Identification of a Selfing Compatible Genotype and Mode of Inheritance in Switchgrass

Switchgrass (Panicum virgatum L.) is being targeted for use as a dedicated lignocellulosic feedstock crop for producing bioenergy in the United States. The breeding of new switchgrass cultivars with enhanced performance is a research emphasis supporting the targeted use. The species is considered allogamous due to wind facilitated cross-pollination and strong genetic self-incompatibility. Plants typically produce few or no seed when self-fertilized. No attempt has been made to identify selfing-compatible plants that would potentially enable developing inbred lines. Here, using a set of 12 simple sequence repeat-based molecular markers, we identified one lowland plant, ??NL94 LYE 16?×?13?? (NL94), demonstrating high self-compatibility. A large potted plant of NL94 and a similar size plant of ??SL93 7?×?15?? were grown in a growth chamber for the purpose of producing a hybrid full-sib mapping population. Marker analyses of 456 progeny from the NL94 plant indicated that 279 (61.2%) and 177 (38.8%) resulted from self- and cross-fertilization, respectively. SSR marker segregation analyses in both the selfed and hybrid progeny populations conclusively indicated disomic inheritance in the two switchgrass parents. Disomic inheritance of switchgrass is significant to the development of switchgrass inbreds as homozygosity is approached much faster via inbreeding under disomic vs. tetrasomic inheritance. Self-compatibility in switchgrass potentially enables the development of inbred lines for use in producing heterotic F1 hybrid cultivars.     

Development of 1,030 genomic SSR markers in switchgrass

Switchgrass, Panicum virgatum L., a native to the tall grass prairies in North America, has been grown for soil conservation and herbage production in the USA and recently widely recognized as a promising dedicated cellulosic bioenergy crop. A large amount of codominant molecular markers including simple sequence repeats (SSRs) are required for the construction of linkage maps and implementation of molecular breeding strategies to develop superior switchgrass cultivars. The objectives of this study were (1) to identify SSR-containing clones and to design PCR primer pairs (PPs) in SSR-enriched genomic libraries, and (2) to validate and characterize the designed SSR PPs. Five genomic SSR enriched libraries were constructed using genomic DNA of ‘SL93 7 × 15’, a switchgrass genotype selected in an Oklahoma State University (OSU) southern lowland breeding population. A total of 3,046 clones from four libraries enriched in (CA/TG)n, (GA/TC)n, (CAG/CTG)n and (AAG/CTT)n SSR repeats were sequenced at the OSU Core Facility. From the sequences, we isolated 1,300 unique SSR-containing clones, from which we designed 1,398 PPs using SSR Locator V.1 software. Among the designed PPs, 1,030 (73.7%) amplified reproducible and strong bands with expected fragment size, and 802 detected polymorphic alleles, in SL93 7 × 15 and ‘NL94 16 × 13’, two parents of one mapping population. All of the four libraries contained a high rate of perfect SSR repeat types, ranging from 62.7 to 76.2%. Polymorphism of the effective SSR markers was also tested in two lowland and two upland switchgrass cultivars, encompassing ‘Alamo’ and ‘Kanlow’, and ‘Blackwell’ and ‘Dacotah’, respectively. The developed SSR markers should be useful in genetic and breeding research in switchgrass.     

Estimation of genetic variability and heritability for biofuel feedstock yield in several populations of switchgrass

Information on heritability and predicted gains from selection for increased biomass yield for ethanol production in switchgrass is limited and may vary among breeding populations. The purpose of this study was to estimate heritability and predicted gains from selection for higher biomass yield within a lowland ecotype switchgrass population, Southern Lowland 93 (SL‐93), and two upland ecotype switchgrass populations, Southern Upland Northern Upland Early Maturing (SNU‐EM) and Southern Upland Northern Upland Late Maturing (SNU‐LM). Narrow‐sense heritabilities (h_n²) for biomass yield in each of the three populations were estimated via progeny–parent regression analysis. Half‐sib (HS) progeny families from 130 randomly selected plants from the SL‐93 population were evaluated for biomass yield in replicated trials in 2002 and 2003. Clonal parent plants were evaluated for biomass yield in separate environments to provide unbiased h_n² estimates from progeny–parent regression. Yield differences were highly significant among SL‐93 HS progenies within and over years. For the SL‐93 population, h_n² estimates were 0.13 and 0.12 based on individual plant and phenotypic family mean (PFM) selection, respectively. Predicted genetic gains (ΔG) per selection cycle were 0.15 kg dry matter (dm) plant^?1 and 0.10 kg dm plant^?1 for PFM and individual plant selection methods, respectively. For the SNU‐EM and SNU‐LM populations, year and year × HS family effects were highly significant (P < 0.01) and the HS family effect over years was nonsignificant (P < 0.05). However, HS family effects were highly significant within respective years (P < 0.01). Estimates of h_n² for the SNU‐EM and SNU‐LM populations based on PFM and individual plant selection were similar, ranging from 0.44 to 0.47; ΔG per selection cycle ranged from 0.22 to 0.33 kg dm plant^?1. The magnitudes of the estimates of additive genetic variation suggest that selection for higher biomass yield should be possible. The substantial effect of environment on biomass yields in the upland populations and the failure of families to respond similarly over years stress the importance of adequately testing biomass yield over years to assess yield.     

