Homomorphic Self-incompatibility Research Articles

Insights into sexual reproduction and morph compatibility of the distylous Nymphoides fallax Ornduff (Menyanthaceae)

Nymphoides fallax Ornduff (Menyanthaceae) is an aquatic distylous plant endemic to the Mexican and Guatemalan highlands. Although vegetative reproduction is frequently reported for Nymphoides species, evidence suggests a high prevalence of sexual reproduction in N. fallax. This study aimed: (i) to test homomorphic self-incompatibility, (ii) to describe aspects of floral phenology, estimate the pollen/ovule ratio and the viability of pollen grains, (iii) to evaluate floral morph proportion in natural populations, and (iv) to identify the assemblage of floral visitors, estimating their frequency and capacity to transport N. fallax pollen. The studied populations were located in Central Mexico. We carried out a controlled pollination experiment with eight treatments (n = 80), and we also estimated pollen/ovule ratio and observed floral phenology aspects. We counted the proportion of floral morphs in seven natural populations and observed the presence of visitors to 891 flowers in three natural populations. We observed pollen on the insects using scanning electron microscopy. Our findings showed that N. fallax has diurnal flowers with one-day anthesis, high stigma receptivity, high pollen viability, and intramorph self-incompatibility. The average pollen/ovule ratio (5.1 × 103) was consistent with xenogamy. Five natural populations were found to have both morphs in approximately equal proportions, and two populations consisted only of the long-style morph. An assemblage of 28 floral visitor taxa was observed in 944 visits. The most frequent visitors were exotic bees (Apis mellifera: 43.5 %) and native dipterans (Ephydridae family: 33.3 %; Stratiomyidae and Syrphidae: 7.2 %). Pollen grains were observed attached to the bodies of the most frequent visitors, suggesting that they are potentially effective pollinators. This research expands the knowledge of heterostylous syndrome and sexual reproduction in N. fallax.

Female self-incompatibility type in heterostylous Primula is determined by the brassinosteroid-inactivating cytochrome P450 CYP734A50

Most flowering plants are hermaphrodites, with flowers having both male and female reproductive organs. One widespread adaptation to limit self-fertilization is self-incompatibility (SI), where self-pollen fails to fertilize ovules.1,2 In homomorphic SI, many morphologically indistinguishable mating types are found, although in heteromorphic SI, the two or three mating types are associated with different floral morphologies.3-6 In heterostylous Primula, a hemizygous supergene determines a short-styled S-morph and a long-styled L-morph, corresponding to two different mating types, and full seed set only results from intermorph crosses.7-9 Style length is controlled by the brassinosteroid (BR)-inactivating cytochrome P450 CYP734A50,10 yet it remains unclear what defines the male and female incompatibility types. Here, we show that CYP734A50 also determines the female incompatibility type. Inactivating CYP734A50 converts short S-morph styles into long styles with the same incompatibility behavior as L-morph styles, and this effect can be mimicked by exogenous BR treatment. Invitro responses of S- and L-morph pollen grains and pollen tubes to increasing BR levels could only partly explain their different invivo behavior, suggesting both direct and indirect effects of the different BR levels in S- versus L-morph stigmas and styles in controlling pollen performance. This BR-mediated SI provides a novel mechanism for preventing self-fertilization. The joint control of morphology and SI by CYP734A50 has important implications for the evolutionary buildup of the heterostylous syndrome and provides a straightforward explanation for why essentially all of the derived self-compatible homostylous Primula species are long homostyles.11.

Genetic mapping of sex and self-incompatibility determinants in the androdioecious plant Phillyrea angustifolia

