Brother-sister Mating Research Articles

Sex determination and inbreeding depression in an ant with regular sib-mating

Haplodiploidy is one of the most widespread mechanisms of sex determination in animals. In many Hymenoptera, including all hitherto investigated social species, diploid individuals, which are heterozygous at the sex locus, develop as females, whereas haploid, hemizygous individuals develop as males (single-locus complementary sex determination, sl-CSD). Inbreeding leads to homozygosity at the sex locus, resulting in the production of diploid males, which are usually sterile and constitute a considerable fitness cost. Nevertheless, regular inbreeding without diploid male production is known from several solitary wasps, suggesting that in these species sex is not determined by sl-CSD but alternative mechanisms. Here, we examine sex determination in an ant with regular inbreeding, Cardiocondyla obscurior. The almost complete absence of diploid males after 10 generations of brother-sister mating in the laboratory documents for the first time the absence of sl-CSD and CSD with two or a few unlinked sex loci in a species of social Hymenoptera. Queens, which mated with a brother, appeared to decrease the number of males in their brood, as expected from the relatedness relationships under inbreeding. In contrast, some colonies began to show signs of an inbreeding depression after several generations of sib-mating, such as shortened queen life span, higher brood mortality, and a shift to more male-biased sex ratios in some colonies, presumably due to lower insemination capability of sperm.

A Tumor Necrosis Factor Receptor 1-Dependent Conversation between Central Nervous System-Specific T Cells and the Central Nervous System Is Required for Inflammatory Infiltration of the Spinal Cord

We examined the role of tumor necrosis factor receptor 1 (TNFR1) in inflammation initiated by the adoptive transfer of central nervous system (CNS)-specific Th1 cells in experimental autoimmune encephalomyelitis, a murine model of multiple sclerosis. This adoptive transfer paradigm eliminates the confounding effects of bacterial adjuvants in the analysis of inflammation. We found that although T cells could reach the meninges and perivascular space in the absence of TNFR1, recruitment of other inflammatory cells from the blood was dramatically reduced. The reduction in the recruitment of CD11b(hi) cells correlated with a dramatic reduction in the production of the chemokines CCL2 (MCP-1) and CXLC2 (MIP-2) in TNFR1-deficient hosts. Bone marrow chimera experiments demonstrated that TNF can be effectively supplied by either the hematopoietic system or the CNS, but the essential TNFR1-responsive cells reside in the CNS. Previous work has demonstrated that microglia produce CCL2, and here we demonstrate that astrocytes and endothelial cells produced CXCL2 in the early stages of inflammation. Therefore, productive inflammation results from a conversation, or mutually responding signals, between the initiating T cells and cells in the parenchyma of the spinal cord.

Small Eye Phenotypes Observed in a Human tau Gene Transgenic Rat

We developed a rat line showing small eye from transgenic rats that were obtained by microinjection of a DNA segment containing the human (h)tau cDNA (GenBank: BC000558: 31-677,774-1180) expressed under control of CAG promoter, which is related to Alzheimer disease, into the pronuclei rat embryos. The rat line was established by selective brother-sister mating of rats showing small eyes. Of 11 offspring in the 11th generation, there were eight animals with microphthalmia and the transgene. The remaining three rats without transgene did not show the small eyes phenotype. The globes of affected rats were 1.2 mm in length compared with normal globes (3.5 mm), and all other ocular structures were normal. The expression of hTau protein was evident immunohistochemically in the ciliary body, extraocular muscle, lens epithelium, and pigment epithelium. Cytogenetic analysis suggested that the chromosome location of the transgene was chromosome 1 (1p12). This region may include genes related to lens development, such as Cat5.

