Ancestor Of Angiosperms Research Articles

The Evolution of the SEPALLATA Subfamily of MADS-Box GenesSequence data from this article have been deposited with the EMBL/GenBank Data Libraries under accession nos. AY850178, AY850179, AY850180, AY850181, AY850182, AY850183, AY850184, AY850185, AY850186.

Members of the SEPALLATA (SEP) MADS-box subfamily are required for specifying the "floral state" by contributing to floral organ and meristem identity. SEP genes have not been detected in gymnosperms and seem to have originated since the lineage leading to extant angiosperms diverged from extant gymnosperms. Therefore, both functional and evolutionary studies suggest that SEP genes may have been critical for the origin of the flower. To gain insights into the evolution of SEP genes, we isolated nine genes from plants that occupy phylogenetically important positions. Phylogenetic analyses of SEP sequences show that several gene duplications occurred during the evolution of this subfamily, providing potential opportunities for functional divergence. The first duplication occurred prior to the origin of the extant angiosperms, resulting in the AGL2/3/4 and AGL9 clades. Subsequent duplications occurred within these clades in the eudicots and monocots. The timing of the first SEP duplication approximately coincides with duplications in the DEFICIENS/GLOBOSA and AGAMOUS MADS-box subfamilies, which may have resulted from either a proposed genome-wide duplication in the ancestor of extant angiosperms or multiple independent duplication events. Regardless of the mechanism of gene duplication, these pairs of duplicate transcription factors provided new possibilities of genetic interactions that may have been important in the origin of the flower.

Gymnosperm Orthologues of Class B Floral Homeotic Genes and Their Impact on Understanding Flower Origin

Class B floral homeotic genes play a key role in specifying the identity of male reproductive organs (stamens) and petals during the development of flowers. Recently, close relatives (orthologues) of these genes have been found in diverse gymnosperms, the sister group of the flowering plants (angiosperms). The fact that such genes have not been found so far, despite considerable efforts, in mosses, ferns or algae, has been taken as evidence to suggest that B genes originated 300–400 million years ago in a lineage that led to extant seed plants. Gymnosperms do not develop petals, and their male reproductive organs deviate considerably from angiosperm stamens. So what is the function of gymnosperm B genes? Recent experiments revealed that B genes from diverse extant gymnosperms are exclusively expressed in male reproductive organs (microsporophylls). At least for some of these genes it has been shown that they can partially substitute for the Arabidopsis B genes AP3 and PI in ectopic expression experiments, or even partially substitute these genes in different class B floral organ identity gene mutants. This functional complementation, however, is restricted to male organ development. These findings strongly suggest that gymnosperm and angiosperm B genes have highly related interaction partners and equivalent functions in the male organs of their different host species. It seems likely that in extant gymnosperms B genes have a function in specifying male reproductive organs. This function was probably established already in the most recent common ancestor of extant gymnosperms and angiosperms (seed plants) 300 million years ago and thus represents the ancestral function of seed plant B genes, from which other functions (e.g., in specifying petal identity) might have been derived. This suggests that the B gene function is part of an ancestral sex determination system in which B gene expression specifies male reproductive organ development, while the absence of B gene expression leads to the formation of female reproductive organs. Such a simple switch mechanism suggests that B genes might have played a central role during the origin of flowers. In the out-of-male and out-of-female hypotheses changes in B gene expression led to the origin of hermaphroditic flower precursors out of male or female gymnosperm reproductive cones, respectively. We compare these hypotheses with other recent molecular hypotheses on the origin of flowers, in which C/D and FLORICAULA/LEAFY-like genes is given a more prominent role, and we suggest how these hypotheses might be tested in the future.

Distinct MADS-box gene expression patterns in the reproductive cones of the gymnosperm Gnetum gnemon.

Expression patterns from in situ hybridization of four MADS-box genes (GGM7, GGM9, GGM11, and GGM15) from the gymnosperm species Gnetum gnemon are presented. Together with previously published data about putative orthologs of floral homeotic genes from G. gnemon (GGM2, GGM3, GGM13), we describe seven temporally and spatially distinct expression patterns in male, female or both types of reproductive units which very likely reflect the diversity of MADS-box gene function in gymnosperm cones. There is evidence that some aspects of the observed differential expression have been conserved since the last common ancestor of extant angiosperms and gymnosperms about 300 million years ago.

