Self-incompatibility: how plants avoid illegitimate offspring

Fine mapping of a major co-localized QTL associated with self-incompatibility identified in two F2 populations (broccoli × cauliflower and cauliflower × Chinese kale)

KEY MESSAGE

A major QTL responsible for self-incompatibility was stably identified in two F2 populations. Through fine mapping and qRT-PCR analysis, ARK3 emerged as the most promising candidate gene, playing a pivotal role in regulating self-incompatibility in Brassica oleracea. Self-incompatibility (SI) is a common phenomenon in Brassica oleracea species, which can maintain genetic diversity but will also limit seed production. Although the S locus has been extensively studied in Arabidopsis and some Brassicaceae crops, map-based cloning of self-incompatibility genes has not been conducted in Brassica oleracea, such as cauliflower and broccoli. In the present study, we identified a major co-localized QTL on chromosome C6 that control SI in two F2 populations derived from intervarietal crosses: broccoli × cauliflower (CL_F2) and cauliflower × Chinese kale (CJ_F2). Subsequently, this QTL was narrowed down to 168.5 Kb through fine mapping using 3,429 F2:3 progenies and 12 available KASP markers. Within this 168.5 Kb region, BolC6t39084H, a homologue of Arabidopsis ARK3, could be a candidate gene that plays a key role in regulating SI in B. oleracea species. This finding can pave the way for an in-depth understanding of the molecular mechanisms underlying SI, and will contribute to the seed production of B. oleracea vegetables.

Evolutionary and structural aspects of Solanaceae RNases T2

Plant RNases T2 are involved in several physiological and developmental processes, including inorganic phosphate starvation, senescence, wounding, defense against pathogens, and the self-incompatibility system. Solanaceae RNases form three main clades, one composed exclusively of S-RNases and two that include S-like RNases. We identified several positively selected amino acids located in highly flexible regions of these molecules, mainly close to the B1 and B2 substrate-binding sites in S-like RNases and the hypervariable regions of S-RNases. These differences between S- and S-like RNases in the flexibility of amino acids in substrate-binding regions are essential to understand the RNA-binding process. For example, in the S-like RNase NT, two positively selected amino acid residues (Tyr156 and Asn134) are located at the most flexible sites on the molecular surface. RNase NT is induced in response to tobacco mosaic virus infection; these sites may thus be regions of interaction with pathogen proteins or viral RNA. Differential selective pressures acting on plant ribonucleases have increased amino acid variability and, consequently, structural differences within and among S-like RNases and S-RNases that seem to be essential for these proteins play different functions.

Plant physiology: ATP at the center of self-recognition

ATP acts as the common currency of metabolic activity in all life forms. A recent study uses inter-specific transfer of the self-recognition module in plants to enable live monitoring of the cellular status in vivo, revealing the pivotal role of ATP in signaling.

Lack of sibling avoidance during mate selection in the butterfly Bicyclus anynana

Species susceptible to inbreeding depression are hypothesized to combat this problem through a number of different mechanisms, including kin recognition. For species with kin recognition, it is unknown if filial recognition is innate or due to prior juvenile experience with siblings. Here, we first test for the presence of kin recognition, and then test these two hypotheses for the development of filial recognition, in the butterfly Bicyclus anynana, a species that suffers from inbreeding depression when forcibly inbred but recovers within a few generations when allowed to breed freely. We evaluate whether the rapid recovery from inbreeding depression is associated with either innate or learned filial recognition. First, we determined whether females innately prefer unrelated males over sibling males using females reared in isolation and then given a choice between an unrelated and a sibling male. Then, we determined if females raised with siblings learned to detect and avoid mating with siblings as adults when provided a choice between an unrelated male and a sibling male. Finally, we determined if females raised with siblings could learn to detect and avoid mating with familiar siblings when given a choice between familiar and unfamiliar siblings. We found that females mated randomly in all three choice combinations. Observed male behavior also did not influence female mating outcome. Our results suggest that adult females do not innately avoid or learn to avoid siblings during mate selection, and that filial detection may not be as critical to reproductive fitness in B. anynana as previously thought.

Transcriptomic Analysis Reveals a Comprehensive Calcium- and Phytohormone-Dominated Signaling Response in Leymus chinensis Self-Incompatibility

Sheepgrass (Leymus chinensis (Trin.) Tzvel.) is an economically and ecologically important forage in the grass family. Self-incompatibility (SI) limits its seed production due to the low seed-setting rate after self-pollination. However, investigations into the molecular mechanisms of sheepgrass SI are lacking. Therefore, microscopic observation of pollen germination and pollen tube growth, as well as transcriptomic analyses of pistils after self- and cross-pollination, were performed. The results indicated that pollen tube growth was rapidly inhibited from 10 to 30 min after self-pollination and subsequently stopped but preceded normally after cross-pollination. Time course comparative transcriptomics revealed different transcriptome dynamics between self- and cross-pollination. A pool of SI-related signaling genes and pathways was generated, including genes related to calcium (Ca2+) signaling, protein phosphorylation, plant hormone, reactive oxygen species (ROS), nitric oxide (NO), cytoskeleton, and programmed cell death (PCD). A putative SI response molecular model in sheepgrass was presented. The model shows that SI may trigger a comprehensive calcium- and phytohormone-dominated signaling cascade and activate PCD, which may explain the rapid inhibition of self-pollen tube growth as observed by cytological analyses. These results provided new insight into the molecular mechanisms of sheepgrass (grass family) SI.

