Comparative effects of inbreeding and outbreeding on the ontogeny of Heliconius erato phyllis (Lepidoptera: Nymphalidae)

Inbreeding depression has been reported in various groups of organisms, including insects. Estimates of inbreeding consequences were obtained by comparing 12 life‐history and morphological traits among nine inbred families (F = 0.25) and 16 outbred families (F = 0) of the Neotropical butterfly Heliconius erato phyllis. A Student's t‐test showed statistically significant differences for pupal weight and right forewing area, both in males and in females, between inbred and outbred families. Survival during development, from egg hatching to adulthood, also differed significantly between inbred and outbred families. The average number of haploid lethal equivalents was 0.17 for pupal weight, 0.15 for forewing area and 0.71 for survival from hatching to adulthood. The results of this study confirm that the consequences of inbreeding are more deleterious to life history traits than to morphological ones.     

Effects of inbreeding and temperature stress on life history and immune function in a butterfly

Theory predicts that inbreeding depression should be more pronounced under environmental stress due to an increase in the expression of recessive deleterious alleles. If so, inbred populations may be especially vulnerable to environmental change. Against this background, we here investigate effects of inbreeding, temperature stress and its interactions with inbreeding in the tropical butterfly Bicyclus anynana. We use a full‐factorial design with three levels of inbreeding (F = 0/0.25/0.38) and three temperature treatments (2 h exposure to 1, 27 or 39 °C). Despite using relatively low levels of inbreeding significant inbreeding depression was found in pupal mass, pupal time, thorax mass, abdomen fat content, egg hatching success and fecundity. However, stress resistance traits (heat tolerance, immune function) were not affected by inbreeding and interactions with temperature treatments were virtually absent. We thus found no support for an increased sensitivity of inbred individuals to environmental stress, and suspect that such patterns are restricted to harsher conditions. Our temperature treatments evidently imposed stress, significantly reducing longevity, fecundity, egg hatching success and haemocyte numbers, while fat content, protein content and lysozyme activity remained unaffected. Males and females differed in all traits measured except pupal time, protein content and phenoloxidase (PO) activity. Correlation analyses revealed, among others, a trade‐off between PO and lysozyme activity, and negative correlations between fat content and several other traits. We stress that more data are needed on the effects of inbreeding, temperature variation and sexual differences on insect immune function before more general conclusions can be drawn.     

Differential effects of offspring and maternal inbreeding on egg laying and offspring performance in the burying beetle Nicrophorus vespilloides

We investigate the effect of offspring and maternal inbreeding on maternal and offspring traits associated with early offspring fitness in the burying beetle Nicrophorus vespilloides. We conducted two experiments. In the first experiment, we manipulated maternal inbreeding only (keeping offspring outbred) by generating mothers that were outbred, moderately inbred or highly inbred. Meanwhile, in the second experiment, we manipulated offspring inbreeding only (keeping females outbred) by generating offspring that were outbred, moderately inbred or highly inbred. In both experiments, we monitored subsequent effects on breeding success (number of larvae), maternal traits (clutch size, delay until laying, laying skew, laying spread and egg size) and offspring traits (hatching success, larval survival, duration of larval development and average larval mass). Maternal inbreeding reduced breeding success, and this effect was mediated through lower hatching success and greater larval mortality. Furthermore, inbred mothers produced clutches where egg laying was less skewed towards the early part of laying than outbred females. This reduction in the skew in egg laying is beneficial for larval survival, suggesting that inbred females adjusted their laying patterns facultatively, thereby partially compensating for the detrimental effects of maternal inbreeding on offspring. Finally, we found evidence of a nonlinear effect of offspring inbreeding coefficient on number of larvae dispersing. Offspring inbreeding affected larval survival and larval development time but also unexpectedly affected maternal traits (clutch size and delay until laying), suggesting that females adjust clutch size and the delay until laying in response to being related to their mate.     

