THE DISTRIBUTION OF SELFING RATES IN HOMOSPOROUS FERNS

The models of Lande and Schemske predict that among species in which the selfing rate is largely under genetic control and not subject to tremendous environmental variation, the distribution of selfing rates should be bimodal. When this prediction was tested empirically using data from the literature for species of angiosperms and gymnosperms, the distribution of outcrossing rates for all species was clearly bimodal. To provide another empirical test of the prediction, we analyzed mating-system data for 20 species of Pteridophyta (ferns). Homosporous ferns and their allies are unique among vascular plants because three types of mating are possible: intragametophytic selfing (selfing of an individual gametophyte); intergametophytic selfing (analogous to selfing in seed plants); and intergametophytic crossing (analogous to outcrossing in seed plants). The distribution of intragametophytic selfing rates among species of homosporous ferns is clearly uneven. Most species of homosporous ferns would be classified as extreme outcrossers. In contrast, a few species are nearly exclusively inbreeding. In only a few populations of Dryopteris expansa and Hemionitis palmata and a single population of Blechnum spicant do we see convincing evidence of a mixed mating system. The uneven distribution of selfing rates we observed for homosporous ferns, coupled with a corresponding bimodality of the magnitude of genetic load, strongly supports the model.     

SEX CHANGE WITH INBREEDING: EXPERIMENTS ON SEPARATE VERSUS COMBINED SEXES

Ipomopsis rubra plants grown in the laboratory initially produced hermaphrodite flowers, but some self- or sib-mated individuals switched to produce large numbers of pistillate (male sterile) flowers. The sex change did not occur with outcrossing. Plants with extreme male sterility were also observed in natural populations, usually in smaller individuals. Male sterility may be compensated by more seeds (resource reallocation), better seeds (avoidance of selfing), or both. Pistillate flowers were smaller, so savings could be used for additional seeds. Selfed seeds had reduced survival and fecundity, so avoidance of selfing could produce better quality offspring. We explored costs and benefits of sex change with two fitness models. The first assumes randomoutcross matings. Estimates of resource reallocation and inbreeding (selfing) depression are sufficient for pistillate inflorescences to have equal or greater fitness than hermaphrodite inflorescences if the selfing rate is high. Frequencies of sex change with intensive self-pollination were consistent with this model. The second model assumes all nonself matings are between sibs in “local mating” groups. Parents may benefit by male sterility in offspring, but gains would be higher if sex change occurred earlier and at higher than observed frequencies.     

Selfing versus outcrossing propensity of the fungal pathogen Microbotryumviolaceum across Silenelatifolia host plants

In the fungal pathogen Microbotryumviolaceum mating (i.e. conjugation between cells of opposite mating type) is indispensable for infection of its host plant Silenelatifolia. Since outcrossing opportunities are potentially rare, selfing may be appropriate to ensure reproduction. On the other hand, outcrossing may create genetic variability necessary in the coevolutionary arms race with its host. We investigated the propensity of M. violaceum to outcross vs. self in different host environments. We used haploid sporidia from each of three strains from five fungal populations for pairwise mixtures of opposite mating type, representing either selfing or outcrossing combinations. Mixtures were exposed to leaf extract from seven S. latifolia plants. The proportion of conjugated sporidia quantified mating propensity. The identity of both fungal strains and host influenced conjugation. First, individual strains differed in conjugation frequency by up to 30%, and strains differed in their performance across the different hosts. Second, selfing combinations produced, on average, more conjugations than did outcrossing combinations. Selfing appears to be the predominant mode of reproduction in this fungus, and selfing preference may have evolved as a mechanism of reproductive assurance. Third, individual strains varied considerably in conjugation frequency in selfing and outcrossing combinations across different hosts. This indicates that conjugation between outcrossing partners could be favoured at least in some hosts. Since the dikaryon resulting from conjugation is the infectious unit, conjugation frequency may correspond with infection probability. This assumption was supported by an inoculation experiment, where high infectious sporidial dosage resulted in higher infections success than did low dosage. We therefore predict that sexual recombination can provide this pathogen with novel genotypes able to infect local resistant hosts.     

