Inbreeding depression in <Emphasis Type="Italic">Solanum carolinense</Emphasis> (Solanaceae), a species with a plastic self-incompatibility response

Background  

Solanum carolinense (horsenettle) is a highly successful weed with a gametophytic self-incompatibility (SI) system. Previous studies reveal that the strength of SI in S. carolinense is a plastic trait, associated with particular S -alleles. The importance of this variation in self-fertility on the ability of horsenettle to found and establish new populations will depend, to a large extent, on the magnitude of inbreeding depression. We performed a series of greenhouse and field experiments to determine the magnitude of inbreeding depression in S. carolinense, whether inbreeding depression varies by family, and whether the estimates of inbreeding depression vary under field and greenhouse conditions. We performed a series of controlled self- and cross-pollinations on 16 genets collected from a large population in Pennsylvania to obtain progeny with different levels of inbreeding. We grew the selfed and outcrossed progeny in the greenhouse and under field conditions and recorded various measures of growth and reproductive output.     

Plant chemistry underlies herbivore‐mediated inbreeding depression in nature

The cost of inbreeding (inbreeding depression, ID) is an important variable in the maintenance of reproductive variation. Ecological interactions such as herbivory could modulate this cost, provided that defence traits harbour deleterious mutations and herbivores are responsible for differences in fitness. In the field, we manipulated the presence of herbivores on experimentally inbred and outcrossed plants of Solanum carolinense (horsenettle) for three years. Damage was greater on inbred plants, and ID for growth and fitness was significantly greater under herbivory. Inbreeding reduced phenolic expression both qualitatively (phytochemical diversity) and quantitatively, indicating deleterious load at loci related to the biosynthesis of defence compounds. Our results indicate that inbreeding effects on plant–herbivore interactions are mediated by changes to functional plant metabolites, suggesting that variation in inbreeding could be a predictor of defence trait variation. The magnitude of herbivore‐mediated, ecological ID indicates that herbivores could maintain outcrossing mating systems in nature.     

Comparison of S-alleles and S-glycoproteins between two wild populations of Brassica campestris in Turkey and Japan

Summary The number of identical S-alleles between two wild populations of B. campestris, one in Turkey, the other in Japan, that have been independent of one another for a long time was investigated. Diallel pollination tests between 38 S-allele homozygotes, i.e., 16 S-allele homozygotes from Turkey and 22 from Japan, revealed that these were 29 different S-alleles only 4 common ones. These S-alleles were differentiated by the iso-electric focusing (IEF) analysis of S-locus glycoproteins (SLGs) stained with an antiserum against SLG⁸. All identical S-alleles had the major SLG band at the same pI value without exception, even though they were collected from different populations. However, the number of minor bands of SLGs varied between the two populations; the S-alleles in Balcesme had generally fewer minor bands than those in Oguni. The 29 independent S-alleles were numbered from S ²¹ to S ⁴⁹ according to the pI value of the major SLG band. The major bands whose pI values were 7.5–8.5 were most common. Blot-hybridization patterns of genomic DNA hybridized with SLG ⁸ cDNA were not always the same among the strains of identical S-alleles obtained from different populations. Because about 20% of the S-alleles were shared between the two populations, it can be inferred that more than hundreds of S-alleles have been accumulated by mutation in B. campestris throughout the world.     

The Sheltered Genetic Load Linked to the S Locus in Plants: New Insights From Theoretical and Empirical Approaches in Sporophytic Self-Incompatibility