The evolutionary history of plant T2/S-type ribonucleases

A growing number of T2/S-RNases are being discovered in plant genomes. Members of this protein family have a variety of known functions, but the vast majority are still uncharacterized. We present data and analyses of phylogenetic relationships among T2/S-RNases, and pay special attention to the group that contains the female component of the most widespread system of self-incompatibility in flowering plants. The returned emphasis on the initially identified component of this mechanism yields important conjectures about its evolutionary context. First, we find that the clade involved in self-rejection (class III) is found exclusively in core eudicots, while the remaining clades contain members from other vascular plants. Second, certain features, such as intron patterns, isoelectric point, and conserved amino acid regions, help differentiate S-RNases, which are necessary for expression of self-incompatibility, from other T2/S-RNase family members. Third, we devise and present a set of approaches to clarify new S-RNase candidates from existing genome assemblies. We use genomic features to identify putative functional and relictual S-loci in genomes of plants with unknown mechanisms of self-incompatibility. The widespread occurrence of possible relicts suggests that the loss of functional self-incompatibility may leave traces long after the fact, and that this manner of molecular fossil-like data could be an important source of information about the history and distribution of both RNase-based and other mechanisms of self-incompatibility. Finally, we release a public resource intended to aid the search for S-locus RNases, and help provide increasingly detailed information about their taxonomic distribution.

Impedance Flow Cytometry: A Novel Technique in Pollen Analysis

INTRODUCTION

An efficient and reliable method to estimate plant cell viability, especially of pollen, is important for plant breeding research and plant production processes. Pollen quality is determined by classical methods, like staining techniques or in vitro pollen germination, each having disadvantages with respect to reliability, analysis speed, and species dependency. Analysing single cells based on their dielectric properties by impedance flow cytometry (IFC) has developed into a common method for cellular characterisation in microbiology and medicine during the last decade. The aim of this study is to demonstrate the potential of IFC in plant cell analysis with the focus on pollen.

METHOD

Developing and mature pollen grains were analysed during their passage through a microfluidic chip to which radio frequencies of 0.5 to 12 MHz were applied. The acquired data provided information about the developmental stage, viability, and germination capacity. The biological relevance of the acquired IFC data was confirmed by classical staining methods, inactivation controls, as well as pollen germination assays.

RESULTS

Different stages of developing pollen, dead, viable and germinating pollen populations could be detected and quantified by IFC. Pollen viability analysis by classical FDA staining showed a high correlation with IFC data. In parallel, pollen with active germination potential could be discriminated from the dead and the viable but non-germinating population.

CONCLUSION

The presented data demonstrate that IFC is an efficient, label-free, reliable and non-destructive technique to analyse pollen quality in a species-independent manner.

The role of reactive oxygen species and nitric oxide in programmed cell death associated with self-incompatibility

Successful sexual reproduction often relies on the ability of plants to recognize self- or genetically-related pollen and prevent pollen tube growth soon after germination in order to avoid self-fertilization. Angiosperms have developed different reproductive barriers, one of the most extended being self-incompatibility (SI). With SI, pistils are able to reject self or genetically-related pollen thus promoting genetic variability. There are basically two distinct systems of SI: gametophytic (GSI) and sporophytic (SSI) based on their different molecular and genetic control mechanisms. In both types of SI, programmed cell death (PCD) has been found to play an important role in the rejection of self-incompatible pollen. Although reactive oxygen species (ROS) were initially recognized as toxic metabolic products, in recent years, a new role for ROS has become apparent: the control and regulation of biological processes such as growth, development, response to biotic and abiotic environmental stimuli, and PCD. Together with ROS, nitric oxide (NO) has become recognized as a key regulator of PCD. PCD is an important mechanism for the controlled elimination of targeted cells in both animals and plants. The major focus of this review is to discuss how ROS and NO control male-female cross-talk during fertilization in order to trigger PCD in self-incompatible pollen, providing a highly effective way to prevent self-fertilization.

Life history mediates mate limitation and population viability in self-incompatible plant species

Genetically controlled self-incompatibility systems represent links between genetic diversity and plant demography with the potential to directly impact on population dynamics. We use an individual-based spatial simulation to investigate the demographic and genetic consequences of different self-incompatibility systems for plants that vary in reproductive capacity and lifespan. The results support the idea that, in the absence of inbreeding effects, populations of self-incompatible species will often be smaller and less viable than self-compatible species, particularly for shorter-lived organisms or where potential fecundity is low. At high ovule production and low mortality, self-incompatible and self-compatible species are demographically similar, thus self-incompatibility does not automatically lead to reduced mate availability or population viability. Overall, sporophytic codominant self-incompatibility was more limiting than gametophytic or sporophytic dominant systems, which generally behaved in a similar fashion. Under a narrow range of conditions, the sporophytic dominant system maintained marginally greater mate availability owing to the production of S locus homozygotes. While self-incompatibility reduces population size and persistence for a broad range of conditions, the actual number of S alleles, beyond that required for reproduction, is important for only a subset of life histories. For these situations, results suggest that addition of new S alleles may result in significant demographic rescue.

The role of late-acting self-incompatibility and early-acting inbreeding depression in governing female fertility in monkshood, Aconitum kusnezoffii

Reduced seed yields following self-pollination have repeatedly been observed, but the underlying mechanisms remain elusive when self-pollen tubes can readily grow into ovaries, because pre-, post-zygotic late-acting self-incompatibility (LSI), or early-acting inbreeding depression (ID) can induce self-sterility. The main objective of this study was to differentiate these processes in Aconitum kusnezoffii, a plant lacking stigmatic or stylar inhibition of self-pollination. We performed a hand-pollination experiment in a natural population of A. kusnezoffii, compared seed set among five pollination treatments, and evaluated the distribution of seed size and seed set. Embryonic development suggested fertilization following self-pollination. A partial pre-zygotic LSI was suggested to account for the reduced seed set by two lines of evidence. The seed set of chase-pollination treatment significantly exceeded that of self-pollination treatment, and the proportion of unfertilized ovules was the highest following self-pollination. Meanwhile, early-acting ID, rather than post-zygotic LSI, was suggested by the findings that the size of aborted selfed seeds varied continuously and widely; and the selfed seed set both exhibited a continuous distribution and positively correlated with the crossed seed set. These results indicated that the embryos were aborted at different stages due to the expression of many deleterious alleles throughout the genome during seed maturation. No signature of post-zygotic LSI was found. Both partial pre-zygotic LSI and early-acting ID contribute to the reduction in selfed seed set in A. kusnezoffii, with pre-zygotic LSI rejecting part of the self-pollen and early-acting ID aborting part of the self-fertilized seeds.

