago species are important genetic sources for forage crops and nitrogen sources for various ecosystems. The ongoing genome sequencing of the model legume, Medicago truncatula, provides a wealth of genetic markers potentially useful for characterizing the population genetic structure and evolutionary history, and the potential of the wild Medicago species. Here we tested the PCR amplification of 92 microsatellites developed from M. truncatula in six other Medicago species, and found that the cross‐species transferability, ranging from 53.26% to 61.96%, is comparable with those reported in other angiosperm genera. This article thus reports a number of microsatellites that are potentially useful for large‐scale ecological and evolutionary genetic studies of wild Medicago species.