Flower-like terminal structures in racemose inflorescences: a tool in morphogenetic and evolutionary research

Terminal flower-like structures (TFLS) occur in many angiosperms that possess indeterminate inflorescences such as spikes, racemes, or spadices. We describe and review TFLS in early-divergent angiosperms, especially the magnoliid order Piperales and the monocot order Alismatales, in which floral interpretation is controversial. Essentially similar TFLS occur in a wide range of taxa. Among magnoliids, they occur in some Piperales (Saururaceae and a few Piperaceae), but are absent from Chloranthaceae. Among monocots, they occur in some early-divergent families such as Acoraceae, Aponogetonaceae, Juncaginaceae, Potamogetonaceae, and Ruppiaceae. Similar TFLS with obscure organ identity are recorded in mutants of Arabidopsis. TFLS can often be interpreted as pseudanthia (close aggregations of reduced flowers), but in some cases the entire terminal pseudanthium is very similar to a true flower. In some cases, elaborated TFLS could therefore have given rise to what are normally termed 'true' (i.e. euanthial) flowers. Data presented here on terminal pseudanthia in Potamogeton and Ruppia support a pseudanthial evolutionary origin of reproductive units in the alismatid families Zannichelliaceae and Cymodoceaceae. Furthermore, in some alismatid species, either the entire inflorescence apex or an individual primordium at or near the inflorescence tip can be transformed into a filamentous or tubular (or intermediate) structure. A tubular structure enclosing stamens and carpels is described in Piper. This indicates that pseudanthium formation can provoke morphological novelties, perhaps due to new patterns of overlap between expression zones of regulatory genes and/or new spatial constraints.     

Morphology of Hydatellaceae, an anomalous aquatic family recently recognized as an early-divergent angiosperm lineage

The family Hydatellaceae was recently reassigned to the early-divergent angiosperm order Nymphaeales rather than the monocot order Poales. This dramatic taxonomic adjustment allows comparison with other early-divergent angiosperms, both extant and extinct. Hydatellaceae possess some monocot-like features that could represent adaptations to an aquatic habit. Ecophysiological parallels can also be drawn from fossil taxa that are known from small achene-like diaspores, as in Hydatellaceae. Reproductive units of Hydatellaceae consist of perianthlike bracts enclosing several pistils and/or stamens. In species with bisexual reproductive units, a single unit resembles an "inside-out" flower, in which stamens are surrounded by carpels that are initiated centrifugally. Furthermore, involucre development in Trithuria submersa, with delayed growth of second whorl bracts, resembles similar delayed development of the second perianth whorl in Cabomba. Several hypotheses on the homologies of reproductive units in Hydatellaceae are explored. Currently, the most plausible interpretation is that each reproductive unit represents an aggregation of reduced unisexual apetalous flowers, which are thus very different from flowers of Nymphaeales. Each pistil in Hydatellaceae is morphologically and developmentally consistent with a solitary ascidiate carpel. However, ascidiate carpel development, consistent with placement in Nymphaeales, is closely similar to pseudomonomerous pistil development as in Poaes.     

Within-carpel and among-carpel competition during seed development, and selection on carpel number, in the apocarpous perennial herb Helleborus foetidus L. (Ranunculaceae)

Apocarpous flowers share opportunities for post-fertilization ovule selection among more functional levels than syncarpous flowers, because the occurrence of a variable number of unfused carpels adds a new source of variation to the likelihood of successful female reproduction. The extent to which post-fertilization events might differ among these unfused carpels may promote variations in the reproductive strategies of plants. We report a study of the variation, within and among carpels and flowers, in seed production and mass in the apocarpous Helleborus foetidus (Ranunculaceae), in relation to the number of carpels per flower. Differences within and among carpels in female reproductive success were affected by carpel number and pollination environment. When analysing whole flowers as functional units we also found that the magnitude of the differences related to carpel number and pollination treatment actually depended on the “distribution” of pollen types within flowers. Thus, variable within-flower pollination environments, more likely to occur in apocarpous than in syncarpous flowers, may affect the strategies of resource allocation for fruit development at different stages of the reproductive process. Regarding seed production, we found that producing more flowers with four carpels was under directional; however, when mean diaspore mass was considered as a measure of fitness, directional selection was found on producing flowers with two and three carpels (the modal carpel number found in wild populations). We discuss ecological and developmental reasons which could explain the observed pattern, and conclude that selection on an optimum carpel number may be very variable across the species range, as the discussed reasons may impose constraints on eventual evolutionary response, thus contributing to the maintenance of the intra-individual variability in carpel number.     