Inbreeding depression and mating systems evolution are closely linked, because the purging of deleterious mutations and the fitness of individuals may depend on outcrossing vs. selfing rates. Further, the accumulation of deleterious mutations may vary among genomic regions, especially for genes closely linked to loci under balancing selection. Sporophytic self-incompatibility (SSI) is a common genetic mechanism in angiosperm that enables hermaphrodite plants to avoid selfing and promote outcrossing. The SSI phenotype is determined by the S locus and may depend on dominance relationships among alleles. Since most individuals are heterozygous at the S locus and recombination is suppressed in the S-locus region, it has been suggested that deleterious mutations could accumulate at genes linked to the S locus, generating a “sheltered load.” In this article, we first theoretically investigate the conditions generating sheltered load in SSI. We show that deleterious mutations can accumulate in linkage with specific S alleles, and particularly if those S alleles are dominant. Second, we looked for the presence of sheltered load in Arabidopsis halleri using CO₂ gas treatment to overcome self-incompatibility. By examining the segregation of S alleles and measuring the relative fitness of progeny, we found significant sheltered load associated with the most dominant S allele (S15) of three S alleles tested. This sheltered load seems to be expressed at several stages of the life cycle and to have a larger effect than genomic inbreeding depression.THE main genetic mechanism causing inbreeding depression is believed to be the expression of recessive mildly deleterious mutations in inbred individuals (Charlesworth and Charlesworth 1999). These deleterious mutations are generally supposed to be distributed throughout the genome. However, some genomic regions where loci under balancing selection are present may be more inclined than others to accumulate deleterious mutations and could lead to the formation of what is generally called a “sheltered load” (Uyenoyama 1997; van Oosterhout 2009). The sheltered load has been suggested as a potential reason why MHC genes, mating-type systems in fungi, and self-incompatibility systems in plants generally show longer terminal branches in their genealogies than expected (Richman 2000). Despite its potential importance, the extent of the sheltered load is still largely unknown.Homomorphic self-incompatibility is widely distributed among angiosperm families (de Nettancourt 2001; Igic et al. 2008). Self-incompatibility (SI) is controlled by genes under strong balancing selection. SI prevents self-fertilization and promotes outcrossing by the presence of a gamete recognition system involving proteins expressed in both the pollen and the pistil. The proteins controlling the recognition system are generally encoded by genes located in a single genomic region, the S locus. Each plant in a self-incompatible population expresses an S specificity and is unable to mate with other plants expressing the same specificity. In species with gametophytic self-incompatibility (GSI), the S specificity is controlled by interactions between protein expressed in the pollen''s haploid genome, the male gametophyte, and the pistil''s diploid genome. In species with sporophytic self-incompatibility (SSI), S specificity is controlled by interactions between gene products of the diploid sporophyte expressed on the pollen coat and those on the stigmatic surface. In this mating system, three reasons may facilitate the accumulation of recessive deleterious mutations in this region, namely a sheltered load (Uyenoyama 1997). First, high heterozygote frequencies are expected in populations at the S locus but also at other linked loci in the S-locus genomic region (Kamau et al. 2007). Second, negative frequency-dependent selection, a form of balancing selection, is the main selective force acting on the S locus and on linked genes (Castric and Vekemans 2004). Third, the recombination rate is low in the S-locus region (Casselman et al. 2000; Charlesworth et al. 2003). Such a sheltered load may have important evolutionary consequences for SI evolution: it can slow down the rate of emergence of new S alleles (Uyenoyama 2003), considerably extend the conditions for the persistence of GSI (Porcher and Lande 2005), and, finally, substantially increase the inbreeding depression in a small population (Glémin et al. 2001), which can have large consequences for endangered species and the viability of their populations.The magnitude of the sheltered load should depend on the size of the genomic region in which heterozygosity is enforced because of linkage to the S locus and also on the number of genes affecting fitness in that region. From an analysis of recombination rates in the S-locus genomic region in Arabidopsis lyrata, a species with SSI, Kawabe et al. (2006) suggested that the number of genes in the S-genomic region is probably not high enough for a large sheltered load to have an impact on fitness compared to the overall genomic load. Dowd et al. (2000) indeed found only 13 genes near the S locus in Petunia inflata. However, two studies have demonstrated the existence of transmission ratio distortion of some S alleles in A. lyrata (Bechsgaard et al. 2004; Leppala et al. 2008). The authors proposed that this could be indirect evidence of the existence of a sheltered load. To the best of our knowledge, the existence of sheltered load in SI species was specifically demonstrated so far only in Solanum carolinense, a species with GSI: Stone (2004) crossed individuals sharing alleles at the S locus, using bud pollination to overcome self-incompatibility. By looking at seed number and genotype of the progeny, a sheltered load linked to only two of seven S alleles investigated was detected. Direct evidence and estimations of the extent of the sheltered load are thus lacking.In SSI, complex dominance interactions among S alleles are usually observed [Ipomoea trifida (Kowyama et al. 1994), Brassica campestris (Hatakeyama et al. 1998), A. lyrata (Mable et al. 2003), and A. halleri (Llaurens et al. 2008a)]. The effect of these dominance interactions on the occurrence of a sheltered genetic load has not been investigated either theoretically or empirically, but may potentially be large. Indeed, recessive S alleles are expected to be more often homozygous in natural populations than dominant alleles (Schierup et al. 1997), and so may rapidly purge strongly deleterious recessive mutations, and thus should limit the sheltering effect. The sheltered load could thus differ depending on the dominance levels of the associated S alleles.In this study, we first investigated the theoretical conditions for the accumulation of a sheltered load in a SSI system, using stochastic simulations. Then, we empirically tested the existence and strength of an S-linked sheltered load in relation to dominance levels in SSI. We focused on A. halleri, a member of the Brassicaceae family. In this family, the S-locus region includes two major genes: SCR (also called SP-11), encoding a cysteine-rich protein of the pollen envelope, and SRK, encoding a receptor kinase located across the membrane of the papilla cells. High heterozygote frequencies at the S locus have been found in several species like B. insularis (Glémin et al. 2005) or A. lyrata (Schierup et al. 2006). The SRK and SCR genes are tightly linked, since they are located close to each other, and recombination suppression in the S-locus region has been suggested in several studies: in Brassica (Casselman et al. 2000) and in A. lyrata (Kamau and Charlesworth 2005; Kawabe et al. 2006). The conditions thus may be suitable for the existence of sheltered genetic load in A. halleri. We performed controlled pollinations in A. halleri to specifically measure the magnitude of the potential sheltered load of three S alleles with different dominance levels: a dominant, an intermediate, and a recessive allele. To evaluate the effect of the sheltered load on these crosses, we looked at the number of seeds produced, as well as at the development and the genotype at the S locus of the progeny.     