Siring success and paternal effects in heterodichogamous Acer opalus

BACKGROUND AND AIMS

Heterodichogamy (a dimorphic breeding system comprising protandrous and protogynous individuals) is a potential starting point in the evolution of dioecy from hermaphroditism. In the genus Acer, previous work suggests that dioecy evolved from heterodichogamy through an initial spread of unisexual males. Here, the question is asked as to whether the different morphs in Acer opalus, a species in which males co-exist with heterodichogamous hermaphrodites, differ in various components of male in fitness.

METHODS

Several components of male fertility were analysed. Pollination rates in the male phase were recorded across one flowering period. Pollen viability was compared among morphs through hand pollinations both with pollen from a single sexual morph and also simulating a situation of pollen competition; in the latter experiment, paternity was assessed with microsatellite markers. It was also determined whether effects of genetic relatedness between pollen donors and recipients could influence the siring success. Finally, paternal effects occurring beyond the fertilization process were tested for by measuring the height reached by seedlings with different sires over three consecutive growing seasons.

KEY RESULTS

The males and protandrous morphs had higher pollination rates than the protogynous morph, and the seedlings they sired grew taller. No differences in male fertility were found between males and protandrous individuals. Departures from random mating due to effects of genetic relatedness among sires and pollen recipients were also ruled out.

CONCLUSIONS

Males and protandrous individuals are probably better sires than protogynous individuals, as shown by the higher pollination rates and the differential growth of the seedlings sired by these morphs. In contrast, the fertility of males was not higher than the male fertility of the protandrous morph. While the appearance of males in sexually specialized heterodichogamous populations is possible, even in the absence of a fitness advantage, it is not clear that males can be maintained at an evolutionary equilibrium with two classes of heterodichogamous hermaphrodites.

A tale of two continents: Baker's rule and the maintenance of self-incompatibility in Lycium (Solanaceae)

Over 50 years ago, Baker (1955, 1967) suggested that self-compatible species were more likely than self-incompatible species to establish new populations on oceanic islands. His logic was straightforward and rested on the assumption that colonization was infrequent; thus, mate limitation favored the establishment of self-fertilizing individuals. In support of Baker's rule, many authors have documented high frequencies of self-compatibility on islands, and recent work has solidified the generality of Baker's ideas. The genus Lycium (Solanaceae) has ca. 80 species distributed worldwide, and phylogenetic studies suggest that Lycium originated in South America and dispersed to the Old World a single time. Previous analyses of the S-RNase gene, which controls the stylar component of self-incompatibility, have shown that gametophytically controlled self-incompatibility is ancestral within the genus, making Lycium a good model for investigating Baker's assertions concerning reproductive assurance following oceanic dispersal. Lycium is also useful for investigations of reproductive evolution, given that species vary both in sexual expression and the presence of self-incompatibility. A model for the evolution of gender dimorphism suggests that polyploidy breaks down self-incompatibility, leading to the evolution of gender dimorphism, which arises as an alternative outcrossing mechanism. There is a perfect association of dimorphic gender expression, polyploidy, and self-compatibility (vs. cosexuality, diploidy, and self-incompatibility) among North American Lycium. Although the association between ploidy level and gender expression also holds for African Lycium, to date no studies of mating systems have been initiated in Old World species. Here, using controlled pollinations, we document strong self-incompatibility in two cosexual, diploid species of African Lycium. Further, we sequence the S-RNase gene in 15 individuals from five cosexual, diploid species of African Lycium and recover 24 putative alleles. Genealogical analyses indicate reduced trans-generic diversity of S-RNases in the Old World compared to the New World. We suggest that genetic diversity at this locus was reduced as a result of a founder event, but, despite the bottleneck, self-incompatibility was maintained in the Old World. Maximum-likelihood analyses of codon substitution patterns indicate that positive Darwinian selection has been relatively strong in the Old World, suggesting the rediversification of S-RNases following a bottleneck. The present data thus provide a dramatic exception to Baker's rule, in addition to supporting a key assumption of the Miller and Venable (2000) model, namely that self-incompatibility is associated with diploidy and cosexuality.

Expression of 10 S-class SLF-like genes in Nicotiana alata pollen and its implications for understanding the pollen factor of the S locus

The S locus of Nicotiana alata encodes a polymorphic series of ribonucleases (S-RNases) that determine the self-incompatibility (SI) phenotype of the style. The pollen product of the S locus (pollen S) in N. alata is unknown, but in species from the related genus Petunia and in self-incompatible members of the Plantaginaceae and Rosaceae, this function has been assigned to an F-box protein known as SLF or SFB. Here we describe the identification of 10 genes (designated DD1-10) encoding SLF-related proteins that are expressed in N. alata pollen. Because our approach to cloning the DD genes was based on sequences of SLFs from other species, we presume that one of the DD genes encodes the N. alata SLF ortholog. Seven of the DD genes were exclusively expressed in pollen and a low level of sequence variation was found in alleles of each DD gene. Mapping studies confirmed that all 10 DD genes were linked to the S locus and that at least three were located in the same chromosomal segment as pollen S. Finally, the different topologies of the phylogenetic trees produced using available SLF-related sequences and those produced using S-RNase sequences suggests that pollen S and the S-RNase have different evolutionary histories.

Influence of genetic dissimilarity in the reproductive success and mate choice of brown trout - females fishing for optimal MHC dissimilarity

We examined the reproductive success of 48 adult brown trout (Salmo trutta L.) which were allowed to reproduce in a stream that was controlled for the absence of other trout. Parentage analyses based on 11 microsatellites permitted us to infer reproductive success and mate choice preferences in situ. We found that pairs with intermediate major histocompatibility complex (MHC) dissimilarity mated more often than expected by chance. It appears that female choice was the driving force behind this observation because, compared with other individuals, males with intermediate MHC dissimilarity produced a larger proportion of offspring, whereas female reproductive output did not show this pattern. Hence, rather than seeking mates with maximal MHC dissimilarity, as found in several species, brown trout seemed to prefer mates of intermediate MHC difference, thus supporting an optimality-based model for MHC-dependent mate choice.

