Developmental studies in orchid flowers III: Neottioid species

The floral development of 19 species of Neottioideae (sensu Rasmussen 1985) was studied by means of scanning electron microscopy, paying special attention to the early differentiation of the organs that constitute the gynostemium. The gynostemium development of the Epipactieae proved to be similar to that of the Epidendroi deae and Vandoideae, in particular in that massive primordia corresponding to inner lateral staminodes are differentiated in early stages and later constitute the lateral appendages of the gynostemium. In the Neottieae a progressive reduction and delayed initiation of these staminodes was observed: the lateral teeth of the gynostemium originate from large staminode primordia in one species ( Corymborkis veratrifolia ), in the remaining species they are initiated in later stages or are missing.     

Developmental studies in orchid flowers IV: Cypripedioid species

The floral development of four species of Cypripediaceae (sensu Rasmussen 1985) was studied by means of scanning electron microscopy, with special attention to the early development of the organs that constitute the gynostemium. At the ventral base of the gynostemium a prominent structure was observed. It is most probably a vestige of the median adaxial stamen a₃ based on its early initiation and place of origin. In Cypripedium calceolus the median carpel primordium is, according to expectation, initiated slightly earlier than the lateral carpel primordia, and later develops into the largest stigma lobe. Interestingly, Cypripedium irapeanurn shows an opposite sequence in the initial phase of the carpel development in that the primordia of the lateral carpels are initiated before the primordium of the median carpel.     

Developmental studies in orchid flowers I: Epidendroid and vandoid species

The floral development of 47 epidendroid and vandoid orchids was studied by means of scanning electron microscopy, paying special attention to the early development of the gynostemium (column) and its appendages. The following main conclusions are drawn: the lateral appendages of the adult gynostemium are homologous with the two lateral stamens of the inner whorl; their primordia are present even in species which lack prominent appendages in the adult gynostemium (incorporation of the sta-minodial primordia into the gynostemium during development). Ventral appendages observed in some species are supposed to be vestiges of the adaxial stamens on account of their early initiation. It is confirmed that the rostellum is the upper part of the median stigma lobe and that the lip corresponds to the inner median tepal. The affinities of the epidendroid and vandoid orchids are briefly discussed.     

MADS about the evolution of orchid flowers

Orchids have unique flowers involving three types of perianth organs: outer tepals, lateral inner tepals, and a lip. Expression studies indicate that the identity of these organs is specified by the combinatorial interaction of four different DEFICIENS-like MADS-box genes. We suggest that clarifying the evolution of these genes provides a rational framework for reconstructing the enigmatic origin and unique diversification of the orchid flower. For example, two rounds of gene duplications during early orchid evolution might have generated the genes that were probably recruited to distinguish the different types of perianth organs. This hypothesis suggests intriguing, experimentally testable mechanisms by which gene duplications followed by sub- and neo-functionalization events might have contributed to the evolutionary origin of morphological novelties in orchids - and well beyond.     

Ethylene production by young Aranda orchid flowers and buds

A high rate of ethylene production was observed in buds and young flowers of Aranda orchid, which increased with bud growth, reaching a high value in half-opened flower. This was followed by a gradual decline but it increased again when the flowers showed sign of senescence. Aminooxylacetic acid (AOA) inhibited ethylene production and bud expansion of Aranda buds.     

Polyploid spore formation in diploid orchid species

Extensive cytological studies into all stages of pollinial development in 10 diploid species of orchids belonging to the Arethuseae tribe revealed four possible mechanisms operating premeiotically and meiotically, resulting in the formation of polyploid spores in the form of monads, dyads and triads. The mechanisms involved are: (1) premeiotic disturbances resulting in tetraploid pollen mother cells; (2) faulty anaphase disjunction giving rise to restitution nuclei at both meiotic divisions; (3) failure to participate in meiosis I or meiosis II; and (4) co-orientation of the two spindles during metaphase II. Why fertile diploid species should possess elaborate pathways for the formation of such spores cannot be explained satisfactorily in view of the low frequencies of such spores formed and the absence of a euploid series in the four genera investigated. Nevertheless, these orchid genera, with high basic numbers, could be highly evolved polyploids and the regular formation of such spores could be viewed as a relic of their evolutionary past.     

Developmental morphology of branching flowers in Nymphaea prolifera

Nymphaea and Nuphar (Nymphaeaceae) share an extra-axillary mode of floral inception in the shoot apical meristem (SAM). Some leaf sites along the ontogenetic spiral are occupied by floral primordia lacking a subtending bract. This pattern of flower initiation in leaf sites is repeated inside branching flowers of Nymphaea prolifera (Central and South America). Instead of fertile flowers this species usually produces sterile tuberiferous flowers that act as vegetative propagules. N. prolifera changes the meristem identity from reproductive to vegetative or vice versa repeatedly. Each branching flower first produces some perianth-like leaves, then it switches back to the vegetative meristem identity of the SAM with the formation of foliage leaves and another set of branching flowers. This process is repeated up to three times giving rise to more than 100 vegetative propagules. The developmental morphology of the branching flowers of N. prolifera is described using both microtome sections and scanning electron microscopy.     

