Inbreeding Process Research Articles

Marker Effects Estimated from Testcrosses of Early and Late Generations of Inbreeding in Maize

In maize (Zea mays L.) breeding programs, early generation testing allows the discarding of lines with poor combining ability early in the inbreeding process. Molecular markers can be used to locate quantitative trait loci (QTL) and measure marker effects in early generations. For marker assisted selection to be useful in improving the efficiency of early generation testing, marker effects estimated in early generations must agree with marker effects in later generations. However, limited attention has been given to comparing marker effects in different generations of the same cross. Therefore, the overall objective of this research was to compare marker effects estimated from testcross performance of 190 S1 and S4:S5 lines from a maize population (BSll(FR)C7) by inbred (FRMo17) cross. A total of 146 probe‐enzyme combinations were used. To be most useful, correlations between marker effects in different generations need to be higher than phenotypic correlations. For all traits, phenotypic correlations were less than genetic correlations that were less than correlations based on marker effects estimated for the best set of markers from the S1. Except for grain yield, test weight, and days to anthesis, the correlations based on all marker effects were higher than the genetic correlations. As an example, the phenotypic correlation for grain yield was 0.36, the genetic correlation was 0.60, the correlation using all markers was 0.50, and that for the best set of markers was 0.86. Overall, marker effects estimated in S1 and S4:S5 testcrosses had reasonable agreement. Thus, marker information from early generations should improve the prediction of later generation performance.

Inbreeding process and establishment of new inbred lines derived from the Japanese house mouse, Mus musculus molossinus

Inbreeding of 8 groups in molossinus mouse began in (Miura female x Urawa male) F1 (MU) from the stock captured in the Kanto district of Japan and were accomplished by each primiparous littermates. The animals were kept under a clean conventional condition at 23.5 +/- 2.5 degrees C and 50 +/- 5% in relative humidity. Pellet feed, CRF-1, and tap water were provided ad libitum. The respective number of inbreeding generations of MUA, MUF and MUG now exceeds F21, F20 and F22, respectively. Inbreeding of MUD and MUE have reached to F18 up to now, but the other 3 groups became extinct at F10. Time required for inbreeding from F0 to F20 was 2,280 days for MUA, 2,454 days for MUF and 2,143 days for MUG. Generation time was 114.0 +/- 47.2 days for MUA, 122.7 +/- 54.6 days for MUF and 107.2 +/- 48.5 days for MUG. Though the generation time varied from a minimum of 60 days to a maximum of 258 days from F0 to F10 in the inbreeding process (mean: 150.2 +/- 41.7 days), which were shortened from F10 and the mean was 79.0 +/- 29.1 days. In the breeding of the molossinus mouse, primiparous age was 124.5 +/- 29.7 days from F0 to F9 and 77.8 +/- 9.3 days from F10 to F19. The litter size was 5.1 +/- 1.3 for MUA and 5.0 +/- 1.3 for MUG in offspring, 5.0 +/- 1.2 and 4.9 +/- 1.2 in weanling, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of rapid inbreeding and of crossing of inbred lines on conception rate, prolificacy and ewe survival in sheep

AbstractThe effects of four generations of inbreeding, to achieve inbreeding coefficients (F) of 0·25, 0·375, 0·50 and 0·59, on conception rate, prolificacy, litter weight at birth and survival of breeding females, have been studied in sheep. Crosses of inbred lines were also examined. Three breeds, Scottish Blackface, Cheviot and Welsh Mountain, and the crosses among these breeds, were involved. Analyses were carried out both for the data overall and within inbred lines.Conception rate at the first oestrus at which the ewe was mated (in each year) declined almost linearly from 0·71 (F2 generation - inbreeding F = 0·0) to 0·44 (F = 0·59) with line crosses showing 0·78 conception rate. The corresponding results following up to three rounds of services at successive oestrous periods were 0·91, 0·74 and 0·97 respectively. Analyses within lines showed a steeper rate of decline in conception rate to 0·33 (F = 0·59) following first services and 0·63 following up to three successive periods. This indicates that the lines with the best conception rates survived the inbreeding process better.Prolificacy, the number of lambs born per ewe lambing, dropped from 1·73 (F = 0·0) to 1·37 (F = 0·25) and thereafter more slowly to 1·24 (F = 0·59). Line crosses were no better than the initial non-inbred generation (F2 and the purebred equivalent O2). Within-line analysis gave the same results.The effects of inbreeding on litter weight at birth mirror closely those for prolificacy.Ewe survival declined linearly with inbreeding. There was a 1·2-fold increase in the risk of death for every increase in the inbreeding coefficient (F) of 0·1 (analysed by using a proportional hazard model).In general, there were no significant differences between purebred and crossbred sheep in the effects of inbreeding. For conception rate, prolificacy and litter weight (for which the point was examined) the changes with inbreeding could not be explained, statistically, in terms of dominance alone, suggesting, therefore, that epistasis is also involved.