The diversity of mating and sexual systems in angiosperms is spectacular, but the factors driving their evolution remain poorly understood. In plants of the Oleaceae family, an unusual self-incompatibility (SI) system has been discovered recently, whereby only two distinct homomorphic SI specificities segregate stably. To understand the role of this peculiar SI system in preventing or promoting the diversity of sexual phenotypes observed across the family, an essential first step is to characterize the genetic architecture of these two traits. Here, we developed a high-density genetic map of the androdioecious shrub P. angustifolia based on a F1 cross between a hermaphrodite and a male parent with distinct SI genotypes. Using a double restriction-site associated digestion (ddRAD) sequencing approach, we obtained reliable genotypes for 196 offspring and their two parents at 10,388 markers. The resulting map comprises 23 linkage groups totaling 1,855.13 cM on the sex-averaged map. We found strong signals of association for the sex and SI phenotypes, that were each associated with a unique set of markers on linkage group 12 and 18 respectively, demonstrating inheritance of these traits as single, independent, mendelian factors. The P. angustifolia linkage map shows robust synteny to the olive tree genome overall. Two of the six markers strictly associated with SI in P. angustifolia have strong similarity with a recently identified 741kb chromosomal region fully linked to the SI phenotype on chromosome 18 of the olive tree genome, providing strong cross-validation support. The SI locus stands out as being markedly rearranged, while the sex locus has remained relatively more collinear between the two species. This P. angustifolia linkage map will be a useful resource to investigate the various ways by which the sex and SI determination systems have co-evolved in the broader phylogenetic context of the Oleaceae family.

Bud pollination and other techniques are ineffective in breaking late-acting self-incompatibility in Ceiba chodatii (Malvaceae - Bombacoideae)

There are two homomorphic self-incompatibility (SI) mechanisms among flowering plants - gametophytic self-incompatibility (GSI) and sporophytic self-incompatibility (SSI). Whilst SI has the advantage of promoting outbreeding in natural populations, it can also be a problem for horticulturists and agronomists. Thus, a number of techniques have been successfully employed to break both GSI and SSI, including bud pollination, saline pre-treatment of the stigmas and simply selfing ageing flowers. We applied these three techniques to determine whether they would be effective at breaking SI in Ceiba chodatii, a species with late-acting self-incompatibility (LSI). We self- or cross-pollinated buds ranging from 1-4 days prior to anthesis and checked ageing by selfing or crossing flowers at 1-4 days after the onset of anthesis, with those at four days showing petal wilt. None of the selfed buds or ageing flowers set fruit, unlike the majority of crossed flowers. Likewise, all flowers to which 1 % saline was applied to the stigmas prior to selfing failed to set fruit. We conclude that unlike GSI and SSI, which have pre-zygotic control of self-pollen function, LSI is likely to be immune to these techniques since the focus of rejection occurs after self pollen tubes have reached the ovary.

Do s genes or deleterious recessives control late-acting self-incompatibility in Handroanthus heptaphyllus (Bignoniaceae)? A diallel study with four full-sib progeny arrays.

Genetically controlled self-incompatibility (SI) mechanisms constrain selfing and thus have contributed to the evolutionary diversity of flowering plants. In homomorphic gametophytic SI (GSI) and homomorphic sporophytic SI (SSI), genetic control is usually by the single multi-allelic locus S. Both GSI and SSI prevent self pollen tubes reaching the ovary and so are pre-zygotic in action. In contrast, in taxa with late-acting self-incompatibility (LSI), rejection is often post-zygotic, since self pollen tubes grow to the ovary, where fertilization may occur prior to floral abscission. Alternatively, lack of self fruit set could be due to early-acting inbreeding depression (EID). The aim of our study was to investigate mechanisms underlying the lack of selfed fruit set in Handroanthus heptaphyllus in order to assess the likelihood of LSI versus EID. We employed four full-sib diallels to study the genetic control of LSI in H. heptaphyllus using a precociously flowering variant. We also used fluorescence microscopy to study the incidence of ovule penetration by pollen tubes in pistils that abscised following pollination or initiated fruits. All diallels showed reciprocally cross-incompatible full sibs (RCIs), reciprocally cross-compatible full sibs (RCCs) and non-reciprocally compatible full sibs (NRCs) in almost equal proportions. There was no significant difference between the incidences of ovule penetrations in abscised pistils following self- and cross-incompatible pollinations, but those in successful cross-pollinations were around 2-fold greater. A genetic model postulating a single S locus with four S alleles, one of which, in the maternal parent, is dominant to the other three, will produce RCI, RCC and NRC full sib situations each at 33 %, consistent with our diallel results. We favour this simple genetic control over an EID explanation since none of our pollinations, successful or unsuccessful, resulted in partial embryo development, as would be expected under a whole-genome EID effect.

