Flower biology and evolutionapl. Prof. Claudia Erbar
Flowers serve in the sexual reproduction of plants. Despite their similar basic structure (perianth, androecium, gynoecium, receptacle) there is an enormous variation in detail. Much of this diversity has evolved in co-evolution with the different pollinators.
Flowers play a crucial role in the life-cycle of flowering plants and fulfil elementary functions in their evolution: 1. The passing on of mutations to a part of the descendants. 2. The recombinative processes of meiosis and gamete fusion and the achievement of the highest possible recombination rate by cross-fertilization following cross-pollination. 3. A pre-zygotic selection mainly of the male gametophytes (pollen tube competition). 4. The development of seeds and their presentation for dispersal (fruit ripening).
In general, we are interested in floral ontogenetic features for understanding relationships between structure, function, systematics, and phylogeny. A second focus is on floral ecology, namely on the functions of the flower in the processes of pollination and fertilization.
The study of the development of flowers (scanning electron microscopy is an indispensable tool, nevertheless accompanied by histological studies) allows a new approach to morphological problems and contributes to systematics.
Floral ontogentic patterns of extant archaic angiosperms are an important approach for a better understanding of processes like early radiation and canalisation (e.g. origin of Monocotyledons from archaic Dicotyledons).
Floral ontogentic patterns have been proven as important characters when testing the predictions of relationships of flowering plants by molecular data. In many cases morphological data are up to now not available or have to be newly valued when supporting recently created clades (relationships of different taxonomic rank). Data from floral ontogeny seem to be more valuable than characters from adult flowers. Flowers that are similar at maturity may show considerable variation in some developmental phases.
Formation of corolla tubes - Relationships of the asterids
Especially the formation of corolla tubes is a good example demonstrating that floral ontogenetic patterns are valuable characters for classifications. The patterns “early” and “late sympetaly” (Erbar 1991, Bot. Jahrb. Syst. 112) substantiate presumable affinities based on molecular data (sequencing the chloroplast genome), namely the “late sympetalous” asterids I (= lamiids) and the “early sympetalous” asterids II (= campanulids). Corolla tube are termed “early-sympetalalous” if the petal primordia are already connected during their initiation by a ring-like structure, the latter corolla tube. “Late sympetaly” means that the petals arise separately on the floral apex and only meet at the dorsal bases of the stamens. In the meantime, our distinction between “late” and “early sympetalous asterids” has been established in several text books.
In all recent sequence based analyses Apiales come out to be placed within broadly defined Asteridae. Within “euasterids II” Apiales (Apiaceae, Araliaceae, Pittosporaceae, Aralidaceae as well as some former cornaceous taxa) form a monophyletic group in a position close to Asterales and Dipsacales. According to our results on floral ontogeny, also from this point of view the mostly choripetalous Apiales are not out of place among these sympetalous orders: In members of Apiales (Araliaceae: Aralia, Hedera, Hydrocotyle; Pittosporaceae: Sollya, Pittosporum) the corollas are initiated from a continuous ring primordium corresponding exactly the development in Asterales and Dipsacales.
In particular the disintegration of the Scrophulariaceae due to molecular data resulted in recircumscriptions, resurrection and new descriptions of families to encompass the monophyletic lineages. In some of these families floral morphological/ontogenetic data, however, are largely missing. Besides the formation of sympetaly, we are searching for further non-molecular diagnostic features. Our floral ontogenetic/morphological study confirms, for example, the resurrection of the family Paulowniaceae and its position within the Lamiales. Distinctive characters are the peculiar shape of the specially multi-winged seeds and the tubular stigma with papillae inside a dilated chamber. In the latter character Paulownia differs from all other members of the Lamiales-clade! Thus it can be used as an easily observable diagnostic feature.
Further studies on corolla tube formation will be made in scarcely investigated asterids.
Pollen tube transmitting tissue
In angiosperm reproduction, pollen tubes elongate from the stigma through the stylar transmitting tissue to the ovary to deliver the male gametes for fertilization. From the stigma towards the ovary, near the ovules, the pollen tubes grow over a long distance in the pollen tube transmitting tissue, a specially differentiated tract between stigma and ovary.
There are different forms of pollen tube transmitting tracts within the style as well as possibilities to bridge the gap from the gynoecial transmitting tissue to the ovule. We are interested in the shape (histology) and extent of the pollen tube transmitting tissue because this tissue is the place of possible pollen tube competition.
Nectaries are nectar glands which can occur at different sites in the flower, i.e. they may occur anywhere in the flower. The histology of nectary tissues can differ, too. Thus, to some degree these data may contribute to systematics.