The major clades of MADS-box genes and their role in the development and evolution of flowering plants

MADS-box genes encode a family of transcription factors which control diverse developmental processes in flowering plants ranging from root to flower and fruit development. Sequencing of (almost) the complete Arabidopsis genome enabled the identification of (almost) all of the Arabidopsis MADS-box genes. MADS-box genes have been divided in two large groups, termed type I and type II genes. The type II genes comprise the MEF2-like genes of animals and fungi and the MIKC-type genes of plants. The majority of MIKC-type genes are of the MIKC c-type, which includes all plant MADS-box genes for which expression patterns or mutant phenotypes are known. By phylogeny reconstruction, almost all of the MIKC c-type genes can be subdivided into 12 major gene clades, each clade comprising 1–6 paralogs from Arabidopsis and putative orthologs from other seed plants. Here we first briefly describe the deep branching of the MADS-box gene tree to place the MIKC c-type genes into an evolutionary context. For every clade of MIKC c-type genes we then review what is known about its members from Arabidopsis and well-studied members from other phylogenetically informative plant species. By gene sampling and phylogeny reconstructions we provide minimal estimates for the ages of the different clades. It turns out that 7 of the 12 major gene clades, i.e., AG-, AGL6-, AGL12-, DEF+GLO- (B), GGM13- (B s), STMADS11- and TM3-like genes very likely existed already in the most recent common ancestor of angiosperms and gymnosperms about 300 MYA. Three of the other clades, i.e., AGL2-, AGL17-, and SQUA-like genes, existed at least already in the most recent common ancestor of monocots and eudicots about 200 MYA. Only for two gene clades, AGL15-like genes (2 genes in Arabidopsis) and FLC-like genes (6 genes) members from plants other than Brassicaceae have not been reported yet. Similarly, only one ancient clade known from other flowering plant species, TM8-like genes, is not represented in Arabidopsis. These findings reveal that the diversity of MADS-box genes in Arabidopsis is rather ancient and representative for other flowering plants. Our studies may thus help to predict the set of MADS-box genes in all other flowering plants, except for relatively young paralogs. For the different gene clades we try to identify ancestral and derived gene functions and review the importance of these clades for seed plant development and evolution. We put special emphasis on gene clades for which insights into their importance has rapidly increased just recently.

Genomic Comparison of P-Type ATPase Ion Pumps in Arabidopsis and Rice

Members of the P-type ATPase ion pump superfamily are found in all three branches of life. Forty-six P-type ATPase genes were identified in Arabidopsis, the largest number yet identified in any organism. The recent completion of two draft sequences of the rice (Oryza sativa) genome allows for comparison of the full complement of P-type ATPases in two different plant species. Here, we identify a similar number (43) in rice, despite the rice genome being more than three times the size of Arabidopsis. The similarly large families suggest that both dicots and monocots have evolved with a large preexisting repertoire of P-type ATPases. Both Arabidopsis and rice have representative members in all five major subfamilies of P-type ATPases: heavy-metal ATPases (P1B), Ca2+-ATPases (endoplasmic reticulum-type Ca2+-ATPase and autoinhibited Ca2+-ATPase, P2A and P2B), H+-ATPases (autoinhibited H+-ATPase, P3A), putative aminophospholipid ATPases (ALA, P4), and a branch with unknown specificity (P5). The close pairing of similar isoforms in rice and Arabidopsis suggests potential orthologous relationships for all 43 rice P-type ATPases. A phylogenetic comparison of protein sequences and intron positions indicates that the common angiosperm ancestor had at least 23 P-type ATPases. Although little is known about unique and common features of related pumps, clear differences between some members of the calcium pumps indicate that evolutionarily conserved clusters may distinguish pumps with either different subcellular locations or biochemical functions.

The molecular characterization of PaHB2, a homeobox gene of the HD-GL2 family expressed during embryo development in Norway spruce.