A serum protein polymorphism determinant on chromosome 9 of Mus musculus

Summary Genetic polymorphism for a previously undescribed serum protein has been found among inbred strains of Mus musculus. The new serum protein locus, gene symbol Sep-1, has been located on Chromosome 9, gene order Lap-1-Sep-1-Mpi-1-d-Mod-1, by utilizing information obtained from 52 recombinant inbred strains together with standard genetic backcrosses. The strain distribution pattern for this locus, supernatant malic enzyme, and transferrin, also on Chromosome 9, are given for 67 inbred strains. Because the genotype of SEP-1 can be determined for individual mice without killing them, Sep-1 is a very useful gene in linkage studies and experimental biology.     

Analysis of the disassortative mating pattern in a heterodichogamous plant, <Emphasis Type="Italic">Acer mono</Emphasis> Maxim. using microsatellite markers

Heterodichogamy is a form of sex expression in which protandrous and protogynous individuals coexist, and is considered to be a mechanism that avoids selfing and promotes disassortative mating. We examined mating patterns in a heterodichogamous maple, Acer mono, using microsatellite markers. Parentage analysis revealed a selfing rate of only 9.8%. Disassortative mating between flowering types significantly exceeded within-type mating, but the mating patterns were better explained by flowering phenology (i.e., the temporal overlap between the female and male stages). Heterodichogamy in A. mono thus appears to promote outcrossing without requiring obligate self- or cross-incompatibility systems, although it did not guarantee disassortative mating. Multiple-regression analysis suggested that successful reproduction of pollen parents significantly increased with increased flower production and reciprocal flowering synchrony, but decreased only marginally with mating distance, although the distribution of mating distances suggested leptokurtic dispersal of pollen.     

Sex ratio variation in female-biased populations of Notostracans

Females from female-biased populations of the notostracan Triops newberryi produce viable eggs when reared in isolation. Clutches produced under such conditions exhibit sex-ratio polymorphism. One category of females (monogenics) produces only female offspring; the second category (amphigenics) produces clutches with a sex ratio of 3 females: 1 male. The relative proportions of the two categories of females varies significantly between populations and is correlated with population sex ratio. This correlation, and the pattern of offspring distribution in amphigenics, suggests that sex is determined by an autosomal Mendelian gene locus for which the male-determining allele is recessive. Limited pedigree analysis of lineages under selfing support the genetic model.     

Sex allocation and mating systems in pigeonpea,Cajanus cajan (Fabaceae)

Sex allocation theory predicts that: (1) resources allocated to androecium should decrease with an increase in selfing, (2) a decrease in androecium biomass should be accompanied by an increase in the biomass of pistils, and (3) a decrease in androecium biomass should be coupled with a decrease in flower size, specifically corolla biomass. Another predicted change in reproductive traits associated with variation in selfing concerns seed to ovule ratios, but does not directly stem from sex allocation theory. It has been postulated that seed to ovule ratios should be positively correlated with the amount of selfing. These predictions were tested for six accessions of pigeonpea,Cajanus cajan L., that differed in selfing rates. The results were remarkably in accordance with the predictions. We conclude that sex allocation theory provides a powerful tool to understand the evolution of many reproductive traits in plants.     

