Sporophytic Self-incompatibility System Research Articles

The number of loci controlling the sporophytic self-incompatibility system in Sinapis arvensis L.

A breeding programme designed to establish the number of loci controlling the sporophytic self-incompatibility system in Sinapis arvensis has been completed. The programme involved analyses of diallel crosses within F1 and F2 families derived from crosses between plants from geographically remote populations, and within self families derived from the parental plants. Incompatibility reactions were determined cytologically using fluorescence microscopy. Control by a single locus has been established. However, the expression of individual S-alleles may be modified, as shown by incomplete dominance in a self family, and mutual weakening in an F1 family.

Cell biology of the stigma of Brassica campestris in relation to Co2 effects on self-pollination

ABSTRACT The effect of carbon dioxide in blocking the sporophytic self-incompatibility system in Brassica campestris occurs within the first 2h of pollination at the pollen-stigma interface. Percentage germination of self pollen on the stigma was found to be similar in air and in the presence of 5% CO2. The CO2 effect therefore must occur after pollen germination, modifying the interaction between pollen tubes and stigma cells. Lectin binding studies showed the presence of fucosyl but not galactosyl residues on the stigma surface. Gel electrophoresis of plant extracts showed that stigma esterase activity is marked in comparison to other plant tissue. This activity is shown histo-chemically to be localized on the stigma cell surface and in the nucleus. Carbonic anhydrase has been detected on the stigma surface by two different histochemical methods and its possible relationship to the CO2 effect is discussed.

A Self-Compatible Plant of Stephanomeria exigua subsp. Coronaria (Asteraceae) and Its Relevance to the Origin of Its Self-Pollinating Derivative S. malheurensis

Stephanomeria exigua subsp. coronaria (COR) and S. malheurensis (MAL) are related as progenitor and recent derivative and occur sympatrically at the only known site of MAL in eastern Oregon. MAL is predominantly self-pollinating, but COR is obligately outcrossing because it has a sporophytic self-incompatibility system. Self-pollination is an important component that contrib- utes to the reproductive isolation of MAL and evolved following a breakdown in the self-incom- patibility (SI) system it inherited from COR. We found a single COR plant derived from the parent population growing in our greenhouse that was self-compatible and self-pollinating. It was used to study the genetic basis of the change in compatibility within COR. We also analyzed the change in interspecific hybrids between COR and MAL. In both cases, the difference in compatibility was governed by a single gene, probably the S locus which controls the incompatibility reactions between the pollen grain and the stigmatic surface. The discovery of a SC allele and accompanying self- pollination in the progenitor population of COR bolsters the model that the origin of MAL required few genetic changes.

Self-incompatibility and interspecific incompatibility: relationships in intra- and interspecific crosses of Zinnia elegans Jacq. and Z. angustifolia HBK (Compositae)

Intraspecific and reciprocal interspecific crosses involving Zinnia angustifolia clones and Z. elegans lines showed that in both species, sporophytic self-incompatibility (SI) systems were present. Intensity of SI varied among clones and lines, and high self seed set was associated with a concomitant decrease in callose fluorescence in papillae and pollen tubes. Incomplete stigmatic inhibition of pollen germination and tube growth was observed in reciprocal interspecific crosses and associated with callose synthesis, suggesting S-gene activity. Seed set and progeny obtained following Z. angustifolia×Z. elegans matings was comparable to intraspecific compatible matings of Z. angustifolia although the rate of pollen tube growth through the style was slower. In Z. elegans × Z. angustifolia matings, additional prezygotic barriers were present and acted between pollen tube penetration of the stigma and syngamy. SI X SI interspecific incompatibility was essentially unilateral, with no embryos or progeny obtained when Z. elegans was the pistillate parent. It was hypothesized that nonfunctioning of Z. elegans × Z. angustifolia crosses was due to S-gene expression at the stigmatic surface and to other isolating mechanisms in the stylar or ovarian transmitting tissue.