PaHB1 (for Picea abies Homeobox1), an evolutionarily conserved HD-GL2 homeobox gene, specifically expressed in the protoderm during somatic embryogenesis in the gymnosperm Norway spruce has been reported previously. An additional HD-GL2 gene designated PaHB2 is reported here. During somatic embryogenesis, the PaHB2 gene is uniformly ex pressed in proembryogenic masses and in early somatic embryos, but it is not detectably transcribed at the beginning of maturation. In mature embryos, PaHB2 expression was essentially detected in the outermost layer of the cortex and the root cap. A similar PaHB2 expression is detected post-embryonically in both the primary root and the hypocotyl. Phylogenetic reconstructions and intron pattern analyses revealed that the PAHB proteins fall within two distinct subclasses comprising highly similar angiosperm homologues. The PAHB1 subclass consists of protoderm/epiderm-specific members. By contrast, the PAHB2 subclass gathers homologues with a subepidermal and protodermal/epidermal activity. This study suggests that at least two distinct HD-GL2 genes with a layer-specific expression already existed in the last common ancestor of angiosperms and gymnosperms. The conserved protodermal/epidermal and subepidermal expression of HD-GL2 genes could be used to study embryo radial pattern formation across seed plants.

New Insights into Patterns of Nuclear Genome Size Evolution in Plants

In land plants (Embryophyta), comprising bryophytes, pteridophytes, gymnosperms and angiosperms, DNA Cvalue data has increased significantly in recent years. Estimates are available for over 3,800 species, ranging over 2000- fold (1C = 0.06 - 127.4 pg). Evaluating the phylogenetic component of this variation is essential to understand its evolutionary significance. Consequently, C-value data were superimposed onto well-supported phylogenies. This showed: (1) Most angiosperms (52%) had small C-values (defined as 1C £ 3.5 pg) within five times the mode (0.6 pg). Very large C-values (1C ³ 35.0 pg) occurred in only two distantly related groups (monocots and Santalales) suggesting that ancestral angiosperms had small genomes, and that very large genomes represented a derived condition that arose independently at least twice. (2) In contrast, extant gymnosperms (sister to angiosperms) typically had larger genomes whose modal Cvalue (15.8 pg) was over 20 times greater than angiosperms. Large C-values (³ 14.0 pg) were even typical in extant cycads (mean 1C = 14.7 pg), regarded as the basal group of gymnosperms. Thus within extant seed plants (gymnosperms and angiosperms), possession of a small genome seems unique to angiosperms. (3) C-value data in pteridophytes and bryophytes are still too sparse to permit analysis of evolutionary trends. However, low C-values (1C = 0.06 - 2.1 pg) in lycophytes, a pteridophyte group sister to all other extant vascular plants, suggest that first vascular plants had small genomes. Further, the narrow range and small size of C-values in bryophytes and Selaginella and Lycopodium (0.06 - 2.1 pg) seem likely to be under tight nucleotypic control. Nucleotypic correlations between cell size and nuclear DNA Cvalues in fossil plants may reveal major trends of plant genome size evolution over geological time. Knowledge of genome size, its origin and consequences will further unify our understanding of the evolutionary processes, structural components and DNA sequences responsible for p lant genome diversity.

The evolution of embryo size in angiosperms and other seed plants: implications for the evolution of seed dormancy.

Seed dormancy plays an important role in germination ecology and seed plant evolution. Morphological seed dormancy is caused by an underdeveloped embryo that must mature prior to germination. It has been suggested that the presence of an underdeveloped embryo is plesiomorphic among seed plants and that parallel directional change in embryo morphology has occurred separately in gymnosperms and in angiosperms. We test these hypotheses using original data on embryo morphology of key basal taxa, a published dataset, and the generalized least squares (GLS) method of ancestral character state reconstruction. Reconstructions for embryo to seed ratio (E:S) using family means for 179 families showed that E:S has increased between the ancestral angiosperm and almost all extant angiosperm taxa. Species in the rosid clade have particularly large embryos relative to the angiosperm ancestor. Results for the gymnosperms show a similar but smaller increase. There were no statistically significant differences in E:S between basal taxa and any derived group due to extremely large standard errors produced by GLS models. However, differences between reconstructed values for the angiosperm ancestor and more highly nested nodes are large and these results are robust to topological and branch-length manipulations. Our analysis supports the idea that the underdeveloped embryo is primitive among seed plants and that there has been a directional change in E:S within both angiosperms and gymnosperms. Our analysis suggests that dormancy enforced by an underdeveloped embryo is plesiomorphic among angiosperms and that nondormancy and other dormancy types probably evolved within the angiosperms. The shift in E:S was likely a heterochronic change, and has important implications for the life history of seed plants.

