Palynological variation in Balsaminoid Ericales. II. Balsaminaceae, Tetrameristaceae, Pellicieraceae and general conclusions

BACKGROUND AND AIMS: The objective of this study is to examine the palynological diversity of Balsaminaceae (two genera/+/-1000 species), Tetrameristaceae (two genera/two species) and Pellicieraceae (one genus/one species). The diversity found will be used to infer the systematic value of pollen features within the balsaminoid clade. METHODS: Pollen morphology and ultrastructure of 29 species, representing all families of the balsaminoid clade except Marcgraviaceae, are investigated by means of light microscopy, scanning electron microscopy and transmission electron microscopy. KEY RESULTS: Balsaminaceae pollen is small to medium sized with three to four apertures, which can be either colpate or porate, and a sexine sculpturing varying from coarsely reticulate to almost microreticulate. Tetrameristaceae pollen is small sized, 3-colporate, with a heterobrochate reticulate sculpturing and granules present in the lumina. Pellicieraceae pollen is large sized, 3-colporate with long ectocolpi and a perforate sexine sculpturing with large verrucae. Furthermore, Pelliciera is characterized by the occurrence of aggregated orbicules, while orbicules are completely absent in both Balsaminaceae and Tetrameristaceae. Balsaminaceae pollen differs from the other balsaminoid families due to the occurrence of colpate or porate grains with an oblate to peroblate shape, a very thin foot layer and a lamellated endexine. CONCLUSIONS: From a pollen morphological point of view, Balsaminaceae are completely different from the other balsaminoid families. Therefore, no pollen morphological synapomorphies could be defined for the balsaminoid clade. However, various pollen features were observed that could indicate a possible relationship between Tetrameristaceae, Pellicieraceae and Marcgraviaceae. Despite the palynological similarities in the latter three families, it remains unclear to what extent they are related to each other.     

Comparative floral structure and systematics in the balsaminoid clade including Balsaminaceae,Marcgraviaceae and Tetrameristaceae (Ericales)

Based on molecular phylogenetic studies, Balsaminaceae, Tetrameristaceae (including Pellicieraceae) and Marcgraviaceae form the strongly supported first branching clade in the asterid order Ericales. Marcgraviaceae and Tetrameristaceae were proposed to be closely related in pre‐molecular studies, but the systematic position of Balsaminaceae has been controversial for some time and a relationship with the other two families was never suggested in pre‐molecular/pre‐cladistic times. However, interfamilial relationships in the clade are still unclear because of conflicting phylogenetic hypotheses from molecular analyses. In order to assess the validity of these molecular hypotheses from a morphological point of view, the floral morphology, anatomy and histology of Balsaminaceae, Tetrameristaceae and Marcgraviaceae are comparatively studied in detail. In addition, earlier literature is reviewed. The monophyly of the balsaminoid clade is strongly supported by floral structure, and a series of potential floral synapomorphies is identified for the clade. Prominent features shared by the three families include broad and dorsiventrally flattened filaments, thread‐like structures lining the stomia of dehisced anthers, secretory inner morphological surfaces of the gynoecium, ovules intermediate between uni‐ and bitegmic, incompletely tenuinucellar ovules, fruits with persistent style and stigma, seeds lacking endosperm and several anatomical/histological traits. The families are also distinctive because the bracts and/or sepals are petaloid and nectariferous. Further, the floral structure supports a sister group relationship between Balsaminaceae and Tetrameristaceae rather than any of the other possible interfamilial relationships. These two families share a caducous calyx, post‐genital fusion/coherence of filaments and ovary surface, latrorse anther dehiscence, commissural carpel lobes and ovules with a thickened funiculus and a constricted chalazal region. The occurrence of these features in Ericales is discussed. Future structural studies in other ericalean lineages and additional molecular studies are needed to further test these features with respect to their systematic value for the balsaminoid clade. Some may turn out to be true synapomorphies, whereas others may be recognized as plesiomorphies, as they may be more widely spread in Ericales than currently thought. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173 , 325–386.     