Distribution of flower morphs, ploidy level and sexual reproduction of the invasive weed Oxalis pes-caprae in the western area of the Mediterranean region

BACKGROUND AND AIMS

Oxalis pes-caprae is a widespread invasive weed in regions with a Mediterranean climate. In its native habitat (southern Africa) this species has been reported as heterostylous with trimorphic flowers and a self- and morph-incompatible reproductive system. In most of the areas invaded, only a pentaploid short-styled morphotype that reproduces mainly asexually by bulbils is reported, but this has only been confirmed empirically. This study aims to analyse the floral morph proportions in a wide distribution area, test the sexual female success, and explain the causes of low sexual reproduction of this species in the western area of the Mediterranean Basin.

METHODS

Fifty-five populations of O. pes-caprae were sampled in the Iberian Peninsula and Morocco to evaluate the floral morph ratio and individual fruit set. In plants from a dimorphic population, hand-pollination experiments were performed to evaluate the effect of the pollen source on pollen tube growth through the style. The ploidy level and genome size of individuals of each floral morph were analysed using flow cytometry.

KEY RESULTS

From the populations studied 89.1 % were monomorphic, with most of them containing the short-styled (SS) floral morph, and 10.9 % were dimorphic containing long-styled (LS) and SS morphs. In some of these, isoplethy was verified but no fruit production was observed in any population. A sterile form was also recorded in several populations. Hand-pollination experiments revealed that pollen grains germinated over recipient stigmas. In intermorph crossings, pollen tubes were able to develop and fruit initiation was observed in some cases, while in intramorph pollinations, pollen tube development was sporadic and no fruit initiation was observed. All individuals within each floral form presented the same DNA ploidy level: SS plants were pentaploid and LS and the sterile form were tetraploid.

CONCLUSIONS

The low or null sexual reproduction success of this species in the area of invasion studied seems related with the high frequency of monomorphic populations, the unequal proportion of floral morphs in dimorphic populations and the presence of different ploidy levels between SS and LS morphs. The discovery of the occurrence of an LS floral morph and a sterile form, whose invading capacity in these areas is as yet unknown, will be valuable information for management programmes.

The molecular genetic basis of plant adaptation

How natural selection on adaptive traits is filtered to the genetic level remains largely unknown. Theory and quantitative trait locus (QTL) mapping have provided insights into the number and effect of genes underlying adaptations, but these results have been hampered by questions of applicability to real biological systems and poor resolution, respectively. Advances in molecular technologies have expedited the cloning of adaptive genes through both forward and reverse genetic approaches. Forward approaches start with adaptive traits and attempt to characterize their underlying genetic architectures through linkage disequilibrium mapping, QTL mapping, and other methods. Reverse screens search large sequence data sets for genes that possess the signature of selection. Though both approaches have been successful in identifying adaptive genes in plants, very few, if any, of these adaptations' molecular bases have been fully resolved. The continued isolation of plant adaptive genes will lead to a more comprehensive understanding of natural selection's effect on genes and genomes.

Positively selected sites in the Arabidopsis receptor-like kinase gene family

We analyze members of the receptor-like kinase (RLK) gene family in Arabidopsis thaliana for positive selection. Likelihood analyses find evidence for positive selection in 12 of the 52 RLK family sequences groups. These 12 groups represent 97 of the 403 sequences analyzed. The majority of genes in groups subject to positive selection have not been functionally characterized, but sites under selection are predominantly located in the extracellular region. The pattern of selection in the extracellular leucine-rich repeat (LRR) motif of groups 14 and 51 is similar to previous studies where positively selected positions are located in a solvent exposed beta-strand that may determine disease specificity, raising the possibility that some RLK genes function in a similar role.

A new group of plant-specific ATP-dependent DNA ligases identified by protein phylogeny, hydrophobic cluster analysis and 3-dimensional modelling

The eukaryotic ATP-dependent DNA ligases comprise a group of orthologous proteins that have distinct roles in DNA metabolism. In contrast with the well-known DNA ligases of animal cells, the DNA ligases of plant cells are poorly described. Until now, only two DNA ligases (I and IV) genes of Arabidopsis thaliana (L.) Heynh were isolated and characterised. Use of the complete genomic sequences of Oryza sativa L. and A. thaliana, as well as the partially assembled genomic data of Medicago truncatula L. and Brassica spp., allowed us to identify a new family of ATP-dependent DNA ligases that are found only in the Viridiplantae kingdom. An in-depth phylogenetic analysis of protein sequences showed that this family composes a distinct clade, which shares a last universal common ancestor with DNA ligases I. In silico sequence studies indicate that these proteins have distinct physico-chemical properties when compared with those of animal and fungal DNA ligases. Moreover, hydrophobic cluster analysis and 3-dimensional modelling allowed us to map two conserved domains within these DNA ligases I-like proteins. Additional data of microsynteny analysis indicate that these DNA ligases I-like genes are linked to the S and SLL2 loci of Brassica spp. and A. thaliana, respectively. Combining the results of all analyses, we propose the creation of the DNA ligases VI (LIG6) family, which is composed by plant-specific DNA ligases.