Disentangling the effects of breakdown of self-incompatibility and transition to selfing in North American Arabidopsis lyrata

A breakdown of self‐incompatibility (SI) followed by a shift to selfing is commonly observed in the evolution of flowering plants. Both are expected to reduce the levels of heterozygosity and genetic diversity. However, breakdown of SI should most strongly affect the region of the SI locus (S‐locus) because of the relaxation of balancing selection that operates on a functional S‐locus, and a potential selective sweep. In contrast, a transition to selfing should affect the whole genome. We set out to disentangle the effects of breakdown of SI and transition to selfing on the level and distribution of genetic diversity in North American populations of Arabidopsis lyrata. Specifically, we compared sequence diversity of loci linked and unlinked to the S‐locus for populations ranging from complete selfing to fully outcrossing. Regardless of linkage to the S‐locus, heterozygosity and genetic diversity increased with population outcrossing rate. High heterozygosity of self‐compatible individuals in outcrossing populations suggests that SI is not the only factor preventing the evolution of self‐fertilization in those populations. There was a strong loss of diversity in selfing populations, which was more pronounced at the S‐locus. In addition, selfing populations showed an accumulation of derived mutations at the S‐locus. Our results provide evidence that beyond the genome‐wide consequences of the population bottleneck associated with the shift to selfing, the S‐locus of A. lyrata shows a specific signal either reflecting the relaxation of balancing selection or positive selection.     

The divergence history of the perennial plant Linaria cavanillesii confirms a recent loss of self‐incompatibility

Many angiosperms prevent inbreeding through a self‐incompatibility (SI) system, but the loss of SI has been frequent in their evolutionary history. The loss of SI may often lead to an increase in the selfing rate, with the purging of inbreeding depression and the ultimate evolution of a selfing syndrome, where plants have smaller flowers with reduced pollen and nectar production. In this study, we used approximate Bayesian computation (ABC) to estimate the timing of divergence between populations of the plant Linaria cavanillesii that differ in SI status and in which SI is associated with low inbreeding depression but not with a transition to full selfing or a selfing syndrome. Our analysis suggests that the mixed‐mating self‐compatible (SC) population may have begun to diverge from the SI populations around 2810 generation ago, a period perhaps too short for the evolution of a selfing syndrome. We conjecture that the SC population of L. cavanillesii is at an intermediate stage of transition between outcrossing and selfing.     