被子植物花的起源:假说和证据

达尔文的 令人讨厌之谜 ,即被子植物的起源和早期演化 ,一直是植物系统学研究领域的热点 .被子植物区别于其它植物类群的一个显著特征就是花 ,因此 ,解决被子植物的起源之谜很大程度上取决于对被子植物花器官起源的研究 .对被子植物花器官的详尽研究已经在形态、解剖、古植物、形态发生、分子等方面积累了大量的证据 ,植物学家基于这些证据为被子植物花器官的起源提出了各种各样的解释 .综述了迄今为止被子植物花器官起源的主要学说流派 ,如 :真花学说、假花学说、生殖叶学说、生殖茎节学说、生花植物学说、新假花学说、古草本学说和 ANITA学说等 .根据研究手段和获得证据的方式 ,作者将被子植物花器官起源研究划分为 5个阶段 ,并简要阐述了各个阶段的代表学说和主要研究特点     

黄瓜雄花形成过程中心皮的发育

黄瓜(Cucumis sativus L)为重要的经济作物,雌雄同株异花,是研究植物性别分化的经典材料。人们对黄瓜性别分化进行了广泛的研究。Astmon和Galun,任吉君和王艳对黄瓜性别分化的形态特征和器官发生进行了初步研究,表明黄瓜单性花分化和发育过程中经历了无性期、两性期和单性期,最     

The megagametophyte in Anarthria (Anarthriaceae,Poales) and its implications for the phylogeny of the Poales

The development and structure of the megagametophyte of Anarthria (Anarthriaceae), Aphclia, and Centrolepis (Centrolepidaceae) are described. Anarthriaceae has tenuinucellate ovules and the Polygonum type of megagametophyte development, both characters typical of the Poales. However, it lacks the anticlinally elongated nucellar epidermis and numerous large starch bodies observed in the megagametophyte of Centrolepidaceae, both characters also present in Restionaceae. This relatively generalized megagametophyte structure is consistent with data from the chloroplast genome, which suggest that Anarthriaceae are not as closely related to Restionaceae as previously assumed. New data from the megagametophyte are analyzed cladistically together with other available information on the poalean families. The results show that there are two possible positions for Anarthriaceae: either as sister to Poaceae. Joinvilleaceae, Restionaceae, Ecdeiocoleaceae, and Restionaceae, or as sister to only the latter three families. The new data also allow a critical reevaluation of the phylogenetic position of Centrolepidaceae, which is either basal to the poalean clade (based on microgametophyte data), or embedded in the Restionaceae (based on anther structure and megagametophyte data).     

黄瓜雄花形成过程中心皮的发育

黄瓜(Cucumis sativus L.)为重要的经济作物,雌雄同株异花,是研究植物性别分化的经典材料。人们对黄瓜性别分化进行了广泛的研究。Astmon和Galun、任吉君和王艳对黄瓜性别分化的形态特征和器官发生进行了初步研究,表明黄瓜单性花分化和发育过程中经历了无性期、两性期和单性期,最终只有一种性别的性器官原基发育成有功能的性器官,从而形成单性花,而对单性花中未形成有功能器官的相反性别原基的研究报道甚少。我们对雄花发育过程进行了连续的形态学分析,并对不同时期雄花中的心皮进行了细胞计数和同工酶电泳分析,以期从性器官发育的角度探讨黄瓜性别表现的机理。     

Evolution of novel morphological and reproductive traits in a clade containing Antirrhinum majus (Scrophulariaceae)

Phylogenetic analysis of DNA sequences of the chloroplast genes rcbL and ndhf revealed a highly supported clade composed of the families Plantaginaceae, Callitrichaceae, and Hippuridaceae in close association with the model organism Antirrhinum majus and other members of family Scrophulariaceae. Plantago has miniature actinomorphic wind-pollinated flowers that have evolved from zygomorphic animal-pollinated precursors. The aquatic Hippuridaceae have reduced windpollinated flowers with one reproductive organ per whorl, and three, rather than four, whorls. In monoecious aquatic Callitrichaceae, further reduction has occurred such that there is only one whorl per flower containing a single stamen or carpel. Optimization of character states showed that these families descended from an ancestor similar to Antirrhinum majus. Recent studies of plant developmental genetics have focused on distantly related species. Differences in the molecular mechanisms controlling floral development between model organisms are difficult to interpret due to phylogenetic distance. In order to understand evolutionary changes in floral morphology in terms of their underlying genetic processes, closely related species exhibiting morphological Variation should be examined. Studies of genes that regulate morphogenesis in the clade described here could aid in the elucidation of a general model tot such fundamental issues as how changes in floral symmetry, organ number, and whorl number are achieved, as well as providing insight on the evolution of dicliny and associated changes in pollination syndrome.     