The eukaryotic Pso2/Snm1/Artemis proteins and their function as genomic and cellular caretakers

DNA double-strand breaks (DSBs) represent a major threat to the genomic stability of eukaryotic cells. DNA repair mechanisms such as non-homologous end joining (NHEJ) are responsible for the maintenance of eukaryotic genomes. Dysfunction of one or more of the many protein complexes that function in NHEJ can lead to sensitivity to DNA damaging agents, apoptosis, genomic instability, and severe combined immunodeficiency. One protein, Pso2p, was shown to participate in the repair of DSBs induced by DNA inter-strand cross-linking (ICL) agents such as cisplatin, nitrogen mustard or photo-activated bi-functional psoralens. The molecular function of Pso2p in DNA repair is unknown, but yeast and mammalian cell line mutants for PSO2 show the same cellular responses as strains with defects in NHEJ, e.g., sensitivity to ICLs and apoptosis. The Pso2p human homologue Artemis participates in V(D)J recombination. Mutations in Artemis induce a variety of immunological deficiencies, a predisposition to lymphomas, and an increase in chromosomal aberrations. In order to better understand the role of Pso2p in the repair of DSBs generated as repair intermediates of ICLs, an in silico approach was used to characterize the catalytic domain of Pso2p, which led to identification of novel Pso2p homologues in other organisms. Moreover, we found the catalytic core of Pso2p fused to different domains. In plants, a specific ATP-dependent DNA ligase I contains the catalytic core of Pso2p, constituting a new DNA ligase family, which was named LIG6. The possible functions of Pso2p/Artemis/Lig6p in NHEJ and V(D)J recombination and in other cellular metabolic reactions are discussed.

Pollen and sperm heteromorphism: convergence across kingdoms?

Sperm competition theory predicts that males should produce many, similar sperm. However, in some species of animals and plants, males exhibit a heteromorphism that results in the production of at least two different types of sperm or pollen grains. In animals, sperm heteromorphism typically corresponds to the production of one fertile morph and one (or more) sterile morph(s), whereas in plants two or more pollen morphs (one of which can be either sterile or fertile) are produced in all flowers but sometimes in different anthers. Heteromorphism has arisen independently several times across phyla and at different phylogenetic levels. Here, we compare and contrast sperm and pollen heteromorphism and discuss the evolutionary hypotheses suggested to explain heteromorphism in these taxa. These hypotheses include facilitation, nutritive contribution, blocking, cheap filler, sperm flushing or killing for animals; outcrossing and precise cross-pollen transfer or bet-hedging strategy for plants; cryptic female choice for both. We conclude that heteromorphism in the two phyla is most likely linked to a general evolutionary response to sexual selection, either to increase one male's sperm or pollen success in competition with other males, or mediate male/female interactions. Therefore, although sperm and pollen are not homologous, we suggest that heteromorphism represents an example of convergence across kingdoms.

The self-incompatibility response in Papaver rhoeas pollen causes early and striking alterations to organelles

Self-incompatibility (SI) in Papaver rhoeas is accompanied by a cascade of signalling events that result in the rapid arrest and eventual death of the pollen tube. We have used rapid freeze fixation, freeze substitution and transmission electron microscopy to provide the first description of changes to pollen at the ultrastructural level during SI in this species. Our studies reveal that dramatic alterations to the morphology of mitochondria, Golgi bodies and ER occur within 1 h of SI induction. Similar symptoms have also been observed during programmed cell death (PCD) in some cell types. These include: the conspicuous condensation of the vegetative and generative nuclei, the swelling and loss of cristae in mitochondria and the disappearance of Golgi bodies. Some of the early alterations to the mitochondria and Golgi bodies observed at 1 h, almost certainly occur when cells are still alive. Other events, such as nuclear condensation, occur later and coincide with DNA fragmentation and the loss of cell viability. Our observations suggest that the SI response in P. rhoeas pollen may potentially involve a type of PCD.

The pollination of Trimenia moorei (Trimeniaceae): floral volatiles, insect/wind pollen vectors and stigmatic self-incompatibility in a basal angiosperm

Trimenia moorei (Oliv.) Philipson is an andromonoecious liane with >0.40 of the total flower buds maturing as bisexual flowers. Male and bisexual flowers are strongly scented with pollen, anther sacs and receptacle scars testing positively for volatile emissions. Scent analyses detect over 20 components. The major fatty acid derivative is 8-heptadecene, and 2-phenylethanol dominates the benzenoids. While hover-flies in the genera Melangyna and Triglyphus contact the stigma with their probosces, the stigma secretes no free-flowing, edible fluids. Copious pollen is the only edible reward consumed by hover-flies (Syprhidae), sawflies (Pergidae) and bees in the families Apidae, Colletidae and Halictidae. All these insects carried pollen of T. moorei on their heads, legs and thoraces and female bees in the genera Apis, Exoneura, Leioproctus and Lasioglossum stored pollen on their hind legs. Pollen traps also indicate that pollen is shed directly into the air, permitting wind pollination. When bisexual flower buds are bagged (isolated from insect foragers) on the liane then subjected to a series of hand-pollination experiments after perianth segments open, the structural analyses of pollen-carpel interactions indicate that T. moorei has a trichome-rich dry-type stigma with an early-acting self-incompatibility (SI) system. Bicellular pollen grains deposited on stigmas belonging to the same plant germinate but fail to penetrate intercellular spaces, while grains deposited following cross-pollination reach the ovule within 24 h. Fluorescence analyses of 76 carpels collected at random from unbagged (open-pollinated) flowers on five plants indicates that at least 64% of carpels are cross-pollinated in situ. Trimenia moorei is the first species within the ANITA group, and second within reilictual-basal angiosperm lineages, to exhibit stigmatic SI in combination with dry-type stigma and bicellular pollen, a condition once considered to be atypical for angiosperms as a whole but now known to be present in numerous taxa.

Evidence for ovarian self-incompatibility as a cause of self-sterility in the relictual woody angiosperm, Pseudowintera axillaris (Winteraceae)

Species within the genus Pseudowintera exhibit high rates of self-sterility. Self-sterility in the genus has been previously posited-but not confirmed-to be the result of late-acting ovarian self-incompatibility (OSI) functioning within nucellar tissue of the ovule to prevent self pollen tubes from entering the embryo sac. Structural and functional aspects of pollen-carpel interactions and early seed development following cross- and self-pollination were investigated in P. axillaris to determine the site, timing and possible mechanisms of self-sterility. No significant differences were observed between pollen tube growth, ovule penetration and double fertilization following cross- and self-pollination. Pollen tubes exhibited phasic growth in an extracellular matrix composed of proteins and carbohydrates, as well as arabinogalactans/arabinogalactan proteins. A uniform failure in embryo sac development prior to division of the zygote was apparent within 15 d following double fertilization by self gametes. Results indicate that SI mechanisms in P. axillaris do not prevent double fertilization from occurring. Instead, mechanisms of self-sterility affect post-zygotic development of the embryo sac. Although self-sterility may be attributed to inbreeding depression, given the post-zygotic nature of failure in embryo sac development, the possibility of late-acting OSI is discussed.