Effect of recurrent selfing on inbreeding depression and mating system evolution in an autopolyploid plant

Genome duplication resulting in polyploidy can have significant consequences for the evolution of mating systems. Most theory predicts that self‐fertilization will be selectively favored in polyploids; however, many autopolyploids are outcrossing or mixed‐mating. Here, we examine the hypothesis that the evolution of selfing is restricted in autopolyploids because the genetic cost of selfing (i.e., inbreeding depression) increases monotonically with successive generations of inbreeding. Using the herbaceous, autotetraploid plant Chamerion angustifolium, we generated populations with different inbreeding coefficients (F= 0, 0.17 and 0.36) through three consecutive generations of selfing and compared their magnitudes of inbreeding depression in a common environment. Mating system estimates for four natural populations confirmed that tetraploid selfing rates (s_m= 0.25, SE = 0.02) are similar to those of diploids (s_m= 0.12, SE = 0.12; F_1,2= 1.34, P= 0.37) indicating that both cytotypes are predominantly outcrossing. Compared to an outbred control line, mean inbreeding depression for seed production, survival, and height (vegetative and total) in the inbred line differed among generations (inbreeding coefficients). Across all stages, inbreeding depression (relative to control) was positively related to generation (inbreeding coefficient). Although the initial costs of inbreeding in extant and newly synthesized polyploids may be low compared to diploids, the monotonic increase in inbreeding depression with repeated inbreeding may limit the extent to which selfing variants are favored.     

Genome‐wide regulatory deterioration impedes adaptive responses to stress in inbred populations of Drosophila melanogaster*

Inbreeding depression is often intensified under environmental stress (i.e., inbreeding–stress interaction). Although the fitness consequences of this phenomenon are well‐described, underlying mechanisms such as an increased expression of deleterious alleles under stress, or a lower capacity for adaptive responses to stress with inbreeding, have rarely been investigated. We investigated a fitness component (egg‐to‐adult viability) and gene‐expression patterns using RNA‐seq analyses in noninbred control lines and in inbred lines of Drosophila melanogaster exposed to benign temperature or heat stress. We find little support for an increase in the cumulative expression of deleterious alleles under stress. Instead, inbred individuals had a reduced ability to induce an adaptive gene regulatory stress response compared to controls. The decrease in egg‐to‐adult viability due to stress was most pronounced in the lines with the largest deviation in the adaptive stress response (R² = 0.48). Thus, we find strong evidence for a lower capacity of inbred individuals to respond by gene regulation to stress and that this is the main driver of inbreeding‐stress interactions. In comparison, the altered gene expression due to inbreeding at benign temperature showed no correlation with fitness and was pronounced in genomic regions experiencing the highest increase in homozygosity.     

Effects of inbreeding on aversive learning in Drosophila

Inbreeding adversely affects life history traits as well as various other fitness‐related traits, but its effect on cognitive traits remains largely unexplored, despite their importance to fitness of many animals under natural conditions. We studied the effects of inbreeding on aversive learning (avoidance of an odour previously associated with mechanical shock) in multiple inbred lines of Drosophila melanogaster derived from a natural population through up to 12 generations of sib mating. Whereas the strongly inbred lines after 12 generations of inbreeding (0.75 < F < 0.93) consistently showed reduced egg‐to‐adult viability (on average by 28%), the reduction in learning performance varied among assays (average = 18% reduction), being most pronounced for intermediate conditioning intensity. Furthermore, moderately inbred lines (F = 0.38) showed no detectable decline in learning performance, but still had reduced egg‐to‐adult viability, which indicates that overall inbreeding effects on learning are mild. Learning performance varied among strongly inbred lines, indicating the presence of segregating variance for learning in the base population. However, the learning performance of some inbred lines matched that of outbred flies, supporting the dominance rather than the overdominance model of inbreeding depression for this trait. Across the inbred lines, learning performance was positively correlated with the egg‐to‐adult viability. This positive genetic correlation contradicts a trade‐off observed in previous selection experiments and suggests that much of the genetic variation for learning is owing to pleiotropic effects of genes affecting functions related to survival. These results suggest that genetic variation that affects learning specifically (rather than pleiotropically through general physiological condition) is either low or mostly due to alleles with additive (semi‐dominant) effects.     