In the Caryophyllales, the nectariferous tissue seems to be in many, if not all, families to be associated with the base of the androecium, namely the base of the filaments. In literature, the description of the nectaries in these families – except the Caryophyllaceae – is sketchy and suffers from the misuse of the term "disc". Because of complex androecia, staminal tubes and hypanthia the picture may be blurred. Thus careful investigations, also of young and older developmental stages, are needed.
In the monophyletic group of euasterids I, superior ovaries dominate. Intrastaminal discs as well as more commonly the base of the ovary may be the site of nectary production. There may be transitions between a gynoecial and a receptacular disc. Then a delimitation is not possible. The early developmental stages, however, have not yet been carefully or comprehensively studied. Within the euasterids II, nectaries are mostly formed by the gynoecium: If this is inferior (as common), the nectary is situated on the ovary roof. In most Dipsacales, however, epidermis hairs take over the nectar secretion.
Style diversity in Asteracae – Function and Phylogeny
In this project we want to combine functional and systematic aspects. The different mechanisms of secondary pollen presentation in Asteraceae can be aligned with the position of hairs on the style (primitive deposition-brushing mechanism in Barnadesioideae without stylar hairs, brushing mechanism in Cichorioideae with a long hairy part of the style, pump mechanism in the majority of Asteroideae with hairs only at the very tips of the stylar branches). Sometimes there are combinations of pump and brushing mechanisms, especially in Asteroideae. In general, from the relative length of the hairy zone of the style we can predict the proportion of pumping and brushing. The hairy part is as long as the style is embedded within the anther tube at the beginning of anthesis.
At present, mainly based on molecular data, 12 subfamilies and 43 tribes are recognized (Panero & Funk 2008, Mol. Phyl. Evol. 47). Samples of all tribes will be studied with SEM (to demonstrate shape, size and stylar hairs and stigma arrangement) and histological sections (to demonstrate stigma tissue and pollen tube transmitting tissue). In addition, the course of secondary pollen presentation will we pursued in as many tribes as material is available.
Flower ecology: Quantitative aspects in reproductive biology
Within a very complicated net of correlations controlled by the principle of economy, the quantity of pollen grains delivered to a pollinator is an important factor. Therefore pollen portioning and its mechanisms especially those of secondary pollen presentation are in the focus of our work.
An important prezygotic factor of selection is the pollen tube competition. Quantitative studies of pollen load on the stigma and pollen tube attrition (the reduction of the number of pollen tubes from the stigma to the ovary) are undertaken in different taxonomic groups. A further interesting question is: How economically deal the flowering plants with their pollen and other resources with regard to reproductive success?
Most members of the Asterales-clade are characterized by secondary pollen presentation (deposition, brushing, pump, cup mechanism). In these mechanisms pollen grains are not presented directly out of the anthers but are transferred to certain other flower structures just before or at the outset of anthesis. Secondary pollen presentation is a special kind of pollen portioning. The wide distribution of portioned pollen release, mostly organized in the flowers by a non-simultaneous opening of the anthers, indicates that it is connected with optimization of pollination. The wide distribution of portioned pollen release (e.g. by non-simultaneous opening of the anthers, secondary pollen presentation), indicates that it is connected with optimization of pollination.
With Spigelia anthelmia (Gentianales) we have found a species that uses a simple deposition mechanism for self-pollination by not separating male and female phase of anthesis. In the autogamous Spigelia anthelmia we discovered an increased pollen tube competition presumably compensating a possible inbreeding depression (Erbar & Leins 1999).
Another example is presented with Cyphia stenopetala. Members of the African subfamily Campanulaceae-Cyphioideae are characterized by a pollen-presenting box formed by the five (empty) anthers as walls and the stylar tip as the bottom. As usual, the mechanism of secondary pollen presentation promotes cross-pollination or limits self-pollination, respectively. Cyphia stenopetala, however, uses the pollen box for self-fertilization: Pollen is shed into the pollen box, where it germinates, and the pollen tubes form a dense felt: an area for an increased pollen tube competition presumably compensating for possible inbreeding depression. Flowers of outbreeding Cyphia species are distinctly proterandrous, but in the selfing Cyphia stenopetala, flowers are proterogynous. Self-fertilization in Cyphia stenopetala appears to be a recent evolutionary event because all flowers investigated produce nectar, and different populations can differ in time and degree of pollen germination (Leins & Erbar 2003, 2005).
