Ancestral inbreeding reduces the magnitude of inbreeding depression in Drosophila melanogaster

The influence of natural selection on the magnitude of inbreeding depression is an important issue in conservation biology and the study of evolution. It is generally expected that the magnitude of inbreeding depression in small populations will depend upon the average homozygosity of individuals, as measured by the coefficient of inbreeding (F). However, if deleterious recessive alleles are selectively purged from populations during inbreeding, then inbreeding depression may differ among populations in which individuals have the same inbreeding coefficient. In such cases, the magnitude of inbreeding depression will partly depend on the ancestral inbreeding coefficient (fa), which measures the cumulative proportion of loci that have historically been homozygous and therefore exposed to natural selection. We examined the inbreeding depression that occurred in lineages of Drosophila melanogaster maintained under pedigrees that led to the same inbreeding coefficient (F = 0.375) but different levels of ancestral inbreeding (fa = 0.250 or 0.531). Although inbreeding depression varied substantially among individual lineages, we observed a significant 40% decrease in the median level of inbreeding depression in the treatment with higher ancestral inbreeding. Our results demonstrate that high levels of ancestral inbreeding are associated with greater purging effects, which reduces the inbreeding depression that occurs in isolated populations of small size.     

Effects on heterozygosity and reproductive fitness of inbreeding with and without selection on fitness in Drosophila melanogaster

The effects of inbreeding, with (IS) and without selection (IO) for reproductive fitness, on inbreeding depression and heterozygosity were evaluated in 20 lines of each treatment inbred over seven generations using full-sib mating. The survival of lines was significantly greater in IS (20/20) than in IO (15/20). The competitive index measure of reproductive fitness was significantly lower in the inbred lines than in the outbred base population, but not significantly different in surviving IS and IO lines. There was a trend for higher fitness in the IS treatment as relative fitnesses were 19% higher in IS than IO for surviving lines and 59% higher for all lines. Heterozygosities were lower in the inbred lines than in the base population, and significantly higher in the IS than the IO lines. Consequently, the reduction of inbreeding depression in IS has been achieved, at least in part, by slowing the rate of fixation.     

Prediction of heterosis in crosses between inbred lines of Drosophila melanogaster

Summary The aim of the experiment was to determine if the estimated genetic distance between two populations could be used to predict the amount of heterosis that would occur when they were crossed. Eight lines of known relatedness to each other were produced by eight generations of sib mating and sub-lining. This produced lines that varied in coefficient of coancestry from zero to 0.78. Fourteen reciprocal crosses of these lines were used to measure heterosis for larval viability and adult fecundity. Gene frequencies at six polymorphic enzyme loci were used to estimate the genetic distances between lines, which were then compared with the known degrees of coancestry. The estimated genetic differences were poorly correlated with the known coancestry coefficients (r=0.4), possibly due to the small number of loci typed. Also genetic distances were only about 1/3 of what was expected. Selection acting on blocks of genes linked to the enzyme loci probably prevented the expected increase in homozygosity. Coancestry coefficient was correlated with heterosis (r=0.44–0.71). This level of correlation implied differences in heterosis among parent lines with the same level of coancestry. This variability is expected if a small number of loci explain most of the heterosis. The average level of heterosis was less than expected after eight generations of sib mating. This is most likely due to selection opposing the increase in homozygosity caused by inbreeding. The combination of these two imperfect correlations resulted in no significant correlation between genetic distance estimated from markers and heterosis.     