Self-compatibility in aLycopersicon peruvianum variant (LA2157) is associated with a lack of style S-RNase activity

A series of crosses between a naturally-occurring self-compatible accession ofLycopersicon peruvianum and a closely-related self-incompatible accession were used to demonstrate that the mutation to self-compatibility is located at the S-locus. Progeny of the crosses contain abundant style proteins of about 30 kDa that segregate with the S₆and S₇-alleles from the SI parent and the S_c-allele from the SC parent. The S₆and S₇-associated proteins have ribonuclease activity whereas the S_c-associated protein is not an active ribonuclease. This finding indicates that S-RNases are determinants of self-incompatibility in the style and that the ribonuclease activity is essential for their function.     

Loss of gametophytic self-incompatibility with evolution of inbreeding depression

Gametophytic self-incompatibility (SI) in plants is a widespread mechanism preventing self-fertilization and the ensuing inbreeding depression, but it often evolves to self-compatibility. We analyze genetic mechanisms for the breakdown of gametophytic SI, incorporating a dynamic model for the evolution of inbreeding depression allowing for partial purging of nearly recessive lethal mutations by selfing, and accounting for pollen limitation and sheltered load linked to the S-locus. We consider two mechanisms for the breakdown of gametophytic SI: a nonfunctional S-allele and an unlinked modifier locus that inactivates the S-locus. We show that, under a wide range of conditions, self-compatible alleles can invade a self-incompatible population. Conditions for invasion are always less stringent for a nonfunctional S-allele than for a modifier locus. The spread of self-compatible genotypes is favored by extremely high or low selfing rates, a small number of S-alleles, and pollen limitation. Observed parameter values suggest that the maintenance of gametophytic SI is caused by a combination of high inbreeding depression in self-incompatible populations coupled with intermediate selfing rates of the self-compatible genotypes and sheltered load linked to the S-locus.     

S-related protein can be recombined with self-compatibility in interspecific derivatives ofLycopersicon

Stylar proteins involved in the self-incompatible (SI) response ofLycopersicon hirsutum have been identified and mapped to the locus that controls SI (S locus).L. esculentum, a self-compatible (SC) species of cultivated tomato, does not display these proteins. Hybrids between SCL. esculentum and SIL. hirsutum are self-sterile despite these individuals bearing pollen containing theS allele ofL. esculentum. In progeny derived from backcrossing the hybrids toL. esculentum, there was a strong correlation between the presence of theS allele fromL. hirsutum and self-infertility. However, this relationship was uncoupled in a number of backcross (BC) progeny. The SI response appeared to be nonexistent in two self-fertile BC individuals that were heterozygous for theS allele ofL. hirsutum, based on Mendelian segregation of a tightly linked DNA marker,CD15, in selfed progeny. Among these progeny self-fertile individuals that were homozygous for theL. hirsutum allele of the linked marker were also determined to be homozygous for anS-related protein ofL. hirsutum through test crosses withL. esculentum. Therefore, plants were produced that were homozygous for a functionalS allele but were self-fertile. This result and other evidence suggest that theS-related proteins are not sufficient to elicit a self-incompatible response inL. esculentum and that there is a mutation(s) inL. esculentum somewhere other than theS locus that leads to self-compatibility.     

Isolation of S-alleles from a wild population of Brassica campestris L. at Balcesme,Turkey and their characterization by S-glycoproteins

Summary S-alleles of self-incompatibility were isolated from a wild population of Brassica campestris growing at Balcesme, Turkey. Out of 88 plants observed, 73 were self-incompatible and 4 were self-compatible. In certain families, selfed progenies from a self-incompatible plant segregated into fewer than three incompatibility classes, which is consistent with a one-locus sporophytic genetic control of self-incompatibility. Out of 25 combinations of S-alleles tested, dominance interactions were observed in 6 of them on the pollen side and on 5 of them on the stigma side. The 35 S-homozygotes thus isolated consisted of 18 independent S-alleles. The number of S-alleles in this population was estimated to be more than 30. The S-locus glycoproteins (SLGs) corresponding to the respective S-alleles were identified by iso-electric focusing (IEF)-gel immunoblotting with a polyclonal antiserum against SLG⁸. SLGs in a stigma were generally composed of several bands, one major and a few minor ones, whose molecular weight was similar to each other, and the major and minor bands were heritable in correlation with each other. SDS-PAGE analysis of SLGs differentiated a few juxtaposed bands between 50 and 60 kDa, and the variations in these bands were considered to be due to differences in the number of polysaccharide residues. General features of the variation of S-genes and their SLGs between the populations in Balcesme, Turkey and Oguni, Japan, were comparatively similar to one another, despite the different surroundings and history of these populations.     