Early floral development and androecium organization in the sarracenioid clade (Actinidiaceae,Roridulaceae and Sarraceniaceae) of Ericales

The early floral development of Actinidia (A. arguta, A. callosa, A. chinensis and A. kolomikta; Actinidiaceae), Saurauia (S. montana, S. oldhamii, S. pittieri and S. subspinosa; Actinidiaceae), Roridula gorgonias (Roridulaceae) and Heliamphora nutans (Sarraceniaceae) was studied comparatively using scanning electron microscopy. Late stages of androecium development are additionally presented for Clematoclethra scandens (Actinidiaceae), Darlingtonia californica and Sarracenia leucophylla (Sarraceniaceae). Flowers are typically pentamerous and share a number of developmental features: perianth organs emerge in a clockwise or anticlockwise spiral sequence on the floral apex with relatively long plastochrons between successive organs, resulting in conspicuous size differences among perianth organs in early development; the perianth always consists of two differentiated whorls (unlike earlier interpretations of the perianth in Heliamphora); the androecium is polystemonous in most species and is initiated with leading stamens in alternipetalous positions; successive stamen primordia appear in a lateral succession until a ring‐like structure is formed; and the anthers become inverted shortly before anthesis. Later androecial development differs conspicuously between taxa and further proliferation may be centrifugal, centripetal and/or lateral. For Ericales, unusual features of floral development include: petals initiated in a spiral sequence (but later organized in a whorl) with comparatively long plastochrons between individual petals (except Saurauia); common occurrence of perianth organs in double positions in Actinidiaceae; and anthers that become inverted close to anthesis. The floral development in the sarracenioids is additionally compared with that of other families and clades in Ericales, further emphasizing the highly variable patterns of androecium development in the order.     

Pollinator adaptation and the evolution of floral nectar sugar composition

A long‐standing debate concerns whether nectar sugar composition evolves as an adaptation to pollinator dietary requirements or whether it is ‘phylogenetically constrained’. Here, we use a modelling approach to evaluate the hypothesis that nectar sucrose proportion (NSP) is an adaptation to pollinators. We analyse ~ 2100 species of asterids, spanning several plant families and pollinator groups (PGs), and show that the hypothesis of adaptation cannot be rejected: NSP evolves towards two optimal values, high NSP for specialist‐pollinated and low NSP for generalist‐pollinated plants. However, the inferred adaptive process is weak, suggesting that adaptation to PG only provides a partial explanation for how nectar evolves. Additional factors are therefore needed to fully explain nectar evolution, and we suggest that future studies might incorporate floral shape and size and the abiotic environment into the analytical framework. Further, we show that NSP and PG evolution are correlated – in a manner dictated by pollinator behaviour. This contrasts with the view that a plant necessarily has to adapt its nectar composition to ensure pollination but rather suggests that pollinators adapt their foraging behaviour or dietary requirements to the nectar sugar composition presented by the plants. Finally, we document unexpectedly sucrose‐poor nectar in some specialized nectarivorous bird‐pollinated plants from the Old World, which might represent an overlooked form of pollinator deception. Thus, our broad study provides several new insights into how nectar evolves and we conclude by discussing why maintaining the conceptual dichotomy between adaptation and constraint might be unhelpful for advancing this field.     

Floral development of Hydrocera and Impatiens reveals evolutionary trends in the most early diverged lineages of the Balsaminaceae

Background and Aims

Balsaminaceae consist of two genera, the monospecific Hydrocera and its species-rich sister Impatiens. Although both genera are seemingly rather similar in overall appearance, they differ in ecology, distribution range, habitat preference and morphology. Because morphological support for the current molecular phylogenetic hypothesis of Impatiens is low, a developmental study is necessary in order to obtain better insights into the evolutionary history of the family. Therefore, the floral development of H. triflora and I. omeiana was investigated, representing the most early-diverged lineage of Impatiens, and the observations were compared with the literature.

Methods

Flowers at all developmental stages were examined using scanning electron microscopy and light microscopy.