The mixed mating system of Impatiens capensis and infection by a foliar rust pathogen: patterns of resistance and fitness consequences

Outcrossing by hosts may offer protection from natural enemies adapted to parental genotypes by creating diverse progeny that differ from their parents through genetic recombination. However, past experimental work addressing the relationship between mating system and disease in offspring has given conflicting results, suggesting that outcrossing might also cause the dissolution of resistant genotypes. To determine if selfed progeny are more susceptible to disease caused by the heteroecious rust, Puccinia recondita, or if selfing preserves existing resistant genotypes, we used a factorial design to compare levels of infection of selfed and outcrossed progeny of Impatiens capensis, a woodland annual with a mixed mating system. We compared the level of host infection when exposed to three pathogen sources in the field: the sympatric rust population, and two allopatric rust populations. Outcrossed progeny exposed to sympatric rust had higher infection scores than selfed progeny exposed to the same rust, suggesting that outcrossing breaks up resistant genotypes. In addition, there was a trend for the rust to be more infective on sympatric rather than allopatric hosts. We also examined whether rust infection differentially alters the fitness of selfed and outcrossed progeny. Outcrossed plants that escaped infection had higher fitness, as measured by fruit production, than selfed plants, but there was no difference in fitness between infected selfed and infected outcrossed plants. Thus, outcrossing was advantageous in the absence of disease, but there was no fitness difference between selfed and outcrossed progeny in the presence of disease. In sum, our results indicate that interactions with pathogens can eliminate or reverse the advantage of outcrossing.     

Development and integration of EST–SSR markers into an established linkage map in switchgrass

Switchgrass (Panicum virgatum L.) is a model cellulosic biofuel crop in the United States. Simple sequence repeat (SSR) markers are valuable resources for genetic mapping and molecular breeding. A large number of expressed sequence tags (ESTs) of switchgrass are recently available in our sequencing project. The objectives of this study were to develop new SSR markers from the switchgrass EST sequences and to integrate them into an existing linkage map. More than 750 unique primer pairs (PPs) were designed from 243,600 EST contigs and tested for PCR amplifications, resulting in 538 PPs effectively producing amplicons of expected sizes. Of the effective PPs, 481 amplifying informative bands in NL94 were screened for polymorphisms in a panel consisting of NL94 and its seven first-generation selfed (S1) progeny. This led to the selection of 117 polymorphic EST–SSRs to genotype a mapping population encompassing 139 S1 individuals of NL94. Of 83 markers demonstrating clearly scorable alleles in the mapping population, 79 were integrated into a published linkage map, with three linked to accessory loci and one unlinked. The newly identified EST–SSR loci were distributed in 17 of 18 linkage groups with 27 (32.5 %) exhibiting distorted segregations. The integration of EST–SSRs aided in reducing the average marker interval (cM) to 3.7 from 4.2, and reduced the number of gaps (each >15 cM) to 10 from 23. Developing new EST–SSRs and constructing a higher density linkage map will facilitate quantitative trait locus mapping and provide a firm footing for marker-assisted breeding in switchgrass.     

