Christian Lacroix

The correlation between development of atypical bisexual flowers and phylogeny in the Aroideae (Araceae)

Abstract. In the intermediate zone of the inflorescence of genera of Aroideae one can find flowers with male and female characteristics. Until now, two types of developmental sequences of atypical bisexual flowers (ABFs) have been recognized: the Philodendron type and the Cercestis type. In the Philodendron type, bisexual flowers generally consist of functional carpels and staminodes inserted on the same whorl. In the Cercestis type, the gynoecium and stamens are inserted on two different whorls. These different ontogenetic patterns represent two different pathways in the evolution of unisexual flowers in this subfamily. A molecular phylogenetic analysis of 33 genera of Araceae, based on the chloroplast trnL intron and trnL–F intergenic spacer sequences was carried out. We use this phylogenetic analysis and those published by French et al. (1995) and Mayo et al. (1997) to examine the distribution of the two types of ABFs in selected genera. Our results suggest that the two developmental patterns of ABFs in Aroideae sensu Mayo et al. (1997) do not correspond to two separate evolutionary lineages but rather are more or less consistent within clades. Although this new molecular phylogeny does not include all aroid genera, it corroborates in general, at the subfamily level, the molecular analysis of French et al. (1995) based on chloroplast DNA restriction site data and the analysis of Mayo et al. (1997) based on morphological and anatomical data.

FLORAL DEVELOPMENT AND PHYLOGENETIC POSITION OF SCHISMATOGLOTTIS (ARACEAE)

In the inflorescence of Schismatoglottis, atypical sterile flowers are present between the male and female zones. Contrary to what occurs in Cercestis and Philodendron, where the atypical flowers are bisexual, in Schismatoglottis the atypical flowers consist of aberrant male or female flowers. These atypical unisexual flowers have different forms depending on their position in the inflorescence. The atypical flowers located near the male zone share characteristics with staminate flowers, and those located near the female zone have features in common with pistillate flowers. The developmental pathway of atypical flowers in Schismatoglottis is already channeled in the direction of female or male flowers before the appearance of aberrant appendages. Interpistillar sterile structures located in the female zone correspond to modified female flowers. The systematic position of Schismatoglottis is not fully resolved on a molecular phylogenetic analysis of 45 genera of Araceae, on the basis of chloroplast trnL intron and trnL‐F intergenic spacer sequences. The floral ontogenetic pattern in Schismatoglottis represents a different pathway in the evolution of unisexual flowers in the subfamily Aroideae.

Homeosis, morphogenetic gradient and the determination of floral identity in the inflorescences ofPhilodendron solimoesense (Araceae)

A comparative developmental study of the inflorescence ofPhilodendron solimoesense was conducted using scanning electron microscopy. The spadix ofP. solimoesense is characterized by unisexual flowers. Staminate flowers are initiated on the upper portion of the spadix while pistillate flowers develop on the lower portion of the spadix. An intermediate zone located between the upper male and lower female portion of the inflorescence consists of sterile male flowers. Within this intermediate zone a row of flowers exhibit polarity with respect to the identity of sexual organs. Stamens are initiated on the flank of the floral meristem facing the upper male zone and carpels are initiated on the portion of the floral meristem facing the lower female zone. The resulting flowers therefore assume a bisexual identity. At the level of the inflorescence, all floral buds are initiated along a series of contact parastichies and the continuity of these parastichies is not disrupted at any level in the male, intermediate, and female zones on the spadix. Results from this study support the presence of a morphogenetic gradient acting at the level of the inflorescence and appears to be independent of the boundaries of floral primordia.

Developmental Potential of Galls Induced by Diplolepis rosaefolii (Hymenoptera: Cynipidae) on the Leaves of Rosa virginiana and the Influence of Periclistus Species on the Diplolepis rosaefolii Galls

The larval stages of the cynipid wasp Diplolepis rosaefolii induce the formation of single‐chambered, lenticular galls on the leaves of the wild shrub rose, Rosa virginiana. The development of galls induced by D. rosaefolii was studied using light and scanning electron microscopy. The gall consists of four tissue layers, which surround a centrally located larval chamber. These include an outermost dermal layer, underlying parenchyma, sclerenchyma, and nutritive tissue. Development in the D. rosaefolii galls involves a number of characteristics that are unique to this gall and differ markedly from development in other Diplolepis galls studied. These characteristics include the presence of double sclerenchymal layers and vascularization embedded between the sclerenchyma. Periclistus is an inquiline in the galls induced by D. rosaefolii. Under the influence of Periclistus larvae, a number of morphological changes, including an increase in the number of larval chambers, are observed in the galls. Changes in tissue type, proportion, and overall morphology exhibited by Periclistus‐modified galls are also studied using conventional light microscopy techniques.

FLORAL DEVELOPMENT OF DIOECIOUS STAGHORN SUMAC, RHUS HIRTA (ANACARDIACEAE)

The organogenesis of floral development in the dioecious clonal tree species Rhus hirta (Anacardiaceae) offers an excellent model system to study gender expression in dioecious plants. Pistillate and staminate flowers from developing inflorescences were collected from Grover Island, located off the northern shore of Prince Edward Island, Canada. The developmental morphology of both floral types was studied using scanning electron microscopy. The early stages of floral development of R. hirta are essentially the same in the two sexes. Both the stamen and carpel primordia are initiated in male and female flowers. Furthermore, the primordial sex organs on both flower types show striking similarities in size and symmetry as well as sequence of initiation in the early stages of development. During the later stages of development, the stamens in the female flowers and the gynoecium in the male flowers abort, resulting in very different morphologies at maturity. Statistical analyses revealed that the height of the mature gynoecium and stamens as well as the length of the petals differed significantly between sexes. This species may be reflecting its relationship to hermaphroditic ancestors during the early stages of development.