Key results

In Hydrocera, two whorls of five free perianth primordia develop into a less zygomorphic perianth compared with its sister genus. The androecial cap originates from five individual stamen primordia. Post-genital fusion of the upper parts of the filaments result in a filament ring below the anthers. The anthers fuse forming connivent anther-like units. The gynoecium of Hydrocera is pentamerous; it is largely synascidiate in early development. Only then is a symplicate zone formed resulting in style and stigmas. In I. omeiana, the perianth is formed as in Hydrocera. Five individual stamen primordia develop into five stamens, of which the upper part of the filaments converge with each other. The gynoecium of I. omeiana is tetramerous; it appears annular in early development.

Conclusions

Comparison of the present results with developmental data from the literature confirms the perianth morphocline hypothesis in which a congenital fusion of the parts of the perianth results in a shift from pentasepalous to trisepalous flowers. In addition, the development of the androecial cap and the gynoecium follows several distinct ontogenetic sequences within the family.     

The evolution of floral biology in basal angiosperms

In basal angiosperms (including ANITA grade, magnoliids, Choranthaceae, Ceratophyllaceae) almost all bisexual flowers are dichogamous (with male and female functions more or less separated in time), and nearly 100 per cent of those are protogynous (with female function before male function). Movements of floral parts and differential early abscission of stamens in the male phase are variously associated with protogyny. Evolution of synchronous dichogamy based on the day/night rhythm and anthesis lasting 2 days is common. In a few clades in Magnoliales and Laurales heterodichogamy has also evolved. Beetles, flies and thrips are the major pollinators, with various degrees of specialization up to large beetles and special flies in some large-flowered Nymphaeaceae, Magnoliaceae, Annonaceae and Aristolochiaceae. Unusual structural specializations are involved in floral biological adaptations (calyptras, inner staminodes, synandria and food bodies, and secretory structures on tepals, stamens and staminodes). Numerous specializations that are common in monocots and eudicots are absent in basal angiosperms. Several families are poorly known in their floral biology.     

Early floral development and androecium organization in Fouquieriaceae (Ericales)

Early floral development with focus on the androecium was studied with the help of scanning electron microscopy and serial microtome sectioning in Fouquieria columnaris and F. splendens. Perianth organs appear in a spiral pattern on the floral apex. The spiral may be a clockwise or anti-clockwise. The androecium is best interpreted as two-whorled with all the stamens arranged in a single series. In F. splendens, two or more of the five epipetalous stamen positions are doubled, i.e. they are occupied by stamen pairs. Unusual features in the floral development of Fouquieriaceae include (1) a strong spiral component even in whorled organ categories and (2) a pronouncedly asymmetric floral apex during an early phase of floral development. From a phylogenetic point of view, it seems plausible that the common ancestor of Fouquieriaceae and its sister family Polemoniaceae was characterized by two alternating, pentamerous stamen-whorls.     

Number of floral organs in Circaeaster agrestis (Circaeasteraceae) and possible homeosis among floral organs

98.9% of 5092 flowers from 1041 individuals of Circaeaster agrestis have five floral organs, the formula is P₃A₁G₁ (73.13%), P₂A₂G₁ (25.59%), and P₂A₁G₂ (0.22%). Only 0.4% of the flowers have six floral organs and the formula is P₃A₁G₂ (20 flowers) or P₃A₂G₁ (one flower). All these flowers have one vascular bundle in the pedicel and were considered to be normal ones. There are 33 flowers (0.65%) with six or more floral organs and two vascular bundles in the pedicel and we found traces of fusion of different degree of two flowers into one. These flowers were considered as abnormal. Therefore the normal number of floral organs of C. agrestis is five and occasionally six, and the floral formulas are P₃A₁G₁ or P₂A₂G₁, sometimes P₂A₁G₂, and occasionally P₃A₁G₂ or P₃A₂G₁. A tepal in P₃A₁G₁ may be replaced by a stamen in P₂A₂G₁ or by a carpel in P₂A₁G₂ or in reverse. A carpel in P₃A₁G₂ may be replaced by a stamen in P₃A₂G₁ or in reverse. We hypothesize that there are two possibilities for the number of the floral organs to be five (six), the result of reduction from P₃A₂G₂, or there exists homeosis among floral organs.     

