Ovule morphogenesis in Ranunculaceae and its systematic significance

BACKGROUND AND AIMS: Ranunculaceae has a prominent phylogenetic position in Ranunculales which appears at the base of eudicots. The aims of the present paper are to reveal the features of ovule morphogenesis in different taxa and gain a better understanding of the systematics of Ranunculaceae. METHODS: Flowers of 17 species from three subfamilies, nine tribes and 16 genera of Ranunculaceae, at successive developmental stages, were collected in the wild and studied with a scanning electron microscope. KEY RESULTS: The integuments in the unitegmic ovules in Helleborus, Ranunculus and Oxygraphis, as well as the inner integuments in the bitegmic genera, initiate annularly and eventually become cup-shaped. However, the integuments in the unitegmic ovules in Anemone and Clematis, as well as the outer integuments in the bitegmic genera, arise semi-annularly and eventually become hood-shaped. Different kinds of appendages appear on the ovules during development. In Coptis of subfamily Coptidoideae, a wrap-shaped appendage arises outside the ovule and envelopes the ovule entirely. In the genera of subfamily Thalictroideae and tribe Anemoneae of subfamily Ranunculoideae, appendages appear on the placenta, the funicle or both. In tribe Helleboreae of subfamily Ranunculoideae, an alary appendage is initiated where the integument and the funicle join and becomes hood-shaped. CONCLUSIONS: Ovule morphogenesis characteristics are significant in classification at the levels of subfamilies and tribes. The initiation patterns of the integuments and the development of appendages show diversity in Ranunculaceae. The present observations suggest that the bitegmic, hood-shaped outer integument and endostomic micropyle are primitive while the unitegmic, cupular-shaped outer integument and bistomic micropyle are derivative.     

Developmental morphology of the ovules of Amborella trichopoda (Amborellaceae) and Chloranthus serratus (Chloranthaceae)

The developmental morphology of the outer integument in the pendent orthotropous ovules of Amborella trichopoda (Amborellaceae) and Chloranthus serratus (Chloranthaceae) was studied. In both species the outer integument is semiannular at an early stage and becomes cup-shaped but dorsiventrally somewhat asymmetric at later stages. The outer integument, which is initiated first on the concave and lateral sides of the ovule, differs from that of the anatropous ovules of other basal families with the outer integument semiannular at an early stage or throughout development. The bilateral symmetry of the outer integument is shared by these orthotropous and anatropous ovules. The developmental pattern of the outer integument and ovule incurving characterize the ovule of the Amborellaceae and Chloranthaceae, which is not equivalent to typical orthotropous ovules of eudicots. A phylogenetic analysis of ovule characters in basal angiosperms suggests that anatropous ovules with cup-shaped outer integuments and orthotropous ovules were derived independently in several clades and that the ovules of Amborella and Chloranthus might also be derivative.     