Inference on microsatellite mutation processes in the invasive mite, Varroa destructor, using reversible jump Markov chain Monte Carlo

Varroa destructor is a parasitic mite of the Eastern honeybee Apis cerana. Fifty years ago, two distinct evolutionary lineages (Korean and Japanese) invaded the Western honeybee Apis mellifera. This haplo–diploid parasite species reproduces mainly through brother–sister matings, a system which largely favors the fixation of new mutations. In a worldwide sample of 225 individuals from 21 locations collected on Western honeybees and analyzed at 19 microsatellite loci, a series of de novo mutations was observed. Using historical data concerning the invasion, this original biological system has been exploited to compare three mutation models with allele size constraints for microsatellite markers: stepwise (SMM) and generalized (GSM) mutation models, and a model with mutation rate increasing exponentially with microsatellite length (ESM). Posterior probabilities of the three models have been estimated for each locus individually using reversible jump Markov Chain Monte Carlo. The relative support of each model varies widely among loci, but the GSM is the only model that always receives at least 9% support, whatever the locus. The analysis also provides robust estimates of mutation parameters for each locus and of the divergence time of the two invasive lineages (67,000 generations with a 90% credibility interval of 35,000–174,000). With an average of 10 generations per year, this divergence time fits with the last post-glacial Korea–Japan land separation.

Inbreeding depression and family variation in a social insect,Bombus terrestris(Hymenoptera: Apidae)

In bumble bees, only a few colonies dominate reproduction at the end of a season such that successful families are likely to experience episodic inbreeding. We compared the fitness of 33Bombus terrestriscolonies resulting from 188 brother–sister matings (“inbred”) to that of 43 outbred colonies (from 183 outbred matings). Large among‐family variation appears to be a key feature that characterizes all main steps in the life cycle ofB. terrestris. Hibernation survival and colony foundation of queens varied strongly among maternal lines (P>0.001). Mating with a brother significantly reduced a queen's hibernation survival and colony foundation success by varying degrees (6% vs 23%). The effect on hibernation survival was not found when including the date of mating into the analysis: queens that mated and hibernated early in the season survived hibernation more frequently than those that mated later (P=0.001) irrespective of mating treatment. Furthermore, maternal family lines differed significantly in their response to inbreeding with respect to colony size, caloric investment into sexuals, and the Shaw‐Mohler fitness index. Some family lines even showed outbreeding depression. On average, inbred colonies were significantly smaller (coefficient of depression δ=0.20) but there was no inbreeding depression for reproductive output or cumulative fitness. Thus, as a population,B. terrestrisis likely to cope with inbreeding episodes. However, among‐family variation in the effects of inbreeding intensifies the differential representation of families in the population.

Altered Epithelial Cell Lineage Allocation and Global Expansion of the Crypt Epithelial Stem Cell Population Are Associated with Ileitis in SAMP1/YitFc Mice

Crohn's disease is characterized by cycles of mucosal injury and ulceration followed by epithelial regeneration and restoration of normal epithelial function. In this study, we examined whether ileitis in SAMP1/YitFc mice, a recombinant-inbred line that spontaneously develops ileitis resembling human Crohn's disease, was associated with alterations in normal patterns of epithelial differentiation or changes in epithelial regeneration after experimental injury. Increased numbers of Paneth, goblet, and intermediate cells were present focally in the ileum of SAMP1/YitFc mice by 4 weeks of age, before any histological evidence of acute or chronic inflammation. This increase in secretory cells became more pronounced at sites of ileitis with increasing age and inflammation. Additionally, there was mispositioning of Paneth and intermediate cells along the crypt-to-villus unit. A concomitant reduction in the number of absorptive enterocytes was observed. In contrast to the ileal-specific changes in lineage allocation, crypt stem cell numbers began to increase in both the ileum and proximal jejunum at the onset of inflammation in SAMP1/YitFc mice. These data suggest that the alterations in epithelial cell differentiation and increases in the size of the crypt stem cell population observed in SAMP1/YitFc mice are regulated by distinct mechanisms. We speculate that these epithelial alterations may play a role in the pathogenesis of ileitis in this murine model of Crohn's disease.

