Significant Inbreeding Depression Research Articles

Maintenance of Inbreeding Depression in a Highly Self-Fertilizing Tree, Magnolia obovata Thunb

Inbreeding depression is a major selective force that maintains outcrossing in flowering plants. If the long life and large mature size of trees cause high inbreeding depression via mitotic mutations and half-sib competition, these characteristics may increase inbreeding depression sufficiently to maintain traits that facilitate outcrossing even with high primary selfing rates (proportion of selfed ovules). Here, I report the maintenance of inbreeding depression in a population of a tree (Magnolia obovata Thunb.) with primary selfing rates greater than 0.8 resulting from geitonogamy. The progenies exhibited inbreeding depression for germination, seedling survival, and seedling mass (δ = 0.29–0.38), but no significant difference between crossing type in seedling height. Cumulative inbreeding depression for early survival (from zygote to 2-year-old stage) estimated from these results and from prior data on embryonic survival was high (δe = 0.91). The fixation index at maturity based on six allozyme loci was low (Fis = 0.08), indicating that significant inbreeding depression for late survival results in a low level of inbreeding with respect to gene transmission to the next generation. From these results, I estimated that inbreeding depression for late and lifetime survival equaled 0.69 and 0.97, respectively. These results suggest that M. obovata trees maintain high inbreeding depression at both early and late life stages, resulting in a low level of inbreeding despite a high primary selfing rate. The high inbreeding depression can be explained by previous theories and is consistent with the predicted maintenance of inbreeding depression in highly self-fertilizing tree populations. The inbreeding load due to the high primary selfing rate represents a cost of this tree’s pollination system for outcrossing, which is based on automimicry and mass flowering.

Influence of inbreeding depression on a lake population of Nymphoides peltata after restoration from the soil seed bank

The negative effects of inbreeding depression on fragmented small populations are likely to be expressed more strongly after restoration efforts if regeneration processes have been highly restricted in degraded habitats. We examined the potential influences of inbreeding depression on a population of Nymphoides peltata (Menyanthaceae) restored from the remnant soil seed bank. A hand-pollination experiment demonstrated self-compatibility of a single remaining homostyle genet and significant inbreeding depression in selfed progeny, especially in parameters related to seedling growth (\(\updelta=0.5\)–0.6 for biomass, and \(\updelta=0.3\)–0.4 for relative growth rate). Our genetic analysis indicated that the presumed number of parents contributing to the current soil seed bank was only 2–8 genets and that a single sib-family dominated at each of three sampling sites. The results also showed that the selfed progeny of the homostyle genet were overwhelmingly dominant at two sites (86.8 and 94.7%). As a result, the growth performance of the seed bank seedlings was significantly reduced, to a level as low as that of the selfed progeny. Active restoration efforts to minimize the negative effects of the genetic bottleneck and continuous monitoring based on genetic and demographic study are recommended.

Inbreeding depression and selfing rate of Ipomoea hederacea var. integriuscula (Convolvulaceae)

Inbreeding depression and selfing rate were investigated in the self-compatible vine Ipomoea hederacea to assess the variability of the breeding system. Inbreeding depression differed between populations and the magnitude varied at germination, growth (as measured by aboveground biomass), and reproductive potential. Plants from Macon County, Alabama, USA, had significant inbreeding depression (31%) at germination, but no significant inbreeding depression for aboveground biomass or number of reproductive structures (buds and flowers) at 45 d post germination in the greenhouse or in the field. Plants from Morgan County, Alabama, however, had significant inbreeding depression (>50%) for all three stages in the greenhouse. In allozyme comparisons, five of the 11 I. hederacea populations surveyed had high selfing rates (66.66-92.53%) and high levels of homozygosity (F(IS) = 0.500-0.861) in 2003, and three of four populations surveyed in 2004 had selfing rates that exceeded 50%. High selfing rates, high levels of homozygosity, and low levels of inbreeding depression suggest that inbreeding depression may not present a significant barrier to the transmission of selfing alleles in some populations of I. hederacea, but does not account for the maintenance of a mixed mating system in other populations.

