Duplicated phosphoglucose isomerase genes in avocado

Summary Avocado (Persea americana) cultivars were assayed for phosphoglucose isomerase (PGI) isozymes using starch gel electrophoresis. Three PGI genes were identified: one monomorphic locus, Pgi-I, coding for the plastid isozyme and two independently assorting loci, Pgi-2 and Pgi-3, coding for the cytosolic isozymes. The genetic analysis was based on comparisons of PGI zymograms from somatic and pollen tissue and on Mendelian analysis of progeny from selfed trees. The isozymic variability for PGI can be used for cultivar identification and for differentiating between hybrid and selfed progeny in avocado breeding.     

Inbreeding depression and selfing rates in a self-compatible,hermaphroditic species,Schiedea membranacea (Caryophyllaceae)

Inbreeding depression and selfing rates were investigated in Schiedea membranacea (Caryophyllaceae), a hermaphroditic species endemic to the Hawaiian Islands. Most theoretical models predict high inbreeding depression in outcrossing hermaphroditic species and low inbreeding depression in inbreeding species. Although high outcrossing rates and high levels of inbreeding depression are characteristic of many species of Schiedea, self- fertilization is common among relatives of hermaphroditic S. membranacea, and high selfing rates and low levels of inbreeding depression were predicted in this species. Sixteen individuals grown in the greenhouse were used to produce selfed and outcrossed progeny. Inbreeding depression, which was evident throughout the stages measured (percentage viable seeds per capsule, mean seed mass, percentage seed germination, percentage seedling survival, and biomass after 8 mo), averaged 0.70. Inbreeding depression among maternal families varied significantly for all measured traits and ranged from −0.12 to 0.97. Using isozyme analysis, the multilocus selfing rate varied from 0.13 to 0.38 over 4 yr. Contrary to the initial prediction of high selfing and low inbreeding depression based on phylogenetic relationships within Schiedea, low selfing rates and high levels of inbreeding depression were found in S. membranacea. These results indicate that outcrossing is stable in this species and maintained by high levels of inbreeding depression.     

Isozyme gene markers in Vicia faba L.

Summary This study was conducted to assess the genetic basis of the variability observed for the glutamate oxaloacetate transaminase (GOT), Superoxide dismutase (SOD), esterase (EST), and malate dehydrogenase (MDH) isozyme systems in different open-pollinated Vicia faba varieties. Individual plants showing contrasting zymogram patterns were simultaneously selfed and cross-combined. Crossing was unsuccessful in producing progeny, and only selfed progenies were suitable for genetical analysis of isozyme variability. Three zones of GOT activity were made visible. The isozyme of GOT-2 and GOT-3 zones were dimeric and under the control of three alleles at the Got-2 locus and two alleles at the Got-3 locus, respectively. The isozymes of the GOT-1 zone did not show any variability. Three zones of SOD isozyme activity were made visible. The isozymes occurring in the SOD-1 (chloroplastic isozyme form) and SOD-2 (cytosol isozyme form) zones were dimeric and under the control of two alleles at the Sod-1 and Sod-2 loci. The isozyme visualized in the SOD-3 zone (mitochondrial isozyme form) were tetrameric and under the control of two alleles at the Sod-3 locus. Apparently the isozymes made visible in the most anodal esterase zones EST-1, EST-2, and EST-3 were monomeric, and the occurrence of two alleles at each of two different loci explained the variability observed in the EST-2 and EST-3 zones. For MDH, only two five-banded zymogram pattern types were found, and every selfed progeny showed only one of the two zymogram type, indicating that each individual possessed fixed alleles at the loci controlling MDH isozyme. Got-2, Got-3, Sod-1, Sod-2, and Sod-3 appear to be five new isozyme gene markers that can be useful in Vicia faba breeding for linkage study, varietal fingerprinting, outcrossing rate estimate, and indirect selection for quantitative characters.     