Dissecting the distyly response to pollination using metabolite profiling in heteromorphic incompatibility system interactions of Plumbago auriculata Lam

Heteromorphic self-incompatibility (HetSI) is a strategy used to prevent selfing while favouring outcrossing in angiosperm. Although the genome, proteome and metabolome of homomorphic self-incompatibility (HomSI) have been investigated, metabolome findings have shown that energy deficiency is one of the factors leading to HomSI. However, the mechanism of HetSI remains unclear. This study investigates the alterations of metabolite fluxes in HetSI. Here, metabolic changes present in dimorphic styles as well as between self-incompatibility (SI) and self-compatibility (SC) pollinations of Plumbago auriculata Lam were simultaneously determined by gas chromatography–mass spectrometry (GC–MS). Fifty-three metabolites, mainly including other organic acids, amino acids, carbohydrates/glycosides, fatty acids/lipids, amines, amides, flavonoids and alcohols, were detected. The overall results showed that carbohydrates, which may affect pollen germination and pollen tube growth rate on stigmas, were abundant in S (short styles without pollination). Moreover, energy deficiency may not be a reason for HetSI because we found inflated levels of metabolites in SI pollinations. This study provided detailed insight into metabolic changes in HetSI and evidence for further studies of HetSI.

Self-incompatibility: a pollination control mechanism in plants

Mode of pollination is very important in plant breeding because it determines the genetic constitution, nature of gene action, ease in pollination control and stability of varieties after release. There are several mechanisms that promote cross pollination, among these self-incompatibility (SI) is of special significance as it is used in hybrid seed production. SI is defined as the prevention of fusion of fertile (functional) male and female gametes of the same plant (Gowers, 1989). SI is a system where self-recognition and rejection is the rule that prevents inbreeding depression. Bateman (1952) classified self-incompatibility based on the interaction between pollen grains and pistil as complementary and oppositional system. Lewis (1954) has classified SI into homomorphic and heteromorphic systems. Homomorphic SI is again subdivided into gametophytic (determined by the genotype of gametes) and sporophytic (determined by the genotype of the plant) systems. Molecular studies after 1980’s revealed that at least two genes within S-locus control the SI, among these one unit function as male and the other as female determinant. In Brassicaceae family, the determinant gene encodes a pollen ligand and its stigmatic receptor kinase and their interaction induces incompatible signaling within the stigma papilla cells. In the Solanaceae, Rosaceae, and Scrophulariaceae, the female determinant is ribonuclease and F-box protein, suggesting the involvement of RNA degradation and protein degradation within the system. In the Papaveraceae, the female determinant induces Ca 2+ dependent signaling network that ultimately results in the death of incompatible pollen (Takayama and Isogai, 2005). Genes controlling the SI is multiallelic in nature and number of alleles varies depending upon the crop. Number of alleles reported are five in Theobroma cacao (Knight and Rogers, 1953), 30 in Brassica campestris (Singh, 2012), 32 alleles in Raphanussativus (Karron et al ., 1989). SI is commercially exploited for the production hybrid seeds. Pusa Hybrid-2, Snow Queen and Snow King hybrids of cauliflower, BRH-5, H-44 of cabbage and CCRP8 to CCRP15 (Minimol et al ., 2015a) of cocoa are some of the examples. Kucera et al . (2006) has compared the quality between SI and male sterility hybrids in cauliflower and it was found that SI hybrids are superior in their performance. Minimol et al . (2015b) emphasized the importance of polyclonal garden in cocoa for production of F 1 hybrid seeds by utilizing the self-incompatibility. Rego and Rego (2013) evaluated the efficiency of three methods of overcoming self-incompatibility in passion fruit and found fruit set of 16.67 and 10 per cent in bud and double pollination, respectively. The main limitations in exploiting SI is the maintenance of inbreds, however, it can be overcome by some temporary methods such as bud pollination, salt sprays and irradiation methods.

The impact of self-incompatibility systems on the prevention of biparental inbreeding.