Smells like aphids: orchid flowers mimic aphid alarm pheromones to attract hoverflies for pollination

Most insects are dependent on chemical communication for activities such as mate finding or host location. Several plants, and especially orchids, mimic insect semiochemicals to attract insects for unrewarded pollination. Here, we present a new case of pheromone mimicry found in the terrestrial orchid Epipactis veratrifolia. Flowers are visited and pollinated by several species of aphidophagous hoverflies, the females of which also often lay eggs in the flowers. The oviposition behaviour of these hoverflies is mainly guided by aphid-derived kairomones. We show that the flowers produce α- and β-pinene, β-myrcene and β-phellandrene, and that these compounds attract and induce oviposition behaviour in female hoverflies. This floral odour profile is remarkably similar to the alarm pheromone released by several aphid species, such as Megoura viciae. We therefore suggest that E. veratrifolia mimics aphid alarm pheromones to attract hoverflies for pollination; this is the first time, to our knowledge, that such a case of mimicry has been demonstrated.     

Influence of ethylene on enzyme activities and mobilization of materials in pollinatedArachnis orchid flowers

PollinatedArachnis flowers exhibit morphological changes that are typical of the postpollination phenomena. Associated with these visible changes was the movement of soluble amino nitrogen and phosphorus from the perianth to the column and ovary. A rapid increase in protease and acid phosphatase activities was observed in the perianth, column, and ovary. However, it appeared that the enhanced enzyme activities were not responsible for the mobilization of materials from the perianth to the ovary, though both were induced by ethylene produced following pollination.     

Cloning and characterization of a novel chalcone synthase gene from Phalaenopsis hybrida orchid flowers

Chalcone synthase (CHS, EC 2.3.1.74) is the key enzyme involved in flavonoid and anthocyanin biosynthesis. A complete DNA sequence of chalcone synthase gene designated Pchs1 was isolated by means of usual and then inverse polymerase chain reactions from genomic DNA of an orchid, Phalaenopsis hybrida, cv. Formosa rose. Nucleotide sequence analysis based on alignment with published Phalaenopsis chs cDNA revealed that Pchs1 contained an intact open reading frame of 1173-bp with one 109-bp intron at the conserved site. The deduced polypeptide (PCHS1) from Pchs1 comprised 390 amino acids with a predicted mol wt of 42.5 kD. PCHS1 showed 61–65% identities with CHS from other plants and retained most of the conserved residues. Some putative cis-regulatory elements were present at the 5′ and 3′ flanking regions of Pchs1. Southern blot analysis predicted at least four chs-like genes, thus indicating the presence of a small multigene chs family in P. hybrida. Relative quantitative RT-PCR showed that Pchs1 is expressed in petals at early flower development as well as in lip tissue when the flower has just opened. Published in Russian in Fiziologiya Rastenii, 2006, Vol. 53, No. 2, pp. 250–258. The text was submitted by the authors in English.     

The strength of reproductive isolation in two hybridizing food-deceptive orchid species

Reproductive isolation is of fundamental importance for maintaining species boundaries in sympatry. In orchids, the wide variety of pollination systems and highly diverse floral traits have traditionally suggested a prominent role for pollinator isolation, and thus for prezygotic isolation, as an effective barrier to gene flow among species. Here, we examined the nature of reproductive isolation between Anacamptis morio and Anacamptis papilionacea, two sister species of Mediterranean food-deceptive orchids, in two natural hybrid zones. Comparative analyses of the two hybrid zones that are located on soils with volcanic origin and have different and well-dated ages consistently revealed that all hybrid individuals were morphologically and genetically intermediate between the parental species, but had strongly reduced fitness. Molecular analyses based on nuclear ITS1 and (amplified fragment length polymorphism) AFLP markers clearly showed that all examined hybrids were F1 hybrids, and that no introgression occurred between parental species. The maternally inherited plastid DNA markers indicated that hybridization between A. morio and A. papilionacea was bidirectional, as confirmed by the molecular analysis of seed families. The genetic architecture of the two hybrid zones suggests that the two parental species easily and frequently hybridize in sympatry as a consequence of partial pollinator overlap but that strong postzygotic barriers reduce hybrid fitness and prevent gene introgression. These results corroborate that chromosomal divergence is instrumental for reproductive isolation between these food-deceptive orchids and suggest that hybridization is of limited importance for their diversification.     

The underground phase: a special challenge in studies of terrestrial orchid populations

Orchid seeds are minute and the first seedling stages of terrestrial species are underground, so there is little knowledge about the biology of the early life history and the size of seed/seedling populations relative to the number of plants that eventually emerge above ground. A recently developed field sowing technique makes it possible to explore a variety of topics such as the length of time that seeds spend as part of the soil seed bank, the phenology of seed germination, substrate requirements for germination, dependence of germination on fungal availability, substrate requirements and growth rate of seedlings, seedling phenology and development, and seedling-fungus specificity patterns in the soil. Several of these applications are relevant for conservation issues. Studies of Goodyera pubescens, Corallorhiza odontorhiza and Liparis lilifolia show considerable variation in germination strategies, patterns of seedling development and establishment, substrate requirements, and levels of specificity with the fungi.