The evolution of key functional floral traits in the early divergent angiosperm family Annonaceae

Potential key functional floral traits are assessed in the species‐rich early divergent angiosperm family Annonaceae. Pollinators (generally beetles) are attracted by various cues (particularly visual, olfactory, and thermogenic), with pollinators rewarded by nectar (generally as stigmatic exudate), heat, and protection within the partially enclosed floral chamber. Petals sometimes function as pollinator brood sites, although this could be deceptive. Annonaceae species are self‐compatible, with outcrossing promoted by a combination of protogyny, herkogamy, floral synchrony, and dicliny. Pollination efficiency is enhanced by pollen aggregation, changes in anthesis duration, and pollinator trapping involving a close alignment between petal movements and the circadian rhythms of pollinators. Most Annonaceae flowers are apocarpous, with syncarpy restricted to very few lineages; fertilization is therefore optimized by intercarpellary growth of pollen tubes, either by stigmatic exudate (suprastylar extragynoecial compitum) or possibly the floral receptacle (infrastylar extragynoecial compitum). Although Annonaceae lack a distinct style, the stigmas in several lineages are elongated to form “pseudostyles” that are hypothesized to function as sites for pollen competition. Flowers can be regarded as immature fruits in which the ovules are yet to be fertilized, with floral traits that may have little selective advantage during anthesis theoretically promoting fruit and seed dispersal. The plesiomorphic apocarpous trait may have been perpetuated in Annonaceae flowers as it promotes the independent dispersal of fruit monocarps (derived from separate carpels), thereby maximizing the spatial/temporal distance between seedlings. This might compensate for the lack of genetic diversity among seeds within fruits arising from the limited diversity of pollen donors.     

Fossil floral and fruit evidence for the evolution of unusual developmental characters in Fagales

Flowers of many living Fagales exhibit unusual developmental characteristics. At anthesis, ovulate flowers have carpels bearing immature orthotropous ovules. After pollination, the ovules increase in size and become anatropous and the ovary enlarges. Simultaneously, the pollen tubes extend from the stigma to the ovules with several phases of growth and quiescence. Finally, after the first fertilization, the remaining ovules abort, resulting in a single‐seeded fruit. Three‐dimensionally preserved potentially fagaceous mesofossil flowers from the Campanian of Massachusetts, USA, provide evidence on the evolution of these characters. The fossils share putative synapomorphies with the Fagales (six tepals, mostly inferior, three‐carpellate ovary with each locule initially containing two pendant ovules, punctate‐rugulate, tricolporate pollen and fruit with a single seed). However, the fossil is bisexual and has nectaries, characters shared with the sister order Cucurbitales, and both lack the fagalean immature orthotropous developmental stage. The fossil shares synapomorphies of an inferior ovary and a single‐seeded indehiscent fruit with both living orders and appears to be transitional. Comparison of ontogenetic changes between the fossil and related fagalean taxa suggests independent stepwise changes in development in which some characters of the modern clades were in place at ～ 75 Myr and others evolved more recently. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 168 , 353–376.     

A two-pollinator model for the evolution of floral complexity

For a new, more complex floral form to become established in a population it must overcome the problem of frequency-dependent constancy to successfully attract pollinators. This may be achieved by complex floral forms offering absolute greater rewards than the simpler forms, or by complex flowers offering a higher probability of being rewarding because fewer pollinators are able to visit them. In this paper we examine the effect of three pollinator foraging strategies on the ratio of flights within and between floral morphs and hence on the probability of a new morph establishing in a population without offering a greater reward. We incorporate pollinator behaviour based around observations of two pollinator species systems into three models of competition for pollinators. In the first model the constancy of the pollinator of the new floral morph is a function only of the foraging strategy of the existing pollinator of the original floral morph. In the next model the constancy of the second pollinator is determined by the number of rewarding flowers of each floral morph left by the original pollinator and in the third model it is determined by the ratio of rewarding flowers of each morph left by the original pollinator. The results demonstrate that under conditions of intense competition for pollinators, new, more complex floral forms are indeed able to attract high levels of constant pollinators without offering intrinsically higher rewards. However, for this to occur constancy in one of the pollinators must be a function of the ratio of rewarding to non-rewarding flowers of both floral forms. One prediction from our results is that sympatric speciation of floral complexity based on a higher probability of reward is more likely to occur in flowers offering rewards of pollen rather than nectar. This is because the cost of visiting non-rewarding flowers is usually higher where the reward is pollen rather than nectar. We also predict that complex flowers occurring at low frequency, which offer rewards of nectar, may need intrinsically greater rewards if they are to successfully attract pollinators.     