Incest versus abstinence: reproductive trade‐offs between mate limitation and progeny fitness in a self‐incompatible invasive plant

Plant mating systems represent an evolutionary and ecological trade‐off between reproductive assurance through selfing and maximizing progeny fitness through outbreeding. However, many plants with sporophytic self‐incompatibility systems exhibit dominance interactions at the S‐locus that allow biparental inbreeding, thereby facilitating mating between individuals that share alleles at the S‐locus. We investigated this trade‐off by estimating mate availability and biparental inbreeding depression in wild radish from five different populations across Australia. We found dominance interactions among S‐alleles increased mate availability relative to estimates based on individuals that did not share S‐alleles. Twelve of the sixteen fitness variables were significantly reduced by inbreeding. For all the three life‐history phases evaluated, self‐fertilized offspring suffered a greater than 50% reduction in fitness, while full‐sib and half‐sib offspring suffered a less than 50% reduction in fitness. Theory indicates that fitness costs greater than 50% can result in an evolutionary trajectory toward a stable state of self‐incompatibility (SI). This study suggests that dominance interactions at the S‐locus provide a possible third stable state between SI and SC where biparental inbreeding increases mate availability with relatively minor fitness costs. This strategy allows weeds to establish in new environments while maintaining a functional SI system.     

The mating system and relative performance of selfed and outcrossed progeny in Arabis fecunda (Brassicaceae)

We estimated rates of self-fertilization and inbreeding depression in the hermaphroditic perennial Arabis fecunda. Crosses were made on mesh-bagged wild plants, yielding 12 plants producing self-, outcross-, and naturally fertilized seeds that were then grown in a greenhouse. Analysis of variance indicated aboveground dry weight at 160 days differed among pollination treatments, but mean seed weight, number of seeds per fruit, and days to germination did not. For dry weight, selfed progeny have 9.4% reduction and naturally pollinated progeny a 3.7% reduction compared to outcrossed progeny, suggesting a 39% selling rate in Arabis. Significant inbreeding depression and a mixed mating system are evidence that the deleterious effects of self-fertilization are maintained through high rates of mildly deleterious mutation. Significant maternal-parent-by-pollination-treatment interactions for mean seed weight and dry weight are consistent with inbreeding depression caused by deleterious recessives and varying past maternal inbreeding.     

Numbers of sporophytic self-incompatibility alleles in populations of wild radish

Summary To estimate the numbers of sporophytic S-alleles in two adjacent populations of wild radish, we performed 701 reciprocal crosses among 50 individuals. Each cross was replicated five times in each direction. Sixteen plants were fully intercompatible, indicating the presence of at least 32 S-alleles in the two populations. A minimum of 22 S-alleles occur in a single population. The frequency of incompatibility was significantly higher for within-population crosses (14.5%) than for between-population crosses (7.8%). This suggests that the two populations differ in the composition and frequency of alleles at the S-locus.     

Identification of self-incompatibility genotypes of almond by allele-specific PCR analysis