Inbreeding in hermaphroditic plants can occur through two different mechanisms: biparental inbreeding, when a plant mates with a related individual, or self-fertilization, when a plant mates with itself. To avoid inbreeding, many hermaphroditic plants have evolved self-incompatibility (SI) systems which prevent or limit self-fertilization. One particular SI system—homomorphic SI—can also reduce biparental inbreeding. Homomorphic SI is found in many angiosperm species, and it is often assumed that the additional benefit of reduced biparental inbreeding may be a factor in the success of this SI system. To test this assumption, we developed a spatially-explicit, individual-based simulation of plant populations that displayed three different types of homomorphic SI. We measured the total level of inbreeding avoidance by comparing each population to a self-compatible population (NSI), and we measured biparental inbreeding avoidance by comparing to a population of self-incompatible plants that were free to mate with any other individual (PSI). Because biparental inbreeding is more common when offspring dispersal is limited, we examined the levels of biparental inbreeding over a range of dispersal distances. We also tested whether the introduction of inbreeding depression affected the level of biparental inbreeding avoidance. We found that there was a statistically significant decrease in autozygosity in each of the homomorphic SI populations compared to the PSI population and, as expected, this was more pronounced when seed and pollen dispersal was limited. However, levels of homozygosity and inbreeding depression were not reduced. At low dispersal, homomorphic SI populations also suffered reduced female fecundity and had smaller census population sizes. Overall, our simulations showed that the homomorphic SI systems had little impact on the amount of biparental inbreeding in the population especially when compared to the overall reduction in inbreeding compared to the NSI population. With further study, this observation may have important consequences for research into the origin and evolution of homomorphic self-incompatibility systems.

Paternal-specific S-allele transmission in sweet cherry (Prunus avium L.): the potential for sexual selection.

Homomorphic self-incompatibility is a well-studied example of a physiological process that is thought to increase population diversity and reduce the expression of inbreeding depression. Whereas theoretical models predict the presence of a large number of S-haplotypes with equal frequencies at equilibrium, unequal allele frequencies have been repeatedly reported and attributed to sampling effects, population structure, demographic perturbation, sheltered deleterious mutations or selection pressure on linked genes. However, it is unclear to what extent unequal segregations are the results of gametophytic or sexual selection. Although these two forces are difficult to disentangle, testing S-alleles in the offspring of controlled crosses provides an opportunity to separate these two phenomena. In this work, segregation and transmission of S-alleles have been characterized in progenies of mixed donors and fully compatible pollinations under field conditions in Prunus avium. Seed set patterns and pollen performance have also been characterized. The results reveal paternal-specific distorted transmission of S-alleles in most of the crosses. Interestingly, S-allele segregation within any given paternal or maternal S-locus was random. Observations on pollen germination, pollen tube growth rate, pollen tube cohort size, seed set dynamics and transmission patterns strongly suggest post-pollination, prezygotic sexual selection, with male-male competition as the most likely mechanism. According to these results, post-pollination sexual selection takes precedence over frequency-dependent selection in explaining unequal S-haplotype frequencies.

Initial invasion of gametophytic self-incompatibility alleles in the absence of tight linkage between pollen and pistil S alleles.

In homomorphic self-incompatibility (SI) systems of plants, the loci controlling the pollen and pistil types are tightly linked, and this prevents the generation of compatible combinations of alleles expressing pollen and pistil types, which would result in self-fertilization. We modeled the initial invasion of the first pollen and pistil alleles in gametophytic SI to determine whether these alleles can stably coexist in a population without tight linkage. We assume pollen and pistil loci each carry an incompatibility allele S and an allele without an incompatibility function N. We assume that pollen with an S allele are incompatible with pistils carrying S alleles, whereas other crosses are compatible. Ovules in pistils carrying an S allele suffer viability costs because recognition consumes resources. We found that the cost of carrying a pistil S allele allows pollen and pistil S alleles to coexist in a stable equilibrium if linkage is partial. This occurs because parents that carry pistil S alleles but are homozygous for pollen N alleles cannot avoid self-fertilization; however, they suffer viability costs. Hence, pollen N alleles are selected again. When pollen and pistil S alleles can coexist in a polymorphic equilibrium, selection will favor tighter linkage.