Gynoecium diversity and systematics of the basal eudicots

Gynoecium and ovule structure was compared in representatives of the basal eudicots, including Ranunculales (Berberidaceae, Circaeasteraceae, Eupteleaceae, Lardizabalaceae, Menispermaceae, Papaveraceae, Ranunculaceae), Proteales (Nelumbonaceae, Platanaceae, Proteaceae), some families of the former ‘lower’ hamamelids that have been moved to Saxifragales (Altingiaceae, Cercidiphyllaceae, Daphniphyllaceae, Hamamelidaceae), and some families of uncertain position (Gunneraceae, Myrothamnaceae, Buxaceae, Sabiaceae, Trochodendraceae). In all representatives studied, the carpels (or syncarpous gynoecia) are closed at anthesis. This closure is attained in different ways: (1) by secretion without postgenital fusion (Berberidaceae, Papaveraceae, Nelumbonaceae, probably Circaeaster); (2) by a combination of postgenital fusion and secretion; (2a) with a complete secretory canal and partly postgenitally fused periphery (Lardizabalaceae, Menispermaceae, some Ranunculaceae, Sabiaceae); (2b) with an incomplete secretory canal and completely fused periphery (Tro-chodendron); (3) by complete postgenital fusion without a secretory canal (most Ranunculaceae, Eupteleaceae, Platanaceae, Proteaceae, all families of Saxifragales and incertae sedis studied here). Stigmas are double-crested and decurrent in most of the non-ranunculalian taxa; unicellular-papillate in most taxa, but with multicellular protuberances in Daphniphyllaceae and Hamamelidaceae. Carpels predominantly have three vascular bundles, but five in Proteales (without Nelumbonaceae), Myrothamnaceae and Trochodendraceae. The latter two also share ‘oil’ cells in the carpels. Stomata on the outer carpel surface are present in the majority of Ranunculales and Proteales, but tend to be lacking in the saxifragalian families. In basal eudicots, especially in the non-ranunculalian families there is a trend to form more than one ovule per carpel but to develop only one seed, likewise there is a trend to have immature ovules at anthesis. Ovule number per carpel is predominantly one or two. Proteales (without Nelumbonales) mainly have orthotropous ovules, the other groups have anatropous (or hemitropous or campylotropous) ovules. The outer integument is annular in the groups with orthotropous or hemitropous ovules, and also in a number of saxifragalian families with anatropous ovules. In Proteales the integuments are predominantly lobed but there is no distinct pattern in this feature among the other groups. Among Ranunculales two pairs of families (Lardizabalaceae/Menispermaceae and Bcrberidaceae/Papaveraceae) due to similarities in gynoecium structure can be recognized, which are not apparent in molecular analyses. The close relationship of Platanaceae and Proteaceae is supported by gynoecium structure but gynoecial features do not support their affinity to Nelumbonaceae. The alliance of Daphniphyllaceae with Hamamelidaceae s.l. is also supported.     

Development of ovule and seed in Rapateaceae

VENTURELLI, M. & BOUMAN, F., 1988. Development of ovule and seed in Rapateaceae. The structure of the ovules and/or seeds of twelve species of Rapateaceae were studied, some additional embryological characters also being recorded. The ovules are always anatropous, bitegmic and crassinucellate, but they differ in the shape, size and in thickness of the outer integument. In Rapateaceae the outer integument is initiated subdermally. The seed coat of the Rapateaceae shows two mechanical layers: an endotesta with silica present as bodies or as incrustations in cell walls, in conjunction with an exotegmen with a jigsaw cell pattern complicated by a labyrinth-like sculpturing of the outer cell walls. The innermost layer of the inner integument is tanniniferous. Large hilar scars with tracheidal plates on the corresponding fruit wall and a persistent obturator are recorded in Rapateaceae. On the basis of embryological characters the family fits well into the Commelinales. Testa structure most closely resembles that of the Commelinaceae. The differences in ovule and seed structure agree with the currently accepted tribal classification.     

The histogenesis of integuments in some species of menispermanceae

The mode of initiation and development of integuments was investigated in six species of five genera in Menispermanceae, which have bitegmic and unitegmic ovules. The species investigated have similar integumentary structures at maturity in each of the bitegmic and unitegmic ovules. In bitegmic ovules (e.g.Cocculus), both integuments are for the most part two-cell layered. The initiation of inner integument (ii) begins with divisions of dermal cells of the nucellar primordium. The initiation of the outer integument (oi) commences with divisions of subdermal cells. In unitegmic ovules (e.g.Stephania), the integument is initiated by periclinal divisions of dermal cells, and cells of subdermal origin (which may represent the oi in case of bitegmy) form a small swelling on the raphal side and, on the antiraphal side, are included in the base of the single integument. Unitegmy of Menispermanceae (at least in the case of the genera investigated) seems to have been derived through elimination of oi, rather than through “integumentary shifting” (Bouman and Calis, 1977), a process suggested for explanation of unitegmy as in Ranunculaceae.     