Segregation of genes from donor strain during the production of recombinant congenic strains

Recombinant congenic strains (RCS) constitute a set of inbred strains which are designed to dissect the genetic control of multigenic traits, such as tumour susceptibility or disease resistance. Each RCS contains a small fraction of the genome of a common donor strain, while the majority of genes stem from a common background strain. We tested at two stages of the inbreeding process in 20 RCS, derived from BALB/cHeA and STS/A, to see whether alleles from the STS/A donor strain are distributed over the RCS in a ratio as would theoretically be expected. Four marker genes (Pep-3; Pgm-1; Gpi-1 and Es-3) located at 4 different chromosomes were selected and the allelic distribution was tested after 3-4 and after 12 generations of inbreeding. The data obtained do not significantly deviate from the expected pattern, thus supporting the validity of the concept of RCS.

Difference of Average Standard Length in Heterozygous or Homozygous Individuals at The Isozymic Loci in Cultured Stocks of Rainbow Trout in Japan.

Relationship between fitness advantage of isozymic genotype and a state of genetic constitution in a stock was examined in cultured stocks of rainbow trout in Japan. A number of stocks exhibited a larger average standard length in heterozygouse individuals under a state of “heterozygote excess” stocks at the Idh-3, Pgm-2 and Sod, significantly. However, an additive effect of advantage of heterozygotes at the three loci could not be observed. It sug-gests a fitness advantage of individuals heterozygous at each locus in cultured stocks of rainbow trout. On the other hand, a few stocks exhibited a larger standard length in homo-zygous individuals under a state of “homozygote excess” at the three loci, suggesting a fitness advantage of homozygous individuals. The mechanism of such phenomena could be explained by the relationship between the state of genetic constitution and quantity of dereterious genes in such a stack. An appearance of a larger average standard length in heterozygotes than in homozygotes could be explained by the resulting of a low fitness in homozygote individuals including many deleterious genes in the stocks. An appearance of larger average standard length in homozygotes than in heterozygotes could be explained by the elimination of deleterious genes during the process of inbreeding

Effect of rate of inbreeding on inbreeding depression in Drosophila melanogaster

This experiment was designed to study the relationship between rate of inbreeding and observed inbreeding depression of larval viability, adult fecundity and cold shock mortality in Drosophila melanogaster. Rates of inbreeding used were full-sib mating and closed lines of N=4 and N=20. Eight generations of mating in the N=20 lines, three generations in the N=4 lines and one generation of full-sib mating were synchronised to simultaneously produce individuals with an expected level of inbreeding coefficient (F) of approximately 0.25. Inbreeding depression for the three traits was significant at F=0.25. N=20 lines showed significantly less inbreeding depression than full-sib mated lines for larval viability at approximately the same level of F. A similar trend was observed for fecundity. No effect of rate of inbreeding depression was found for cold shock mortality, but this trait was measured with less precision than the other two. Natural selection acting on loci influencing larval viability and fecundity during the process of inbreeding could explain these results. Selection is expected to be more effective with slow rates of inbreeding because there are more generations and greater opportunity for selection to act before F=0.25 is reached. Selection intensities seem to have been different in the three traits measured. Selection was most intense for larval viability, less intense for fecundity and, perhaps, negligible at loci influencing cold shock mortality.

Selection in highly inbred populations of Drosophila melanogaster polymorphic at a single locus

The polymorphic Esterase-6 system in Drosophila melanogaster has been studied in laboratory populations made up from highly inbred lines. The Est-6S and Est-6F homozygous lines were obtained after 100 generations of inbreeding, selecting heterozygotes at each generation. The level of homogeneity in the genetic background of the lines has been estimated by computer simulation of the inbreeding process and confirmed by electrophoresis. The experimental populations were cultured on a control medium and on media containing different concentrations of naturally occurring esters known to be rapidly utilised by Esterase-6. The results indicate significant genotype-environment interactions. The high level of genetic homogeneity of the lines and the specific pressure applied to the locus analysed suggest that the fitness effects observed could be reasonably attributed to Est-6 allozymes.