A genetic perspective on mating systems and sex ratios of parasitoid wasps

Parasitoid sex ratios are influenced by mating systems, whether complete inbreeding, partial inbreeding, complete inbreeding avoidance, or production of all-male broods by unmated females. Population genetic theory demonstrates that inbreeding is possible in haplodiploids because the purging of deleterious and lethal mutations through haploid males reduces inbreeding depression. However, this purging does not act quickly for deleterious mutations or female-limited traits (e.g., fecundity, host searching, sex ratio). The relationship between sex ratio, inbreeding, and inbreeding depression has not been explored in depth in parasitoids. The gregarious egg parasitoid, Trichogramma pretiosum Riley, collected from Riverside, CA (USA) produced a female-biased sex ratio of 0.24 (proportion of males). Six generations of sibling mating in the laboratory uncovered considerable inbreeding depression (∼ 20%) in fecundity and sex ratio. A population genetic study (based upon allozymes) showed the population was inbred (F _it = 0.246), which corresponds to 56.6% sib-mating. However, average relatedness among females emerging from the same host egg was only 0.646, which is less than expected (0.75) if ovipositing females mate randomly. This lower relatedness could arise from inbreeding avoidance, multiple mating by females, or superparasitism. A review of the literature in general shows relatively low inbreeding depression in haplodiploid species, but indicates that inbreeding depression can be as high as that found in Drosophila. Finally, mating systems and inbreeding depression are thought to evolve in concert (in plants), but similar dynamic models of the joint evolution of sex ratio, mating systems, and inbreeding depression have not been developed for parasitoid wasps. Received: November 13, 1998 /Accepted: January 8, 1999     

Simultaneous inbreeding modifies inbreeding depression in a plant–herbivore interaction

Because inbreeding is common in natural populations of plants and their herbivores, herbivore‐induced selection on plants, and vice versa, may be significantly modified by inbreeding and inbreeding depression. In a feeding assay with inbred and outbred lines of both the perennial herb, Vincetoxicum hirundinaria, and its specialist herbivore, Abrostola asclepiadis, we discovered that plant inbreeding increased inbreeding depression in herbivore performance in some populations. The effect of inbreeding on plant resistance varied among plant and herbivore populations. The among‐population variation is likely to be driven by variation in plant secondary compounds across populations. In addition, inbreeding depression in plant resistance was substantial when herbivores were outbred, but diminished when herbivores were inbred. These findings demonstrate that in plant–herbivore interactions expression of inbreeding depression can depend on the level of inbreeding of the interacting species. Furthermore, our results suggest that when herbivores are inbred, herbivore‐induced selection against self‐fertilisation in plants may diminish.     

Intergenerational effects of inbreeding in Nicrophorus vespilloides: offspring suffer fitness costs when either they or their parents are inbred

Inbreeding depression is the reduction in fitness caused by mating between related individuals. Inbreeding is expected to cause a reduction in offspring fitness when the offspring themselves are inbred, but outbred individuals may also suffer a reduction in fitness when they depend on care from inbred parents. At present, little is known about the significance of such intergenerational effects of inbreeding. Here, we report two experiments on the burying beetle Nicrophorus vespilloides, an insect with elaborate parental care, in which we investigated inbreeding depression in offspring when either the offspring themselves or their parents were inbred. We found substantial inbreeding depression when offspring were inbred, including reductions in hatching success of inbred eggs and survival of inbred offspring. We also found substantial inbreeding depression when parents were inbred, including reductions in hatching success of eggs produced by inbred parents and survival of outbred offspring that received care from inbred parents. Our results suggest that intergenerational effects of inbreeding can have substantial fitness costs to offspring, and that future studies need to incorporate such costs to obtain accurate estimates of inbreeding depression.     

Inbreeding and inbreeding depression in a threatened endemic plant,the African violet (Saintpaulia ionantha ssp. grotei), of the East Usambara Mountains,Tanzania

Mating among closely‐related individuals in small and isolated plant populations may result in reduced vigour of the inbred offspring, i.e. inbreeding depression, especially in naturally outbreeding plants. Occurrence of inbreeding and inbreeding depression was studied in Saintpaulia ionantha ssp. grotei, a threatened endemic plant species with a narrow ecological amplitude from the East Usambara Mountains. The level of inbreeding (measured as the fixation index, F) was investigated in twelve populations by analyzing variation at one microsatellite marker locus. The effect of one generation of selfing and outcrossing on the progeny fitness was studied by controlled crosses in two small patches that differ in the level isolation. The fixation index (F) across the populations was on the average 0.21 and varied among the populations from substantial inbreeding (F = 0.58) to surplus heterozygosity (F = −0.29). High inbreeding depression (δ) was observed at early and late stages of the life‐cycle. The isolated patch exhibited lower inbreeding depression than did the non‐isolated patch. The results of this study suggest that inbreeding and subsequent inbreeding depression are potential threats to the survival of Saintpaulia populations.     