Interactions among Genes Regulating Ovule Development in Arabidopsis Thaliana

The INNER NO OUTER (INO) and AINTEGUMENTA (ANT) genes are essential for ovule integument development in Arabidopsis thaliana. Ovules of ino mutants initiate two integument primordia, but the outer integument primordium forms on the opposite side of the ovule from the normal location and undergoes no further development. The inner integument appears to develop normally, resulting in erect, unitegmic ovules that resemble those of gymnosperms. ino plants are partially fertile and produce seeds with altered surface topography, demonstrating a lineage dependence in development of the testa. ant mutations affect initiation of both integuments. The strongest of five new ant alleles we have isolated produces ovules that lack integuments and fail to complete megasporogenesis. ant mutations also affect flower development, resulting in narrow petals and the absence of one or both lateral stamens. Characterization of double mutants between ant, ino and other mutations affecting ovule development has enabled the construction of a model for genetic control of ovule development. This model proposes parallel independent regulatory pathways for a number of aspects of this process, a dependence on the presence of an inner integument for development of the embryo sac, and the existence of additional genes regulating ovule development.     

Ovular development and morphology in some magnoliaceae species

Floral phenology and ovular development ofLiriodendron tulipifera are described. The ovule primordia are initiated in December, followed by prominent development in March, and the ovules are mature in May. The inner integument is formed as an annular rim on the incurving ovule primordia, but the outer integument develops as a semi-annular rim interrupted on the concave side of the funicle. Later, an outgrowth, which is interpreted here as an obturator, arises on the concave side of the funicle. The funicular outgrowth arises far from the inner integument, while the outer integument is close to the inner. The outer integument and the funicular outgrowth together form an envelope complex. Later the outer integument produces two distal lobes, which disappear at maturity. Mature ovules of the threeMagnolia species examined have similar lobes. It is suggested that the hood-shaped outer integument is primitive in angiosperms.     

Gynoecium diversity and systematics of the Laurales

Carpel and ovule structure was comparatively studied in representatives of all eight families of the Laurales: Amborellaceae, Calycanthaceae, Chloranthaceae, Gomortegaceae, Hernandiaceae, Lauraceae, Monimiaceae, and Trimeniaceae. In all representatives the carpels are closed at anthesis. As in Magnoliales/winteroids, closure takes place in three different modes: (1) by postgenital fusion of the stylar (and ovarial) ventral slit (Calycanthaceae, Gomortegaceae, Lauraceae, Hernandiaceae); (2) by occlusion of the inner space by secretion (Amborellaceae, Chloranthaceae, Trimeniaceae, Mollinedioideae of Monimiaceae), all having extremely ascidiate carpels; (3) by a combination of (1) and (2), whereby the ventral slit in the style is postgenitally fused but a central canal remains open, which is filled by secretion (Monimiaceae except Mollinedioideae). The carpels have a single ovule in ventral median placentation; only Calycanthaceae have two lateral ovules, although the upper ovule degenerates. In contrast to Magnoliales/winteroids, several representatives have orthotropous or almost orthotropous ovules (Amborellaceae, Chloranthaceae, Gomortegaceae). Mature ovules vary in length between 425 μm (some Monimiaceae) and 1500 urn (some Calycanthaceae, Trimeniaceae). Although all ovules are crassinucellar, nucellus breadth varies between 60 μm (Chimonanthus, Calycanthaceae) and 500 μm (Hemandia, Hernandiaceae). In almost all representatives the single ovule (two in Calycanthaceae) tightly fills out the ovarial cavity. The micropyle is mostly formed by the inner integument. In a few cases there is no micropyle and the nucellar apex makes direct contact with the inner ovary surface or the funicle (Lauraceae p.p., Calycanthaceae p.p., Hernandiaceae p.p., Monimiaceae p.p.). The ovule is pachychalazal (or perichalazal) in Lauraceae, some Hernandiaceae, and Gomortegaceae. Both integuments are variously lobed or unlobed. The outer integument is semiannular or annular, and this may vary within a family (Calycanthaceae, Hernandiaceae, Monimiaceae); it is also exceedingly diverse in thickness (2–23 cell layers). Gynoecial traits support the association of Chloranthaceae, Trimeniaceae, and Amborellaceae, and also separately Gomortegaceae, Hernandiaceae, and Lauraceae. In addition, affinities of the first group with Schisandraceae, Illiciaceae and Austrobaileyaceae may also be supported.     