臭椿花器官分化的初步研究

利用扫描电镜(SEM)和光镜(LM)对臭椿花序及花器官的分化和发育进行了初步研究,表明:1)臭椿花器官分化于当年的4月初,为圆锥花序;2)分化顺序为花萼原基、花冠原基、雄蕊原基和雌蕊原基。5个萼片原基的发生不同步,并且呈螺旋状发生;5个花瓣原基几乎同步发生且其生长要比雄蕊原基缓慢;雄蕊10枚,两轮排列,每轮5个原基的分化基本是同步的;雌蕊5,其分化速度较快;3)在两性花植株中,5个心皮顶端粘合形成柱头和花柱,而在雄株中,5个心皮退化,只有雄蕊原基分化出花药和花丝。本研究着重观察了臭椿中雄花及两性花发育的过程中两性花向单性花的转变。结果表明,臭椿两性花及单性花的形成在花器官的各原基上是一致的(尽管时间上有差异),雌雄蕊原基同时出现在每一个花器官分化过程中,但是,可育性结构部分的形成取决于其原基是否分化成所应有的结构:雄蕊原基分化形成花药与花丝,雌蕊原基分化形成花柱、柱头和子房。臭椿单性花的形成是由于两性花中雌蕊原基的退化所造成,其机理有待于进一步研究。     

Morphology and development of the reproductive shoots of Lilaea scilloides (Poir.) Hauman (Alismatidae)

Shoots of adult plants of Lilaea scilloides have a sympodial form. Each unit of the sympodium bears a single sheathing prophyll (which is the only kind of foliage leaf produced in the adult) and terminates in an inflorescence. The prophyll subtends the next unit of the sympodium. A further accessory bud can form in association with each unit. This bud repeats the pattern of the main sympodium, giving the plant a tufted habit. Five different kinds of flower can be identified in the inflorescence: a unisexual male flower with a single perianth member and adnate stamen; a bisexual flower, with a single perianth member and adnate stamen, and a single carpel with an anatropous bitegmic ovule; a unisexual female flower with a single perianth member and carpel; a unisexual female flower comprising only a single carpel; and a female flower comprising only a single carpel with a very long filamentous style. The first four kinds occur in the upper part of the inflorescence which is normally elevated on a scape, while the last kind is restricted to the base of the inflorescence. In the position of the basal flowers several variations have been observed in cultivated material. These include branching associated with the basal flowers, which results in the development of additional basal flowers or inflorescences, and even total replacement of a basal flower by an inflorescence or a branching structure bearing flowers. A review of past literature includes a clarification of some persistent errors which have confused the taxonomic position of the plant and the morphological interpretation of the reproductive appendages.     

Ontogeny of Staminate and Carpellate Flowers of Schisandra sphenanthera(Schisandraceae)

The ontogenetic process of the staminate and carpellate flowers of Schisandra sphenanthera Rehd. et Wils., an endemic species to China, was observed for the first time under the scanning electron microscope (SEM). In the staminate flowers, the perianth units and stamens were initiated acropetally in a continuous fasion with 2/5 spiral phyllotaxis, while no female structures were formed. Anthers were differentiated prior to the filaments formation. Throughout all the stages were the stamens arranged spirally on a columniform receptacle. In the carpellate flowers, the initiation sequence of the perianth units and carpels were similar to that of the staminate flowers. In contrast, no male structures were formed. Shortly after initiation, the carpel primordia began their marginal growth besides the apical growth and then appresses were formed on the adaxial surfaces of the primordia. However the lower margins of these appresses were inconspicuous, resulting in conduplicate carpels. Two ovules were developed on the inner surface near either lateral margins of the carpel, shaping laminar placentae. Compared with S. glabra (Brickell) Rehd., a related American species, the evolutionary trend of phyllotaxis of androecia is considered that stamens may change from spiral to approximately whorled arrangement, accompanying with the change of receptacle from a column to a flattened shield. It was also suggested that the stamens being numerous and uncertain in number become certain and decrease in number to 5 (4－7). Sterile stamens are observed and the unisexual nature of the flowers is discussed. Two types of carpel primordia are categorized, corresponding to two types of carpels, namely, ascidiate and conduplicate carpels, respectively.     