Testing for stress‐dependent inbreeding depression in Impatiens capensis (Balsaminaceae)

The relevance of inbreeding depression to the persistence of plant populations can depend upon whether stress magnifies inbreeding depression for fitness-related traits. To examine whether drought stress exacerbates inbreeding depression in gas exchange traits and biomass, we grew selfed and outcrossed progeny of inbred lines from two populations of Impatiens capensis in a greenhouse experiment under water-limited and moist soil conditions. Drought stress did not magnify the degree of inbreeding depression for any of the traits measured. In fact, in one population there was a trend for stronger inbreeding depression under well-watered, benign conditions. Furthermore, significant inbreeding depression for carbon assimilation rate and stomatal conductance was only detected in the lines from one population. In contrast, inbreeding depression for biomass was detected within both populations and differed among lines. Drought stress exerted significant selection on physiological traits, favoring increased carbon assimilation rates and decreased stomatal conductance in drought-stressed plants. Patterns of selection did not differ between inbred and outcrossed plants but did differ marginally between populations. Thus, estimates of selection were not biased by the mixed mating system per se, but may be biased by combining individuals from populations with different histories of selection and inbreeding.

The Economic Rationale of Recycling Hybrid Seeds in Northern Tanzania

A survey of 126 farmers was conducted with the objective of evaluating the economic rationale for recycling hybrid maize seed in northern Tanzania. About 80% of the sample farmers recycled hybrid and open pollinated maize seeds for three to six years as response to cash constraint and to hedge against production and financial risks. Farmers who recycled improved seed realized profits and remunerative returns to labor even when the producer price of maize fell by 46%. Hence, seed recycling is an economically rational behavior. The implications of these findings are researchers in Tanzania need to develop hybrid maize varieties that may be recycled without significant inbreeding depression. The extension services also need to advise farmers on seed selection methods that minimize inbreeding depression. Key words: maize hybrid, risk, seed recycling, Tanzania. Eastern African Journal of Rural Development Vol.20(1) 2004: 113-124

Consequences of a One-male Harem Reproductive System and Inbreeding in a Captive Group of Cercopithecus solatus

In closed captive populations, where dispersal is not possible, kin recognition and behavioral avoidance are the only mechanisms by which closely related individuals can avoid inbreeding. In the absence of avoidance, a loss of genetic diversity is inevitable in successive generations. In the 1980s, the CIRMF in Gabon established a small breeding group of sun-tailed monkeys (Cercopithecus solatus) with 4 individuals, and subsequently 17 births have been registered. We aimed to describe via microsatellite genotyping the reproductive system in the colony of Cercopithecus solatus, to evaluate the loss of genetic diversity with succeeding generations, and to evaluate consequences of inbreeding depression on a measure of the lifespan reproductive success of females giving birth to inbred vs. noninbred offspring. During the 11-yr period for which data are available, only alpha males sired offspring, confirming a one-male social organization. They reproduced only during their period of tenure. Two of the 3 alpha males were responsible for all the infants born. Genetic diversity decreased and inbreeding coefficients increased with successive generations. Interbirth interval was increased following the birth of an inbred infant, indicating possible increased maternal costs of rearing inbred infants. Loss of genetic variability in this captive group of sun-tailed monkeys has led to significant inbreeding depression and demonstrates the importance of male-mediated gene flow in restricted one-male harem breeding groups.

Inbreeding in Pinus Radiata – V. The Effects of Inbreeding on Fecundity

AbstractA successful inbreeding and hybrid breeding strategy in tree improvement requires that 1) inbreeding (selfing) can produce superior inbred lines (effective purging of deleterious alleles), 2) there is heterosis among crosses of inbred lines, 3) early selection between lines is effective, and 4) inbreeding will not substantially reduce reproductive ability. We have previously reported that inbreeding depression on growth was lower in radiata pine relative to other conifers and segregation in the first two-generations of selfs generated superior inbred trees. In addition, we have observed that early selection among inbred trees (lines) was more effective than in out-crossed populations and there was an apparent heterosis in radiata pine. In this study, the effect of inbreeding on the reproductive ability in young and adult trees of radiata pine has been quantified from five populations of varied inbreeding levels (F =0, 0.125, 0.25, 0.5, and 0.75). It was observed that the effects of inbreeding depression on fecundity was higher at a young age than at older age and inbreeding depression at a young age is due to two factors: 1) a delay of reproductive age (about 8.3, and 8.5% of trees delayed for F =0.5 and F =0.75 populations, respectively) and 2) a true reduction of flowering trees (6.7 and 13.1% more trees having no flowers for F =0.5 and F =0.75 populations than F =0 population, respectively). Despite significant inbreeding depression on the percentage of female reproductive trees and the number of cones on adult trees, overall inbreeding depression on fecundity was low in radiata pine. One founder clone contributed most of the significant inbreeding depression observed for the population of eight founder clones. It was observed that fecundity varied more widely among the eight clones than among the inbreeding level (self and outcross).