Comparative embryology of Bambusa tulda Roxb. and Thyrsostachys siamensis Gamble (Poaceae: Bambuseae)

Bambusa tulda and Thyrsostachys siamensis resemble each other in having an obovate ovary which is hairy and thickened along the apex, a pseudo-crassinucellate ovule with a wide region of attachment, poorly-developed and ephemeral outer integument, an inner integument which fails to grow beyond the nucellus, 'Polygonum' type of embryo sac ontogeny, parallel orientation of embryo sac to the long axis of the ovule, multiple antipodals which retain apical position in the embryo sac even during post-fertilization phase of development, an ephemeral nucellus, relatively small bambusoid embryos, and many-layered and apically thickened pericarp. However, they differ from each other in their gynoecial structure, the extent of the development of the outer integument, organization of megaspore tetrads and development-stage-related behaviour of the inner integument in the fertilized ovules. These taxa also differ from other members of the subfamily Bambusoideae in the structure of the mature ovule, endosperm and pericarp.     

Gametophytes,embryogeny and pericarp ofMicrostylis wallichii lindl. (Orchidaceae)

The anther wall is 4-layered thick. Its development is of the Monocotyledonous type. Simultaneous cytokinesis results in decussate, isobilateral, linear and tetrahedral tetrads. At anthesis, the microspores are 2-celled. The mature ovules are anatropous, bitegmic and tenuinucellate. Both the integuments are dermal in origin and 2-layered. The inner integument alone forms the micropyle. Development of the female gametophyte is of the Monosporic type. Double fertilization occurs but the primary endosperm nucleus degenerates without any division. Development of embryo corresponds to the variation of the Onagrad type. The mature embryo lacks differentiation. The seeds are minute and non-endospermic. The seed coat is formed entirely by the outer layer of outer integument. There are three sterile and three fertile valves in the ovary. In the prefertilization stages valves consist of parenchymatous cells. After fertilization, the sterile valves become sclerenchymatous whereas the fertile valves remain parenchymatous.     

A structural investigation of the ovule in sugar beet, Beta vulgaris: Integuments and micropyle

The micropyle and the integuments of sugar beet (Beta vulgaris) ovules have been investigated by light and electron microscopy during differentiation and maturation of the ovule. The micropyle itself is formed by the inner integument which is surrounded by the outer integument at its base. The micropyle containts a fibrillar PAS+ substance and is often covered by a thin sheet or hymen. Both integuments are cuticle-covered thin sheets, each 2-few cell layers in thickness. In the outer integument an increase in starch accumulation occurs during ovule maturation and probably functions as nutrient storage for embryo development. The inner epidermis of the inner integument differentiates as the most conspicuous cell layer of the beet ovule. During growth and maturation of the ovule a system of small perinuclear vacuoles containing dense material increases steadily in these cells. At maturity this system fills up more than half of each cell and very dense material has accumulated in each vacuole. This vacuole content is highly refractive and contains tannins and/or polyphenols.     

Integument initiation and testa development in some Cruciferae

This study has shown for the first time that the middle layer (or layers) of the outer integument is (are) of subdermal derivation in at least some taxa of the Cruciferae. The outer integument is initiated in the Cruciferae in three different ways, viz. subdermally (Brassica, Sinapis) , partly subdermally and partly dermally (Lunaria) , or completely dermally (Capsella). These differences in initiation are reflected in the structure of the mature testa. The inner integument is completely of dermal derivation and originally two cell-layers thick, but may become more than two-layered during the ovule and seed maturation by periclinal divisions of the inner cell layer. The consequences of the ontogeny of the integuments for the terminology and interpretation of the mature testa is discussed.     