Aspects of Floral Development in Philodendron grandifolium and Philodendron megalophyllum (Araceae)

This study deals specifically with floral organogenesis and the development of the inflorescence of Philodendron grandifolium and Philodendron megalophyllum. Pistillate flowers are initiated on the lower portion of the inflorescence, and the staminate flowers are initiated on the distal portion. The presence of extracellular calcium oxalate is observed on the surface of immature stamens of P. megalophyllum. An intermediate zone consisting of sterile male flowers and bisexual flowers with fused or free carpels and staminodes is also present on the inflorescences. This zone is located between the male and female floral zones. In general, the portion of bisexual flowers facing the male zone forms staminodes, and the portion facing the female zone develops an incomplete gynoecium with few carpels. In P. grandifolium, the incomplete separation of some staminodes from the gynoecial portion of the whorl shows that they belong to the same whorl as the carpels. The bisexual flowers of P. grandifolium and P. megalophyllum are believed to be a case of homeosis in which carpels have been replaced by sterile stamens on the same whorl.

Developmental morphology of the androecium and gynoecium in Ruppia maritima L.: considerations for pollination

Abstract Ruppia maritima , widgeon grass, is an aquatic angiosperm common in salt marshes. Although previous studies have covered general aspects of pollination and floral development, little information is available on the functional aspects and development of the androecium and gynoecium in relation to pollination. Using the techniques of scanning electron microscopy and thin sectioning, this study focuses on the functional morphology of the gynoecium, anthers, and pollen grains. The funnel-like arrangement of the carpels directing pollen to the stigmatic regions, the buoyancy offered by presence of stomates and underlying spongy tissue in the carpellary outgrowth, the boomerang-like morphology of the pollen grains which facilitates their aggregation, and the sheath enclosing the anther are examined from the perspective of this specialized, hydrophilous pollination system.

Floral development, fruit set, and dispersal of the Gulf of St. Lawrence Aster (Symphyotrichum laurentianum) (Fernald) Nesom

The Gulf of St. Lawrence aster, Symphyotrichum laurentianum (Fernald) Nesom is listed as “threatened” according to the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). This rare halophyte is found in only a few locations in New Brunswick, Prince Edward Island, and the Magdalen Islands in Quebec. Developmental evidence confirms that there are two types of florets within each flower head and that each floral type has a biseriate pappus. The centrally located “disk” florets are distinctly larger than the more numerous peripheral “pistillate” florets throughout their development. The disk florets are bisexual and consist of an ovule, a style with a bifid stigma, and four to five stamens. The peripheral florets are pistillate and consist of an ovule and a style with a bifid stigma but no stamens. The main goals of this study were to assess fruit set in both types of florets, and the wind dispersal potential of their fruit (achenes). Pistillate flowers had a lower percentage of embryo-contain...

Quantitative Developmental Analysis of Homeotic Changes in the Inflorescence of Philodendron (Araceae)

BACKGROUND AND AIMS The inflorescence of Philodendron constitutes an interesting morphological model to analyse the phenomenon of homeosis quantitatively at the floral level. The specific goals of this study were (1) to characterize and quantify the range of homeotic transformation in Philodendron billietiae, and (2) to test the hypothesis that the nature of flowers surrounding atypical bisexual flowers (ABFs) channel the morphological potentialities of atypical bisexual flowers. METHODS Inflorescences of P. billietiae at different stages of development were observed using SEM. The number of appendices in male, female and sterile flowers were counted on 11 young inflorescences (5-6 flowers per inflorescence). The number of staminodes and carpels on ABFs were counted on 19 inflorescences (n = 143). These data were used for regression and ANOVA analyses. RESULTS There was an average of 4.1 stamens per male flower, 9.8 carpels per female flower and 6.8 staminodes per sterile male flower. There was an average of 7.3 floral appendices per atypical flower. Staminodes and carpels are inserted on the same whorl in ABFs. A negative exponential relationship was found between the average number of staminodes and the number of carpels in ABFs. If only the ABFs consisting of less than six carpels are considered, there is a linear relationship between the number of carpels and the average number of staminodes. The value of the slope of the regression equation indicates that on average, in P. billietiae, 1.36 carpels are replaced by one staminode. CONCLUSIONS In P. billietiae, the number of appendages in female flowers imposes a constraint on the maximum total number of appendages (carpels and staminodes) that can develop on ABFs. The quantitative analyses indicate that the average number of different types of floral appendages on an ABF and the number of organs involved in a homeotic transformation are two independent phenomena.

Early development and viability testing of embryos of Scirpus acutus Muhl.

Abstract Scirpus acutus Muhl. is a hardstem bulrush found throughout North American wetlands. The seeds of most species of Scripus have a relatively low germination potential. Seed viability and the developmental morphology of the embryo upon germination were studied using standard tetrazolium (TZ) testing, scanning electron microscopy (SEM), and standard light microscopy techniques. TZ indicates the majority of seeds are viable. Upon germination, a leaf is the first structure to elongate, emerge from the seed coat, and finally break through the pericarp. Following this stage, the radicle develops from an extension of the embryo axis located between the scutellum and the shoot tip of the embryo. The next leaf eventually arises at the junction of the sheath of the first leaf. The pericarp may act as a physical barrier and be a factor contributing to low germination in Scirpus acutus .