Dissection of conjugants and mating type plus and minus cells in selfing clones of the isogamous green alga Closterium ehrenbergii

In the sexual reproduction of the green alga Closterium ehrenbergii, two sexually competent cells that are morphologically indistinguishable from the vegetative cells first come close to each other to form a sexually interacting pair. Each then divides into two gametangial cells. Isogamous conjugation occurs between nonsister gametangial cells of the two resulting pairs. With unusual selfing clones derived from a certain cross of heterothallic strains, we dissected apart a pair of gametangial cells that had already been united together by a delicate transparent tube, into which each gametangial cell was going to develop its conjugation papilla. In spite of such a degree of differentiation, when each was cultured in fresh medium, individual gametangial cells could dedifferentiate into vegetative cells and form subclones. By crossing such subclones with standard stable heterothallic mating-type strains, we show that each selfing clone of this alga actually produces both stable mt ⁺ and stable mt ^- cells, in addition to unstable mt ^- cells with selfing potency, during its mitotic vegetative growth. Although the selfing in C. ehrenbergii studied here differs in certain points from true homothallism, the results of the present study provide insight into how homothallism might have evolved from heterothallism.     

Early evolution in a hybrid swarm between outcrossing and selfing lineages in Geum

Although often considered as evolutionary dead ends, selfing taxa may make an important contribution to plant evolution through hybridization with related outcrossing lineages. However, there is a shortage of studies examining the evolutionary dynamics of hybridization between outcrossing and selfing taxa. On the basis of differential pollinator attractiveness, production and competitive ability of pollen, as well as levels of inbreeding depression, we predict that the early products of hybridization between outcrossing and selfing lineages will be F1s and first-generation backcrosses sired mainly by the outcrossing lineage, together with selfed F2s containing a limited genetic contribution from the outcrosser. These predictions were tested using amplified fragment length polymorphism and chloroplast markers to analyze the composition of a recent hybrid swarm between predominantly outcrossing Geum rivale and predominantly selfing Geum urbanum. In line with predictions, the hybrid swarm comprised both parental species together with F1s and first-generation backcrosses to G. rivale alone. Chloroplast data suggested that G. rivale was the pollen parent for both observed hybrid classes. However, there was no evidence for F2 individuals, despite the fact that the F1 was fully self-compatible and able to auto-pollinate. The pollen fertility of F1s was only 30% lower than that of the parental taxa, and was fully restored in backcross hybrids. Predicting future evolution in the hybrid swarm will require an understanding of the mating patterns within and among the mix of parental, F1 and backcross genotypes that are currently present. However, these results support the hypothesis that introgression is likely to be asymmetrical from selfing to outcrossing lineages.     

Generation and mapping of Mus spretus strain-specific markers for rapid genomic scanning

We describe here a set of genetic markers, based on IRS–PCR amplification difference, that are specifically designed for efficient, high throughput genetic mapping in [(M. domesticus× wild-derived) F₁×M. domesticus] interspecific backcrosses. 146 new genetic loci have been mapped, and strain distribution for these markers has been determined in 96 mouse strains. 103 (81%) of 127 tested markers are present only in one or more wild-derived strains, but absent in 76 other commonly used strains, demonstrating their utility in a variety of mouse pair combinations. Because of the ease of genotyping with this marker set, rapid genome scans for complex genetic trait loci involving crosses between wild-derived strains and other commonly used strains can now be carried out efficiently with large numbers of animals. Received: 2 October 1995 / Accepted: 12 December 1995     

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.     

Mouse microsatellites from a flow-sorted 4:6 Robertsonian chromosome

Twenty microsatellites were generated from a previously characterized gt10 library containing C57BL/6J mouse DNA from a flow-sorted 4:6 Robertsonian chromosome. These sequences were analyzed for size variation between different strains of mice with the polymerase chain reaction (PCR) and mapped by use of either strain distribution patterns (SDPs) in recombinant inbred (RI) strains, or intra- and interspecific backcrosses. Eighty-five percent of the sequences showed allelic variations between different inbred strains of mice and the wild mouse, Mus spretus, and 70% were variant between inbred strains. Eight (62%) of the 13 repeats that have been mapped lie on Chromosomes (Chr) 4 and 6. This approach is an effective way of generating informative markers on specific chromosomes.     