The transition to gender dimorphism on an evolutionary background of self-incompatibility: an example from Lycium (Solanaceae)

Populations of three North American species of Lycium (Solanaceae) are morphologically gynodioecious and consist of male-sterile (i.e., female) and hermaphroditic plants. Marked individuals were consistent in sexual expression across years and male sterility was present throughout much of the species' ranges. Controlled pollinations reveal that L. californicum, L. exsertum, and L. fremontii are functionally dioecious. Fruit production in females ranged from 36 to 63%, whereas hermaphrodites functioned essentially as males. Though hermaphrodites were mostly male, investigation of pollen tube growth reveals that hermaphrodites of all dimorphic species were self-compatible. Self-fertilization and consequent inbreeding depression are commonly invoked as important selective forces promoting the invasion of male-sterile mutants into cosexual populations. A corollary prediction of these models is that gender dimorphism evolves from self-compatible ancestors. However, fruit production, seed production, and pollen tube number following outcross pollination were significantly higher than following self-pollination for three diploid, cosexual species that are closely related to the dimorphic species. The data presented here on incompatibility systems are consistent with the hypothesis that polyploidy disrupted the self-incompatibility system in the gynodioecious species leading to the evolution of gender dimorphism.

Evolutionary relationships among self-incompatibility RNases

T2-type RNases are responsible for self-pollen recognition and rejection in three distantly related families of flowering plants-the Solanaceae, Scrophulariaceae, and Rosaceae. We used phylogenetic analyses of 67 T2-type RNases together with information on intron number and position to determine whether the use of RNases for self-incompatibility in these families is homologous or convergent. All methods of phylogenetic reconstruction as well as patterns of variation in intron structure find that all self-incompatibility RNases along with non-S genes from only two taxa form a monophyletic clade. Several lines of evidence suggest that the best interpretation of this pattern is homology of self-incompatibility RNases from the Scrophulariaceae, Solanaceae, and Rosaceae. Because the most recent common ancestor of these three families is the ancestor of approximately 75% of dicot families, our results indicate that RNase-based self-incompatibility was the ancestral state in the majority of dicots.

Inheritance of tristyly in Oxalis tuberosa (Oxalidaceae)

Frequencies of floral morphs in progenies obtained from a complete set of diallelic crosses among three accessions of tristylous, octoploid oca (Oxalis tuberosa) were used for a Mendelian analysis of floral morph inheritance. The frequencies observed had the best fit to a model of tetrasomic inheritance with two diallelic factors, S, s and M, m, with S being epistatic over M. No explanation could be found for the unexpected formation of a small percentage of short-styled individuals in crosses between the mid-styled and the long-styled parent. For the acceptance of models of disomic and octosomic inheritance several additional assumptions would have to be made and therefore these modes of inheritance are less likely. Dosage-dependent inheritance of floral morph was rejected. Only a small frequency (36%) of the cross progenies flowered, in contrast to the greater propensity for flowering of O. tuberosa accessions held at gene banks.

Low neighbourhood size and high interpopulation differentiation in the endangered shrub Grevillea iaspicula McGill (Proteaceae)

Mating system parameters and genetic diversity were examined for five populations of the endangered shrub Grevillea iaspicula (Proteaceae). Controlled pollinations show that G. iaspicula has an effective self-incompatibility system and little potential for agamospermy. This is reflected in uniformly high multilocus outcrossing rates (tm=0.96-1.00). However, average paternal diversity within open-pollinated sibships is low (rp=0.31-0.54), suggesting that mating within populations is quite restricted. Despite the small size of most populations (four of the five populations studied have fewer than 20 reproductive individuals) the species still possesses moderate to high allelic richness (A=1.6-2.5). Interpopulation genetic differentiation is high (D=0.04-0.32), suggesting that gene flow is limited, even among populations separated by only a few kilometres.

Programmed cell death in plant reproduction

Reproductive development is a rich arena to showcase programmed cell death in plants. After floral induction, the first act of reproductive development in some plants is the selective killing of cells destined to differentiate into an unwanted sexual organ. Production of functional pollen grains relies significantly on deterioration and death of the anther tapetum, a tissue whose main function appears to nurture and decorate the pollen grains with critical surface molecules. Degeneration and death in a number of anther tissues result ultimately in anther rupture and dispersal of pollen grains. Female sporogenesis frequently begins with the death of all but one of the meiotic derivatives, with surrounding nucellar cells degenerating in concert with embryo sac expansion. Female tissues that interact with pollen undergo dramatic degeneration, including death, to ensure the encounter of compatible male and female gametes. Pollen and pistil interact to kill invading pollen from an incompatible source. Most observations on cell death in reproductive tissues have been on the histological and cytological levels. We discuss various cell death phenomena in reproductive development with a view towards understanding the biochemical and molecular mechanisms that underlie these processes.

Mate selection and the evolution of highly polymorphic self/nonself recognition genes

Multicellular organisms use the products of highly polymorphic genes to distinguish self from conspecific nonself cells or tissues. These allorecognition polymorphisms may regulate somatic interactions between hosts and pathogens or between competitors (to avoid various forms of parasitism), as well as reproductive interactions between mates or between gametes (to avoid inbreeding). In both cases, rare alleles may be advantageous, but it remains unclear which mechanism maintains the genetic polymorphism for specificity in self/nonself recognition. Contrary to earlier reports, we show that mate selection cannot be a strong force maintaining allorecognition polymorphism in two colonial marine invertebrates. Instead, the regulation of intraspecific competitive interactions appears to promote the evolution of polymorphisms in these species.