Pollinator response to female and male floral display in a monoecious species and its implications for the evolution of floral dimorphism

Pollinator-mediated selection has been hypothesized as one cause of size dimorphism between female and male flowers. Flower number, ignored in studies of floral dimorphism, may interact with flower size to affect pollinator selectivity. In the present study, we explored pollinator response, and estimated pollen receipt and removal, in experimental populations of monoecious Sagittaria trifolia, in which plants were manipulated to display three, six, nine or 12 female or male flowers per plant. In this species, female flowers are smaller but have a more compressed flowering period than males, creating larger female floral displays. Overall, pollinators preferred to visit male rather than female displays of the same size. Both first visit per foraging bout and visitation rates to female displays increased with display size. However, large male displays did not show increased attractiveness to pollinators. A predicted relationship that pollen removal, rather than pollen receipt, is limited by pollinator visitation was confirmed in the experimental populations. The results suggest that the lack of selection on large male displays may affect the evolution of floral dimorphism in this species.     

Comparative floral development in the tribe Mentheae (Nepetoideae: Lamiaceae) and its bearing on the evolution of floral patterns in asterids

A comparative developmental study of flowers was carried out using epi-illumination light microscopy on four genera of Lamiaceae (Nepeta, Rosmarinus, Salvia, andZiziphora), representing all three subtribes of Mentheae. All species examined share unidirectional (adaxial to abaxial) sepal initiation, except Rosmarinus, which has the reverse unidirectional sequence, starting abaxially. Initiated but suppressed bracteoles were detected only in Rosmarinus. In Rosmarinus, Salvia, and Ziziphora, initiation of petals and stamens proceeds unidirectionally from the abaxial side. Floral initiation of Nepeta has bidirectional inception of petals and unidirectional stamen initiation from the adaxial side. Temporal overlap in organ initiation between petal and stamen whorls occurs in all taxa, though this feature is more prominent in Rosmarinus. Significant structural and developmental features that distinguish the four genera include: (1) polysymmetric calyx tube, highly tomentose corolla and deeply four-partitioned ovary in Nepeta; (2) monosymmetric two-lipped calyx and shallowly four-partitioned ovary in Ziziphora; and (3) suppression of adaxial stamens in Salvia and Rosmarinus. Adaxial stamens are absent from Rosmarinus, but reduced stamens remain as staminodia in Salvia. In a phylogenetic context, the late monosymmetry of Nepeta and very early monosymmetry of Rosmarinus could both be regarded as derived conditions compared with the early monosymmetry ofSalvia and Ziziphora.     

Multigene analyses resolve early diverging lineages in the Rhodymeniophycidae (Florideophyceae,Rhodophyta)

Multigene phylogenetic analyses were directed at resolving the earliest divergences in the red algal subclass Rhodymeniophycidae. The inclusion of key taxa (new to science and/or previously lacking molecular data), additional sequence data (SSU, LSU, EF2, rbcL, COI‐5P), and phylogenetic analyses removing the most variable sites (site stripping) have provided resolution for the first time at these deep nodes. The earliest diverging lineage within the subclass was the enigmatic Catenellopsis oligarthra from New Zealand (Catenellopsidaceae), which is here placed in the Catenellopsidales ord. nov. In our analyses, Atractophora hypnoides was not allied with the other included Bonnemaisoniales, but resolved as sister to the Peyssonneliales, and is here assigned to Atractophoraceae fam. nov. in the Atractophorales ord. nov. Inclusion of Acrothesaurum gemellifilum gen. et sp. nov. from Tasmania has greatly improved our understanding of the Acrosymphytales, to which we assign three families, the Acrosymphytaceae, Acrothesauraceae fam. nov. and Schimmelmanniaceae fam. nov.     