DNA C-values in seven families fill phylogenetic gaps in the basal angiosperms

The evolutionary significance of the c. 1000-fold range of DNA C-values in angiosperms (1C = c. 0.1–127.4 pg) has often attracted interest. A recent analysis, which superimposed available C-value data onto the angiosperm phylogeny, that placed Ceratophyllaceae as the most basal angiosperm family led to the conclusion that ancestral angiosperms were characterized by small genomes (defined as 1C £ 3.5 pg). However, with the recent increase in DNA sequence data and large-scale phylogenetic analyses, strong support is now provided for Amborellaceae and/or Nymphaeaceae as the most basal angiosperm families, followed by Austrobaileyales (comprising Schisandraceae, Trimeniaceae and Austrobaileyaceae). Together these five families comprise the ANITA grade. The remaining basal angiosperm families (Ceratophyllaceae, Chloranthaceae and magnoliids), together with monocotyledons and eudicotyledons, form a strongly supported clade. A survey showed that C-value data were scarce in the basal angiosperm families, especially the ANITA grade. The present paper addresses these phylogenetic gaps by providing C-value estimates for each family in ANITA, together with C-values for species in Chloranthaceae, Ceratophyllaceae and a previously unrepresented family in the magnoliids, the Winteraceae. © The Linnean Society of London, Botanical Journal of the Linnean Society, 2002, 140, 175–179.

The evolution of gymnosperms redrawn by phytochrome genes: the Gnetatae appear at the base of the gymnosperms.

Gymnosperms possess two to four phytochrome types which apparently are the result of successive gene duplications in the genomes of their common ancestors. Phytochromes are nuclear-encoded proteins whose genes, contrary to chloroplast, mitochondrion, and rRNA genes, have hitherto rarely been used to examine gymnosperm phylogenies. Since the individual phytochrome gene types implied phylogenies that were not completely congruent to one another, conflicting branching orders were sorted by the number of gene lineages present in a taxon. The Gnetatae (two gene types) branched at the base of all gymnosperms, a position supported by bootstrap sampling (distance and character state trees, maximum likelihood). The Gnetatae were followed by Ginkgo, Cycadatae, and Pinaceae (three gene types) and the remaining conifers (four gene types). Therefore, in phytochrome trees, the most ancient branch of the conifers (Pinatae) seems to be the Pinaceae. The next split appears to have separated Araucariaceae plus Podocarpaceae from the Taxaceae/Taxodiaceae/Cupressaceae group. Structural arrangements in the plastid genomes (Raubeson and Jansen 1992) corroborate the finding that there is no close connection between Pinaceae and Gnetatae as suggested by some publications. The analyses are based on 60 phytochrome genes (579 positions in an alignment of PCR fragments) from 28 species. According to rough divergence time estimates, the last common ancestor of gymnosperms and angiosperms is likely to have existed in the Carboniferous.

Genes for two mitochondrial ribosomal proteins in flowering plants are derived from their chloroplast or cytosolic counterparts.

Often during flowering plant evolution, ribosomal protein genes have been lost from the mitochondrion and transferred to the nucleus. Here, we show that substitution by a duplicated, divergent gene originally encoding the chloroplast or cytosolic ribosomal protein counterpart accounts for two missing mitochondrial genes in diverse angiosperms. The rps13 gene is missing from the mitochondrial genome of many rosids, and a transferred copy of this gene is not evident in the nucleus of Arabidopsis, soybean, or cotton. Instead, these rosids contain a divergent nuclear copy of an rps13 gene of chloroplast origin. The product of this gene from all three rosids was shown to be imported into isolated mitochondria but not into chloroplasts. The rps8 gene is missing from the mitochondrion and nucleus of all angiosperms examined. A divergent copy of the gene encoding its cytosolic counterpart (rps15A) was identified in the nucleus of four angiosperms and one gymnosperm. The product of this gene from Arabidopsis and tomato was imported successfully into mitochondria. We infer that rps13 was lost from the mitochondrial genome and substituted by a duplicated nuclear gene of chloroplast origin early in rosid evolution, whereas rps8 loss and substitution by a gene of nuclear/cytosolic origin occurred much earlier, in a common ancestor of angiosperms and gymnosperms.