In almond, gametophytic self-incompatibility is controlled by a single multiallelic locus (S-locus). In styles, the products of S-alleles are ribonucleases, the S-RNases. Cultivated almond in California have four predominant S-alleles (S ^a, S ^b, S ^c, S ^d). We previously reported the cDNA cloning of three of these alleles, namely S ^b, S ^c and S ^d. In this paper we report the cloning and DNA sequence analysis of the S ^a allele. The S^a-RNase displays approximately 55% similarity at the amino-acid level with other almond S-RNases (S^b, S^c, and S^d) and this similarity was lower than that observed among the S^b, S^c and S^d-RNases. Using the cDNA sequence, a PCR-based identification system using genomic DNA was developed for each of the S-RNase alleles. Five almond cultivars with known self-incompatibility (SI) geno-types were analyzed. Common sequences among four S-alleles were used to create four primers, which, when used as sets, amplify DNA bands of unique size that corresponded to each of the four almond S-alleles; S ^a (602 bp), S ^b (1083 bp), S ^c (221 bp) and S ^d (343 bp). All PCR products obtained from genomic DNA isolated from the five almond cultivars were cloned and their DNA sequence obtained. The nucleotide sequence of these genomic DNA fragments matched the corresponding S-allele cDNA sequence in every case. The amplified products obtained for the S ^a- and S ^b-alleles were both longer than that expected for the coding region, revealing the presence of an intron of 84 bp in the S ^a-allele and 556 bp in the S ^b-allele. Both introns are present within the site of the hypervariable region common in S-RNases from the Rosaceae family and which may be important for S specificity. The exon portions of the genomic DNA sequences were completely consistent with the cDNA sequence of the corresponding S-allele. A useful application of these primers would be to identify the S-genotype of progeny in a breeding program, new varieties in an almond nursery, or new grower selections at the seedling stage. Received: 21 June 1999 / Accepted: 15 November 1999     

Self-incompatibility is codominant in intraspecific hybrids of self-compatible and self-incompatible Lycopersicon peruvianum and L. hirsutum based on protein and DNA marker analysis

Self-compatibility was investigated separately in two species of tomato, Lycopersicon peruvianum and L. hirsutum. The codominant expression of self-compatibility (SC)/self incompatibility (SI) was established using intraspecific hybrids of SC and SI hybrids. In SC L. peruvianum, a major stylar protein of approximately 29 kDa cosegregates with self-compatibility in the progeny of SC/SI hybrids. The SC/SI hybrids are self-fertile, but only partially so, since the SI allele present in the hybrids is capable of eliminating certain genotypes in the resultant progeny. In L. hirsutum, the majority of hybrids between one accession of SI L. hirsutum f. hirsutum and one of SC L. hirsutum f. glabratum are self-fertile. Analysis of the progeny revealed that the SC and SI alleles are codominant in this species as well. A protein product for the SC allele is not obvious in style extracts of L. hirsutum f. glabratum. Segregating progeny from SC/SI hybrids of L. hirsutum were used to map the S locus against five RFLP markers on chromosome 1, and estimated map distances are given. In addition, evidence is presented that indicates that one of the DNA markers, CD15, is duplicated in L. hirsutum f. glabratum, and the duplication is not linked to the S locus.     

Compensation for floral herbivory in <Emphasis Type="Italic">Solanum carolinense</Emphasis>: identifying mechanisms of tolerance

While a plant’s capacity to tolerate damage by herbivores can be studied as a single trait, it is important to recognize that tolerance is generally a result of the combined action of several different traits. Here, we report on a pair of experiments to identify mechanisms for tolerating floral herbivory in Solanum carolinense, an andromonoecious perennial herb that regularly suffers from high levels of florivory. We measured the effect of actual and simulated florivory on host-plant fitness and assessed which plant traits exhibited plasticity in response to florivory. In addition, for each of nine plant genets, we calculated tolerance indices and determined which traits were genetically correlated with tolerance. Traits that served to help S. carolinense tolerate florivory in terms of sexual reproduction included initiating more inflorescences, aborting fewer buds prior to anthesis and fewer ovaries after fertilization, and increasing the ratio of perfect:male flowers. In addition, the greater the levels of florivory, the more the plants allocated to root growth, which may promote tolerance through greater potential future reproduction. The plant population contained significant genetic variation for tolerance itself and for nearly all of the putative tolerance mechanisms, which suggests that S. carolinense has the potential to evolve greater tolerance through a variety of different routes in response to natural selection.     

COMPETITION-DEPENDENT ABSCISSION OF SELF-POLLINATED FLOWERS OF PHORMIUM TENAX (AGAVACEAE): A SECOND ACTION OF SELF-INCOMPATIBILITY AT THE WHOLE FLOWER LEVEL?