Profile of a flower: How rates of morphological evolution drive floral diversification in Ericales and angiosperms

Premise

Recent studies of floral disparity in the asterid order Ericales have shown that flowers vary strongly among families and that disparity is unequally distributed between the three flower modules (perianth, androecium, gynoecium). However, it remains unknown whether these patterns are driven by heterogeneous rates of morphological evolution or other factors.

Methods

Here, we compiled a data set of 33 floral characters scored for 414 species of Ericales sampled from 346 genera and all 22 families. We conducted ancestral state reconstructions using an equal-rates Markov model for each character. We estimated rates of morphological evolution for Ericales and for a separate angiosperm-wide data set of 19 characters and 792 species, creating “rate profiles” for Ericales, angiosperms, and major angiosperm subclades. We compared morphological rates among flower modules within each data set separately and between data sets, and we compared rates among angiosperm subclades using the angiosperm data set.

Results

The androecium exhibits the highest evolutionary rates across most characters, whereas most perianth and gynoecium characters evolve more slowly in both Ericales and angiosperms. Both high and low rates of morphological evolution can result in high floral disparity in Ericales. Analyses of an angiosperm-wide floral data set reveal that this pattern appears to be conserved across most major angiosperm clades.

Conclusions

Elevated rates of morphological evolution in the androecium of Ericales may explain the higher disparity reported for this floral module. Comparing rates of morphological evolution through rate profiles proves to be a powerful tool in understanding floral evolution.     

Pollinator parasites and the evolution of floral traits

The main selective force driving floral evolution and diversity is plant–pollinator interactions. Pollinators use floral signals and indirect cues to assess flower reward, and the ensuing flower choice has major implications for plant fitness. While many pollinator behaviors have been described, the impact of parasites on pollinator foraging decisions and plant–pollinator interactions have been largely overlooked. Growing evidence of the transmission of parasites through the shared‐use of flowers by pollinators demonstrate the importance of behavioral immunity (altered behaviors that enhance parasite resistance) to pollinator health. During foraging bouts, pollinators can protect themselves against parasites through self‐medication, disease avoidance, and grooming. Recent studies have documented immune behaviors in foraging pollinators, as well as the impacts of such behaviors on flower visitation. Because pollinator parasites can affect flower choice and pollen dispersal, they may ultimately impact flower fitness. Here, we discuss how pollinator immune behaviors and floral traits may affect the presence and transmission of pollinator parasites, as well as how pollinator parasites, through these immune behaviors, can impact plant–pollinator interactions. We further discuss how pollinator immune behaviors can impact plant fitness, and how floral traits may adapt to optimize plant fitness in response to pollinator parasites. We propose future research directions to assess the role of pollinator parasites in plant–pollinator interactions and evolution, and we propose better integration of the role of pollinator parasites into research related to pollinator optimal foraging theory, floral diversity and agricultural practices.     

Phylogenetic relationships and floral evolution in the papilionoid legume clade Amorpheae

Amorpheae (Fabaceae: Papilionoideae) was first considered a natural group by Rupert Barneby in his illustrated monographDaleae Imagines. Amorpheae currently comprise eight genera, ca. 250 spp., and extensive floral diversity, including loss of corolla and addition of a stemonozone. The Amorpheae and many of Barneby’s subtribal groups are supported as monophyletic by previous phylogenetic analysis of nuclear ribosomal and chloroplast sequence data. However, some relationships remain unclear. A nuclear marker derived from a genomic study inMedicago CNGC 4, was sequenced in selected Amorpheae This is one of the first applications of this marker. for phylogenetic study. The new data confirm some relationships inferred usingtrnK and ITS, but also provide evidence for new arrangements. Combined data were used to explore several aspects of Barneby’s taxonomic framework. The phylogeny, in concert, with data on floral morphology, implies that simplification of the complex papilionoid flower has occurred several times in the history of the Amorpheae.