A major quantitative trait locus on mouse chromosome 3 is involved in disease susceptibility in different colitis models

Mice with a disrupted gene for the G-protein alpha inhibitory 2 chain ( Gnai2 -/- ) develop a spontaneous colitis resembling human inflammatory bowel disease. Disease expression differs markedly between inbred strains of mice, indicating genetic control of disease susceptibility. We performed a genome-wide screen to localize the chromosomal regions regulating disease expression. A total of 284 F2 mice derived from resistant C57BL/6J Gnai2 -/- mice and susceptible C3H/HeN Gnai2 -/- mice were analyzed in a genome-wide screen for colitis susceptibility and severity. A highly significant locus on chromosome 3 (Gpdc1) contributed to colitis susceptibility and severity (likelihood ratio statistics [LRS] = 32.4; LOD score = 7; P < 1.0 x 10(-5)). The peak linkage of this locus at 62 cM colocalizes exactly with a previously identified locus controlling colitis susceptibility in interleukin-10-deficient mice. In addition, evidence for linkage with a locus on chromosome 1 (Gpdc2 ; LRS = 19.7; LOD = 4.3) was found, and the 2 loci interacted epistatically (combined LRS = 68.2). A third locus (Gpdc3) was found on chromosome 9 and this locus interacted epistatically with a locus on chromosome 7, which by itself did not have an effect on the trait. The identification of a major locus on chromosome 3 that controls susceptibility to spontaneous colitis in 2 different gene-knockout models indicates that this locus harbors a gene(s) that plays a key role in maintaining mucosal homeostasis. Identification of this gene(s) may contribute to further understanding of the mechanisms underlying human inflammatory bowel disease.

Enhanced Egg-Induced Immunopathology Correlates With High IFN-γ in Murine Schistosomiasis: Identification of Two Epistatic Genetic Intervals

The genetic basis of dissimilar immunopathology development among mouse strains infected with Schistosoma mansoni is not known. We performed a multipoint parametric linkage analysis on a cohort of F(2) mice, offspring of brother-sister mating between (high pathology CBA x low pathology BL/6)F(1) mice, to examine whether the observed differences in the type of immune response or the extent of hepatic immunopathology are linked to any particular genomic intervals. The F(2) mice exhibited cytokine responses and immunopathologies that revealed a statistically significant correlation between prominent egg Ag-stimulated IFN-gamma production by mesenteric lymph node cells and hepatic egg granuloma size. Increased IFN-gamma production showed suggestive linkage to a dominant CBA locus on chromosome 1 and a recessive CBA locus on chromosome 5; significantly, there was an epistatic interaction between the two IFN-gamma loci. An additional locus with suggestive linkage to granuloma formation and a CBA-recessive mode of inheritance was mapped to centromeric chromosome 13. Our analysis identified the first three genetic regions that appear to influence the immunopathology in murine schistosomiasis; however, further congenic dissection studies will furnish a more precise understanding of the genetic control of this disease.

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.

Functionally reproductive diploid and haploid males in an inbreeding hymenopteran with complementary sex determination.

It has become a matter of orthodoxy that among wasps, ants, bees, and other insects in the order Hymenoptera, only uniparental haploid males that arise from unfertilized eggs are capable of reproduction. This idea is of interest because the best understood and perhaps most widespread sex determination system among these insects [known as single locus complementary sex determination (sl-CSD)] does not depend on ploidy alone and, paradoxically, consistently results in small numbers of diploid biparental males. To date, the reproductive potential of diploid males has been studied in 13 of the perhaps 200,000 hymenopterans world-wide; in each of these instances, the diploid males are genetic dead ends because they are inviable or sterile. The data from these species have resulted in a general conclusion that has been invoked for virtually all species with sl-CSD and has become the basis for assumptions regarding conservation biology, sex ratio analysis, and the evolution of social behavior. Here, we report that in the solitary vespid wasp Euodynerus foraminatus, both diploid and haploid males are fertile, which documents normal fertility in diploid males of a hymenopteran with sl-CSD. This wasp has high levels of inbreeding because of frequent brother-sister mating in nature; therefore, diploid males are more frequently produced and thus more likely exposed to selection favoring their fertility. Because inbreeding and diploid male production may be important features of the population biology of many hymenopterans, we sound a cautionary note regarding ideas about the evolutionary ecology of these insects.

