Leonard B. Thien

New Perspectives on the Pollination Biology of Basal Angiosperms

Coleoptera and Diptera are the primary pollinators of extant basal angiosperms (wind pollination is rare); lineages of these insects were established by the Late Jurassic. Contemporary examples of insect pollination of nonangiosperm plants are present in Gnetales (flies, moths) and Cycadales (beetles). The breeding systems of extant basal angiosperms are dominated by bisexual, protogynous, fragrant flowers that may form chambers in the female phase. Floral thermogenesis is widely distributed throughout the extant basal angiosperms (Nymphaeaceae, Illiciaceae, Aristolochiaceae, Magnoliaceae, and Annonaceae), into the basal monocots (Araceae) and extends into the advanced monocots (Arecaceae and Cyclanthaceae) and eudicots (only one family, Nelumbonaceae). The cycads are the only other plant group with heat‐producing reproductive structures (male and female cones). Flower temperatures of thermogenic plants are in the range required by endothermic insects for purposes of mating and flight, and it is hypothesized that floral heat is a direct energy reward to insects (a resource). Floral fragrance and heat played major roles in early plant reproductive systems, and fragrance could signal not only food and sex but also heat.

Molecular phylogeny of the Magnoliaceae: the biogeography of tropical and temperate disjunctions

The boreotropical flora concept suggests that relictual tropical disjunctions between Asia and the Americas are a result of the expansion of the circumboreal tropical flora from the middle to the close of the Eocene. Subsequently, temperate species diverged at high latitudes and migrated to other continents. To test this concept, we conducted a molecular phylogenetic analysis (using cpDNA) of the Magnoliaceae, a former boreotropical element that currently contains both tropical and temperate disjuncts. Divergence times of the clades were estimated using sequences of matK and two intergenic regions consisting of psbA-trnH and atpB-rbcL. Results indicate the tropical American section Talauma branched first, followed by the tropical Asian clade and the West Indies clade. Within the remaining taxa, two temperate disjunctions were formed. Assuming the temperate disjunction of Magnolia acuminata and Asian relatives occurred 25 mya (late Oligocene; based on seed fossil records), section Talauma diverged 42 mya (mid-Eocene), and tropical Asian and the West Indies clades 36 mya (late Eocene). These events correlate with cooling temperatures during the middle to late Eocene and probably caused the tropical disjunctions.

Self-isolation and insect pollination in the primitive angiosperms: New evaluations of older hypotheses

A review of the literature, compiled over the past 40 years suggests new directions for theories discussing the evolution of reproductive isolation and entomophily in angiosperms. Data on breeding systems suggests that the protoangiosperms may have developed gametophytic self-incompatibility. Protogyny is probably ancestral to protandry and herkogamy. Although the proto-flowers were bisexual wide variation in the number of sexual organs within proto-flowers probably led to labile sexuality and early trends towards dicliny. Beetle-pollination in extant relicts appears too specialized to represent an ancestral condition. Rather, the proto-flowers may have been generalist entomophiles incorporating some beetles,Plecoptera, thrips, micropterigid moths and proto-dipterans into their fluctuating spectra of opportunistic pollinators. Bee-pollination is probably polyphyletic in origin evolving repeatedly from angiosperms showing these generalist syndromes. There is still no correlation between primitive bees (especiallyColletidae) and the relictual angiosperms. Pollen that is usually retained within the anthers following dehiscence and the presence of staminal filaments and styles characterizes most of the flowers of those relictual angiosperms pollinated by specialized, modernApoidea.

Molecular Phylogeny of Magnolia (Magnoliaceae) Inferred from cpDNA Sequences and Evolutionary Divergence of the Floral Scents

Magnolia (disjuncts), however, have similar chemical profiles. A molecular phylogeny of Magnoliaceae was constructed to reveal phylogenetic relationships of taxa by sequencing the trnK intron (including the matK coding region), psbA-trnH, and atpB-rbcL intergenic spacer regions of chloroplast DNA from 25 Magnolia, two Michelia, and two Liriodendron taxa. The psbA-trnH spacer region showed twice the sequence divergence (0.0157) of the trnK intron (0.0073) or the matK coding region (0.0077). The strict consensus tree constructed from the combined data set (ca. 3,700 bp) indicated the genus Magnolia was polyphyletic containing Michelia species as ingroup. The clade of Magnolia liliifera var. obovata, M. coco, and M. delavayi formed the first branch. Among the remaining species, two additional large clades were recognized, i.e., one comprised of American evergreen Magnolia species and another of subgenus Yulania. The relationship among sect. Rytidospermum taxa was not clearly resolved. Parsimonious mapping of the floral scent chemical characters was performed onto the molecular phylogenetic tree to discuss evolutionary trends of the floral scent chemistries.

A Conserved Motif in the 5.8S Ribosomal RNA (rRNA) Gene is a Useful Diagnostic Marker for Plant Internal Transcribed Spacer (ITS) Sequences

The nuclear ribosomal DNA (rDNA) internal transcribed spacer (ITS) region has become an important nuclear locus for molecular systematic investigations of angiosperms at the intergenic and interspecific levels. Universal PCR primers are positioned on the conserved rRNA genes (18S, 5.8S, 26S) to amplify the entire ITS spacer region. Recent reports of fungal and algal contaminants, first described as plant ITS sequences, stress the need for diagnostic markers specific for the angiosperm ITS region. This report describes a conserved 14 base pair (bp) motif in the 5.8S rRNA gene that can be used to differentiate between flowering plants, bryophytes, and several orders of algae and fungi, including common plant pathogenic and non-pathogenic fungi. A variant of the motif (found in fungi and algae) contains a convenient EcoRI restriction site that has several applications for eliminating problematic contaminants from plant ITS preparations.