OUTCROSSING RATE AND INBREEDING DEPRESSION IN TWO ANNUAL MONOECIOUS HERBS,BEGONIA HIRSUTA AND B. SEMIOVATA

Most models of mating-system evolution predict inbreeding depression to be low in inbred populations due to the purging of deleterious recessive alleles. This paper presents estimates of outcrossing rates and inbreeding depression for two highly selfing, monoecious annuals Begonia hirsuta and B. semiovata. Outcrossing rates were estimated using isozyme polymorphisms, and the magnitude of inbreeding depression was quantified by growing progeny in the greenhouse produced through controlled selfing and outcrossing. The estimated single-locus outcrossing rate was 0.03 ± 0.01 (SE) for B. hirsuta and 0.05 ± 0.02 for B. semiovata. In both species, the seed production of selfed flowers was on average 12% lower than that of outcrossed flowers (B. hirsuta P = 0.07, B. semiovata P < 0.05, mixed model ANOVAs). There was no significant effect of crosstype on germination rate or survival, but selfed offspring had a lower dry mass than outcrossed offspring 18 weeks after planting in both species (on average 18% lower in B. hirsuta and 31% lower in B. semiovata). Plants that were the products of selfing began flowering later than plants produced through outcrossing in B. semiovata, but not in B. hirsuta. The effects of crosstype on seed production (B. semiovata), days to first flower and offspring dry mass (both species) varied among maternal parents, as indicated by significant crosstype x maternal parent interactions for these characters. Both species showed significant inbreeding depression for total fitness (estimated as the product of seed production, germination rate, survival and dry mass at 18 weeks). In B. hirsuta, the average total inbreeding depression was 22% (range -57%-98%; N = 23 maternal parents), and in B. semiovata, it was 42% (-11%-84%; N = 21). This study demonstrates that highly selfing populations can harbor substantial inbreeding depression. Our findings are consistent with the hypothesis that a high mutation rate to mildly deleterious alleles contributes to the maintenance of inbreeding depression in selfing populations.     

Different levels of inbreeding depression between outcrossing and selfing Serapias species

We quantified inbreeding depression for fruit production, embryo vitality and seed germination in three deceptive orchids, Serapias vomeracea, S. cordigera and S. parviflora, which do not provide any reward to their pollinators, and are predicted to experience high outcrossing. Of the three species examined only S. parviflora was autonomously selfing. Both S. vomeracea and S. cordigera showed highly significant differences in fitness between selfed and outcrossed progenies, resulting in high levels of inbreeding depression, which increased in magnitude from seed set to seed germination. Inbreeding depression may promote outcrossing in Serapias by acting as a post-pollination barrier to selfing. Cumulative inbreeding depression across three stages in S. parviflora was lower that in both outcrossing species. The large difference in germination between selfed and outcrossed seeds is an important issue in conservation biology.     

Experimental and genetic analyses reveal that inbreeding depression declines with increased self‐fertilization among populations of a coastal dune plant

Theory predicts that inbreeding depression (ID) should decline via purging in self‐fertilizing populations. Yet, intraspecific comparisons between selfing and outcrossing populations are few and provide only mixed support for this key evolutionary process. We estimated ID for large‐flowered (LF), predominantly outcrossing vs. small‐flowered (SF), predominantly selfing populations of the dune endemic Camissoniopsis cheiranthifolia by comparing selfed and crossed progeny in glasshouse environments differing in soil moisture, and by comparing allozyme‐based estimates of the proportion of seeds selfed and inbreeding coefficient of mature plants. Based on lifetime measures of dry mass and flower production, ID was stronger in nine LF populations [mean δ = 1?(fitness of selfed seed/fitness of outcrossed seed) = 0.39] than 16 SF populations (mean δ = 0.03). However, predispersal ID during seed maturation was not stronger for LF populations, and ID was not more pronounced under simulated drought, a pervasive stress in sand dune habitat. Genetic estimates of δ were also higher for four LF (δ = 1.23) than five SF (δ = 0.66) populations; however, broad confidence intervals around these estimates overlapped. These results are consistent with purging, but selective interference among loci may be required to maintain strong ID in partially selfing LF populations, and trade‐offs between selfed and outcrossed fitness are likely required to maintain outcrossing in SF populations.     

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.     

INFERENCES ABOUT INBREEDING DEPRESSION BASED ON CHANGES OF THE INBREEDING COEFFICIENT

In a mixed-mating population, the fitness of selfed individuals, relative to outcrossed individuals, can be estimated from observed changes of the inbreeding coefficient F. In general, three measurements of F, or two measurements of F and one of the selfing rate, spanning two generations are needed. If however, adult F is assumed constant among generations, the adult F and selfing rate of one generation are sufficient to estimate relative fitness. Estimates of the relative fitness of selfed individuals for 14 Mimulus populations, assuming equilibrium of adult F, averaged 0.38, which is significantly lower than the 0.50 threshold needed to favor selfing genotypes. However, estimates for some populations showed large variance and over the 14 populations, relative fitness did not correlate with selfing rate. Violations of the equilibrium assumption causes a positive bias of the estimate and a spurious negative correlation of estimates with selfing rate. Estimates of relative fitness based on two generations of F do not suffer these problems. The need for large sample sizes, multiallelic loci, and moderate levels of natural selfing limits the usefulness of using changes of F to infer inbreeding depression.     