Overcoming Self-incompatibility through the Use of Lectins and Sugars in Petunia and Eruca

Treatment of stigma with a lectin (Con A/PHA) before pollination was effective in overcoming self-incompatibility in Petunia hybrida, a gametophytic self-incompatible system, and Eruca sativa, a sporophytic self-incompatible system. Treatment of pollen with glucose/N-acetyl-D-galactosamine (tested only with Petunia) was also effective. These results suggest the involvement of pollen lectins and specific sugar components of the pistil in self-incompatibility recognition.

Is the ?rejection reaction? inducing ability in sporophytic self-incompatible systems restricted only to pollen and tapetum?

The present investigation describes the stigmatic rejection response, induced by anther or pollen at developmental stages ranging from anther primordia to mature pollen, as well as in other floral and vegetative tissues. This has been studied in a sporophytic self-incompatible system, Brassica campestris (Brassicaceae). The implications of these observations have been discussed.

Studies on heteromorphic selfincompatibility systems: The cytochemistry and ultrastructure of the tapetum of Primula obconica

This paper describes the cytochemistry and ultrastructure of the developing tapetum in Primula obconica, a plant with a heteromorphic, sporophytic self-incompatibility system. The tapetum is of the secretory type and cytochemical tests have shown that when it breaks down proteinaceous (esterase) and lipidic components are deposited on the developing pollen grains. Acid phosphatase, a marker of gametophytic enzyme activity, is confined to the cytoplasm and intine of the developing pollen. Ultrastructural studies show that prior to its dissolution the tapetum undergoes a number of changes. In the early stages of development the tapetum is rich in ribosomes and rough endoplasmic reticulum, but following the breakdown of the tapetal cell wall the main components of the cytoplasm are densely staining spherical bodies surrounded by ribosomes and orbicular bodies, which appear to be confined to the cell periphery. As the cells break down, rod-like fibrils can be seen amongst the degenerate organelles and within the bacular cavities of the pollen. On dehiscence the pollen has a lipidic coating in addition to the fibrillar material in the pollen wall and the remnants of the tapetum can be seen adhering to the fibrous layer of the anther wall. Thus the mature, binucleate pollen of P. obconica is demonstrated to carry wall materials of sporophytic origin.

Sporophyte Versus Gametophyte: A Note on the Origin of Self-Incompatibility in Flowering Plants

The evolution of gametophytic and sporophytic self-incompatibility systems in flowering plants may be the result of a conflict created by dissimilar reproductive requirements of the gametophytic and sporophytic generations. Stig- matic recognition and rejection of self-pollen is proposed as an evolutionary ad- vancement by the sporophyte in this conflict. An increased number of incompatibility groups and flower heteromorphisms are breeding system characteristics that prob- ably evolved to increase mating efficiency. Although the genetic benefits of outcrossing must be the ultimate force behind the evolution of self-incompatibility in flowering plants, still un- resolved are the selective factors that have promoted the development of two common, yet distinctively different incompatibility mechanisms. The cytological and physiological derivation of sporophytic self-incom- patibility (SSI) from gametophytic self-incompatibility (GSI) has been discussed (Brewbaker 1957; Pandey 1958, 1960; Heslop-Harrison 1978) and Heslop-Harrison (1978) has pointed out that SSI may be more met- abolically e-fficient than GSI, but the selective basis for the two incom- patibility mechanisms remains unknown. The selective forces responsible for the evolution of these two incom-. patibility systems is clarified by considering the reproductive biology of the male gametophyte. After emphasizing the evolutionary identity of the gametophytic generation, we will discuss the reproductive goals of the angiosperm male gametophyte, point out that these goals are dissim- ilar to those of the sporophyte, and propose that gametophytic and spo- rophytic incompatibility mechanisms are adaptations of the sporophytic generation in response to this conflict of reproductive requirements. We assume GSI is the primitive and fundamental type of self-incompatibility system in the angiosperms and SSI is the derived system (Arasu 1968;

The biology of heterostyly

Abstract Heterostyly is a genetically controlled floral polymorphism that is known to occur in 24 families of flowering plants. The floral morphs differ reciprocally in stamen and style lengths. They often differ in pollen grain size and production, and may differ in pollen exine sculpturing, pollen colour, presence of starch in pollen, stigmatic papillae, or corolla size or morphology. There are two morphs in distylous plants and three morphs in tristylous plants. Distyly is much more common than tristyly. Tristyly is known to occur only in the Lythraceae, Oxalidaceae, and Pontederiaceae, although there are unconfirmed reports in the Connaraceae and in Hugonia of the Linaceae. In distylous plants the supergene determining floral morphology also controls a diallelic sporophytic self-incompatibility system, so that only pollinations between morphs are compatible. Tristylous plants usually possess a two-locus diallelic sporophytic self-incompati-bility system associated with the floral trimorphism, but whet...