Genetic compatibility, mate choice and patterns of parentage: invited review

There is growing interest in the possibility that genetic compatibility may drive mate choice, including gamete choice, particularly from the perspective of understanding why females frequently mate with more than one male. Mate choice for compatibility differs from other forms of choice for genetic benefits (such as 'good genes') because individuals are expected to differ in their mate preferences, changing the evolutionary dynamics of sexual selection. Recent experiments designed to investigate genetic benefits of polyandry suggest that mate choice on the basis of genetic compatibility may be widespread. However, in most systems the mechanisms responsible for variation in compatibility are unknown. We review potential sources of variation in genetic compatibility and whether there is any evidence for mate choice driven by these factors. Selfish genetic elements appear to have the potential to drive mate compatibility mate choice, though as yet there is only one convincing example. There is abundant evidence for assortative mating between populations in hybrid zones, but very few examples where this is clearly a result of selection against mating with genetically less compatible individuals. There are also numerous cases of inbreeding avoidance, but little evidence that mate choice or differential fertilization success driven by genetic compatibility occurs between unrelated individuals. The exceptions to this are a handful of situations where both the alleles causing incompatibility and the alleles involved in mate choice are located in a chromosome region where recombination is suppressed. As yet there are only a few potential sources of genetic compatibility which have clearly been shown to drive mate choice. This may reflect limitations in the potential for the evolution of mate choice for genetic compatibility within populations, although the most promising sources of such incompatibilities have received relatively little research.

A breakdown of Brassica self-incompatibility in ARC1 antisense transgenic plants

Self-incompatibility, the rejection of self pollen, is the most widespread mechanism by which flowering plants prevent inbreeding. In Brassica, the S receptor kinase (SRK) has been implicated in the self-incompatibility response, but the molecular mechanisms involving SRK are unknown. One putative downstream effector for SRK is ARC1, a protein that binds to the SRK kinase domain. Here it is shown that suppression of ARC1 messenger RNA levels in the self-incompatible Brassica napus W1 line is correlated with a partial breakdown of self-incompatibility, resulting in seed production. This provides strong evidence that ARC1 is a positive effector of the Brassica self-incompatibility response.

Genetic Isolation among Host Races of the Anther Smut Fungus Microbotryum violaceum on Three Host Plant Species

Genetic isolation among strains of the plant pathogenic fungus Microbotryum violaceum on three species of its host plants was examined. Fungal strains collected from a sympatric population of all three host plant species were examined for their ability to infect the other host plant species and to cross-conjugate among each other. Genetic isolation was investigated from the distribution of neutral microsatellite alleles. Since this is a pollinator-transmitted disease, we examined movement patterns and flower-visitation behavior of pollinators to investigate whether they transfer fungal spores between different host species. Low infection success from the cross-inoculation experiment limits interpretability of the results, but fungus collected from Silene vulgaris was capable of infecting Dianthus carthusianorum. Different fungal strains were able to conjugate and form the infectious dikaryon in most combinations, so hybridization between different fungal host races is possible. The distribution of neutral genetic variation, however, revealed little successful genetic exchange among the fungal host races that were clearly differentiated by host plant species. Pollinators, while showing partial constancy, moved between plants of different host species. Pollinator behavior is therefore not adequate to explain the lack of gene flow among the different fungal races. This indicates that the divergence among these fungal races that has produced incipient species sharing almost no alleles may have occurred in allopatry, unless disruptive selection can outweigh gene flow among fungal races in sympatry.

Fertilization and wounding of the style induce the expression of a highly conserved plant gene homologous to a Plasmodium falciparum surface antigen in the wild potato Solanum chacoense Bitt

Pistil tissues are actively involved in pollen tube growth and respond to the presence of the growing pollen tubes by modulating the expression of specific genes. Once fertilization has occurred, complex developmental programs lead to embryogenesis, ovary maturation, and seed set. In order to understand the early events that follow pollination and fertilization we have used a subtractive hybridization approach to characterize genes which are related to pollination and fertilization events. One cDNA clone isolated and named SPP30 (Solanum pollinated pistil) was found to share significant sequence identities with a Plasmodium falciparum (malaria parasite) surface antigen and a yeast gene of unknown function. Searches in recent EST databases also revealed that SPP30 homologues are found in both monocot and dicot species. The presence of this conserved gene in evolutionarily distant organisms such as yeast, Plasmodium, and plants suggests that it codes for an essential cellular function. This is also strengthened by its extremely high sequence conservation in both monocots and dicots where virtually all substitutions tolerated are conservative.

Differential ovule development following self- and cross-pollination: the basis of self-sterility in Narcissus triandrus (Amaryllidaceae)

Self-pollination results in significantly lower seed set than cross-pollination in tristylous Narcissus triandrus. We investigated structural and functional aspects of pollen-pistil interactions and ovule-seed development following cross- and self-pollination to assess the timing and mechanism of self-sterility. Ovule development within an ovary was asynchronous at anthesis. There were no significant differences in pollen tube behavior following cross- vs. self-pollination during the first 6 d of growth, regardless of style morph type. Double fertilization was significantly higher following cross- vs. self-pollination. Aborted embryo development was not detected following either pollination type up to seed maturity. Prior to pollen tube entry, a significantly greater number of ovules ceased to develop following self- vs. cross-pollination. These results indicate that self-sterility in N. triandrus operates prezygotically but does not involve differential pollen tube growth typical of many self-incompatibility (SI) systems. Instead, low seed set following self-pollination is caused by a reduction in ovule availability resulting from embryo sac degeneration. We hypothesize that this is due to the absence of a required stimulus for normal ovule development. If this is correct, current concepts of SI may need to be broadened to include a wider range of pollen-pistil interactions.