Polymorphism of C lambda genes and units of duplication in the genus Mus.

The number of Ig C lambda genes in nine geographically widespread species from the four subgenera in the genus Mus was estimated from the number of Bam HI and Eco RI restriction fragments that hybridize under high stringency conditions to cDNA probes of BALB/c inbred mouse origin (Mus musculus domesticus). Three closely related species in the subgenus Mus, M. musculus, M. spretus, and M. spicelegus, show considerable variation in the number of C lambda genes. Estimates of gene numbers in these animals range from two C lambda genes in M. spretus from Puerto Real, Spain to 12 C lambda genes in M. musculus musculus from Studenec, Czechoslovakia. Strains of mice carrying either six or 10 C lambda genes were derived from a single population of M. musculus domesticus from Centreville, MD. The hybridization patterns of mice exhibiting C lambda gene amplification indicate that duplications are of relatively recent origin and probably occurred by reiteration of a DNA segment closely related to the 6.5 kb [C lambda 3 - C lambda 1] unit found in BALB/c inbred mice. Three more distantly related species in the subgenus Mus, and a species representing the Nannomys subgenus all appear to carry only four C lambda genes. DNA of species representing the Coelomys and Pyromys subgenera hybridized weakly to the C lambda cDNA probes, but these animals also have no more than four C lambda genes. Thus, there may be a base number of four C lambda genes in most species in the genus Mus. All inbred strains of mice so far examined also have only four C lambda genes, but no feral M. musculus examined have fewer than six C lambda genes. One explanation of the discrepancy in the number of genes between inbred and feral M. musculus is that C lambda genes were deleted during the process of inbreeding.

Variation in V lambda genes in the genus Mus.

The complement of Ig V lambda genes in nine species of feral mice representing the four extant subgenera of the genus Mus was examined and compared with that of BALB/c inbred mice. Although all inbred strains examined have two V lambda genes, there is variation in the number of copies of V lambda genes in the wild mice. All feral representatives of M. musculus domesticus, from which inbred strains are derived, have at least three V lambda genes, indicating that a V lambda gene may have been lost during the inbreeding process. At least three V lambda genes are also found in representatives of three other M. musculus subspecies, including the stock of M. musculus musculus "Czech II" shown to have at least 12 C lambda genes. In comparing the complement of V lambda and C lambda genes in these animals, evidence is found that supports a mechanism of lambda gene reiteration involving duplication of a unit containing a V lambda and two C lambda genes. However, the possibility that C lambda gene amplification occurred independent of V lambda gene evolution cannot be ruled out. M. spicelegus and M. spretus, species that are semifertile with M. musculus, have one to three V lambda genes. Species more distantly related to M. musculus, such as M. cookii and M. platythrix, appear to have more (four to six) V lambda genes. Greater V lambda gene heterogeneity is also found in these animals. We propose that the ancestors of the subgenus Mus had more V lambda genes than are seen in modern species and that the paucity of V lambda genes in M. musculus, M. spicelegus, and M. spretus may be the result of V lambda gene deletion events that occurred since the divergence of the ancestor of these three species and those of the distantly related species.

Pathological lesions in the spontaneously diabetic BB Wistar rat: A comprehensive autopsy study

A total of 145 BB Wistar diabetic rats, 46 of their nondiabetic siblings, and 43 outbred Wistar rats were autopsied and the frequency of lesions in all organ systems were determined. Common strain-related lesions included pulmonary infections, granulomas, lymphoid hyperplasia, lymphomas, lymphocytopenia, eosinophilia, supradiaphragmatic accessory lobes of the liver, and prostatic atrophy. These suggest some basic strain-related abnormalities of the immune system that were selected by the process of inbreeding. Diabetes-related lesions were insulitis, testicular atrophy, cataracts, hepatic fatty change, pancreatitis, lymphocytic thyroiditis, hypoglycemic brain damage, central pontine myelinolysis, stomach erosions, and idiopathic megacolon. Many of these are sequelae of human juvenile-onset diabetes and support the validity of the BB Wistar rat as an animal model for human diabetes mellitus. The absence of several important sequelae of the human disease (i.e., diabetic nephropathy, atherosclerosis, and severe microangiopathy) suggests a degree of infidelity as a model for human diabetes mellitus. Nonspecific lesions occurring in all three groups of rats included myocardial degeneration and fibrosis, splenic extramedullary hematopoiesis, and chronic progressive glomerulonephropathy.