Effects of FUT1 gene mutation on resistance to infectious disease

Alpha-(1,2)-fucosyltransferase (FUT1) gene has been identified as a candidate gene for regulating the expression of Escherichia coli F18 receptor gene (ECF18R) which promotes adherence of Enterotoxigenic (ETEC) and Verotoxigenic (VTEC) Escherichia coli (E. coli) via F18 fimbriae. In order to illustrate the polymorphisms of FUT1 and their effects on resistance to natural infection by Porcine Respiratory and Reproductive Symdrome Virus (PRRSV) and Haemophilus parasuis, the distributions of different genotypes and the relative risks of disease incidence in pigs were investigated. A total of 1,041 pigs representing three European breeds (Duroc, Landrace and LargeWhite), five Chinese local breeds (Wild pig, Small MeiShan, QinPing, JinHua, and JianLi) and three commercial populations (LargeWhite?×?JianLi, Duroc?×?Landrace?×?LargeWhite and Duroc?×?wild pig) were selected to analyze the genotype of the FUT1 gene by PCR-RFLP. Only the GG genotype associated with susceptibility to ECF18 bacteria was detected in Chinese local pig breeds and a population of LargeWhite?×?JianLi, while the AA genotype which confers resistance to ECF18 was detected in two European breeds (Duroc and LargeWhite), two populations of Duroc?×?wild pig and Duroc?×?Landrace?×?LargeWhite. Regarding relative risk of incidence, Duroc?×?Landrace?×?LargeWhite with genotypes GG or AG showed greater relative risk (OR?=?2.040, P?=?0.025; OR?=?1.750, P?=?0.081, respectively) than those with genotype AA during natural infection by both PRRSV and Haemophilus parasuis. It can be concluded that the mutation of FUT1 gene might play a role in pig infection by multi-pathogens, and that AA may be a favourable genotype for increasing the resistance to disease.     

Inbreeding depression from selfing and mating between relatives in the Neotropical tree <Emphasis Type="Italic">Cariniana legalis</Emphasis> Mart. Kuntze

Selfing or mating between related individuals in self-compatible hermaphroditic tree species may lead to inbreeding depression (ID) due to homozygosis in recessive, identical by descent alleles. In general, studies of ID in tree species have been based on comparisons of selfed individuals (produced by controlled pollination) with outcrossed individuals for quantitative traits in progeny tests. However, this approach requires a long time to quantify the extent of ID. Thus, we used an approach based on genetic markers to estimate coancestry coefficients between assigned parents from paternity analysis in two populations of the Neotropical tree Cariniana legalis. Using this method, we were able to determine which seedlings in a nursery trial originated from; (i) outcrossing between un-related trees, (ii) mating between related trees and (iii) selfing. We detected a low selfing rate (<10 %), but a substantial quantity of seedlings from mating between related parents (minimum of 35.7 %). In general, the outcrossed seedlings from unrelated parents exhibited significantly greater genetic diversity than those resulting from selfing and mating among relatives. The extent of ID varied among traits and populations. Outcrossed seedlings originating from unrelated trees generally showed greater survival than seedlings originating from selfing and related parents. Inbreeding depression was greater in the selfed seedlings than in those from mating among related parents. The results are discussed in terms of implications for genetic conservation, breeding and environmental restoration using the species.     

The cost of selfing inEncyclia cochleata (Orchidaceae)

To determine whether inbreeding depression accounts for the maintenance of outcrossing in populations of the self-compatible orchidEncyclia cochleata, the estimated selective advantage of selfing was compared to a measure of inbreeding depression. Individuals from three populations ofE. cochleata and some of their progeny were phenotyped using isozyme analysis. The electrophoretic data were used to estimate the outcrossing rate and the theoretical cost of outcrossing. Inbreeding depression was estimated by comparing the fitness of the progeny resulting from both types of pollinations. The seeds from outcrossed and selfed hand-pollinations and naturally pollinated seeds from a population of the triandrous form ofE. cochleata were grown aseptically on culture media, and their development over the next three years recorded. Inbreeding was common, particularly in one population (outcrossing rate 40%). However, the level of inbreeding depression was only 1–2%, considerably less inbreeding depression than expected.     

Endogamy costs and reproductive biology of <Emphasis Type="Italic">Laelia autumnalis</Emphasis>, an endemic orchid of Mexico

It has been hypothesized that the fitness of selfed progeny must be lowered by inbreeding depression. Most research into the breeding systems of orchids shows a similar fruit set from outcrossing and self-pollination, but few studies have measured seed production and viability. In five populations of Laelia autumnalis, in central México, we studied the species reproductive system and the early costs of endogamy. We performed spontaneous self-pollination, apomixis, assisted self-pollination, cross-pollination, and exogamous pollination (using pollen from a different population) treatments of bagged flowers and measured fruit set, seed production, and germination. No fruits were produced in the apomixis and spontaneous self-pollination treatments. Reproductive success from self-pollination was lower than that from cross- and exogamous pollination and no difference was found between the latter two treatments. The ratio between self-pollination and cross-pollination success in different traits ranged from 0.80?±?0.18 (fruit set) to 0.22?±?0.13 (seeds with embryo). The latter value suggests a high endogamy costs in the production of viable seeds. The concatenated success of the different traits studied showed that the relative fitness of self-pollination was 63% lower than with cross-pollination. Laelia autumnalis is a self-compatible non-autogamous species in which the cost of endogamy occurs at seed production and cannot be compensated for at other early stages. This also occurs in other orchid species and is likely to be a general pattern.     