Meiosis and fertility of interspecific hybrids betweenPhaseolus vulgaris L. andP. acutifolius A. Gray

Summary Meiosis and fertility of interspecific hybrids obtained from reciprocal crosses between Phaseolus vulgaris and P. acutifolius were examined. Bivalents as well as univalents were found at Metaphase I. The majority of the microsporocytes had four or more univalents and the average was 6.3 univalents per cell. The average number of lagging chromosomes at Anaphase I was 2.3 per cell and the most frequent chromosome distribution at late Anaphase I was 10–12. The lower than expected number of lagging chromosomes as compared with the number of univalents at Metaphase I suggests the possible occurrence of precocious separation of bivalents. The male fertility as measured by pollen stainability was 17%, however, the frequency of pollen germination in selfing was 3.5%. Upon selfing of the interspecific hybrids, no dividing embryos were found even though 7 and 26% of the ovules were fertilized at 12 hours and four days after pollination. In backcrosses to P. vulgaris (male), 6 and 20% of the ovules were fertilized and 0 and 4% of the ovules contained dividing embryos at the same sampling times. When P. acutifolius was the male parent, respective values were 8 and 31% for fertilization and 0 and 13% for ovules with dividing embryos. The frequencies of backcross embryos recovered at 14–26 days were in agreement with the frequencies of dividing embryos at four days. The ability to obtain backcross plantlets demonstrates the feasibility to further utilize interspecific hybrids for the improvement of P. vulgaris Technical paper No. 5311 of the Oregon Agricultural Experiment Station. Research was supported by the Oregon Agricultural Experiment Station, the Science and Education Administration of the U.S. Department of Agriculture under Grant 5901-0410-8-0028-0 from the Competitive Research Grants Office, the Research Council of Oregon State University (NIH Biomedical Research Support Grant RR 07079) and the Processor Research Council of Oregon, A.R. and C T.S. are respectively supported by an African Graduate Fellowship from the African-American Institute and a fellowship from the National Science Council of the Republic of China     

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.     

Localization of growth arrest-specific genes on mouse Chromosomes 1, 7, 8, 11, 13, and 16

Growth arrest in NIH3T3 cells is associated with increased expression of a variety of mRNAs, several of which have been isolated as cDNA clones. Six of these growth arrest-specific (Gas) genes were mapped by following the inheritance of DNA restriction fragment length variants (RFLVs) associated with them in panels of recombinant inbred (RI) strains of mice and in the progeny of backcrosses both between laboratory mouse strains and between a laboratory strain and Mus spretus. The six genes are unlinked. Gas-1 maps to Chromosome (Chr) 13, Gas-2 to Chr 7, Gas-3 to Chr 11, Gas-4 to Chr 16, Gas-6 to Chr 8, and Gas-10 to Chr 1.     

Sex determination and evolution of unisexuality in the Conchostraca

Field collected or laboratory-reared samples of 60 species of conchostracans (representing all extant genera) indicate that males and females are equally common in most species. Deviations from this pattern occur in four lineages.Cyzicus andLeptestheria each include at least one unisexual species; many species of Limnadiinae are either unisexual or characterized by female-biased sex ratios; and Cyclestheriidae are either unisexual or express males in the later generations of their life cycles. Laboratory studies indicate that species with sex ratios near unity are gonochoric (obligately sexual), whereas females in species with female-biased sex ratios are capable of both outcrossing and selfing modes of reproduction. Phylogenetic analysis of patterns of reproduction suggest that sexual reproduction is the primitive condition. Genetic analysis of sexual species indicate that gender is determined by one or a few genetic factors and that the male-determining allele is recessive. The inheritance of gender in androdioecious species (where females are capable of self-fertilization) is similar to that in sexual species. Androdioecy is likely to be the intermediate stage between obligately sexual reproduction and unisexuality in the Limnadiinae. The phylogenetic distribution of sex ratio variation suggests that unisexuality in Cyzicidae, Leptestheriidae, and Cyclestheriidae has arisen independently of that in the Limnadiinae and that these cases have evolved by different evolutionary pathways.     