The Arabidopsis SUPERMAN Gene Mediates Asymmetric Growth of the Outer Integument of Ovules

Arabidopsis superman (sup, also referred to as floral mutant10) mutants have previously been shown to have flowers with supernumerary stamens and reduced carpels as a result of ectopic expression of the floral homeotic gene APETALA3 (AP3). Here, we report that sup mutations also cause specific alterations in ovule development. Growth of the outer integument of wild-type ovules occurs almost exclusively on the abaxial side of the ovule, resulting in a bilaterally symmetrical hoodlike structure. In contrast, the outer integument of sup mutant ovules grows equally on all sides of the ovule, resulting in a nearly radially symmetrical tubular shape. Thus, one role of SUP is to suppress growth of the outer integument on the adaxial side of the ovule. Genetic analyses showed that the effects of sup mutations on ovule development are independent of the presence or absence of AP3 activity. Thus, SUP acts through different mechanisms in its early role in ensuring proper determination of carpel identity and in its later role in asymmetric suppression of outer integument growth.     

Translocation of Uranin within the Living Ovules of Vanilla

In the ovules of Vanilla (Vanilla planifolia Andr.) before fertilization, outer integument surrounded the lower part of ovule. Uranin got into ovule through funiculus, forming, the first center of fluorescence at the chalaza zone of ovule. Then uranin was transported to micropyle end along inner integument, forming the second center of fluorescence at micropyle end of inner integument. Soon, fluorescence appeared in the egg apparatua. After fertilization, the outer integument ovule extended upward, forming micropyle ogerber with inner integument. After getting into ovule through funiculus, uranin spreads to- ward several directions: l. transported to outer integument at the entrance of micropyle; 2. transported downward to chalaza zone along outer integument at the side of funiculus; 3. extended from chalaza zone to the inside and to the outer integument at the side far from funiculus The ovules of Vanilla had no vascular bundles. On transporting in inner integument, however, the cells in inner layer next to the embryo sac appeared to be the major passage. In mature embryo sac, there was cuticle between inner integument and embryo sac at the half of micropyle end. But between embryo sac at the half of chalaza end and nucellus, cuticle was absent. Nutrient could get into embryo sac from chalaza end undoubtedly. As egg apparatus showed the fluorescence after formation of fluorescence center of inner integument at micropylar end, the possibility that nutrient got into embryo sac from micropyle could not be excluded.     

北美香柏雌球果的发育

用扫描电镜(SEM)观察了北美香柏 Thuja occidentalis 雌球果的发育过程。在北京,北美香柏的雌球果是在八月初由营养芽转变而来,雌球果一般有4~6对苞片,中间2~3对可育,每一苞片腋部着生两枚胚珠,在可育苞片腋部最先观察到一扁平的隆起,并在其上分化出两个胚珠原基,接着分化出珠被和珠心,最后形成扁平而两侧对称的胚珠。在北美香柏雌球果发育过程中,约一半的雌球果在2~3对可育苞片中位于下面的1~2对的腋部产生3个胚珠原基,中间一个较小,并在以后的发育中逐渐退化。由此推测北美香柏的雌球果可能是由祖先类群中每一苞片具多于2个胚珠的雌球果演化而来。在光镜下对雌球果维管系统的观察发现,传粉前幼小雌球果的苞片内仅有一束维管束,传粉后随着苞片基部的居间生长,有4—8束维管束在苞片内形成,但是新发育的维管束木质部和韧皮部相对位置与正常叶性器官一致,这与在以往报道的柏科植物成熟雌球果的苞片中均有反向维管束的发育不同。北美香柏雌球果早期发育和维管束分析结果支持傅德志和杨亲二提出的解释裸子植物生殖器官形态演化的“苞鳞-种鳞复合体”理论。关键词北美香柏;雌球果的发育;胚珠分化;SEM     

Studies in the embryology of the diandrous orchidCypripedium cordigerum (Cypripedieae,Orchidaceae)