The mating system of the tropical understory shrub Ardisia escallonioides (Myrsinaceae)

Electrophoretic analysis of progeny arrays was used to determine whether Ardisia escallonioides (Myrsinaceae). a self-compatible tropical understory shrub in south Florida, exhibits a mixed-mating system. Ten seedlings from seven to 16 maternal plants from four populations were analyzed using five polymorphic enzyme systems. Multilocus population out-crossing rates ranged from 0.39 to > 1, with three populations showing high levels of selfing. Both single and multilocus inbreeding coefficients (F) indicated excess homozygosity in seedlings but not adults for three of the four populations. Population outcrossing rate was not positively correlated with increased numbers of flowering plants. The importance of annual variation in population outcrossing rates is discussed with regard to the high temporal variability in seedling recruitment in this species.     

Wild sorghum from different eco-geographic regions of Kenya display a mixed mating system

Knowledge of mating systems is required in order to understand the genetic composition and evolutionary potential of plant populations. Outcrossing in a population may co-vary with the ecological and historical factors influencing it. However, literature on the outcrossing rate is limited in terms of wild sorghum species coverage and eco-geographic reference. This study investigated the outcrossing rates in wild sorghum populations from different ecological conditions of Kenya. Twelve wild sorghum populations were collected in four sorghum growing regions. Twenty-four individuals per population were genotyped using six polymorphic simple sequence repeat (SSR) markers to compute their indirect equilibrium estimates of outcrossing rate as well as population structure. In addition, the 12 populations were planted in a field in a randomised block design with five replications. Their progeny (250 individuals per population) were genotyped with the six SSR markers to estimate multi-locus outcrossing rates. Equilibrium estimates of outcrossing rates ranged from 7.0 to 75.0%, while multi-locus outcrossing rates (t _m) ranged from 8.9 to 70.0% with a mean of 49.7%, indicating that wild sorghum exhibits a mixed mating system. The wide range of estimated outcrossing rates in wild sorghum populations indicate that environmental conditions may exist under which fitness is favoured by outcrossing and others under which selfing is more advantageous. The genetic structure of the populations studied is concordant with that expected for a species displaying mixed mating system.     

Effect of Potassium and Nitrogen Fertilizer on Switchgrass Productivity and Nutrient Removal Rates under Two Harvest Systems on a Low Potassium Soil

Biomass demand for energy will lead to utilization of marginal, low fertility soil. Application of fertilizer to such soil may increase switchgrass (Panicum virgatum L.) biomass production. In this three-way factorial field experiment, biomass yield response to potassium (K) fertilizer (0 and 68 kg?K?ha^?1) on nitrogen (N)-sufficient and N-deficient switchgrass (0 and 135 kg?N?ha^?1) was evaluated under two harvest systems. Harvest system included harvesting once per year after frost (December) and twice per year in summer (July) at boot stage and subsequent regrowth after frost. Under the one-cut system, there was no response to N or K only (13.4 Mg?ha^?1) compared to no fertilizer (12.4 Mg?ha^?1). Switchgrass receiving both N and K (14.6 Mg?ha^?1) produced 18 % greater dry matter (DM) yield compared to no fertilizer check. Under the two-cut harvest system, N only (16.0 Mg?ha^?1) or K only (14.1 Mg?ha^?1) fertilizer produced similar DM to no fertilizer (15.1 Mg?ha^?1). Switchgrass receiving both N and K in the two-cut system (19.2 Mg?ha^?1) produced the greatest (P?<?0.05) DM yield, which was 32 % greater than switchgrass receiving both N and K in the one-cut system. Nutrient removal (biomass?×?nutrient concentration) was greatest in plots receiving both N and K, and the two-cut system had greater nutrient removal than the one-cut system. Based on these results, harvesting only once during winter months reduces nutrient removal in harvested biomass and requires less inorganic fertilizer for sustained yields from year to year compared to two-cut system.     