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.

Heterosis studies for yield and its components in bread wheat over environments

A set of diallel crosses involving 10 parents was made to have information on the extent of heterosis over mid-parent and better parent and inbreeding depression for yield and yield contributing characters under three different environments. Marked heterobeltiosis for grain yield and its important components were observed. For grain yield, 83 crosses showed significant positive heterobeltiosis in all the three sowing dates, however, twenty crosses showed significant consistent heterobeltiosis for grain yield per plant over all the three environments. The maximum heterobeltiosis for grain yield per plant observed was 50.94% (Raj 3765 x HD 2285), 121.08% (PBW 373 x HD 2329) and 93.96% (PBW 373 x HD 2329) under early, normal and late sowing conditions, respectively. Cross PBW 373 x HD 2329 in both early and normal plantings and cross Raj 3765 x HD 2285 under late planting were observed most heterotic for grain yield. The crosses showing heterosis for grain yield were not heterotic for all the characters. Heterosis for grain yield per spike followed by tillers per plant and 1000-grain weight was independently associated with heterosis for grain yield in early and normal plantings. However, heterosis for grain yield per spike, dwarf plant height and tillers per plant contributed maximum towards yield heterosis. Significant inbreeding depression was recorded frequently for yield and yield contributing traits, however, in a few traits it was observed significant negative indicated that F(2) was superior to F(1) considered desirable combination for trait(s). The study reveals good scope for commercial exploitation of heterosis as well as isolation of pure lines among the progenies of heterotic F(1) for improvement of yield levels in bread wheat.

Genetic variability and founder effect in the pitcher plant Sarracenia purpurea (Sarraceniaceae) in populations introduced into Switzerland: from inbreeding to invasion.

The long-lived and mainly outcrossing species Sarracenia purpurea has been introduced into Switzerland and become invasive. This creates the opportunity to study reactions to founder effect and how a species can circumvent deleterious effects of bottlenecks such as reduced genetic diversity, inbreeding and extinction through mutational meltdown, to emerge as a highly invasive plant. A population genetic survey by random amplified polymorphism DNA markers (RAPD) together with historical insights and a field pollination experiment were carried out. At the regional scale, S. purpurea shows low structure (thetast=0.072) due to a recent founder event and important subsequent growth. Nevertheless, multivariate statistical analyses reveal that, because of a bottleneck that shifted allele frequencies, most of the variability is independent among populations. In one population (Tenasses) the species has become invasive and genetic analysis reveals restricted gene flow and family structure (thetast=0.287). Although inbreeding appears to be high (Fis >0.410 from a Bayesian estimation), a field pollination experiment failed to detect significant inbreeding depression upon F1 seed number and seed weight fitness-traits. Furthermore, crosses between unrelated individuals produced F1 seeds with significantly reduced fitness, thus showing local outbreeding depression. The results suggest that, under restricted gene flow among families, the species may not only have rapidly purged deleterious alleles, but also have undergone some form of selection for inbreeding due to co-adaptation between loci.

Two classes of deleterious recessive alleles in a natural population of zebrafish, Danio rerio.