Reduced inbreeding depression due to historical inbreeding in Drosophila melanogaster: evidence for purging

An important issue in conservation biology and the study of evolution is the extent to which inbreeding depression can be reduced or reversed by natural selection. If the deleterious recessive alleles causing inbreeding depression can be 'purged' by natural selection, outbred populations that have a history of inbreeding are expected to be less susceptible to inbreeding depression. This expectation, however, has not been realized in previous laboratory experiments. In the present study, we used Drosophila melanogaster as a model system to test for an association between inbreeding history and inbreeding depression. We created six 'purged' populations from experimental lineages that had been maintained at a population size of 10 male-female pairs for 19 generations. We then measured the inbreeding depression that resulted from one generation of full-sib mating in the purged populations and in the original base population. The magnitude of inbreeding depression in the purged populations was approximately one-third of that observed in the original base population. In contrast to previous laboratory experiments, therefore, we found that inbreeding depression was reduced in populations that have a history of inbreeding. The large purging effects observed in this study may be attributable to the rate of historical inbreeding examined, which was slower than that considered in previous experiments.     

Inbreeding depression and dominance-suppression competition after inbreeding in rapeseed (Brassica napus)

Rapeseed plants, of the summer annual variety Topas, that had been selfed twice consecutively were compared to outcrossed half-sibs for inbreeding depression in a rapeseed population at mating equilibrium. The effect of dominance-suppression competition was included in the effect of inbreeding. Both female-and male-fitness characters showed significant inbreeding depression. Biomass decreased 17% with inbreeding and was highly correlated with seed weight. The total number of flowers decreased 15% with inbreeding. There was a significant effect of lines. The possible importance of experimental design in studies that estimate inbreeding depression is discussed.     

Mating system and inbreeding depression in whitebark pine (Pinus albicaulis Engelm.)

Mating system and inbreeding depression in quantitative traits of whitebark pine (Pinus albicaulis Engelm.) was determined using isozymes and a seedling common garden experiment. Simultaneous isozyme analysis of embryo and haploid megagametophyes from progeny arrays of families in three distinct geographic regions (Oregon, Montana, and southern British Columbia) was used to estimate parental and progeny inbreeding coefficients, as well as regional and family mean multilocus outcrossing rates (t _m). Quantitative trait family means of seedlings from the same families growing in two temperature treatments in a common garden experiment were regressed on the estimated inbreeding coefficient to determine the presence and magnitude of inbreeding depression. Regional estimates of t _m ranged from 0.73 to 0.93, with a mean over all regions of 0.86. Family mean t _m values indicated predominant outcrossing; however, some individuals experienced substantial inbreeding. The Oregon region had a significant excess of heterozygotes in the parental generation relative to Hardy–Weinberg equilibrium, while both the Oregon and southern BC regions had a heterozygote deficiency in progeny, suggesting selection against inbred individuals. Biomass in the ambient temperature treatment for the southern BC region was the only trait significantly related to inbreeding coefficient. The mean inbreeding coefficient for this region was 0.25, and based on this relationship, mean predicted biomass would be reduced by 19.6% in this region if inbred individuals are not removed by selection. The estimated outcrossing rate of whitebark pine is slightly lower than most wind-pollinated conifers, and while most individuals are highly outcrossing, some experience substantial inbreeding.     

Relationship between inbreeding depression and inbreeding coefficient in maritime pine (Pinus pinaster)

The relationship between inbreeding depression and inbreeding coefficient (F) for several important traits was investigated in an 11-year trial of maritime pine (Pinus pinaster). Five levels of inbreeding (F=0; 0.125; 0.25; 0.5; 0.75) were obtained in a mating design involving ten plus-trees, or their progenies, as parents (total of 51 families). For F=0.75, the mean inbreeding depressions were 27% for height, 37% for circumference at breast height (63% for bole volume), 23% for basal straightness (better straightness of the inbred trees), and 89% for female fertility (number of cones). Large differences were observed among inbred families for the same level of inbreeding. The evolution of depression with F was more or less linear, depending on the traits. Significant differences among F-levels appeared very early for height (from 5-years of age). Inbreeding depression was much more expressed during unfavorable years than during favorable years for yearly height growth. When compared with other Pinus species, maritime pine appears to be less affected by inbreeding, especially for the percentage of filled seeds and general vigor. A reduced genetic load in maritime pine may result from the evolutionary history of the species and its scattered distribution.     