Relationships between plant density,outcrossing rates and seed set in natural and experimental populations of Scabiosa columbaria

Outcrossing rates were estimated in both natural and experimental populations of Scabiosa columbaria, a self-compatible, entomophilous, gynodioecious, protandrous perennial. In natural populations, estimates of the outcrossing rate in hermaphrodites were near to one and ranged from 0.84 ± 0.07 to 1.12 ± 0.11. The effect of plant density on outcrossing rates was studied in two experimental populations of 27 individuals. Contrary to expectation the estimates of the outcrossing rate in hermaphrodites were about 100% for both densities. However, in the sparse population, the fraction of developed seeds of plants used to estimate outcrossing rates was significantly lower than of plants in the dense population (0.41 ± 0.06 and 0.68 ± 0.08, respectively). Artificial pollinations of these plants in the greenhouse showed that the fraction of developed seeds was 0.60 ± 0.01 and 0.83 ± 0.05 after self- and cross-pollination, respectively. The combined results suggested that the differential success of self- and cross-pollination might have caused equalization of the outcrossing rates in the experimental populations, despite different plant densities. The implications of the results for conservation biology are discussed.     

RELATIVE FITNESS OF SELFED AND OUTCROSSED PROGENY IN A SELF-COMPATIBLE,PROTANDROUS SPECIES,SABATIA ANGULARIS L. (GENTIANACEAE): A COMPARISON IN THREE ENVIRONMENTS

The consequences of selfing were examined for a population of self-compatible, protandrous, Sabatia angularis L. (Gentianaceae). Field-collected plants were hand-pollinated in the greenhouse to produce selfed progeny and outcrossed progeny from parents separated by a maximum of 5 m (near-outcross) and 85 m (far-outcross) in the field. Self, near-outcross, and far-outcross half sib progeny were grown in the greenhouse, a garden plot, and their native habitat. Progeny were compared by multiplicative fitness functions based on seed production per hand-pollination, seed germination, rosette formation, survival to reproduction, and reproduction in each environment. Variation in reproduction among progeny groups was influenced by the environment in which they were grown. Significant inbreeding depression was detected between the self and far-outcross progeny in each environment. Only the natural environment demonstrated a greater than 50% reduction in relative fitness of self compared to near-outcross progeny. This is of biological relevance since near-outcross hand-pollinations occurred within the range of pollen and seed dispersal suggesting that inbreeding depression in S. angularis is strong enough to maintain outcrossing in the study population. In the field, the far-outcross progeny outperformed the near-outcross progeny suggesting local population substructure. The magnitude of the inbreeding depression expressed among the self progeny was the greatest in the field, intermediate in the garden plot, and the least in the greenhouse.     

Inbreeding depression in self-incompatible and self-compatible populations of Leavenworthia alabamica

Inbreeding depression is one of the leading factors preventing the evolution of self-fertilization in plants. In populations where self-fertilization evolves, theory suggests that natural selection against partially recessive deleterious alleles will reduce inbreeding depression. The purpose of this study was to evaluate this hypothesis by comparing the magnitude of inbreeding depression in self-incompatible and self-compatible populations of Leavenworthia alabamica. Within-population crosses were conducted to compare the quantity and quality of offspring produced by outcrossing and self-fertilization. These progeny were grown in a common greenhouse and inbreeding depression was measured in germination, survival, biomass, transition rate to flowering, flower number, petal length, pollen grains/anther, pollen viability, and ovule number. In comparison to outcrossing, self-fertilization led to the production of fewer and smaller seeds within self-incompatible populations. Moreover, inbreeding depression was observed in eight of 11 offspring traits within self-incompatible populations of L. alabamica. In contrast, there was significant inbreeding depression only in flower number within self-compatible populations. The results of this study are consistent with the idea that self-fertilization selectively removes partially recessive deleterious alleles causing inbreeding depression in natural plant populations. However, in plant species such as L. alabamica where self-compatibility may evolve in small populations following long-distance dispersal, declines in inbreeding depression may also be facilitated by genetic drift.     

Genetic mapping of seed shape in three populations of recombinant inbred lines of soybean (Glycine max L. Merr.)