Behavioral characterization of alcohol-tolerant and alcohol-nontolerant rat lines and an f(2) generation.

The Alcohol Tolerant (AT) and Alcohol Nontolerant (ANT) rats, selectively bred for ethanol-induced ataxia on the inclined plane at ALKO in Finland, were moved to the University of Colorado in 1998. The selection phenotype was tested on generation 60 animals in Colorado. In week one, ataxia was measured on the inclined plane 30 minutes after an intraperitoneal dose of 2 g/kg 15% w/v ethanol. Differences in ethanol-induced ataxia between the AT and ANT lines at the University of Colorado were similar to those in the original lines in Finland. In week two, ataxia was measured on the inclined plane at 5 and 30 minutes, and tolerance was measured as the time to regain the original angle of sliding. The AT rats rapidly developed tolerance to 2 g/kg ethanol on the inclined plane; tolerance development was significantly slower in the ANT rats. In week three, the animals were tested for the duration of loss of righting reflex (LORR) and blood ethanol concentration at regain of the righting reflex (BECRRR) following a dose of 3.5 g/kg. The AT rats had a significantly higher BECRRR than did the ANT rats, but did not differ in LORR. A separate experiment with previously untreated rats demonstrated that naïve animals of the two lines did not differ in BECRRR or LORR. AT and ANT rats were genotyped for the mutation that occurs in the gene for the alpha6 subunit of the GABAA receptor, a natural mutation that is known to affect benzodiazepine responses. All ANT animals tested carried the mutant allele, whereas some AT families carried the mutation and others were wild type. There was no effect of the mutation in AT rats for any of the phenotypes that were tested. After several generations of brother-sister mating, the AT and ANT lines were more than 90% inbred as determined by genotyping. One AT (wild-type) line and one ANT (mutant) line were selected for breeding an F2 intercross generation of 1200 animals. They were phenotyped for sensitivity and tolerance to ethanol on each of three consecutive weeks. Order of testing had a modest effect on some of the phenotypes: when tested during the third week as compared to weeks one or two, BECRRR was increased, 30-minute sensitivity was increased, and development of acute tolerance was increased. Statistically significant correlations were found between tolerance and sensitivity at both 5 and 30 minutes, and between LORR and BECRRR. The smaller (or absence of) significant correlations between others of the phenotypes indicate(s) that they are most likely controlled by different sets of genes.

DNA Sequence Error Rates in Genbank Records Estimated using the Mouse Genome as a Reference

We estimate DNA sequence error rates in Genbank records containing protein-coding and non-coding DNA sequences by comparing sequences of the inbred mouse strain C57BL/6J, sequenced as part of the mouse genome project and independently by other laboratories. C57BL/6J was produced by more than 100 generations of brother-sister mating, and can be assumed to be virtually free of residual polymorphism and mutational variation, so differences between independent sequences can be attributed to error. The estimated single nucleotide error rate for coding DNA is 0.10% (SE 0.012%), which is substantially lower than previous estimates for error rates in Genbank accessions. The estimated single nucleotide error rate for intronic DNA sequences (0.22%; SE 0.051%) is significantly higher than the rate for coding DNA. Since error rates for the mouse genome sequence are very low, the vast majority of the errors we detected are likely to be in individual Genbank accessions. The frequency of insertion-deletion (indel) errors in non-coding DNA approaches that of single nucleotide errors in non-coding DNA, whereas indel errors are uncommon in coding sequences.

Effects of inbreeding on immune response and body size in a social insect, Bombus terrestris

Summary Inbreeding can negatively affect various fitness components. Here we examine how immune response and body size of a social insect are affected by inbreeding, sex and ploidy. In the bumble‐bee, Bombus terrestris (L.), the offspring of colonies resulting from brother–sister matings were compared with that of outbred colonies. Immune response was measured as the degree of encapsulation of a novel antigen, body size as the length of the radial cell in the forewings. Inbreeding affected neither immune response nor body size in either workers or haploid males under laboratory conditions. However, fitness characteristics varied significantly among maternal families and colonies. The lack of detectable inbreeding depression for two fitness components might help explain why B. terrestris is a good colonizer in nature. In addition, sex and ploidy strongly affected the fitness components studied: diploid males had a significantly lower immune response than haploid males, who in turn had a significantly lower immune response than workers of the same colony. The body size of diploid males was intermediate between the body size of workers and haploid males.