Nuclear DNA C-values Complete Familial Representation in Gymnosperms

The gymnosperms are a monophyletic yet diverse group of woody trees with approx. 730 extant species in 17 families. A recent survey showed that DNA C-values were available for approx. 16% of species, but for only 12 of the 17 families. This paper completes familial representation reporting first C-values for the five remaining families: Boweniaceae, Stangeriaceae, Welwitschiaceae, Cephalotaxaceae and Sciadopityaceae. C-values for nine Ephedra and two Gnetum species are also reported. C-values are now available for 152 (21%) species. Analysis confirms that gymnosperms are characterized by larger C-values than angiosperms (modal 1C of gymnosperms=15.8pg compared with 0.6pg in angiosperms) although the range (1C=2.25–32.20pg) is smaller than that in angiosperms (1C=0.05–127.4pg). Given complete familial coverage for C-values and increasing consensus in gymnosperm phylogeny, the phylogenetic component of C-value variation was also investigated by comparing the two datasets. This analysis revealed that ancestral gymnosperms (represented by cycads and/orGinkgo ; mean genome size=14.71pg) probably had larger genomes than ancestral angiosperms.

First Nuclear DNA C-values for Another 25 Angiosperm Families

Nuclear DNA C-value is an important genomic biodiversity character with many uses. An international workshop sponsored by Annals of Botany and held at the Royal Botanic Gardens, Kew, UK, in 1997 identified major gaps in our knowledge of plant DNA C-values and recommended targets for new work. Improved taxonomic coverage was highlighted as a key need for angiosperms, especially at the familial level. In 1997 C-values were known for only approx. 32% of angiosperm families; a goal of complete familial representation by 2002 was recommended. A review published in 2000 (Bennett et al.;Annals of Botany86: 859–909) noted poor progress towards this aim: of the 691 first C-values for species only 12 (1.7%) were for unrepresented families. We began new work to address this in 1999, reporting first DNA C-values for 25 angiosperm families in 2001 (Hanson et al.;Annals of Botany87: 251–258). Here we report first DNA C-values for a further 25 angiosperm families, increasing familial coverage in angiosperms to approx. 45%. Such targeting remains essential to approach the goal set by the 1997 workshop of familial coverage for angiosperms within 5 years. The 4C DNA amounts presented here range from 0.76pg (similar toArabidopsis thaliana ) in Roridula gorgonias(Roridulaceae) to 29.74pg in Gunnera manicata(Gunneraceae). 1C values were<3.5pg in 23 of the 25 families; these data provide further support for the view that ancestral angiosperms almost certainly had small genomes (defined as 1C⩽3.5pg). Chromosome counts are reported for 20 taxa, including first records for one genus and five species.

First Nuclear DNA C-values for 25 Angiosperm Families

DNA amount is a widely used biodiversity character. As known DNA C-values represent the global angiosperm flora poorly, better coverage of taxonomic groups is needed, including at the familial level. A workshop, sponsored byAnnals of Botany , was held at the Royal Botanic Gardens, Kew in 1997. Its key aim was to identify major gaps in our knowledge of plant DNA C-values and recommend targets for new work to fill them by international collaboration. In 1997 C-values were known for approx. 150 families, meaning there was no estimate for most angiosperm families (approx 68%). The workshop recommended a goal of complete familial representation by 2002, as a main target for angiosperms. Bennett et al. (Annals of Botany86: 859–909, 2000) presented a fifth supplementary list of angiosperm C-values from 70 original sources which included first C-values for 691 species. Only 12 (1.7%) of these were first C-values for unrepresented families, so the need to improve familial representation was substantially unmet. We began new work to address this in September 1999, and now report first DNA C-values for 25 angiosperm families. Such targeting seems essential to achieve the goal of familial coverage set by the 1997 workshop within 5 years. 4C values range from 0.67pg (similar toArabidopsis thaliana ) in Amoreuxia wrightii(Cochlospermaceae) to 7.49pg in Deutzia prunifolia(Hydrangeaceae). These data support the view that ancestral angiosperms almost certainly had small genomes (defined as 1C⩽3.5pg). Chromosome counts are reported for 19 taxa, including first records for three genera plus four species.