Interspecific hybridization between Vigna radiata (L.) Wilczek and V. glabrescens

Summary Interspecific hybrids of the mungbean, Vigna radiata (L.) Wilczek (2n=22) and V. glabrescens (2n=44) were generated with the aid of embryo culture. V. glabrescens x V. radiata hybrids were recovered via germination of the immature embryos. Reciprocal hybrids were obtained via shoot formation from embryonic callus. The authenticity of the hybrids was determined by morphological characteristics, chromosome number, and isozyme patterns. The hybrids were highly sterile upon selfing, but backcrossing to the diploid parent yielded viable seeds. Some of the plants resembled the diploid parent morphologically while others resembled neither parent. The backcross plants were sufficiently fertile to give a large number of mature, selfed seeds. Plants obtained differed morphologically and in their isozyme patterns from either parent, indicating introgression. These progeny populations will be used as bridging materials to transfer pest resistance from the wild tetraploid to the cultivated mungbean.     

Patterns of self compatibility, inbreeding depression, outcrossing, and sex allocation in a marine bryozoan suggest the predominating influence of sperm competition

Sex allocation by simultaneous hermaphrodites is theoretically influenced by selfing rate, which is in turn influenced by the benefits of enhanced genomic transmission and reproductive assurance relative to the cost of inbreeding depression. The experimental investigation of these influences in seed plants has a rich pedigree, yet although such an approach is equally relevant to colonial invertebrates, which globally dominate subtidal communities on firm substrata, such studies have been scarce. We reared self‐compatible genets of the marine bryozoan Celleporella hyalina s.l. in the presence and absence of allosperm, and used molecular genetic markers for paternity analysis of progeny to test theoretical predictions that: (1) genets from focal populations with high selfing rates show less inbreeding depression than from focal populations with low selfing rates; (2) genets whose selfed progeny show inbreeding depression prefer outcross sperm (allosperm); and (3) genets bias sex allocation toward female function when reared in reproductive isolation. Offspring survivorship and paternity analysis were used to estimate levels of inbreeding depression and preference for outcrossing or selfing. Sex allocation was assessed by counting male and female zooids. As predicted, inbreeding depression was severe in selfed progeny of genets derived from the populations with low self‐compatibility rates, but, with one exception, was not detected in selfed progeny of genets derived from the populations with higher self‐compatibility rates. Also, as predicted, genets whose selfed progeny showed inbreeding depression preferred outcrossing, and a genet whose selfed progeny did not show inbreeding depression preferred selfing. Contrary to prediction, sex allocation in the majority of genets was not influenced by reproductive isolation. Lack of economy of male function may reflect the over‐riding influence of allosperm‐competition in typically dense breeding populations offering good opportunity for outcrossing. We suggest that hermaphroditism may be a plesiomorphic character of the crown group Bryozoa, prevented by phylogenetic constraint from being replaced by gonochorism and therefore not necessarily adaptive in all extant clades. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 519–531.     

THE SIGNIFICANCE OF OUTCROSSING IN AN INTIMATE PLANT-HERBIVORE RELATIONSHIP. I. DOES OUTCROSSING PROVIDE AN ESCAPE FROM HERBIVORES ADAPTED TO THE PARENT PLANT?

Sexual reproduction may be advantageous for hosts that are preyed on or parasitized by enemies that are highly adapted to them. Sexual reproduction can create rare genotypes that may escape predation by virtue of rarity and can create variable progeny that may escape predation if enemies are specialized to only one genotype of host. Populations of the herbivorous thrips, Apterothrips apteris, have been shown to be adapted to individual Erigeron glaucus clones. Here, we show that thrips adapted to the parental clone could better use plant progeny of the “home” clone produced through selfing than progeny derived from selfing of other clones. Thus, despite recombination, progeny produced by selfing presented a resource that was similar to the parental phenotype with respect to use by adapted thrips. We also show that E. glaucus susceptibility to thrips has a genetic basis and then ask whether outcrossing provides a means for E. glaucus clones to escape attack by adapted thrips. When we compared the success of thrips on progeny produced by selfing or outcrossing of the home clone, we found that the merits or disadvantages associated with outcrossing were dependent on the susceptibility to infestation of the parental clones. Selfing by clones characterized by low infestations of thrips appeared to preserve resistant genotypes; all outcrossed progeny had, on average, higher infestation levels than selfed progeny. In contrast, outcrossed progeny of clones characterized by high infestations of thrips had either the same thrips density as progeny from selfing, when the pollen donor was a highly infested clone, or lower density, when the pollen donor was a low infestation clone. The advantages of outcrossing were caused by the alleles contributed to progeny rather than to progeny variability or rarity.     