The Receptive Surface of the Angiosperm Stigma

This paper reports upon a study of the characteristics of the receptive surfaces of angiosperm stigmas, covering almost 1000 species of about 900 genera of some 250 families. In both monocotyledons and dicotyledons the major subdivision is into those stigmas which are 'dry' at maturity, having a hydrated, proteinaceous extracuticular layer or pellicle but no free-flowing secretion, and those which are 'wet' bearing such a fluid secretion when in the receptive state. Further subdivision may be made talcing into account the nature of the surface cells-whether or not the receptive surface is papillate or smooth, and if papillate, whether the papillae are unicellular or multicellular. When the genera are classified according to these criteria, various taxonotnic regularities emerge. Many families are homogeneous in stigma type, but some prove to be diverse. Among these are families such as the Liliaceae, a fact that may have phylogenetic significance. The physiological importance of stigma type is shown by the correlations that exist between the characteristics of the receptive surface and self-incompatibility system. Sporophytic self-incompatibility systems are associated with dry, papillate stigmas; most gametophytic systems with wet stigmas. Further relationships exist with pollen type and physiology. Trinucleate pollen, not readily germinated in vitro, tends to be associated with dry stigmas, while wet-stigma forms tend to have binucleate pollen, easily germinated in liquid or semi-solid media; binucleate pollen, however, occurs with both wet and dry stigmas.

DYNAMICS OF UNSTABLE SPOROPHYTIC SELF-INCOMPATIBILITY SYSTEMS

Sporophytic self-incompatibility systems were studied by using deterministic computer simulation. Frequencies of individual S alleles, or combinations of S alleles, were set at 0.001 of their equilibrium frequency and observations were made on the speed with which equilibrium was restored. Ten cases were studied, each of which had three variants regarding the site of dominance, i.e., in the pollen, in the stigma, and in both sexes. Dominant alleles regained equilibrium much faster than did recessive alleles, but both types were slower than the 11th case in which all alleles acted individually. Alleles that acted independently of the dominance series were almost as slow to regain equilibrium as were the recessive alleles. Models with dominance confined to the pollen were unstable in that all of the recessive alleles except one could become extinct. The results are discussed in terms of known sporophytic systems and the evolutionary strategies that would enable a species to acquire several pollen recessive S alleles.

EVOLUTIONARY RELATIONSHIPS IN THE OUTCROSSING DIPLOID ANNUAL SPECIES OF STEPHANOMERIA (COMPOSITAE).

The annual taxa of the western North American genus Stephanomeria (Compositae) provide excellent material for the comparative study of genetic differentiation and speciation in plants. They are a closely related assemblage, have broad and narrow distributions, occupy numerous habitats, and, in their center of variability in California, they are actively differentiating in response to sharp ecological gradients as well as the activities of man. They have long been known for their complex morphological intergradation (Ferris, 19660), which has stymied taxonomic efforts to separate the taxa. They exhibit a remarkable array of different evolutionary strategies involving changes in breeding system, polyploidy, hybridization, adaptation to particular substrates, and aggressive colonization of disturbed roadside habitats. As a result of the studies reported herein, the annual Stephanomerias are now known to include nine diploid (2n = 16) and two allotetraploid (2n = 32) taxa. Tetraploid chromosome numbers were first reported in Stephanomeria by Stebbins et al. (1953). They constitute both a polyploid complex and an homogamic complex in which the same divergent diploid species are connected by morphologically intermediate tetraploid and diploid species. Seven of the diploid taxa have a sporophytic selfincompatibility system and are consequently obligate outcrossers. The other two diploids and both tetraploids are selfcompatible and self-pollinating in the greenhouse. Their morphological, cyto-