The Stomatal Ontogeny and Structure of the Liassic Pteridosperm Sagenopteris (Caytoniales) from Hungary.

Stomatal ontogeny is often inferred but rarely documented for extinct fossil plants because it requires observations from young leaves that are rarely preserved as fossils. The discovery of several very young leaves of the Jurassic plant Sagenopteris (Caytoniales) in the Mecsek Mountains (southern Hungary) in a good state of preservation provides the opportunity for studying the stomatal ontogenesis of this genus. The specimens show perigenous anomocytic stomata. This feature confirms the evolutionarily high position of Sagenopteris among fossil gymnosperms and supports the opinion that the ancestors of angiosperms and some groups of pteridosperms might be closely related. Such clear examples of stomatal development have not previously been documented for fossil material.

Retention of lignin in seagrasses:angiosperms that returned to the sea

Using Curie-point Pyrolysis Gas Chromatography Mass Spectrometry (Py-GCMS) and Direct Temperature-resolved Mass Spectrometry (DT-MS), lignin was detected in highly purified preparations (Milled Wood Lignin = MWL) of various tissues of the seagrasses Zostera marina and Posidonia oceanica. The results indicate that P. oceanica contains more lignin than Z. marina and that roots and rhizomes generally contain more lignin than leaves. It is concluded that the ability to produce lignin is not lost by the angiosperm ancestors of extant seagrasses upon their colonization of the marine environment. Relative lignin abundances in the different tissues appear to be positively correlated with life span. It is suggested that lignification contributes to the longevity of a tissue by protecting it against microbial attack, but that deposition of lignin in seagrasses is restricted to tissues that show limited growth. [KEYWORDS: lignin; seagrass; pathogenic resistance; pyrolysis; gas chromatography; mass spectrometry Pyrolysis mass-spectrometry; zostera-marina; curie-point; discriminant-analysis; fungal degradation; oxidation-products; phenolic-acids; beech wood; growth; decomposition]

Chloroplast small heat shock proteins: evidence for atypical evolution of an organelle-localized protein.

Knowledge of the origin and evolution of gene families is critical to our understanding of the evolution of protein function. To gain a detailed understanding of the evolution of the small heat shock proteins (sHSPs) in plants, we have examined the evolutionary history of the chloroplast (CP)-localized sHSPs. Previously, these nuclear-encoded CP proteins had been identified only from angiosperms. This study reveals the presence of the CP sHSPs in a moss, Funaria hygrometrica. Two clones for CP sHSPs were isolated from a F. hygrometrica heat shock cDNA library that represent two distinct CP sHSP genes. Our analysis of the CP sHSPs reveals unexpected evolutionary relationships and patterns of sequence conservation. Phylogenetic analysis of the CP sHSPs with other plant CP sHSPs and eukaryotic, archaeal, and bacterial sHSPs shows that the CP sHSPs are not closely related to the cyanobacterial sHSPs. Thus, they most likely evolved via gene duplication from a nuclear-encoded cytosolic sHSP and not via gene transfer from the CP endosymbiont. Previous sequence analysis had shown that all angiosperm CP sHSPs possess a methionine-rich region in the N-terminal domain. The primary sequence of this region is not highly conserved in the F. hygrometrica CP sHSPs. This lack of sequence conservation indicates that sometime in land plant evolution, after the divergence of mosses from the common ancestor of angiosperms but before the monocot-dicot divergence, there was a change in the selective constraints acting on the CP sHSPs.