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.

Emergence of perianal fistulizing disease in the SAMP1/YitFc mouse, a spontaneous model of chronic ileitis

Background & Aims: SAMP1/Yit mice spontaneously develop chronic terminal ileitis, reminiscent of the human disease described by Crohn et al. in 1932. Several new phenotypic features have appeared in our colony after more than 20 generations of brother-sister mating. In this report, we describe the distinguishing features of the SAMP1/YitFc substrain at the University of Virginia, compared with the Japanese SAMP1/Yit parental strain. Methods: A colony of SAMP1/Yit mice was established at the University of Virginia in 1996, from 2 breeding pairs obtained from Japan. A systematic characterization of their phenotypic and immunologic characteristics was performed at 4, 10, 40, and more than 60 weeks of age. Results: The following differences were observed: (1) SAMP1/YitFc mice displayed established ileitis as early as 10 weeks of age, (2) the incidence of skin lesions inversely correlated with the occurrence of intestinal inflammation, (3) mice develop chronic ileitis with prominent muscular hypertrophy and focal collagen deposition in inflamed segments, (4) mesenteric lymph node lymphocytes acquired an activated phenotype coincident with disease progression, (5) high interferon-γ production was detected by 4 weeks of age and preceded the onset of ileitis, and (6) a subgroup of mice (~5%) developed perianal disease with ulceration and fistulae. Conclusions: The SAMP1/YitFc substrain exhibits unique characteristics when compared with the original Japanese strain. Of particular interest is the emergence of perianal fistulizing disease, to our knowledge the first report of such occurrence in an animal model of inflammatory bowel disease.

Histocompatible miniature swine: an inbred large-animal model.

Three herds of miniature swine, each homozygous for a different set of alleles at the major histocompatibility complex (MHC), and five intra-MHC recombinant strains, have been reported by the authors' laboratory. One herd (SLAdd) was selected for further inbreeding to achieve a histocompatible line. It has undergone seven additional generations of sequential brother-sister or father-daughter matings (termed G7). To determine the level of histocompatibility of these animals, the authors performed skin and heart transplantation without immunosuppression. In contrast to MHC-matched, minor antigen-mismatched animals that rejected skin in 11 days (median survival time [MST], n=6) and hearts in 35 days (MST, n=4), G7 animals accepted skin for greater than 340 days (>340, >448, and >677 days) and hearts for greater than 265 days (>265 and >269 days). Nevertheless, rejection of second grafts could be induced by sensitization, indicating that weak minor antigens remain, requiring further inbreeding to achieve full histocompatibility. We conclude that G7 animals are sufficiently inbred to accept first set skin and heart grafts indefinitely.

Elongation Factor 1 α resolves the monophyly of the haplodiploid ambrosia beetles Xyleborini (Coleoptera: Curculionidae)

Elongation Factor 1-alpha was used to test the monophyly of the wood boring beetle tribe Xyleborini, where all species are haplodiploid and perform regular inbreeding by brother-sister mating. Due to their feeding requirements, being highly dependent on ophiostomatoid fungi which they cultivate in wood tunnels, monophyly may be expected due to nutritional constraints. During the course of analyses, two copies of EF-1alpha were amplified in these beetles, differing in intron structure. The high similarity between paralogous amino acid sequences (93-94%) indicates a rather recent duplication in beetles, but phylogenetic analyses of different copies in insects rejected this hypothesis. Subsequent phylogenetic analyses of eighty orthologous sequences from Xyleborini and allied taxa, using the single-intron bearing copy, were greatly improved in resolution and node support by including the intron sequences (c. 60 bp). Most analyses resulted in a monophyletic Xyleborini, implying one origin of fungus feeding in this tribe. However, clear evidence for a polyphyletic Xyleborus and three more xyleborine genera calls for further revision of xyleborine classification.