Linkage between an incompatibility locus and a peroxidase isozyme locus (Prx 7) in rye

Summary Under controlled growth chamber conditions of 30 °C, seed set after selfing is possible in normally self-incompatible rye plants. Within selfed progenies produced by this method, plants homozygous at the peroxidase isozyme locus Prx 7 were crossed to heterozygous individuals. Segregation at the Prx 7 locus in progenies of these crosses provides clear evidence of a close linkage between Prx 7 and one of the two incompatibility loci in rye. A recombination fraction in the range of 0–2% was calculated from the segregation data. In rye, Prx 7 is linked with a phosphoglucoisomerase locus (Pgi). The similarity between the observations in Secale cereale and those made in Lolium perenne is discussed.     

On sex allocation and selfing in higher plants

Summary Sex allocation (male allocation/female allocation) as a function of selfing rate is studied in the wild riceOryza perennis. Using dry weight measures, the male/female ratio is linearly related to the selfing rate. This linear relationship may have a fairly radical interpretation in terms of current sex allocation theory. It suggests that the intermediate selfing rates are themselves maintained by a form of frequency dependence. In particular, the linearity suggests: (i) the relative fitness of a selfed versus outcrossed offspring decreases with increased selfing; (ii) in equilibrium, a selfed offspring is approximately half as fit as an outcrossed offspring; (iii) the frequency dependence, being the opposite of that proposed in most selfing models, may result from the same forces thought to be involved in the maintenance of sex itself, and (iv) the position of the fitted line contains information about the plant's use of wind pollination for male reproduction. It suggests that wind shows much less mixing of pollen than previously assumed, and/or that there are severe morphological constraints on pollen presentation. The above interpretations are clearly speculative and tentative. Possible problems in the analysis, and some alternatives for data interpretation are discussed.     

INBREEDING DEPRESSION IN TWO MIMULUS TAXA MEASURED BY MULTIGENERATIONAL CHANGES IN THE INBREEDING COEFFICIENT

In mixed-mating plant populations, one can estimate the relative fitness of selfed progeny w by measuring the inbreeding coefficient F and selfing rate s of adults of one generation, together with F of adults in the following generation (after selection). In the first application of this multigenerational method, we estimated F and s for adults over three consecutive generations in adjacent populations of two annual Mimulus taxa: the outbreeding M. guttatus and the inbreeding M. platycalyx. This gave estimates of w for the last two generations. Although average multilocus selfing rates were high in both taxa (0.63 in M. guttatus; 0.84 in M. platycalyx), the relative fitness of selfed progeny averaged only 0.19 in M. guttatus and 0.32 in M. platycalyx. An alternative estimator for w that incorporates biparental inbreeding gave even lower estimates of w. These values are significantly below the 0.5 threshold thought to favor selfing, and show that partially selfing populations can harbor substantial genetic load. In accordance with the purging hypothesis, the more highly selfing M. platycalyx showed marginally lower inbreeding depression than M. guttatus in both years (P = 0.08). Inbreeding depression and selfing rates also varied among years in concert among taxa. Several sources of bias are discussed, but computer simulations indicate it is unlikely that w is biased downwards by linkage of marker loci to load loci.     

Selfing and inbreeding depression in seeds and seedlings of Neobalanocarpus heimii (Dipterocarpaceae)

We evaluated the degree of selfing and inbreeding depression at the seed and seedling stages of a threatened tropical canopy tree, Neobalanocarpus heimii, using microsatellite markers. Selection resulted in an overall decrease in the level of surviving selfed progeny from seeds to established seedlings, indicating inbreeding depression during seedling establishment. Mean seed mass of selfed progeny was lower than that of outcrossed progeny. Since the smaller seeds suffered a fitness disadvantage at germination in N. heimii, the reduced seed mass of selfed progeny would be one of the determinants of the observed inbreeding depression during seedling establishment. High selfing rates in some mother trees could be attributed to low local densities of reproductive individuals, thus maintenance of a sufficiently high density of mature N. heimii should facilitate regeneration and conservation of the species.     

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.     

THE SIGNIFICANCE OF OUTCROSSING IN AN INTIMATE PLANT-HERBIVORE RELATIONSHIP. II. DOES OUTCROSSING POSE A PROBLEM FOR THRIPS ADAPTED TO THE HOST-PLANT CLONE?