Floral development in the legume tree Colophospermum mopane, Caesalpinioideae: Detarieae

Floral ontogeny of Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Leonard, an apetalous member of the Crudia group with four sepals and a large number (20–25) of stamens, was studied as part of a larger project on reproductive biology of this much-utilized tree. The flowers have been described as being inserted in the axil of a bract, but lacking lateral bracteoles. Four outer, light cream or white 'sepals' are present. The first two sepals are initiated in a lateral position, where the bracteoles, if present, develop in other members of the Caesalpinioideae. The inner sepals arise simultaneously adaxially and abaxially. These four structures, conventionally regarded as sepals, enclose the bud. The outer two 'sepals' should be regarded as lateral bracteoles inserted at the apex of the pedicel. The inner structures represent the only two sepals. The large number of stamens arise on a large meristematic surface and different whorls were not observed. The filaments elongate within the bud and after anthesis become exposed outside the flowers. The filaments are of equal length and the large anthers form a suspended cluster. One carpel develops terminally and gives rise to an indehiscent one-seeded fruit.     

Floral development of dioecious species and trends of floral evolution in Piper sensu lato

The initiation of the floral parts (mainly stamens and carpels) is described for the four dioecious species of Piper: Piper polysyphorum C. DC, P. bavinum C. DC., P. pedicellatum C. DC., P. pubicatulum C. DC. The initiation order resembles that in the perfect flowers of some species, such as P. amalago. The carpels are initiated simultaneously, in most cases, as three primordia. In P. polysyphorum , carpel tips split into two lobes, so that finally a four- or five-lobed stigma will be formed when the ovary is fully developed. The staminodes (exactly, staminodial primordia) in the female flowers are initiated in the same order as the stamens in the male flowers and remain until the ovaries are enclosed. The unisexual flowers have stamens reduced to three or two. The reduction of stamen or staminode (staminodial primordium) number is accompanied by the change of their positions from opposite the carpels to alternate. After the initiation of the staminodes, or, exactly staminodial primordia, in the female flowers, the central part of the floral apex forms a ring meristem which is triangular. The carpel primordia (often three) are initiated on the three points of the ring meristem. The evolutionary trends of the flowers of Piper sensu lato are discussed.     

Is LEAFY a useful marker gene for the flower-inflorescence boundary in the Euphorbia cyathium?

The flower-like reproductive structure of Euphorbia s.l. (Euphorbiaceae) is widely believed to have evolved from an inflorescence, and is therefore interpreted as a special type of pseudanthium, termed a cyathium. However, fuzzy morphological boundaries between the inflorescence, individual flowers, and organs have fuelled the suggestion that the cyathium does not merely superficially resemble a flower but could actually share developmental genetic pathways with a conventional flower. To test this hypothesis, immunolocalizations of FLORICAULA/LEAFY (LFY), a protein associated with floral identity in many angiosperm species, were performed in developing cyathia of different species of Euphorbia. Expression of the LFY protein was found not only in individual floral primordia (as predicted from results in the model organisms Arabidopsis and Anthirrhinum), but also in the cyathium primordium and in the primordia of partial male inflorescences. These results provide further evidence that the evolution of floral traits in pseudanthial inflorescences often involves expression of floral development genes in the inflorescence apex. This finding blurs the conventional rigid distinction between flowers and inflorescences.     

Floral ontogeny in legume genera Petalostylis, Labichea, and Dialium (Caesalpinioideae: Cassieae), a series in floral reduction

Floral ontogeny of taxa of two subtribes (Labicheinae, Dialiinae) of caesalpinioid tribe Cassieae, characterized by reduced number of floral organs, was compared. All three taxa studied are distichous; Petalostylis labicheoides flowers are solitary in leaf axils, Labichea lanceolata has few-flowered racemes, and Dialium guineense has numerous-flowered cymes. The first sepal primordium in each is initiated abaxially and nonmedianly. Order of organogenesis in Petalostylis is: five sepals bidirectionally, five petals and carpel simultaneously, then five stamens bidirectionally, starting abaxially. The order in Labichea is: five sepals helically (one lagging in time), five petals unidirectionally starting abaxially, the carpel and petals concurrently, then two stamens successively, starting laterally. Order in Dialium is: five sepals bidirectionally, the single petal adaxially, and lastly the carpel and two stamens concurrently. Specializations include (1) reduction of the five sepals to four by fusion in Petalostylis and Labichea; (2) reduction of petal number to one in Dialium; (3) reduction of stamen number to two in Labichea and Dialium, and reduction of functional stamens to three in Petalostylis; and (4) an elaborate, late-developing style in Petalostylis. Floral asymmetry, another specialization, characterizes Labichea, expressed by dissimilar stamens, while the other genera have zygomorphic flowers. Floral ontogenies are compared with other taxa of Cassieae.     