Medieval plant names
Some of the Codices Palatinae germanicae, kept in the University Library, contain Latin-German glossaries that are not studied until now. Based on the German and Latin medieval plant names the herbs are identified and described with today's nomenclature. To secure the provision more medieval herbal books are included in the comparison, namely the "German Macer", the “Physica” of Hildegard von Bingen and the so-called “Speyerer Kräuterbuch”
Flower and fruit
This textbook deals with the fascinating pivotal players of the evolution of flowering plants: flowers and fruits. It is not only a concise introduction to the basics, but also discusses the latest knowledge to date. Flower and Fruit is the first English language edition of “Blüte und Frucht” (2nd ed.), the successful standard textbook on floral and fruit biology in Germany. Unique in its breadth, Flower and Fruit covers virtually all aspects of this field.
The authors start out by describing the origin of flowers and by delineating the crucial role they play in the life cycle of plants. They go on to explain and illustrate basic terminology and introduce the genetic approach to the determination of fl oral organs.
On the basis of ontogeny, a morphological analysis of the manifold flower structures follows, constituting the basis for discussing further phylogenetic and diverse functional and ecological questions: the problem of hermaphroditism, optimization of pollination, pollen tube competition, adaptations of flowers to specific pollinators, patterns of seed dispersal, modes of diaspore dispersal and adaptations of the plants and their diaspores to the dispersing agents.
The textbook includes a useful classification of fruits, a comprehensive updated appendix, which serves as reference for placing the mentioned taxa in a classification of the flowering plants (with more than 400 fl oral formulas), and a glossary.
Popular scientific books (in German)
Das Motto dieses Buches heißt: „vergnügliches Kennenlernen“ von Holzgewächsen an Merkmalen, die uns in Herbst und Winter zur Verfügung stehen, z.B. das verdorrte Laub, die Borke und eventuell die Früchte, vor allem aber die Überwinterungsknospen in ihrer überwältigenden Vielfalt.
Die Bestimmung von Bäumen, Sträuchern und weiteren Holzgewächsen erfolgt in diesem Buch nicht wie üblich mit „trockenen“ Bestimmungstabellen, sondern mit zu Bildgruppen zusammengefassten Bilderschlüsseln und großen farbigen Bildtafeln mit diagnostischen „Steckbriefen“ zur anschließenden Überprüfung der Richtigkeit.
In leicht verständlicher Form bietet ein einführendes Kapitel Grundkenntnisse über den vegetativen Aufbau und die Funktion, die Bestimmung und die Eigenschaften von vielen in Europa wachsenden Holzgewächsen. Darauf aufbauend ist das eigentliche Ziel die Aneignung einer gewissen Artenkenntnis dieser Pflanzen. Insgesamt werden 77 heimische und durch den Menschen eingeführte exotische und eingebürgerte Baum- und Straucharten vorgestellt.
Das Buch ist auch als unterhaltsamer Wegbegleiter auf Spaziergängen für den interessierten Laien, Gehölzliebhaber, Lehrer und Schüler, Eltern und Großeltern angelegt. Als Besonderheit gibt es zu jeder vorgestellten Baum- und Strauchart eine kleine Erzählung, entweder aus Naturwissenschaft, Geschichte, Mythologie, Sprachwissenschaft, dem täglichen Gebrauch oder zur jeweiligen Verwendbarkeit.
Das erfolgreiche Buch mit dem Titel „Bäume und Sträucher in Herbst und Winter erkennen“ hat ein Pendant bekommen. Darin werden nun die gleichen Bäume und Sträucher – insgesamt 77 heimische, eingeführte und eingebürgerte Arten – in der gleichen Reihenfolge und mit denselben Kennnummern in Frühjahr und Sommer vorgestellt. Während dieser Jahreszeiten bieten die Holzgewächse mit Blättern, Blüten, Blütenständen und reifenden Früchten eine Fülle von Merkmalen. Die zu ihrer Erkennung wichtigen Merkmale sind auf großen farbigen Bildtafeln mit knappen Steckbriefen zusammengestellt. Die Bildtafeln dienen der Überprüfung einer ersten schnellen Bestimmung nach Blattformen. Durch einen direkten Formvergleich in einem Bilderschlüssel ist die Bestimmung vergnüglicher als unter Zuhilfenahme nicht bebilderter, „trockener“ Bestimmungstabellen. Eine leicht verständliche Einführung mit eingefügten Graphiken vermittelt dem Benutzer wichtige Grundkenntnisse zur Biologie von Holzgewächsen und regt zur eigenen Beobachtung an. Neue Begleitgeschichten zu jeder Baum- und Strauchart aus Biologie, Mythologie und Verwendbarkeit machen auch dieses Buch zu einem unterhaltsamen Wegbegleiter auf Spaziergängen.
Das Buch ist in erster Linie für den interessierten Laien und Gehölzliebhaber, für Lehrer und Schüler, Eltern und Großeltern konzipiert. Auch für Studierende der Biologie mag es als erste „Tuchfühlung“ mit der Biodiversität der Pflanzen eine willkommene Lektüre sein.