Slow inbred lines of <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> express as much inbreeding depression as fast inbred lines under semi-natural conditions

Selection may reduce the deleterious consequences of inbreeding. This may be due to purging of recessive deleterious alleles or balancing selection favouring heterozygote offspring. Such selection is expected to be more efficient at slower compared to at faster rates of inbreeding. In this study we tested the impact of inbreeding and the rate of inbreeding on fitness related traits (egg productivity, egg-to-adult viability, developmental time and behaviour) under cold and benign semi-natural thermal conditions using Drosophila melanogaster as a model organism. We used non-inbred control and slow and fast inbred lines (both with an expected inbreeding level of 0.25). The results show that contrary to expectations the slow inbred lines do not maintain higher average fitness than the fast inbred lines. Furthermore, we found that stressful environmental conditions increased the level of inbreeding depression but the impact of inbreeding rate on the level of inbreeding depression was not affected by the environmental conditions. The results do not support the hypothesis that inbreeding depression is less severe with slow compared to fast rates of inbreeding and illustrate that although selection may be more efficient with slower rates of inbreeding this does not necessary lead to less inbreeding depression.     

Effects of inbreeding on life history and thermal performance in the tropical butterfly Bicyclus anynana

Human-induced loss and fragmentation of natural habitats reduces population size and thereby presumably genetic diversity through inbreeding or genetic drift. Additionally, many species are confronted with increased temperature stress due to climate change, with reduced genetic diversity potentially interfering with a species’ ability to cope with such conditions. While in general the detrimental impact of inbreeding has often been documented, its consequences for the ability to cope with temperature stress are still poorly understood. Against this background we here investigate the effects of inbreeding on egg hatching success, development and temperature stress tolerance in the tropical butterfly Bicyclus anynana. Specifically we test for an increased sensitivity to environmental stress in inbred individuals. Our results revealed that even comparatively low levels of inbreeding yield negative consequences for reproduction and development under beneficial conditions. Inbreeding also reduced cold tolerance in adult butterflies, while heat tolerance remained unaffected. We therefore conclude that acute stress tolerance may not be generally impaired by inbreeding.     

Effects of embryonic responses to clutch mates on egg hatching patterns of Pentatomidae (Heteroptera)

Stink bugs and shield bugs of the family Pentatomidae (Heteroptera) generally produce a clutch of densely deposited eggs. In a few species of this family, embryos hatch in response to some form of cues associated with the preceding hatching to synchronize egg hatching with clutch mates. The aim of the present study is to obtain a family‐wide understanding of the extent to which the hatching response to clutch mates accelerates hatching within egg clutches. Accordingly, the hatching patterns in intact egg clutches and eggs individually detached from egg clutches are compared in eight species among different genera. In Halyomorpha halys, hatching is significantly and highly synchronized by the effect of the hatching response: when eggs are not attached to each other, the hatching rate is only 3.8% at 15 min and exceeds 95% at 200 min. By contrast, when eggs are attached to each other, the hatching rate reaches more than 95% at 15 min. Hatching is also significantly synchronized by the hatching response in Nezara viridula (which shows relatively high hatching synchronization) and in Piezodorus hybneri and Plautia stali (both of which show milder hatching synchronization). Synchronization of hatching is not found to be promoted by a hatching response in Aelia fieberi, Dolycoris baccarum, Eurydema rugosum or Palomena angulosa. These findings reveal that the hatching response varies depending on the species in Pentatomidae, with a wide spectrum of effects on the hatching patterns of the egg clutches.     