Natural populations carry deleterious recessive alleles which cause inbreeding depression. We compared mortality and growth of inbred and outbred zebrafish, Danio rerio, between 6 and 48 days of age. Grandparents of the studied fish were caught in the wild. Inbred fish were generated by brother-sister mating. Mortality was 9% in outbred fish, and 42% in inbred fish, which implies at least 3.6 lethal equivalents of deleterious recessive alleles per zygote. There was no significant inbreeding depression in the growth, perhaps because the surviving inbred fish lived under less crowded conditions. In contrast to alleles that cause embryonic and early larval mortality in the same population, alleles responsible for late larval and early juvenile mortality did not result in any gross morphological abnormalities. Thus, deleterious recessive alleles that segregate in a wild zebrafish population belong to two sharply distinct classes: early-acting, morphologically overt, unconditional lethals; and later-acting, morphologically cryptic, and presumably milder alleles.

Genetics of grains per spike in durum wheat under normal and late planting conditions

Parental, F1, F2, BC1, BC2, BC11, BC12, BC21, BC22, BC1 self and BC2 selfed generations of three crosses involving six cultivars of durum wheat (Triticumdurum Desf.) were studied for grains per spike under normal and late sown environments to analyze the nature of gene effects. A 10-parameter model did not fully account for the differences among the generation means. In two cases more complex interactions or linkage were involved in the inheritance of grains per spike in durum. Both digenic and trigenic epistatic interactions had a role in controlling the inheritance of grains per spike, however, trigenic interactions contributed more than digenic interactions. Non-fixable gene effects were many times higher than fixable ones in all three crosses and in both sowing environments indicating a major role of non-additive gene effects in the inheritance of this trait. Duplicate epistasis between sets of three genes under both environments was recorded for the cross Raj 911 × DWL 5002. Epistatic interactions, particularly the trigenic ones, contributed the maximum significant heterosis. Epistatic interactions involving dominance in the F2 generations caused significant inbreeding depression. Selective diallel mating and/or biparental mating could be used for amelioration of grains per spike in durum wheat.

Inbreeding and inbreeding depression of female reproductive traits in two populations of Coho salmon selected using BLUP predictors of breeding values

Levels of inbreeding and inbreeding depression were studied in two populations of Coho salmon (Oncorhynchus kisutch) in Chile. The two populations, termed even year, and odd year were artificially selected by weight at harvest over four generations, using the best linear unbiased prediction (BLUP) of breeding values. Also, general linear models (GLM) were used to analyze the effects of inbreeding on reproductive traits of the females and on survival of the progeny. The selection resulted in 56–76% of the parents of the base population not contributing with descendents in the fourth generation. The inbreeding rate was greater in the even population (ΔF=2.45% per generation) than the odd population (ΔF=1.10% per generation) as a direct consequence of the smaller number of founder individuals in the former population (Ne=61 and 106, respectively). The level of inbreeding in the last generation was 9.5% (S.D.=2.7, range 5–19%) for year-class 2000 and 4.3% (S.D.=2.6, range 1–12%) for year-class 2001. Significant inbreeding depression was estimated for the gonadosomatic index (−5.3% per each 10% increase in inbreeding) in population year 2000, and for body length at spawning (−1.56%) in population year 2001. The inbreeding did not significantly reduce other traits such as weight body at spawning, weight of the gonad, number of green eggs, or relative fecundity. No significant inbreeding depression was observed in either population regarding the survival of eggs in the eyed stage. Given the deleterious effects of inbreeding on reproductive traits, salmon selection programs should employ methods which limit the rate of increase of this factor.

Multiple genetic contributions to plant performance in Thymus vulgaris

Summary 1 Diverse genetic factors can affect plant performance. Here we evaluate how maternal and paternal origin, maternal and offspring sexual phenotype and degree of outcrossing jointly influence performance of gynodioecious Thymus vulgaris . 2 In uniform conditions we quantified size and survival of second-generation (G 2 ) offspring produced by selfing and outcrossing on the open-pollinated offspring (G 1 ) of G 0 females from four populations that differ in sex ratio, selfing rate and chemical type. We controlled for environmental variation, maternal effects and plant age. 3 Offspring with maternal parents from populations dominated by plants that synthesize phenolic monoterpenes had greater survival and size than those from populations with non-phenolic chemotypes. In addition, outcrossing among populations with pollen donors from non-phenolic populations significantly reduced progeny performance. These results suggest chemotype-mediated local adaptation. 4 The progeny of hermaphrodites showed significant inbreeding depression, which despite the pollination treatment × family and pollination treatment × population interactions, showed no consistent variation in relation to population inbreeding levels. 5 For a given level of outcrossing, the progeny of hermaphrodites showed greater among-family and among-population variation in plant size than those of females. Variation in nuclear restorer gene composition across cytoplasmic backgrounds may cause this variation. 6 In T. vulgaris , diverse genetic factors jointly affect performance, a result with general implications for understanding fitness variation in natural populations.