Enzymatic variation at seven loci in nine natural populations of Drosophila melanogaster

Allozyme polymorphisms at seven loci have been studied in nine natural populations of Drosophila melanogaster from the Saône and Rhône valleys sampled in 1973 and 1974. A great deal of polymorphism was observed; an individual was on the average heterozygous at 20.2% of its loci. The populations were genetically very homogeneous throughout the region sampled. The number of ovariolae per female varied from one group of populations to another depending on their geographical separation. Yet the number of ovariolae remained constant from one year to the next. The results show that migration alone cannot explain the homogeneity of the allozyme frequencies. It seems reasonable to conclude that selection plays a major role in maintaining the homogeneity of populations living in proximal biotopes.E.R.A. No. 406: Analyse et mécanismes de maintien du polymorphisme.     

Spatial differences in patterns of modification: selection on hairy in Drosophila melanogaster wings

Artificial selection was carried out for over 45 generations to enhance and suppress expression of the mutation hairy on the Drosophila melanogaster wing. Whole chromosome mapping of X‐linked and autosomal modifiers of sense organ number displayed regional differences in magnitude and direction of their effects. Regional specificity of modifier effects was also seen in some interchromosomal interactions. Scanning electron microscopy allowed precise measurement of sense organ size and position along the L3 longitudinal wing vein. Sense organ size varied in a predictable fashion along the proximal–distal axis, and the dorsal pattern differed from the ventral pattern. The high and low selection lines differed most in the proximal portion of the L3 vein. Extra sense organs in the High line were often associated with vein fragments at locations predicted from ancestral vein patterns. Thus, regional specificity of polygenic or quantitative trait locus modifier effects was identified in several different parts of the wing. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of ancestral and current-generation inbreeding in an experimental strawberry breeding population

Progenies from first-generation self, half-sib, full-sib, and cross fertilizations were generated to evaluate the magnitude of inbreeding depression for vegetative and production traits in strawberry. Tests were conducted to determine the linearity of trait mean depression with inbreeding rate (F) over this range of inbreeding values, as an indication of the presence of non-additive epistasis. A control population, for which a similar range of coancestry had accumulated over several cycles of breeding and selection, was also generated to compare the consequences of ancestral and current-generation inbreeding. Trait means for crosses among current-generation half-sibs, full-sibs, and selfs were 2–17%, 3–12%, and 14–45% lower than for unrelated crosses among the same set of parents, respectively. Linear regression of progeny means on current generation F was significantly negative for all traits and explained 17–44% of the variance among progeny means. Mean depression was largely linear over the range of inbreeding rates tested in this population, indicating the absence of epistasis for the traits evaluated. Conversely, (F) regressions of progeny means on pedigree inbreeding coefficients, where coancestry had accumulated over several cycles of breeding and selection, were uniformly non-significant and explained 0–10% of the variance among cross means. Further, multiple regression of progeny means for current-generation relatives on pedigree F failed to improve fit significantly over regression on current-generation F alone for all traits. Together, these results suggest that pedigree inbreeding coefficients are poor predictors of changes in homozygosity when populations are developed through multiple cycles of breeding and selection. They also imply that inbreeding depression will be of minor importance for strawberry breeding populations managed with adequate population sizes and strong directional selection.     

Effect of different heavy-metal concentrations on Drosophila melanogaster larval growth and development

The survival percentage of Drosophila melanogaster larvae on synthetic media containing different concentrations of heavy metals, including Cd, Cu, Pb, and Zn, in the first generation indicated no significant reduction in their growth and development up to 500 ppm for all tested heavy-metal concentrations. At 500 ppm, results showed that there was a significant reduction in pupa and adult stages: 65% and 25% for Cd, 50% and 25% for Cu, 100% and 95% for Pb, and 85% and 75% for Zn, respectively. The survival percentages at 1000 ppm were further significantly reduced: 15% and 0% for Cd, 35% and 15% for Cu, 45% and 90% for Pb, and 65% and 35% for Zn, respectively for pupa and adult stages. For the second generation, there was no significant reduction in survival growth and development up to 100 ppm, but above 500 ppm, there was a significant reduction. For most of these heavy-metal concentrations, the survival percentages of the second generation at the pupa stage was higher than the first generation, whereas for the adult, there was a lower survival percentage, indicating some effect on metamorphosis of these heavy-metal concentrations on Drosophila melanogaster Comparing the survival percentage between first and second generations at 500 ppm for pupa indicate a significant increase on Cu only, and for the adult, there was a significant reduction for Cd.     