Hatching asynchrony aggravates inbreeding depression in a songbird (Serinus canaria): An inbreeding–environment interaction

Understanding how the intensity of inbreeding depression is influenced by stressful environmental conditions is an important area of enquiry in various fields of biology. In birds, environmental stress during early development is often related to hatching asynchrony; differences in age, and thus size, impose a gradient in conditions ranging from benign (first hatched chick) to harsh (last hatched chick). Here, we compared the effect of hatching order on growth rate in inbred (parents are full siblings) and outbred (parents are unrelated) canary chicks (Serinus canaria). We found that inbreeding depression was more severe under more stressful conditions, being most evident in later hatched chicks. Thus, consideration of inbreeding‐environment interactions is of vital importance for our understanding of the biological significance of inbreeding depression and hatching asynchrony. The latter is particularly relevant given that hatching asynchrony is a widespread phenomenon, occurring in many bird species. The exact causes of the observed inbreeding‐environment interaction are as yet unknown, but may be related to a decrease in maternal investment in egg contents with laying position (i.e. prehatching environment), or to performance of the chicks during sibling competition and/or their resilience to food shortage (i.e. posthatching environment).     

Accounting for cryptic population substructure enhances detection of inbreeding depression with genomic inbreeding coefficients: an example from a critically endangered marsupial

Characterizing inbreeding depression in wildlife populations can be critical to their conservation. Coefficients of individual inbreeding can be estimated from genome‐wide marker data. The degree to which sensitivity of inbreeding coefficients to population genetic substructure alters estimates of inbreeding depression in wild populations is not well understood. Using generalized linear models, we tested the power of two frequently used inbreeding coefficients that are calculated from genome‐wide SNP markers, F_H and F^^III, to predict four fitness traits estimated over two decades in an isolated population of the critically endangered Leadbeater's possum. F_H estimates inbreeding as excess observed homozygotes relative to equilibrium expectations, whereas F^^III quantifies allelic similarity between the gametes that formed an individual, and upweights rare homozygotes. We estimated F_H and F^^III from 1,575 genome‐wide SNP loci in individuals with fitness trait data (N = 179–237 per trait), and computed revised coefficients, F_H^{by group} and F^^{IIIby group}, adjusted for population genetic substructure by calculating them separately within two different genetic groups of individuals identified in the population. Using F_H or F^^III in the models, inbreeding depression was detected for survival to sexual maturity, longevity and whether individuals bred during their lifetime. F^^{IIIby group} (but not F_H^{by group}) additionally revealed significant inbreeding depression for lifetime reproductive output (total offspring assigned to each individual). Estimates of numbers of lethal equivalents indicated substantial inbreeding load, but differing between inbreeding estimators. Inbreeding depression, declining population size, and low and declining genetic diversity suggest that genetic rescue may assist in preventing extinction of this unique Leadbeater's possum population.     

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.     

Efficiency of selection, as measured by single nucleotide polymorphism variation, is dependent on inbreeding rate in Drosophila melanogaster

It is often hypothesized that slow inbreeding causes less inbreeding depression than fast inbreeding at the same absolute level of inbreeding. Possible explanations for this phenomenon include the more efficient purging of deleterious alleles and more efficient selection for heterozygote individuals during slow, when compared with fast, inbreeding. We studied the impact of inbreeding rate on the loss of heterozygosity and on morphological traits in Drosophila melanogaster. We analysed five noninbred control lines, 10 fast inbred lines and 10 slow inbred lines; the inbred lines all had an expected inbreeding coefficient of approximately 0.25. Forty single nucleotide polymorphisms in DNA coding regions were genotyped, and we measured the size and shape of wings and counted the number of sternopleural bristles on the genotyped individuals. We found a significantly higher level of genetic variation in the slow inbred lines than in the fast inbred lines. This higher genetic variation was resulting from a large contribution from a few loci and a smaller effect from several loci. We attributed the increased heterozygosity in the slow inbred lines to the favouring of heterozygous individuals over homozygous individuals by natural selection, either by associative over‐dominance or balancing selection, or a combination of both. Furthermore, we found a significant polynomial correlation between genetic variance and wing size and shape in the fast inbred lines. This was caused by a greater number of homozygous individuals among the fast inbred lines with small, narrow wings, which indicated inbreeding depression. Our results demonstrated that the same amount of inbreeding can have different effects on genetic variance depending on the inbreeding rate, with slow inbreeding leading to higher genetic variance than fast inbreeding. These results increase our understanding of the genetic basis of the common observation that slow inbred lines express less inbreeding depression than fast inbred lines. In addition, this has more general implications for the importance of selection in maintaining genetic variation.     