Self-sterility in flowering plants: preventing self-fertilization increases family diversification rates

New data are presented on the distribution and frequency of self-sterility (SS) - predominantly pre-zygotic self-incompatibility (SI) systems - in flowering plants and the hypothesis is tested that families with self-sterile taxa have higher net diversification rates (DRs) than those with exclusively self-compatible taxa using both absolute and relative rate tests. Three major forms of SI systems (where pollen is rejected at the stigmatic, stylar or ovarian interface) are found to occur in the oldest families of flowering plants, with times of divergence >100 million years before the present (mybp), while post-fertilization SS and heterostyly appear in families with crown ages of 81 and 87 mybp, respectively. It is also founnd that many (22) angiosperm families exhibit >1 SI phenotype and that the distribution of different types of SS does not show strong phylogenetic clustering, collectively suggesting that SS and SI systems have evolved repeatedly de novo in angiosperm history. Families bearing self-sterile taxa have higher absolute DRs using all available calibrations of the angiosperm tree, and this affect is caused mostly by the high DR of families with homomorphic SI systems (in particular stigmatic SI) or those in which multiple SS/SI phenotypes have been observed (polymorphic). Lastly, using sister comparisons, it is further demonstrated that in 29 of 38 sister pairs (including 95 families), the self-sterile sister group had higher species richness and DR than its self-compatible sister based on either the total number of taxa in the clade with SS or only the estimated fraction to harbour SS based on literature surveys. Collectively, these analyses point to the importance of SS, particularly pre-zygotic SI in the evolution of flowering plants.

Functional fragments of a relictual gametophytic self-incompatibility system are associated with the loci determining flower type of the heterostylous outcrosser Fagopyrum esculentum Moench. and the homostylous selfer F. homotropicum Ohnishi

Functional fragments of presumably a relictual gametophytic self-incompatibility system (GSI) linked with the loci determining flower type were discovered by genetic analysis of an unilateral pre-zygotic barrier between the short-styled (thrum) morph of a heterostylous cross-pollinated species, Fagopyrum esculentum Moench., and a self-pollinator with homostylous flowers, F. homotropicum Ohnishi (asseccion C9139). The relic genes of GSI were revealed only in interspecific crosses. However, this is a direct experimental confirmation of a hypothesis proposed by Lewis (1954) which combined the heterostyly supergene components (G, P and A) with "pistil" and "pollen" parts of the S-locus of homomorphic self-incompatibility systems (I1 and I2). Also, this result provides strong evidence for the evolution of heterostyly upon the ruins of a gametophytic self-incompatibility system.

An experimental study of the S-Allee effect in the self-incompatible plant Biscutella neustriaca

Homomorphic self-incompatibility (SI) evolved in many plant families to enforce selfing avoidance, and is controlled by a single multiallelic locus (the S-locus). In a fragmented landscape, strong variation in population size and in local density is expected to cause strong variation in allelic diversity at the S-locus, which could generate an Allee effect on female reproductive success by constraining compatible pollen availability. In this experimental study, we aimed at detecting this SI-specific Allee effect (or S-Allee effect) in the endangered species Biscutella neustriaca. We demonstrated the occurrence of a SI mating system in the species and determined compatibility relationships among genotypes through a large set of controlled pollinations. For the experiment, we chose three different pollen receptor genotypes, each compatible with respectively 100, 75 and 25% of four other genotypes, which constituted the pollen sources. We placed different ramets of each receptor at different distances from the pollen sources to control for pollen limitation due to low local density, and we measured the seed set on each receptor plant three times consecutively. Analyses performed with generalized linear mixed models showed that both the distance to the pollen sources and the mate availability due to SI had a significant effect on seed set, with a strong reduction observed when mate availability was limited to 25%. Our results suggest that pollen limitation due to a restriction in compatible mate availability could occur in small or scattered populations exhibiting low allelic diversity at the S-locus.