Ether-induced segmentation disturbances in Drosophila melanogaster

Summary Drosophila embryos, exposed to ether between 1 and 4 h after oviposition, develop defects ranging from the complete lack of segmentation to isolated gaps in single segments. Between these extremes are varying extents of incomplete and abnormal segmentation. On the basis of both their temporal and spatial characteristics, five major phenotype classes may be distinguished: headless — unsegmented or incompletely segmented anteriorly; gap — interruptions of segmentation not obviously periodic; alternating segment gaps — interruptions with double segment periodicities; fused segments; and short segments — truncations with single segment periodicities. Many defects resemble known mutant phenotypes. The disturbances in segmentation are predominantly global and frequently accompanied by alterations in segment specification, such that the segments obtained show no resemblance to the normal homologues. These features, together with the distinctive spatiotemporal characteristics of the defects, all point to segmentation as a dynamic process. The regular spacing of the segments and the fact that the entire range of defects is inducible by ether are further consistent with the hypothesis that at least part of the segmentation process may consist of physicochemical reactions coordinated over the whole body. The relationship between our data and data from genetic and other analyses are briefly discussed.     

Ontogeny of sorbitol dehydrogenases in Drosophila melanogaster

It has been shown that crude extracts of Drosophila melanogaster adults contain three distinctly different enzymes which catalyze the oxidation of d-sorbitol into d-fructose. These include (1) a soluble NAD-dependent sorbitol dehydrogenase (NAD-SoDH^s), (2) a mitochondrial NAD-dependent sorbitol dehydrogenase (NAD-SoDH^m), and (3) a soluble NADP-dependent sorbitol dehydrogenase (NADP-SoDH). Developmental studies have shown that the activities of all three of these enzymes are lowest during the larval stages while highest levels are seen during or shortly prior to the adult period. With respect to NAD-SoDH^s, studies of tissue distribution in adults have shown that highest activity is associated with thoracic musculature in both sexes and with organs of the male reproductive system. The developmental profile of this enzyme reveals a significant increase in activity at between 40 and 60 hr after hatching. This time interval corresponds closely to that during which the paternally derived NAD-SoDH^s gene is expressed. An additional increase in activity is seen in male pupae at 160 hr and in female adults at 210 hr. The rapid increase in males takes place immediately following the developmental period during which the testes attach to their respective duct systems. NADP-SoDH activity is concentrated among organs of the thorax and abdomen in both sexes. Males show significantly higher levels of this enzyme during the late pupal and early adult periods. In contrast to the patterns of distribution seen for NAD-SoDH^s and NADP-SoDH, 91–92% of the total NAD-SoDH^m activity in adults is localized to the thoracic musculature. The developmental profile of this enzyme reveals a significant increase in activity during the late pupal and early adult periods, when flight muscle mitochondria are known to be proliferating and undergoing structural maturation.This work was supported in part by Grant No. GY-10830 from the National Science Foundation and a faculty research fellowship from The University of Toledo Board of Trustees.     

Studies on transvection at the bithorax complex in Drosophila melanogaster

Summary We have studied the influence of some mutations in the bithorax complex on the observed synapsis dependent phenotype of the genotypes Cbx ¹Ubx¹/+ and bx ^34e/Ubx¹. The effect of these mutations is similar to that introduced by disruption of pairing or by the z ^a mutation. Among the bx mutations, we find that bx ⁸ behaves differently from most other bx mutations in its influence on the synapsis dependent phenotype. This observation induced us to map the position of bx ⁸ with respect to other bx mutations; we find that it maps between bx ^34e and bx ³. We show how some of the observations reported here can be fitted into a model of activation of the bithorax complex proposed by one of us.     