Fertilization-induced Chemotaxis in the Zygotes of the Watermold Allomyces

Zygotes obtained by self-fertilization of Allomyces macrogynus, strain Burma 3 and of A. arbuscula, strain Ceylon 1, behave chemotactically as do their respective zoospores. All the swarmers respond to an equimolar mixture of L-leucine and L-lysine with the response enhanced by the addition of L-Proline. The A. arbuscula swarmers also respond moderately to a mixture of L-proline with certain amino acids other than leucine and lysine whereas those of A. macrogynus do not. The gametes are not chemotactically responsive to the amino acids. Within no more than five minutes subsequent to fertilization, the zygotes become chemotactically active. The genetically-derived, approximately 95 % male or female isolates do not appear to form zygotes when crossed. The few zygotes observed in a series of attempted crosses appear to be the result of selfing by the contaminating opposite sex in each of the highly unisexual strains.     

Methods for Inducing Selfing, Selfing and Its Role in the Life Cycle of Euplotes woodruffi syngen 3 (Ciliophora)

Methods for inducing selfing, and the relation between selfing and the life cycle of Euplotes woodruffi syngen 3 are reported. Three intercrossing stocks were used in this experiment. Selfing was induced with several treatments as follows: cell-free fluid from the cultures of complementary mating types; intact cells of GI or S phase in the cell cycle; heat-killed cells, and lysed cells of GI-, S-, and D-phase cells which were prepared by freeze-thawing. Stock SJ-27 was used as a parental stock from which Fl clones were originated through selfing. The other two stocks, SJ-8 and SJ-19, were used as testers. The period of immaturity varied from clone to clone. The heterotypic conjugation of clones with cells of stock SJ-8 seems to occur earlier in the life cycle than with cells of stock SJ-19. This result shows that this syngen has an adolescent period in the life cycle. The length of selfing immaturity seems to be different from that of crossing immaturity, and selfing appeared slightly later than crossing with testers. But the clones in which selfing 1st occurred are considered to be in adolescence or maturity, not in senility. Once selfing appeared in any clone, the clone continued to produce selfing pairs till just before clonal death. The viability of selfing and of outcrossing were compared and found not significantly different. Inbreeding depression took place in some of the F2 clones by successive selfing. The viability of F2 clones from young parents was significantly higher than that from old parents (220 to 230 fissions) both in selfing and outcrossing. The total life spans which were studied in three F1 clones were 168 to 264 fissions.     

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.     

Role of parasite transmission in promoting inbreeding: I. Infection intensities drive individual parasite selfing rates

Among parasitic organisms, inbreeding has been implicated as a potential driver of host–parasite co‐evolution, drug‐resistance evolution and parasite diversification. Yet, fundamental topics about how parasite life histories impact inbreeding remain to be addressed. In particular, there are no direct selfing‐rate estimates for hermaphroditic parasites in nature. Our objectives were to elucidate the mating system of a parasitic flatworm in nature and to understand how aspects of parasite transmission could influence the selfing rates of individual parasites. If there is random mating within hosts, the selfing rates of individual parasites would be an inverse power function of their infection intensities. We tested whether selfing rates deviated from within‐host random mating expectations with the tapeworm Oochoristica javaensis. In doing so, we generated, for the first time in nature, individual selfing‐rate estimates of a hermaphroditic flatworm parasite. There was a mixed‐mating system where tapeworms self‐mated more than expected with random mating. Nevertheless, individual selfing rates still had a significant inverse power relationship to infection intensities. The significance of this finding is that the distribution of parasite infection intensities among hosts, an emergent property of the transmission process, can be a key driver in shaping the primary mating system, and hence the level of inbreeding in the parasite population. Moreover, we demonstrated how potential population selfing rates can be estimated using the predicted relationship of individual selfing rates to intensities and showed how the distribution of parasites among hosts can indirectly influence the primary mating system when there is density‐dependent fecundity.