Ultrastructure, chemical composition, and recrystallization of epicuticular waxes: transversely ridged rodlets

Transversely ridged rodlets (Aristolochia-type) are of high systematic significance characterizing the ancestral Aristolochiales, Magnoliales, and Laurales. Sporadically, they also occur in various unrelated derived taxa. The ultrastructure, chemistry, and recrystallization of epicuticular waxes of nine species were investigated by high resolution scanning electron microscopy, gas chromatography, and mass spectrometry. Chemical analyses show that transversely ridged rodlets clearly differ in their composition. Waxes of one group are characterized by ketones, whereas a second group completely lacks ketones and is dominated by alkanes. Hentriacontan-16-one (palmitone) was found to be characteristic for transversely ridged rodlets of Aristolochia, Laurus, and Paeonia. Standard solutions were taken for recrystallization experiments under different conditions of solvent, crystallization velocity, and temperature. It was shown that transversely ridged rodlets or related crystals grow from total waxes of all species but never crystallize from individual compounds such as alkanes or palmitone. We concluded that transversely ridged crystals are formed by self-assembly based on a slow crystallization process and the presence of additives. This paper shows that transversely ridged rodlets occur convergently within angiosperms based on a similar mode of crystallization but a different chemical composition. The role of palmitone as a chemotaxonomic character of ancestral angiosperms is discussed.Key words: plant cuticle, epicuticular waxes, chemistry, ultrastructure, recrystallization, systematics.

A homeobox gene with potential developmental control function in the meristem of the conifer Picea abies.

Many homeobox genes control essential developmental processes in animals and plants. In this report, we describe the first cDNA corresponding to a homeobox gene isolated from a gymnosperm, the HBK1 gene from the conifer Picea abies (L.) Karst (Norway spruce). The sequence shows distinct similarities specifically to the KNOX (knotted-like homeobox) class of homeobox genes known from different angiosperm plants. The deduced amino acid sequence of HBK1 is strikingly similar within the homeodomain (84% identical) to the maize gene Knotted1 (Kn1), which acts to regulate cell differentiation in the shoot meristem. This similarity suggested that the phylogenetic association of HBK1 with the KNOX genes might be coupled to a conservation of gene function. In support of this suggestion, we have found HBK1 to be expressed in the apical meristem in the central population of nondifferentiated stem cells, but not in organ primordia developing at the flanks of the meristem. This pattern of expression is similar to that of Kn1 in the maize meristem. We show further that HBK1, when expressed ectopically in transgenic Arabidopsis plants, causes aberrations in leaf development that are similar to the effects of ectopic expression of angiosperm KNOX genes on Arabidopsis development. Taken together, these data suggest that HBK1 has a role, similar to the KNOX genes in angiosperms, in the control of cellular differentiation in the apical meristem of spruce. The data also indicate that KNOX-gene regulation of vegetative development is an ancient feature of seed plants that was present in the last common ancestor of conifers and angiosperms.

Phylogenetic Analysis of DNA C-values provides Evidence for a Small Ancestral Genome Size in Flowering Plants

DNA C-value is a highly variable aspect of plant biodiversity whose origin and significance has often attracted general interest. Evaluation of the phylogenetic component of genome size variation is essential for a full explanation of its evolutionary significance but was previously prevented by insufficient data and lack of phylogenetic consensus. However, the recent development for the angiosperms of a DNA C-values database for 2802 species and a robust phylogenetic tree based on a three-gene DNA sequence matrix and 252 non-molecular characters allows meaningful new investigations of genome size in a phylogenetic context. Superimposing data from the former onto the latter shows that whereas all 15 higher order groups for which data are available contain species with small C-values, very large C-values occur in only two distantly related groups. At the lower taxonomic levels within these two groups similar trends were detected, with very large C-values restricted to species in the more derived families. The most parsimonious explanation for these observations is that ancestral angiosperms almost certainly had small genomes, and the possession of very large genomes represents a derived condition that has arisen independently at least twice. In contrast, gymnosperms (sister group to the angiosperms) are characterized by larger C-values than angiosperms. Thus within extant seed plants the possession of a small genome is a character unique to the angiosperms that was not only present in the ancestral species but has also been retained in most living taxa.