Ocorrência de auto-incompatibilidade homomórfica em Mitracarpus longicalyx E. B. Souza & M. F. Sales (Rubiaceae - Spermacoceae)

The breeding system of Mitracarpus longicalyx was studied from September to December 2004 at Feira de Santana, BA, Brazil. This species is an annual herb found in the caatinga of northeastern Brazil, flowering between June and December. The inflorescences are glomerules with ca. 90 flowers. Each flower has a white 4-5 mm long hypocrateriform corolla. The flowers are protandrous with a three days long staminate phase followed by a five days long pistilate phase. No overlapping between phases was observed within a flower, but flowers in different phases appear in the same inflorescence. M. longycalyx is psycophilous species, pollinated by Hemiargus hanno hanno (Stoll, 1790) (Lycaenidae). Hand-pollination tests indicated that M. longicalyx is a self-incompatible, non-agamospermous species, with high fruit set under natural conditions (97.8%) in the studied population; fruit set was also high (82.3%) with hand cross-pollinations but geitonogamous hand pollination gave zero fruit set. Out-crossed pollen tubes reached the ovary within 24h after pollination, while selfed pollen grains failed to germinate or selfed pollen tubes failed to penetrate the stigmatic papillae. The results indicate that M. longicalyx has homomorphic self-incompatibility, possibly of the sporophytic type, in addition to protandry. Homomorphic self-incompatibility is very rare in the Rubiaceae, a family with abundant occurrence of heteromorphic self-incompatibility.

Evolution under tight linkage to mating type

Recent large-scale sequencing studies of mating type loci in a number of organisms offer insight into the origin and evolution of these genomic regions. Extensive tracts containing genes with a wide diversity of functions typically cosegregate with mating type. Cases in which mating type determination entails complementarity between distinct transcription units may descend from systems in which close physical linkage facilitated the coordinated expression and cosegregation of the interacting genes. In response to the particular selection pressures associated with the maintenance of more than one mating type, this nucleus of low recombination may expand over evolutionary time, engulfing neighboring tracts bearing genes with no direct role in reproduction. This scenario is consistent with the present-day structure of some mating type loci, including regulators of homomorphic self-incompatibility in angiosperms (S-loci). Recombination suppression and enforced S-locus heterozygosity accelerate the accumulation of genetic load and promote genetic associations between S-alleles and degenerating genes in cosegregating tracts. This S-allele-specific load may influence the evolution of self-incompatibility systems.

Maximum-likelihood estimation of rates of recombination within mating-type regions.

Features common to many mating-type regions include recombination suppression over large genomic tracts and cosegregation of genes of various functions, not necessarily related to reproduction. Model systems for homomorphic self-incompatibility (SI) in flowering plants share these characteristics. We introduce a method for the exact computation of the joint probability of numbers of neutral mutations segregating at the determinant of mating type and at a linked marker locus. The underlying Markov model incorporates strong balancing selection into a two-locus coalescent. We apply the method to obtain a maximum-likelihood estimate of the rate of recombination between a marker locus, 48A, and S-RNase, the determinant of SI specificity in pistils of Nicotiana alata. Even though the sampled haplotypes show complete allelic linkage disequilibrium and recombinants have never been detected, a highly significant deficiency of synonymous substitutions at 48A compared to S-RNase suggests a history of recombination. Our maximum-likelihood estimate indicates a rate of recombination of perhaps 3 orders of magnitude greater than the rate of synonymous mutation. This approach may facilitate the construction of genetic maps of regions tightly linked to targets of strong balancing selection.

Reproductive biology of woody plants in a cerrado community of Central Brazil

The flowering phenology, pollination biology and breeding systems of tree and shrub species were studied in a 40 h area of cerrado vegetation in Central Brazil. The cerrados are Neotropical savanna plant formations in Central Brazil. They represent the second most important vegetation province in Brazil and are nowadays the main agricultural frontier which has resulted in deforestation rates much higher than those occurring in lowland tropical forests. Despite the marked winter/dry − summer/wet seasonality, different species were encountered in flower throughout the year, although with a peak at the end of the dry season. Seven pollination guilds were identified, ranging from very small insects (body size less than 5 mm) to bats, but bee pollination was predominant, and overall pollination biology of the cerrado community was similar to that reported for evergreen neotropical forests. Most species were outbreeding in that self-pollinations yielded very few fruits, but heteromorphic and homomorphic self-incompatibility were rare and control of selfing was commonly exercised by ill-defined post-zygotic mechanisms. The results indicated that the reproductive biology of woody cerrado species is not markedly different from other tropical forest communities and suggested the adaptation to seasonal habitats did not always lead to simplification or changes in breeding features of the cerrado species.