Genetic diversity of threeAristichthys nobilis populations and one inbreeding stock

Genetic diversity of threeAristichthys nobilis populations and one inbreeding stock was studied using random amplified polymorphic DNA (RAPD) method. Materials of natural populations were collected from two counties of Guangxi Province, Hengxian (NH) and Shanglin (NS) and one city of Hubei Province, Wuhan city. (NW). Shanglin and Hengxian's river system respectively belongs to Qianjiang and Yujiang River, two tributaries confluencing to Xunjiang River, main tributary of Xijiang River, which is the biggest tributary of the Zhujiang River system, and Wuhan's water system belongs to the Yangtse River system. The inbreeding stock (RS) was the more than 10 generation descendant by brother-sister mating system whose parents were collected from Shanglin. The results showed that the genetic variety of individuals of RS was very low (0.105 4), whereas that of natural populations was relatively high, which from high to low was 0.158 1 (NW), 0.132 0 (NH) and 0.110 5 (NS). As an index for genetic distance pair-wise populations, the genetic variety between populations was studied, which characterized the genetic distance between populations. The genetic distance of NW and NH, NW and NS, NW and RS were respectively high, whereas that of NH and NS, NS and RS, NH and RS was low. Chi-test (χ2) and the analysis of the genetic variety pair-wise populations was taken as efficient approach for studying population difference.

Seed sex ratio in dioecious plants depends on relative dispersal of pollen and seeds: an example using a chessboard simulation model

Abstract Two principles are important for the optimal sex ratio strategy of plants. (1) Sib mating. Because seed dispersal is restricted, sib mating may occur which selects for a female bias in the seed sex ratio. (2) Local resource competition (LRC). If a plant produces pollen its nuclear genes are dispersed in two steps: first through the pollen and then, if the pollen is successful in fertilizing an ovule on another plant, through the seed. If the plant produces an ovule, its genes are dispersed only through the seed. By making pollen instead of ovules the offspring of a single plant is then spread out over a wider area. This reduces the chance that genetically related individuals are close together and need to compete for the same resource. The effect is the strongest if pollen is dispersed over a much wider area than seeds. Less LRC for paternally vs. maternally derived offspring selects for a male bias in sex allocation. We study the above-mentioned opposite effects in dioecious plants (with separate male and female individuals), with maternal control over the sex ratio (fraction males) in the seeds. In a two-dimensional spatial model female-biased sex ratios are found when both pollen and seed dispersal are severely restricted. If pollen disperses over a wider area than seeds, which is probably the common situation in plants, the seed sex ratio becomes male-biased. If pollen and seeds are both dispersed over a wide area, the sex ratio approaches 0.5. Our results do not change if the offspring of brother–sister matings are less fit because of inbreeding depression.

Inbreeding depression: tests of the overdominance and partial dominance hypotheses.

The two principal theories of the causal mechanism for inbreeding depression are the partial dominance hypothesis and the overdominance hypothesis. According to the first hypothesis, inbreeding increases the frequency of homozygous combinations of deleterious recessive alleles thereby decreasing fitness, whereas the overdominance hypothesis posits that inbreeding increases homozygosity and thus reduces the frequency of the superior heterozygotes. These two hypotheses make different predictions on the effect of crossing inbred lines: the overdominance hypothesis predicts that trait means will be restored to the outbred means, whereas the partial dominance hypothesis predicts that trait means will exceed those of the outbred population. I tested these predictions using seven inbred lines of the sand cricket, Gryllus firmus. Fourteen generations of brother-sister mating resulted in an inbreeding depression of 20-34% in four traits: nymphal weights at ages 14 days, 21 days, 28 days, and early fecundity. An incomplete diallel cross of these lines showed genetic variation among lines and an increase in all trait means above the outbred means, with three being significantly higher. These results provide support for the partial dominance hypothesis and are inconsistent with the overdominance hypothesis.