Comparative reproductive study of two <Emphasis Type="Italic">Streptocephalus sirindhornae</Emphasis> (Branchiopoda: Anostraca) populations and F1 hybrids: a laboratory study

We investigated the effects of inbreeding and crossbreeding on the reproductive biology of two populations of fairy shrimp Streptocephalus sirindhornae from Maha Sarakham (M) and Suphan Buri (S) provinces of Thailand. Four groups of the mature fairy shrimp were experimentally mated; S × S, M × M, M × S and S × M. The data on life span, age at first spawning, number of eggs per brood and total number of eggs per female of the inbred and crossbred parent generations (P) were not significantly different. The spawning frequency and capacity of the crossbred P were significantly more productive than those of the inbred. In both populations, the majority of P had the greatest egg numbers in the 8th to 14th broods. The hatching percentages of eggs and the survival percentages of nauplii from the 10th brood of the F ₁ populations were significantly higher than those of the 20th and 1st broods. The inbreeding in F ₁ had no effect on hatching and survival percentages, sex ratios of males to females, average weight, body length and specific growth percentages. Our findings suggest that the inbred populations in P and F ₁ were able to yield eggs similar to the crossbred populations, with similar growth, hatching and survival percentages.     

Variation in self-fertility,inbreeding depression and levels of inbreeding in four Cyclamen species

The ability to self in the absence of pollinators, i.e. reproductive assurance, and the detrimental consequences of inbreeding, i.e. inbreeding depression, are central factors influencing plant mating system evolution. The purpose of this study was to quantify whether self-fertility and inbreeding depression are related to levels of inbreeding in four Cyclamen species, namely C. balearicum (mean F_is = 0.930), C. creticum (mean F_is = 0.748), C. repandum (mean F_is = 0.658) and C. hederifolium (mean F_is = 0.329). C. balearicum showed a markedly greater capacity to autonomously self-fertilize than the three other species, which may have favoured inbreeding in this species. Levels of inbreeding depression were highest in C. creticum and C. hederifolium at the fruit maturation (δ = 0.18 and 0.20, respectively) and seed number (δ = 0.32 and 0.30, respectively) stages, and for C. repandum at the seed weight stage (δ = 0.23). Although C. balearicum showed inbreeding depression on seed germination (δ = 0.45), this may be an artefact of the generally low levels of seed germination in the experiment. Overall, we observed only limited evidence for the predicted negative relation between inbreeding coefficients and levels of inbreeding since C. creticum had high levels of inbreeding and inbreeding depression. Other factors may thus influence the relationship between inbreeding and inbreeding depression in these species.     

Influences of inbreeding and genetics on telomere length in mice

We measured telomere lengths of blood leukocytes in several inbred and outbred mammalian species, using a telomere-specific fluorescent probe and flow cytometry. Humans, non-human primates, and three outbred populations of Peromyscus mice (Peromyscus leucopus, Peromyscus maniculatus, and Peromyscus polionotus) have short telomeres. Two common strains of laboratory mice, C57BL/6J and DBA/2J, have telomeres several times longer than most other mammals surveyed. Moreover, the two inbred laboratory mouse strains display significantly different telomere lengths, suggesting the existence of strain-specific genetic determinants. To further examine the effects of inbreeding, we studied three Peromyscus leucopus inbred lines (GS109, GS16A1, and GS16B), all derived from the outbred P. leucopus stock. Telomeres of all three inbred lines are significantly lengthened relative to outbred P. leucopus, and the three lines display strain-specific significantly different telomere lengths, much like the C57BL/6J and DBA/2J strains of M. musculus. To further characterize the genetic inheritance of telomere length, we carried out several crosses to obtain hybrid F₁ mice between parental strains displaying the phenotype of long and short telomeres. In all F₁ mice assayed, peripheral blood leukocyte telomere length was intermediate to that of the parents. Additionally, we generated F₂ mice from a cross of the (P. leucopus outbred × GS16B)F₁. Based on the distribution of telomere length in the F₂ population, we determined that more than five loci contribute to telomere length regulation in Peromyscus. We concluded that inbreeding, through unknown mechanisms, results in the elongation of telomeres, and that telomere length for a given species and/or sub-strain is genetically determined by multiple segregating loci.