Population Genetics of the Homomorphic Self-incompatibility Polymorphisms in Flowering Plants

This paper surveys the results obtained from the investigation of the homomorphic self-incompatibility polymorphisms in 19 species of flowering plants in terms of the number and frequency of alleles in their populations, the distribution of alleles between these populations and the total number of alleles in the species, and assesses the extent to which these results conform with theoretical expectations. The survey shows that, contrary to theory, the populations of most species do not contain a very large number of alleles and the same appears to be true for the total number of alleles in the species. Populations of Trifolium repens and T. pratense, however, appear to possess a large number of alleles for reasons which are unknown. Data on the distribution of alleles range from complete overlap to considerable differentiation between populations in respect of the complement of alleles they contain. The frequencies of alleles in populations of most species with gametophytic systems of self-incompatibility appear to be equal, as the theory suggests; frequencies in populations of Papaver rhoeas and Lolium perenne, however, appear to be significantly unequal. Allele frequencies in populations of sporophytic species are, as expected, unequal, those of dominant alleles being lower than those of recessive alleles. The implications of these results are discussed and the need for further, more comprehensive information on the attributes of these polymorphisms in natural populations is stressed.

MULTIPLE ORIGINS OF SELF-COMPATIBILITY IN LINANTHUS SECTION LEPTOSIPHON (POLEMONIACEAE): PHYLOGENETIC EVIDENCE FROM INTERNAL-TRANSCRIBED-SPACER SEQUENCE DATA.

Phylogenetic reconstruction based on sequence variation in the internal transcribed spacer (ITS) region of rDNA was used to investigate the evolutionary dynamics of homomorphic self-incompatibility in Linanthus section Leptosiphon (Polemoniaceae), a group of annual plant species. Hand-pollination experiments revealed that five species were self-incompatible and four were self-compatible. Optimization of breeding systems onto the tree resulting from maximum-likelihood analysis, with no assumptions made about the ancestral condition, indicated that self-incompatibility has been lost four times in this section. An alternative tree rearrangement conforming to the hypothesis of three losses of self-incompatibility did not have a significantly lower likelihood than the maximum-likelihood tree as determined by a paired-sites test, but all rearrangements resulting in fewer than three losses were statistically rejected. Linanthus bicolor, a selfing species, was found to be polyphyletic, with populations from different geographic regions occurring in three well-supported clades. Morphological similarity in these distinct lineages is likely to have resulted from convergent evolution of traits associated with self-fertilization. Selection for reproductive assurance is hypothesized to have played an important role in the recurrent transformations from self-incompatibility to selfing in this group of annual species.

Taxonomy and breeding system in a new species of Scrophularia L. (Scrophulariaceae) from Morocco

Abstract A new species of the genus Scrophularia , S. fontqueri Ortega-Olivencia & Devesa, is described for the Western Rif mountains in Morocco. Similarities and differences with the most closely related species ( S. auriculata Loefl. ex L. and S. viciosoi Ortega-Olivencia & Devesa) are indicated, and ecological and distribution data are provided. The new species displays a high degree of polyploidy (2 n =56), and has palynological characteristics similar to those of other species in this genus. The new species shows protogynous dichogamy and produces considerable amounts of nectar in both sexual phases. Nectar secretion was maximum during the early hours of the morning. Mean longevity of the flowers was 2.66 days. Both nectar secretion and floral longevity correlated strongly with mean daytime air temperature. Pollination experiments showed that interaction with pollinators is necessary for reproduction, and gametophytic homomorphic self-incompatibility was documented. This feature was confirmed by observation of pollen tubes after hand self-pollination. The seed/ovule ratio was very low.