Ovules are complex structures that are present in all seed bearing plants and are contained within the carpels in flowering plants. Ovules are the site of megasporogenesis and megagametogenesis and, following fertilization, develop into seeds. We combined genetic methods with anatomical and morphological analyses to dissect ovule development. Here, we present a detailed description of the morphological development of Arabidopsis ovules and report on the isolation of two chemically induced mutants, bell (bel1) and short integuments (sin1), with altered ovule development. Phenotypic analyses indicated that bel1 mutants initiate a single integument-like structure that develops aberrantly, sin1 mutants initiate two integuments, but growth of the integuments is disrupted such that cell division continues without normal cell elongation. Both mutants can differentiate archesporial cells, but neither forms a normal embryo sac. Genetic analyses indicated that bel1 segregates as a single recessive mutation, and complementation tests showed that the two mutants are not allelic. The phenotypes of the mutants indicate that normal morphological development of the integuments and proper embryo sac formation are interdependent or are governed in part by common pathways. The ovule mutants that we describe in Arabidopsis represent novel genetic tools for the study of this stage of reproductive development.

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Ovule Development in Wild-Type Arabidopsis and Two Female-Sterile Mutants.

To isolate and study genes controlling floral development, we have carried out a large-scale transposon-mutagenesis experiment in Antirrhinum majus. Ten independent floral homeotic mutations were obtained that could be divided into three classes, depending on whether they affect (1) the identity of organs within the same whorl; (2) the identity and sometimes also the number of whorls; and (3) the fate of the axillary meristem that normally gives rise to the flower. The classes of floral phenotypes suggest a model for the genetic control of primordium fate in which class 2 genes are proposed to act in overlapping pairs of adjacent whorls so that their combinations at different positions along the radius of the flower can specify the fate and number of whorls. These could interact with class 1 genes, which vary in their action along the vertical axis of the flower to generate bilateral symmetry. Both of these classes may be ultimately regulated by class 3 genes required for flower initiation. The similarity between some of the homeotic phenotypes with those of other species suggests that the mechanisms controlling whorl identity and number have been highly conserved in plant evolution. Many of the mutations obtained show somatic and germinal instability characteristic of transposon insertions, allowing the cell-autonomy of floral homeotic genes to be tested for the first time. In addition, we show that the deficiens (def) gene (class 2) acts throughout organ development, but its action may be different at various developmental stages, accounting for the intermediate phenotypes conferred by certain def alleles. Expression of def early in development is not necessary for its later expression, indicating that other genes act throughout the development of specific organs to maintain def expression. Direct evidence that the mutations obtained were caused by transposons came from molecular analysis of leaf or flower pigmentation mutants, indicating that isolation of the homeotic genes should now be possible.

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Floral homeotic mutations produced by transposon-mutagenesis in Antirrhinum majus.

We have examined the floral morphology and ontogeny of three mutants of Arabidopsis thaliana, Ap2-5, Ap2-6, and Ap2-7, that exhibit homeotic changes of the perianth organs because of single recessive mutations in the AP2 gene. Homeotic conversions observed are: sepals to carpels in all three mutants, petals to stamens in Ap2-5, and petals to carpels in Ap2-6. Our analysis of these mutants suggests that the AP2 gene is required early in floral development to direct primordia of the first and second whorls to develop as perianth rather than as reproductive organs. In addition, our results support one of the two conflicting hypotheses concerning the structures of the calyx and the gynoecium in the Brassicaceae.

AP2 Gene Determines the Identity of Perianth Organs in Flowers of Arabidopsis thaliana.

Article de synthese decrivant les mutants d'Arabidopsis avec une morphologie modifiee a l'un des 3 niveaux suivants: l'embryogenese, la formation des organes (developpement floral) et le developpement cellulaire (trichomes)

Genetic control of morphogenesis in Arabidopsis

Various criteria have been used in recent literature to decide whether a given character state is primitive or derived. The problems are similar in botany and zoology. The present review emphasizes the botanical literature, in which there has been no general examination of the validity of the criteria since Spome's excellent review (149). Many of the criteria that Spome rejected as being inadequate have since crept back in to use; the criterion that he favored, correlation of characters, is of doubtful utility; and a criterion that he did not discuss, character state distribution, has since become widely used, but in two very different ways. Many of the criteria are interrelated, and several depend on further, generally unstated criteria; criteria are often used in combinations, so that an author's actual procedure becomes unclear. Finally, a criterion rejected in one place in an article may be accepted elsewhere (sometimes in the same paragraph). Hence a reexam­ ination of the validity of the various proposed criteria is overdue. There is now considerable interest in various forms of cladistic or phylo· genetic classification in botanical circles [(22, 72, 91, 166) and references]. Although these approaches to classification are thought beneficial because of their explicit assumptions and procedures, it is ironic that the recent literature hardly mentions the crucial early step: the assignment of evolu­ tionary polarity (101) to character states-i.e. the decision about which character states in the taxa being examined are derived or apomorphous and which are primitive or plesiomorphous (the terms are defined below). That estimates of evolutionary relationships depend on the decision about which character states are derived was shown by Solbrig (145), who obtained different phylogenies when he used three different criteria for this decision.

Evolutionary Polarity of Character States

Summary

In the introduction to this study, the chief phases in the interpretation of the flower, from Goethe's day onwards, are briefly indicated in their historical sequence. Goethe's theory of the equivalence of the vegetative shoot to the flower, in the angiosperms, is then discussed and an attempt is made to evaluate the evidence for it. It is shown that this theory, if understood in a broad sense, harmonizes with the modern holistic trend in morphology. It is suggested that the flower is comparable with a vegetative shoot in a condition of permanent infantilism. Special emphasis is laid upon the inflorescence as offering, in some respects, an intermediate term between the vegetative shoot and the flower.



After a brief consideration of bracts, sepals, petals and stamens, the Candollean theory of the carpel is discussed, and it is concluded that it has been peculiarly successful in providing a framework for the vast plexus of facts which it is its task to correlate. Some of the difficulties which have been felt in regard to this theory are considered, with special reference to recent work on the gynaeceum structure of the Papaveroideae. The stigma and “transmitting tissue” are then discussed, and it is concluded that there is nothing in the behaviour of this tissue which is out of harmony with the Candollean theory of the carpel.



An attempt is made to arrive at a more precise notion of the meaning to be attached to correspondence, equivalence and homology, when these terms are used in connexion with Goethe's comparison of the vegetative and reproductive parts. It is suggested that these terms are best translated into the language of modern thought by the word parallelism, thus avoiding the use of Goethe's type concept, which cannot be safely employed unless its abstractness is constantly borne in mind.



The nineteenth-century phase, in which morphological ideas were lifted bodily into an historical setting, is then discussed, and emphasis is laid upon the danger of thus confusing two irreducible worlds of thought. Certain attempts which have been made to relate the flower of the angiosperm to the reproductive organs of plants of earlier geological periods are briefly criticized.



In the concluding sections, attention is drawn to the current reaction against phylogenetic morphology, and in favour of the purely comparative morphology contemplated by Goethe. A slight sketch is given of Delpino and Troll's theories of the flower, in which “form” is considered as distinct from “organization”. Whether these views are accepted or not, the “Gestaltlehre” is at least an indication that the morphological ideas, which Goethe initiated before the end of the eighteenth century, may even to-day suggest fresh approaches to the problem of the interpretation of the flower.

Agnes Arber

Papers

The interpretation of the flower: a study of some aspects of morphological thought

The interpretation of leaf and root in the angiosperms