The Population Structure and Floral Biology of Amborella Trichopoda (Amborellaceae)

The shrubs and small trees of Amborella trichopoda are functionally unisexual and the populations are dioecious, male biased, and occur primarily in clumps. Floral size dimorphism reported for this species was confirmed by differences in floral biomass. At the level of the inflorescence, there were significantly greater numbers of male versus female flowers/inflorescence. No differences were observed between male and female plants in height, stem number, and diameter at the ground level. Male flowers bear 6 to 21 stamens and female flowers 3 to 6 spirally arranged carpels and staminodes that mimic the fertile androecia in male flowers. Flowering within a population was synchronous, and flowers of Amborella trichopoda are both insect- and wind-pollinated. A wide variety of insects ranging in size from ca. 1 mm to 7 cm in length pollinate the flowers, indicating a generalist pollination system. Beetles involved in pollination dwell in the forest litter but also spend hours on the leaves, flowers, and branches feeding on pollen. Pollen is the reward for insects as there is an absence of detectable floral volatiles and nectars, and anthers lack secretions or food bodies. A free-flowing stigma secretion was occasionally present, but it was not consumed by pollinators. Structural studies indicate that the stigma is of the dry-type, and the pollinators probably visit female flowers because of the mimetic role of the staminodes. The combination of wind and insect pollination exhibited in A. trichopoda is rare in basal angiosperms. Gall midges, parasitoid wasps, and thrips utilize floral tissue as a breeding site, impeding reproduction. Two species of gall-inducing midges (Cecidomyiidae) insert egg(s) into the gynoecia of developing flower buds, converting one or more ovaries into galls. Parasitoid wasps (Chalcidae) lay eggs in the galls that develop into larvae that prey upon the midge maggots. The Cecidomyiidae expanded with the angiosperms, but the earliest fossils of gall-inducing gall midges occur in the Miocene. Deceptive mechanisms involving numerous floral traits in small bisexual and unisexual flowers are common in the ANITA group and other basal angiosperms.

The Pollination of Zygogynum (Winteraceae) by a Moth, Sabatinca (Micropterigidae): An Ancient Association?

The primitive and vesselless angiosperm Zygogynum (Winteraceae), which is restricted to New Caledonia, is pollinated by a moth, Sabatinca (Micropterigidae). Fossil records of both the moth and the plant families extend to the Early Cretaceous. Adult Sabatinca have grinding mandibles and usually feed on the spores of ferns and on pollen. The insects use the flowers as mating sites and eat the pollen which is immersed in a dense pollenkitt. This mode of pollination in which flowers serve as mating and feeding stations with floral odors acting as cues may have been common in the early evolution of flowering plants.

Pollination of New Caledonian Winteraceae: opportunistic shifts or parallel radiation with their pollinators?

Flower visitors on 12 species of New CaledonianWinteraceae were studied. The visitors were two species of ancestral moths (Sabatinca; Micropterigidae), three species of weevils (Palontus; Curculionidae), and a species of thrips. Behavior observations and pollen records suggest that the beetles and occasionally the moths serve as pollinators ofZygogynum and Exospermum, andBelliolum is pollinated primarily by thrips. The floral volatiles are simple in composition, usually dominated by short esters. Preliminary experiments showed that ethyl acetate elicited alighting, and a distinctive huddling behavior was elicited by artificially mixed fragrance. The host associations of otherSabatinca andPalontus spp. do not support the hypothesis that theWinteraceae have radiated in association with their pollinators. The available evidence supports the notion of opportunistic isolated host colonizations at some point in the radiation of the pollinator groups.

Distribution and differential expression of (E)-4,8-dimethyl-1,3,7-nonatriene in leaf and floral volatiles of Magnolia and Liriodendron taxa

Analyses of volatiles emitted from artificially damaged leaves attached to branches of seven Magnolia taxa revealed the presence of (Z)-3-hexenyl acetate, (Z)-3-hexenol (the green odor compounds), and several mono- and sesquiterpenes, e.g., (Z)- and (E)-β-ocimene and caryophyllene. An herbivore-induced leaf volatile, (E)-4,8-dimethyl-1,3,7-nonatriene, known as a predator attractant in agricultural plants, was emitted 4–6 hr after leaves were damaged in M. hypoleuca. The damaged leaves of M. grandiflora, however, immediately released (E)-4,8-dimethyl-1,3,7-nonatriene. Undamaged leaves of Magnolia species examined did not emit volatile compounds. In addition, detached flowers of six Magnolia taxa and Liriodendron tulipifera also emit (E)-4,8-dimemyl-1,3,7-nonatriene as a floral volatile (up to 30% in some species); the chemical was also emitted from the intact flowers of M. heptapeta and M. salicifolia.

Self-Incompatibility in Ipomoea pes-caprae (Convolvulaceae)

-The breeding system of Ipomoea pes-caprae, the goats foot or beach morning glory, was examined in two populations bordering the Gulf of Mexico in order to investigate the self-incompatibility mechanism of the species, especially the number of s alleles in two populations. Ipomoea pes-caprae possesses a sporophytic multi-allelic incompatibility system which is rare in flowering plants. The maximum likelihood estimates of the number of s alleles in the samples of plants from Grand Isle, Louisiana, and Progreso, Yucatan, are four and seven, respectively. Although self-incompatibility appears to be an incongruous feature of a pantropical colonizing species, long-range dispersal of pollen and water-dispersed seeds enhance the self-incompatibility system to promote genetic diversity and a widespread distribution.