Gondwanan evolution of the grass alliance of families (Poales)

Phylogenetic interrelationships among all 18 families of Poales were assessed by cladistic analysis of chloroplast DNA rbcL and atpB sequences from 65 species. There are two well-supported main clades; the graminoid clade with Poaceae (grasses), Anarthriaceae, Centrolepidaceae, Ecdeiocoleaceae, Flagellariaceae, Joinvilleaceae, and Restionaceae; and the cyperoid clade with Cyperaceae, Juncaceae, and Thurniaceae. A sister group relationship between Poaceae and Ecdeiocoleaceae is identified with strong support. The sister group of this pair is Joinvilleaceae. These relationships help in elucidating the evolution of grasses and the grass spikelet. Dating of the tree was done by nonparametric rate smoothing of rbcL molecular evolution. Most Poales families date back to the Cretaceous >65 million years ago (mya). Dispersal-vicariance analysis indicates that the Poales originated in South America, the cyperoid clade in West Gondwana (South America or Africa), and the graminoid clade in East Gondwana (Australia). The Trans-Antarctic connection between South America and Australia, and its breakup about 35 mya, probably influenced the evolution of the Poales and the graminoid clade in particular, leading to vicariance between the continents, but the separation of Africa from the other Gondwanan areas, completed about 105 mya, is too old for such a relation.     

Floral anatomy of Bromeliaceae, with particular reference to the evolution of epigyny and septal nectaries in commelinid monocots

Floral anatomy is described in ten genera of Bromeliaceae, including three members of subfamily Bromelioideae, three Tillandsioideae, and four genera of the polyphyletic subfamily Pitcairnioideae (including Brocchinia, the putatively basal genus of Bromeliaceae). Bromeliaceae are probably unique in the order Poales in possessing septal nectaries and epigynous or semi-epigynous flowers. Evidence presented here from floral ontogeny, vasculature, and the relative positions of nectary and ovules indicates that there could have been one or more reversals to apparent hypogyny in Bromeliaceae, although this hypothesis requires a better-resolved phylogeny. Such evolutionary reversals probably evolved in response to specialist pollinators, and in conjunction with other aspects of floral morphology of Bromeliaceae, such as the petal appendages of some species. The ovary is initiated in an inferior position even in semi-epigynous or hypogynous species. The ovary of all so-called hypogynous Bromeliaceae is actually semi-inferior, because the septal nectary is infralocular; in these species the nectaries have a labyrinthine surface and many vascular bundles. Brocchinia differs from most other fully epigynous species in that each carpel is secretory at the apex and reproductive, rather than secretory, at the base.     

Flower or spikelet? Understanding the morphology and development of reproductive structures in Exocarya (Cyperaceae, Mapanioideae, Chrysitricheae)

Fundamental questions of floral morphology remain unresolved in the grasslike monocots in order Poales, including what constitutes a flower and what constitutes a spikelet. The mapaniid sedges have particularly complex spikeletlike structures, variously interpreted as clusters of flowers or spikelets. Recent phylogenetic studies of Cyperaceae have identified the mapaniid clade as sister to the rest of the family, but the homology of mapaniid reproductive units (RUs) and spikeletlike units (SLUs) to other sedge flowers and spikelets is unclear. We examined reproductive development in the mapaniid Exocarya sclerioides. Inflorescence branches terminated in a SLU with bracts and 1-4 RUs. RUs had four small leaflike structures (LLSs): two lateral LLSs, each associated with a stamen, an abaxial LLS associated with a stamen, and an adaxial LLS. The gynoecium terminated the RU. All RUs were axillary to bracts, and unexpanded bracts and RUs were produced beyond expanded RUs, so SLUs were racemose. RUs developed from a single primordium that initiated two lateral LLSs, then two lateral stamens, then the gynoecium. Initiation of the abaxial LLS and stamen and the adaxial LLS followed. We hypothesize that the RU is a sympodial branch that terminates in a hermaphroditic flower with two stamens and a gynoecium; the two lateral LLSs are halves of a deeply divided prophyll.