The effects of inbreeding on performance traits of adult Pacific oysters (Crassostrea gigas)

Understanding the effects of inbreeding is critical to the long-term viability of shellfish breeding programs, especially as breeders attempt to develop selected lines in hatcheries with small effective population sizes. Inbreeding depression in shellfish is well documented among the offspring of selfed individuals and full-sib crosses. This study was conducted to determine if crossing more distantly related parents would result in measurable inbreeding depression of performance traits in adult Crassostrea gigas raised in a commercial intertidal growing environment. Families were created with expected inbreeding coefficients ( F) of 0, 0.0625, and 0.203. Average family yield, individual growth rate, and survival were recorded after the first and second growing seasons. Significant inbreeding depression in yield, individual growth rate and survival was observed after the first growing season in families with F =0.203 ( P< 0.0001), but not in families with F =0.0625 (P>0.058). After two growing seasons, significant inbreeding depression of yield and individual growth rate was observed in families with F =0.0625 ( P <0.01)and F =0.203 ( P < 0.001). Significant depression of survival at harvest was observed only in families with F =0.203 ( P =0.009). These results emphasize the importance of maintaining pedigree records in shellfish breeding programs to help avoid the deleterious effects of inbreeding depression, even among crosses of distantly related parents.

Effects of inbreeding on male function and self‐fertility in the partially self‐incompatible herb Campanula rapunculoides (Campanulaceae)

We examined the effect of inbreeding on fitness (through both male and female functions) and changes in self-fertility in the partially self-incompatible species Campanula rapunculoides. Individuals in natural populations of C. rapunculoides varied extensively in their strength of self-incompatibility (SI). We crossed 11 individuals that differed in their strength of SI to generate families with four levels of inbreeding (f = 0.0, 0.25, 0.5, and 0.75). Progeny were scored for three traits related to male fitness and for outcrossed and selfed seed production. Analyses of variance revealed significant inbreeding depression for the three male traits and seed set. Families with strong or weak SI differed in their response to inbreeding. Families with weak SI had lower levels of inbreeding depression for most traits than families with strong SI, but strong SI families had a greater increase in selfed seed set, but not self-fertility, with inbreeding. Finally, we found evidence of a significant linear response to inbreeding for all three male reproductive traits and outcrossed seed, indicating that inbreeding depression was primarily caused by partially or fully recessive deleterious alleles. Variation in genetic load was associated with variation in self-fertility, a finding that suggests an evolutionary role for partial self-fertility in natural populations of C. rapunculoides.

Maternal and Fetal Inbreeding Depression for 70-Day Nonreturn and Calving Rate in Holsteins and Jerseys

Inbreeding depression for 70-d nonreturn rate was estimated in 50,613 Holstein and 47,673 Jersey cows with five-generation pedigrees using an animal model. Heritabilities of 70-d nonreturn rate were very low for both breeds (1 to 2%). Maternal inbreeding depression was small (3% reduction for 10% inbreeding) and significant only for Jerseys. Fetal and maternal inbreeding depression was not significant for individual parities in Holsteins, but maternal inbreeding depression was significant in first parity only in Jerseys. Maternal and fetal inbreeding depression of calving rate (verified by a subsequent calving) was estimated on separate datasets by parity from 13,229 to 26,876 Holstein and 7374 to 11,742 Jersey cows. First-parity estimates for heritability of calving rate were 1% or less, whereas estimates for later parities varied from 1 to 6%. Significant inbreeding depression in first-parity Holsteins reduced calving rate by 4% per 10% maternal or fetal inbreeding, but effects, while undesirable, were not consistently significant in other parities. In Jerseys, maternal inbreeding significantly reduced calving rate by 6% per 10% inbreeding in first parity, and was undesirable but not significant for second through fourth parities. Fetal inbreeding depression was not significant in Jerseys. Maternal inbreeding depression of 70-d nonreturn and calving rate was small, undesirable, but not consistently significant across breeds and parities. The cumulative economic impact of maternal or fetal inbreeding on lifetime reproductive performance of Holstein or Jersey cows would be more dramatic than results for a single breeding.