Cloning and characterization of a gibberellin-induced RNase expressed in barley aleurone cells

We cloned a cDNA for a gibberellin-induced ribonuclease (RNase) expressed in barley (Hordeum vulgare) aleurone and the gene for a second barley RNase expressed in leaf tissue. The protein encoded by the cDNA is unique among RNases described to date in that it contains a novel 23-amino acid insert between the C2 and C3 conserved sequences. Expression of the recombinant protein in tobacco (Nicotiana tabacum) suspension-cultured protoplasts gave an active RNase of the expected size, confirming the enzymatic activity of the protein. Analyses of hormone regulation of expression of mRNA for the aleurone RNase revealed that, like the pattern for alpha-amylase, mRNA levels increased in the presence of gibberellic acid, and its antagonist abscisic acid prevented this effect. Quantitative studies at early times demonstrated that cycloheximide treatment of aleurone layers increased mRNA levels 4-fold, whereas a combination of gibberellin plus cycloheximide treatment was required to increase alpha-amylase mRNA levels to the same extent. These results are consistent with loss of repression as an initial effect of gibberellic acid on transcription of those genes, although the regulatory pathways for the two genes may differ.

A relic S-RNase is expressed in the styles of self-compatible Nicotiana sylvestris

We surveyed ribonuclease activity in the styles of Nicotiana spp. and found little or no activity in self-compatible species and in a self-compatible accession of a self-incompatible species. All self-incompatible species had high levels of ribonuclease activity in their style. Interestingly, one self-compatible species, N. sylvestris, had a level of stylar ribonuclease activity comparable to that of some self-incompatible Nicotiana species. A ribonuclease with biochemical properties similar to those of the self-incompatibility (S-)RNases of N. alata was purified from N. sylvestris styles. The N-terminal sequence of this protein was used to confirm the identity of a cDNA corresponding to the stylar RNase. The amino acid sequence deduced from the cDNA was related to those of the S-RNases and included the five conserved regions characteristic of these proteins. It appears that the N. sylvestris RNase may have evolved from the S-RNases and is an example of a 'relic S-RNase'. A number of features distinguish the N. sylvestris RNase from the S-RNases, and the role these may have played in the presumed loss of the self-incompatibility response during the evolution of this species are discussed.

Characterization of the flanking regions of the S-RNase genes of Japanese pear (Pyrus serotina) and apple (Malus x domestica)

Genomic sequences of the self-incompatibility genes, the S-RNase genes, from two rosaceous species, Japanese pear and apple, were characterized. Genomic Southern blot and sequencing of a 4.5-kb genomic clone showed that the S4-RNase gene of Japanese pear is surrounded by repetitive sequences as in the case of the S-RNase genes of solanaceous species. The flanking regions of the S2- and Sf-RNase genes of apple were also cloned and sequenced. The 5' flanking regions of the three alleles bore no similarity with those of the solanaceous S-RNase genes, although the position and sequence of the putative TATA box were conserved. The putative promoter regions of the Japanese pear S4- and apple Sf-RNase genes shared a stretch of about 200bp with 80% sequence identity. However, this sequence was not present in the S2-RNase gene of apple, and thus it may reflect a close relationship between the S4- and Sf-RNase genes rather than a cis-element important in regulating gene expression. Despite the uniform pattern of expression of the rosaceous S-RNase genes, sequence motifs conserved in the 5' flanking regions of the three alleles were not found, implying that the cis-element controlling pistil specific gene expression also locates at the intragenic region or upstream of the analyzed promoter region.

Binding of an arm repeat protein to the kinase domain of the S-locus receptor kinase

Screening of a yeast two-hybrid library for proteins that interact with the kinase domain of an S-locus receptor kinase (SRK) resulted in the isolation of a plant protein called ARC1 (Arm Repeat Containing). This interaction was mediated by the C-terminal region of ARC1 in which five arm repeat units were identified. Using the yeast two-hybrid system and in vitro binding assays, ARC1 was found to interact specifically with the kinase domains from SRK-910 and SRK-A14 but failed to interact with kinase domains from two different Arabidopsis receptor-like kinases. In addition, treatment with a protein phosphatase or the use of a kinase-inactive mutant reduced or abolished the binding of ARC1 to the SRK-910 kinase domain, indicating that the interaction was phosphorylation dependent. Lastly, RNA blot analysis revealed that the expression of ARC1 is restricted to the stigma, the site of the self-incompatibility response.

Identification of active-site histidine residues of a self-incompatibility ribonuclease from a wild tomato

The style component of the self-incompatibility (S) locus of the wild tomato Lycopersicon peruvianum (L.) Mill. is an allelic series of glycoproteins with ribonuclease activity (S-RNases). Treatment of the S3-RNase from L. peruvianum with iodoacetate at pH 6.1 led to a loss of RNase activity. In the presence of a competitive inhibitor, guanosine 3'-monophosphate (3'-GMP), the rate of RNase inactivation by iodoacetate was reduced significantly. Analysis of the tryptic digestion products of the iodoacetate-modified S-RNase by reversed-phase high-performance liquid chromatography and electrospray-ionization mass spectrometry showed that histidine-32 was preferentially modified in the absence of 3'-GMP. Histidine-88 was also modified, but this occurred both in the presence and absence of 3'-GMP, suggesting that this residue is accessible when 3'-GMP is in the active site. Cysteine-150 was modified by iodoacetate in the absence of 3'-GMP and, to a lesser extent, in its presence. The results are discussed with respect to the related fungal RNase T2 family and the mechanism of S-RNase action.

Hypervariable Domains of Self-Incompatibility RNases Mediate Allele-Specific Pollen Recognition

Self-incompatibility (SI) in angiosperms is a genetic mechanism that promotes outcrossing through rejection of self-pollen. In the Solanaceae, SI is determined by a multiallelic S locus whose only known product is an S RNase. S RNases show a characteristic pattern of five conserved and two hypervariable regions. These are thought to be involved in the catalytic function and in allelic specificity, respectively. When the Solanum chacoense S12S14 genotype is transformed with an S11 RNase, the styles of plants expressing significant levels of the transgene reject S11 pollen. A previously characterized S RNase, S13, differs from the S11 RNase by only 10 amino acids, four of which are located in the hypervariable regions. When S12S14 plants were transformed with a chimeric S11 gene in which these four residues were substituted with those present in the S13 RNase, the transgenic plants acquired the S13 phenotype. This result demonstrates that the S RNase hypervariable regions control allelic specificity.