Dynamics of spermiogenesis in Drosophila melanogaster

Summary A morphogenetic process that transforms spermatids from a syncytial state to a state in which each spermatid is invested in its own membrane, is initiated at the head region of the spermatid bundle and traverses through the entire length of the bundle in the testis of Drosophila melanogaster. This process not only eliminates the syncytial bridges between spermatids but also removes unneeded organelles and the excess parts of the nuclear membrane, nucleoplasm and cytoplasm. It also brings about structural modifications to flagellar elements. The propagation of this process is seen as the caudal movement of a fusiform swelling of the spermatid bundle, 100 or more in length. Spermatids are individualized in the basal half of the swelling, whereas they remain syncytial in the apical half. The swelling increases its volume as it accumulates cytoplasmic debris while traversing the sperm bundle, from about 15 in maximum diameter in the basal testicular region to as large as 30 at the apical end where it becomes a bag of wastes. A variation of the process in a mutant stock which is known to inactivate up to half of the products of meiosis is briefly described. The morphological change of interspermatid bridges prior to the individualization is also reported.This work was supported by grants from the National Institutes of Health (USPHS-HD 03015 and GM-15971) and a contract from the Atomic Energy Commission, AT(04-3)-34, P.A. 150.Graduate training grant USPHS GM 00702.     

Embryonic cAMP and developmental potential in Drosophila melanogaster

Summary Measurements of cAMP in early embryos of Drosophila melanogaster demonstrate that the dunce gene plays a major role, and the rutabaga gene a secondary role, in maternal regulation of embryonic cAMP content. Studying the double mutant combination, we find that variability in elevated cAMP content between individual embryos is associated with a wide variability in developmental potential. Embryos with about five times the normal cAMP content define a threshold between apparently normal and abnormal development. Measurements of cAMP content in anterior and posterior halves of embryos indicate that the posterior embryonic region, which is developmentally more sensitive to the effects of elevated cAMP than the anterior region, does not contain more cAMP than the anterior region. The variety of developmental defects observed is discussed in relation to possible targets of cAMP action. Offprint requests to: J.A. Kiger, Jr     

Sustainability of the South China tiger: implications of inbreeding depression and introgression

The South China tiger (Panther tigris amoyensis) is critically endangered with 73 remaining individuals living in captivity, all derived from six wild founders since 1963. The population shows a low level of juvenile survivorship and reproductive difficulties, and faces a huge conservation challenge. In this study, inbreeding depression and genetic diversity decline were examined by using pedigree data and 17 microsatellites. The constant B, which is related to the number of lethal equivalents, was estimated to be 0 for the offspring of noninbred parents, but was >0 for the offspring of inbred parents and for all offspring. Percentage of successfully breeding tigers inversely correlated with inbreeding level (r = −0.626, α = 0.05). Taken together, these findings suggest the population is suffering from inbreeding depression in juvenile survivorship and fecundity. No significant correlation was detectable for the mean litter size with f of either dams (r = −0.305, α = 0.46) or kittens (r = 0.105, α = 0.71), indicating litter size was not strongly subject to inbreeding depression. The average number of alleles per locus was 4.24 ± 1.03 (SE), but effective number of alleles was only 2.53 ± 0.91. Twenty-one alleles carried by early breeders at 13 loci were absent in the present breeders and potential breeders. Multilocus heterozygosity was inversely correlated with inbreeding levels (r = −0.601, α = 0.004). These findings suggest rapid allelic diversity loss is occurring in this small captive population and that heterozygosity is being lost as it becomes more inbred. Our phylogenetic analysis supports past work indicating introgression from northern Indochinese tigers in the population. As no wild representatives of the South China tiger can be added to the captive population, we may consider the alternate scenario of further introgression in the interest of countering inbreeding depression and declining genetic diversity.     