The thrips herbivore Apterothrips apteris reproduces both sexually and parthenogenetically and exhibits fine-scale local adaptation to individual phenotypes of its host plant, Erigeron glaucus. We ask whether we can disrupt the ability of thrips to use progeny of their “home”-plant clone by outcrossing females with males from other plant clones. We compare the performance of sexually produced thrips to that of parthenogenetically produced thrips on plant progeny of the home clone. Because we use thrips from plant clones experiencing both high and low infestations by thrips, we also ask whether the relative performance of “sexuals” versus “parthenogens” differs with the infestation level of the home clone. Plant progeny of 10 E. glaucus clones were either the product of selfing or of outcrossing with the other 9 clones. We have shown in previous work that selfing preserved the parental phenotype with respect to attack by thrips. Because of this result, we predicted that parthenogens should fare better than sexuals on the selfed progeny of the home-plant clone. Our results, however, showed the contrary: sexuals outperformed parthenogens on these selfed plants. We also found that plant characters appear to influence thrips performance more than the mating system of thrips. We found no evidence for outbreeding depression in this system.     

FITNESS CONSEQUENCES OF OUTCROSSING IN IMPATIENS CAPENSIS: TESTS OF THE FREQUENCY-DEPENDENT AND SIB-COMPETITION MODELS

Using field and greenhouse experiments, we tested two hypotheses that could account for the maintenance of outcrossing in Impatiens capensis. Seedlings derived from cleistogamous (CL) and chasmogamous (CH) flowers were grown under competitive conditions while flanked by neighbors that were either related or unrelated. In both experiments, CH progeny sometimes expressed more phenotypic variability than CL progeny. In the greenhouse experiment, CH progeny attained the same weight as CL progeny, and the relatedness of neighboring plants did not affect the growth of either type. In the field experiment, CL and CH progeny performed similarly when grown with related competitors. However, CH progeny were somewhat larger when planted with nonsibs, while CL progeny were somewhat smaller under those conditions. Thus, there is no evidence that either frequency-dependent selection or the avoidance of competition among siblings favors the maintenance of outcrossing in this species. We also modeled the relative variability of selfed and outcrossed progeny under several reproductive systems. When females mate with one male (progeny are full sibs), selfed progeny are often more variable than outcrossed progeny. When females engage in both selfing and outcrossing, variation among progeny is frequently maximized at an intermediate selfing rate. The sib-competition mechanism, under a range of genetic models, is not apt to promote outcrossing, since selfed progeny are commonly more variable than outcrossed progeny.     

Modes of self-fertilization in Mimulus guttatus (Scrophulariaceae): a field experiment

We inferred Lloyd's modes of selfing in a natural population of the common monkeyflower, Mimulus guttatus. Estimates were obtained using floral manipulations combined with seed counts and isozyme analyses of selfing rates. Of the 25% selfing estimated from isozyme markers, about one-half was competing, about one-third was geitonogamous, and at least one-fifth (perhaps twice this) was due to biparental inbreeding. Estimates of prior and delayed selfing were small and did not significantly differ from zero. These results were obtained using plants with the characteristic pair of open flowers at an inflorescence node. The second-opening flower showed twice the rate of selfing, presumably because of protogynous-based geitonogamy differences. Solitary-flowered plants, which have smaller flowers but no geitonogamy, showed about 50% selfing, consisting of about equal components of competing selfing and biparental inbreeding. While geitonogamy and biparental inbreeding might be unavoidable by-products of adaptations for outcrossing, competing selfing is subject to more direct natural selection and warrants adaptive explanations.     

DNA methylation in diploid inbred lines of potatoes and its possible role in the regulation of heterosis

Self-incompatible diploid potatoes were altered to self-compatible ones by a function of S-locus inhibitor gene and continued selfing generated highly homozygous inbreds. In this study, this process was investigated for the status of DNA methylation by a simple method using genomic DNA digested by methylation-sensitive restriction enzymes prior to RAPD analysis. We detected 31 methylation-sensitive RAPD bands, of which 11 were newly appeared in the selfed progenies, and 6 of them stably inherited to subsequent generations. Aberrant segregations and paternal- or atavism-like transmission were also found. Segregating methylation-sensitive bands in initial populations became fixed in the advanced selfed progenies by 75.0–93.8%, of which 41.7% were fixed to all present and 58.3% to all absent. Because DNA methylation is generally recognized to suppress gene expression as regulatory factors, homozygosity/heterozygosity of methylated DNA may be involved in inbreeding depression/heterosis.