Inbreeding depression in fecundity and inbred line extinction in the bulb mite, Rhizoglyphus robini.

This study investigated the magnitude of inbreeding depression in fecundity, and whether the depression is purged during six generations of sib mating in the bulb mite, Rhizoglyphus robini. The progeny resulting from a single generation of brother-sister mating suffered significant inbreeding depression in fecundity. During the following six generations of continuous sib-mating, 58% lines were lost, 45% because of sterility and 13% because of preadult mortality. The lines were then outcrossed, and their inbreeding depression compared with that of the base population. The inbreeding depression for the outcrossed population was 0.15, and for the base population 0.19, but the difference was not significant. The lack of significant purging of inbreeding depression indicates that it was caused either by detrimental genes of small effect, or by the breaking down of overdominant relations between alleles. However, the large proportion of extinct lines points to the former mechanism as a predominant cause of inbreeding depression. Theory predicts that the probability of line extinction with inbreeding increases with its load of mutations. If phenotypic variation in fecundity was partly because of differences in numbers of mutations carried by individuals, the fecundity of the line founder could be expected to correlate with the probability that the line derived from it will survive long-term inbreeding. Indeed, fecundity of founder females was significantly associated with line survival, which suggests that line extinction rate may be used as a method to study individual mutational loads, for example, in studies of sexual selection.

Inbreeding depression in captive bighorn sheep

We estimated the extent of inbreeding depression for juvenile survival in 589 captive-born bighorn sheep, and, unlike an earlier report, found no evidence of significant inbreeding depression. There did not appear to be any overall effect of year of birth, place of birth, subspecies, sex or ancestral inbreeding upon the viability of inbred animals as compared to non-inbred animals. However, for the first few years of data, there was lower survival for inbred than non-inbred offspring. After this period, there was an increase in the viability of inbred offspring, probably reflecting a general improvement in husbandry conditions in the zoos. It should be emphasized that the lack of significant inbreeding depression does not necessarily imply that there may not be inbreeding depression for other traits or in a less benign or more natural environment for juvenile viability.

Overdominant epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice. I. Biomass and grain yield.

To understand the genetic basis of inbreeding depression and heterosis in rice, main-effect and epistatic QTL associated with inbreeding depression and heterosis for grain yield and biomass in five related rice mapping populations were investigated using a complete RFLP linkage map of 182 markers, replicated phenotyping experiments, and the mixed model approach. The mapping populations included 254 F(10) recombinant inbred lines derived from a cross between Lemont (japonica) and Teqing (indica) and two BC and two testcross hybrid populations derived from crosses between the RILs and their parents plus two testers (Zhong 413 and IR64). For both BY and GY, there was significant inbreeding depression detected in the RI population and a high level of heterosis in each of the BC and testcross hybrid populations. The mean performance of the BC or testcross hybrids was largely determined by their heterosis measurements. The hybrid breakdown (part of inbreeding depression) values of individual RILs were negatively associated with the heterosis measurements of their BC or testcross hybrids, indicating the partial genetic overlap of genes causing hybrid breakdown and heterosis in rice. A large number of epistatic QTL pairs and a few main-effect QTL were identified, which were responsible for >65% of the phenotypic variation of BY and GY in each of the populations with the former explaining a much greater portion of the variation. Two conclusions concerning the loci associated with inbreeding depression and heterosis in rice were reached from our results. First, most QTL associated with inbreeding depression and heterosis in rice appeared to be involved in epistasis. Second, most ( approximately 90%) QTL contributing to heterosis appeared to be overdominant. These observations tend to implicate epistasis and overdominance, rather than dominance, as the major genetic basis of heterosis in rice. The implications of our results in rice evolution and improvement are discussed.