Effects of inbreeding on the magnitude of inbreeding depression in a highly self-fertilizing tree, Magnolia obovata

Inbreeding depression is one of the major selective forces driving the evolution of mating systems. Previous theories predict that long-lived plants will show a negative correlation between inbreeding depression and the level of inbreeding (as determined by an inbreeding coefficient) at maturity, but the extent of this correlation may vary among life stages because of variation in the genetic basis for inbreeding depression at different stages. To test this prediction, I used electrophoretic allozyme analysis and pollination experiments to examine the fixation index (F _is) at maturity and inbreeding depression in the early and late life stages of two populations with different outcrossing rates of a highly self-fertilizing tree, Magnolia obovata. The magnitude of inbreeding depression for early survival (δ _e) in an outcrossing population (t _m = 0.51; F _is = −0.015) was higher (δ _e = 0.97) than that in an inbreeding population (t _m = 0.18; F _is = 0.15; δ _e = 0.38). From these results, I estimated that both populations exhibited high inbreeding depression for late survival (δ _l) (0.94 in the outcrossing population and 0.93 in the inbreeding one) and lifetime survival (δ _t) (0.99 and 0.96, respectively). My results and previously published data demonstrate the predicted relationship between inbreeding depression and the level of inbreeding for early survival, but not for late survival. This suggests that there is a differential genetic basis for inbreeding depression at different life stages. The inbreeding depression for late survival appears to play a central role in the maintenance of reproductive traits that promote outcrossing in M. obovata.     

Heterosis in crosses between geographically separated populations of Drosophila melanogaster

Summary An experiment was performed to test the hypothesis that the genetic distance between populations estimated from enzyme loci could be used to predict the amount of heterosis that would occur in crosses between these populations. A partial diallel cross using 11 populations of Drosophila melanogaster from the AustralianPacific region and from England was carried out. Heterosis for larval viability, fecundity, cold shock mortality, and an index of these three traits was recorded. When two populations originating from the same location were crossed, no heterosis occurred, but otherwise heterosis was significant for all traits. For larval viability, a similar low level of heterosis occurred in all crosses. For cold shock mortality, the level of heterosis varied widely and fecundity showed a pattern intermediate between these two. The geographic distance between the sites from which populations originated was not correlated with the amount of heterosis in their crosses. There was a tendency for populations from ecologically different environments to show heterosis in crosses. Genetic distance based on ten enzyme loci was correlated with heterosis for cold shock mortality and the combined trait index. These results can be explained by the hypothesis that genes affecting larval viability are subject to strong, uniform selection in all populations, which limits the extent to which gene frequencies can drift apart. However, genes affecting cold shock mortality and the enzyme loci are subject to different selection pressures in different environments. This divergent selection combined with genetic drift causes divergence in gene frequency and heterosis.     

No evidence for postcopulatory inbreeding avoidance in Drosophila melanogaster

Selection to avoid inbreeding is predicted to vary across species due to differences in population structure and reproductive biology. Over the past decade, there have been numerous investigations of postcopulatory inbreeding avoidance, a phenomenon that first requires discrimination of mate (or sperm) relatedness and then requires mechanisms of male ejaculate tailoring and/or cryptic female choice to avoid kin. The number of studies that have found a negative association between male-female genetic relatedness and competitive fertilization success is roughly equal to the number of studies that have not found such a relationship. In the former case, the underlying mechanisms are largely unknown. The present study was undertaken to verify and expand upon a previous report of postcopulatory inbreeding avoidance in D. melanogaster, as well as to resolve underlying mechanisms of inbreeding avoidance using transgenic flies that express a sperm head-specific fluorescent tag. However, siblings did not have a lower fertilization success as compared to unrelated males in either the first (P(1) ) or second (P(2) ) mate role in sperm competition with a standard unrelated competitor male in our study population of D. melanogaster. Analyses of mating latency, copulation duration, egg production rate, and remating interval further revealed no evidence for inbreeding avoidance.