Round soybean seeds are sought-after for food-type soybean. Also the genetic control of seed geometry is of scientific interest. The objectives of this study were to estimate heritability and map quantitative trait loci (QTLs) responsible for seed shape traits. Three densely mapped recombinant inbred populations each with 192 segregants were used, Minsoy × Archer, Minsoy × Noir1, and Noir1 × Archer. A two rep two location experiment was conducted in Los Andes, Chile, and East Lansing, MI, USA. Seed height (SH), width (SW), length (SL), and seed volume (SV) as width × height × length were measured to determine seed shape. Heritability was estimated by variance component analysis. A total of 19 significant QTLs (LOD ≥ 3.7) in ten linkage groups (LG) were detected for all the traits. Only one QTL was stable across populations and environments and six were stable in at least two populations in both environments. The amount of phenotypic variation explained by a single QTL varied from 7.5% for SH, to 18.5% for SW and at least 30% of the genetic variation for the traits is controlled by four QTL or less. All traits were highly correlated with each other in all populations with values ranging from 0.5 to 0.9, except for SL and SW that were not significantly correlated or had a low correlation in all populations. Narrow sense heritabilities for all traits ranged from 0.42 to 0.88. We note that LG u9, u11, and u14 are hot points of the genome for QTLs for various traits. The number and genomic distribution of the QTLs confirms the complex genetic control of seed shape. Transgressive segregation was observed for all traits suggesting that careful selection of parents with similar phenotypes but different genotypes using molecular markers can result in desirable transgressive segregants.     

Polymorphic microsatellites in Antirrhinum (Scrophulariaceae), a genus with low levels of nuclear sequence variability

In Antirrhinum, reproductive systems range from self-compatible to self-incompatible, but the actual outcrossing rates of self-compatible populations are not known. Thus the extent to which levels of variability and inbreeding differ among Antirrhinum populations is not known. In order to address this issue we isolated nine Antirrhinum nuclear microsatellite loci. In contrast to several nuclear genes that show low levels of sequence variation, six of the microsatellite loci indicate high levels of variability within and between Antirrhinum species. The highly self-compatible Antirrhinum majus ssp. cirrhigerum population has high levels of variability and no significant deviation from Hardy-Weinberg equilibrium, suggesting substantial rates of outcrossing.     

Mating system, sex ratio, and persistence of females in the gynodioecious shrub Daphne laureola L. (Thymelaeaceae)

Although in gynodioecious populations male steriles require a fecundity advantage to compensate for their gametic disadvantage, southern Spanish populations of the long-lived shrub Daphne laureola do not show any fecundity advantage over hermaphrodites in terms of seed production and early seedling establishment. By using allozyme markers, we assess the mating system of this species in five populations differing in sex ratio, and infer levels of inbreeding depression over the whole life cycle by comparing the inbreeding coefficients at the seed and adult plant stages. Extremely low outcrossing rates (0.001相似文献   

Does inbreeding avoidance maintain gender dimorphism in Wurmbea dioica (Colchicaceae)?

The maintenance of females in gender dimorphic populations requires that they have a fitness advantage to compensate for their loss of male reproductive function. We assess whether inbreeding avoidance provides this advantage in two subdioecious Wurmbea dioica populations by estimating seed production, outcrossing rates and inbreeding depression. Fruiting males produced less than half as many seeds as females, owing to low outcrossing rates and early acting inbreeding depression. Inbreeding coefficients of fruiting males demonstrated that progeny were more inbred than their parents, implying that few selfed progeny reach maturity, as confirmed by inbreeding depression estimates that exceeded 0.85. In a glasshouse experiment, open-pollinated females exhibited a fitness advantage of 3.7 relative to fruiting males, but when we increased fruiting male outcrossing rate, female advantage was only 1.4. This reduced advantage is insufficient to maintain females if nuclear genes control sex. Thus, inbreeding avoidance could maintain females at high frequencies, although this is contingent upon high frequencies of fruiting males, which can be altered by environmentally determined gender plasticity.     

Implementation of extensive citrus triploid breeding programs based on 4x?×?2x sexual hybridisations

Seedlessness is one of the most important characteristics for mandarins for the fresh-fruit market, and mandarin triploid hybrids have this trait. Triploid citrus plants can be recovered by 4x?×?2x hybridisations using non-apomictic genotypes as female parents. In this study, we characterise the type of seeds obtained in 4x?×?2x hybridisations and the ploidy level of plants recovered from each type of seed. The majority of the plants recover were triploid (98.3?%), but a few diploid, tetraploid and pentaploid plants were also produced and their genetic origin was analysed by simple sequence repeat (SSR) markers. We also analysed the influence of parents and environmental conditions on the efficiency of recovery triploid hybrids. In this work, we present an effective methodology to recover triploid hybrids from 4x?×?2x hybridisations based on in vitro embryo rescue and determination of ploidy level by flow cytometry that allow us to recover more than 4,400 triploid hybrids from more than 60 parental combinations.