The anther wall layers ofCypripedium cordigerum are six to eight. The glandular tapetum is 2- or 3-layered and its cells are uninucleate. Simultaneous cytokinesis results in decussate, isobilateral and tetrahedral pollen tetrads. Ripe pollen grains are 2-celled. The mature ovules are anatropous, bitegmic and tenuinucellate. Both the integuments are dermal in origin and 2-layered. The inner integument alone forms the micropyle. The female gametophyte is 6-nucleate and bisporic. The reduction of nuclei is due to the strike phenomenon. Double fertilization occurs. The primary endosperm nucleus divides to form two free endosperm nuclei. The mature embryo is undifferentiated. The cells ca, m and n contribute to the embryo. The suspensor is single-celled. The seed coat is formed entirely by the outer layer of the outer integument. There are three sterile and three fertile valves in the ovary. In the prefertilization stages these valves consist of parenchymatous cells with starch and raphides. After fertilization, the sterile valves develop sclerotic cells whereas the fertile valves remain parenchymatous. The pericarp structure and embryological features support the retention of tribeCypripedieae within theOrchidaceae.     

CONE AND OVULE ONTOGENY IN TAXODIUM AND GLYPTOSTROBUS (TAXODIACEAE – CONIFERALES)

Seed cones in Taxodium distichum and Glyptostrobus pensilis occupy the position of permanent shoots and are initiated in the summer preceding spring pollination. Morphological features are similar in the two genera, reflecting their close taxonomic relationship. Ovule complexes originate as two (rarely more) ovule primordia in the axil of each fertile bract but without any indication of a preceding discrete ovuliferous scale. When the nucellus, integument, and micropyle are well developed, a series of up to ll abaxial lobes forms at the base of each ovule pair. They become fused by basal growth. After pollination the common basal meristem of lobes and bract extends by intercalary growth to form the conspicuous “ovuliferous scale” of the mature cone; the lobes enlarge and exceed the ovules. Despite the topographic similarity in the cones of both genera, there are differences in vasculature such that the vascular traces to the axillary complex originate directly from the axial cylinder in Glyptostrobus but from the bract trace in Taxodium. The complex vasculature of the mature cone develops late and primarily as an expression of intercalary growth.     

Homeotic Transformation of Ovules into Carpel-like Structures in Arabidopsis

Ovules are specialized reproductive organs that develop within the carpels of higher plants. In Arabidopsis, mutations in two genes, BELL1 (BEL1) and APETALA2 (AP2), disrupt ovule development. In Bel1 ovules, the inner integument fails to form, the outer integument develops abnormally, and the embryo sac arrests at a late stage of megagametogenesis. During later stages of ovule development, cells of the outer integument of a Bel1 ovule sometimes develop into a carpel-like structure with stigmatic papillae and second-order ovules. The frequency of carpel-like structures was highest when plants were grown under conditions that normally induced flowering and was correlated with ectopic expression in the ovule of AGAMOUS (AG), an organ-identity gene required for carpel formation. Together, these results suggested that BEL1 negatively regulates AG late in ovule development. Likewise, mutants homozygous for the strong AP2 allele ap2-6 sometimes displayed structures with carpel-like features in place of ovules. However, such abnormal Ap2 ovules are much less ovulelike in morphology and form earlier than the Bel1 carpel-like structures. Because one role of the AP2 gene is to negatively regulate AG expression early in flower development, it is possible that AP2 works in a similar manner in the ovule. A novel ovule phenotype observed in Bel1/Ap2-6 double mutants suggested that BEL1 and AP2 genes function independently during ovule development.     

Comparative structure of ovules and seeds inRafflesiaceae

The genera of theRafflesiaceae show a marked diversity in the structure of their ovules and seeds. Evolutionary trends are recognizable in ovule orientation and number of integuments. A change from anatropous ovules inApodantheae andMitrastemoideae towards incomplete anatropy inRafflesieae and orthotropy inCytineae occurs, next to a change from bitegmic ovules inApodantheae towards unitegmy with rudimentary outer integuments inRafflesieae andCytineae and full unitegmy inMitrastemoideae.—The differences in ovule structure are clearly reflected in the seeds. The seeds are essentially exotegmic, have very small embryos and an oily endosperm.—Seed structure strongly confirms the existing subfamilial classification and supports additional arguments for the generic status ofApodanthes. It does not support a separate status of the genusBerlinianche. InRafflesiaceae, seed micromorphology is only of limited use at the species level. As far as known seed dispersal is endo- or exozoochorous in all genera.