The relative success of fruit from paired self- and cross-pollinations was examined in Phormium tenax when the contrasted pollinations were separated by different distances on the same and different inflorescences. We determined whether the retention of selfed fruits differed from that of crossed fruits and whether it depended on the level of competition with crossed fruit, the number of seeds per fruit, and/or the presence of earlier developing fruit. We found that the success of selfed fruits is determined by the degree of competition with crossed fruits and may be an expression of self-incompatibility. Competition-dependence of the abscission of selfed flowers has not been documented previously. It is parallel to cryptic self-incompatibility in which individual self-pollen grains are not as successful as cross-pollen when competing on the same pistil. The competition-dependent abscission of self-pollinations considered here, however, operates at the level of whole flowers. The phenomenon of competition-dependent abscission of selfed flowers in P. tenax also has implications for the measurement and interpretation of self-incompatibility in other species. Self-incompatibility is a quantitative phenomenon. The facultative success of selfing shows that the effective strength of self-incompatibility can be highly susceptible to the conditions of competition under which it is measured. The competition-dependent abscission of selfed flowers allows a high level of outcrossing to be achieved while it assures seed set when pollinations are scarce. Several other causes of intermediate selfing frequencies can also be explained by this “best-of-both-worlds” hypothesis.     

Genetics of self incompatibility in diploid Ageratum houstonianum mill

Summary Diallels and backcrosses among self-incompatible (SI) clones and progeny of Ageratum houstonianum Mill. could be organized into intra-incompatible classes. Four of 5 progenies segregated in expected ratios of S genotypes. Ageratum expressed a one-locus incompatibility system of the sporophytic type with a linear dominance series of multiple alleles and complete allelic dominance in both pollen and stigma. In the second part of the study, a high percentage of self-seed set was observed during the first flowering of a progeny from a pseudo-self compatible (PSC) seed source. Six progenies were derived from the PSC seed source. Five of the 6 segregated PSC SI plants, 4 of which fit a 3 1 ratio of PSC SI plants. All plants of the sixth progeny were SI. Two F₁ progenies with the same PSC pollen parent produced significantly different segregations of PSC SI plants. It appeared that PSC acted as a major gene when the most recessive S allele was also present, but PSC was not expressed when the most dominant S allele was present. Clones propagated from PSC plants were SI and cross incompatible with a related S-allele tester. Thus, PSC was transient in that it was apparent in seed-propagated plants but not in plants clonally propagated from the PSC individuals.Scientific Journal Series Paper Number 12,299 of the Minnesota Agricultural Experiment Station     

LIFETIME INBREEDING DEPRESSION,PURGING, AND MATING SYSTEM EVOLUTION IN A SIMULTANEOUS HERMAPHRODITE TAPEWORM

Classical theory on mating system evolution suggests that simultaneous hermaphrodites should either outcross if they have high inbreeding depression (ID) or self‐fertilize if they have low ID. However, a mixture of selfing and outcrossing persists in many species. Previous studies with the tapeworm Schistocephalus solidus have found worms to self‐fertilize some of their eggs despite ID. The probability for selfing to spread depends on the relative fitness of selfers, as well as the genetic basis for ID and whether it can be effectively purged. We bred S. solidus through two consecutive generations of selfing and recorded several fitness correlates over the whole life cycle. After one round of selfing, ID was pronounced, particularly in early‐life traits, and the conservatively estimated lifetime fitness of selfed progeny was only 9% that of the outcrossed controls. After a second generation of selfing, ID remained high but was significantly reduced in several traits, which is consistent with the purging of deleterious recessive alleles (the estimated load of lethal equivalents dropped by 48%). Severe ID, even if it can be rapidly purged, likely prevents transitions toward pure selfing in this parasite, although we also cannot exclude the possibility that low‐level selfing has undetected benefits.     

Protein expression of a self-compatible allele from Lycopersicon peruvianum: introgression and behavior in a self-incompatible background

Lycopersicon peruvianum (wild tomato) is a gametophytic self-incompatible (SI) species. One natural population has been shown to harbor a self-compatible (SC) allele. A stylar protein associated with the self-compatibility allele has been elucidated using SDS-PAGE. The temporal and spatial expression of this protein is presented and compared with protein expression of two SI alleles. Hybrids containing the SC and SI alleles were used in a backcrossing program to introgress the SC allele into SI backgrounds in six independent lines. Controlled pollinations and SDS-PAGE were used to identify and select classes of progeny. After four backcross generations (approximately 97% recovery of the SI backgrounds) the SC allele still confers self-fertility in lines that contain this allele, providing evidence that the mutation to SC occurred at the S-locus and that the associated protein is likely responsible.