Hematological Characteristics of the BB Wistar Rat

Complete blood counts, differential white blood cell and platelet counts were performed on male and female BB Wistar diabetic rats (BBWd), their nondiabetic siblings (BBWnd) and outbred Wistar rats of the line from which the BB Wistar rats were derived. Most of the observed changes were strain-related (those present in both BBWd and BBWnd but not in control rats) rather than diabetes-related (those in BBWd but neither BBWnd nor control rats) and therefore probably due to the inbreeding process. The BBW strain had significantly lower numbers of white cells and platelets, as well as markedly changed differential white cell counts. Differential counts showed a pattern of lymphopenia, neutrophilia, monocytosis and eosinophilia. It is possible that these white blood cell changes contribute to the increased susceptibility to infection reported for the BBW strain. No significant difference in serum immunoglobulin concentrations was found in any of these three groups of rats. There- fore, hypogammaglobutinemia cannot account for the increased susceptibility to infections, but it is not possible to rule out an abnormality in the distribution of immunoglobulin fractions as an etiological factor.

Spontaneous “primary biliary cirrhosis” in rabbits

A syndrome resembling human Primary Biliary Cirrhosis (PBC) and occurring spontaneously in a strain of domesticated rabbits from the Faenza region in Italy, is reported. The syndrome is characterized by histological liver changes consisting of chronic non-suppurative destructive cholangitis and copper accumulation, biochemical and immunological abnormalities mainly represented by an elevation of serum alkaline phosphatase and a positive antimitochondrial antibody test. The abnormalities were not observed in newborn rabbits from the same area nor in control rabbits from the island of Sardinia. A toxic effect related to some environmental factor may be responsible for the disease. Alternatively, a continuous process of partial inbreeding among consanguineous animals may have resulted in a selection of a strain genetically predisposed to the development of PBC. Whatever the etiology, the domesticated Faenza rabbit proves to be an interesting animal model for a chronic nonsuppurative destructive cholangitis, very similar to the human disease known as PBC.

Studies on an isolated West Indies population: I. Analysis of HLA genotypes.

The transmission of HLA genes was studied in an isolated population of French origin on the lesser Antilles islands in the West Indies. The study of 74 unrelated individuals, 44 of whom were genotyped, was carried out for the alleles of HLA loci: A, B, C and Bf (proactivator factor of properdin). As a result of the founder effect and the inbreeding process, the HLA haplotypes were noted to be less polymorphic than in a French continental population. Two haplotypes: A2, Cw5, B12, BfS and A3, C-, B14, BfF represent 24% of the observed haplotypes, and only 2% of the reference haplotypes in France. No significant excess or deficit of homozygotes was observed at the A and B loci.

Germ-line reinsertions of AKR murine leukemia virus genomes in Akv-1 congenic mice.

Congenic mouse strains NIH,Akv-1 and NIH,Akv-2 carry the two high ecotropic virus-inducing loci of the AKR mouse on the NIH Swiss genetic background. Progeny tests of animals in three separate congenic families show that both Avk-1 and Akv-2 are stably transmitted as classical mendelian loci in these mice. However, during the process of inbreeding, additional chromosomal viral loci were detected in several NIH.Akv-1 sublines. These loci appeared only in the progeny of virus-positive females. They segregate with mendelian ratios, are unlinked to markers on chromsome 7 near Akv-1, and are phenotypically expressed as high-virus-inducing loci. The generation of new loci for viurs induction, no doubt resulting from the rare germ-line reintegration of the endogenous ectropic provirus, represents a unique form of gene duplication and rearrangement.

NATURAL SELECTION RESISTING INBREEDING DEPRESSION IN CAPTIVE WILD HOUSEMICE (MUS MUSCULUS).

The role which heterozygote selection plays in maintaining the genetic variability of natural populations has often been debated, and the resulting controversy has contributed substantially to populationgenetic theory and experimentation (Cain and Sheppard, 1954; Dobzhansky, 1955; Dobzhansky et al., 1963; Berry, 1978). With respect to wild Mus musculus, several studies have demonstrated that natural populations do possess large amounts of genetic variability (Dunn et al., 1960; Selander and Yang, 1969; Berry, 1978). However, it is difficult to determine the role which heterozygote selection plays in maintaining this genetic variability since a variety of obstacles impede the direct measurement of heterozygote selection; for example, linkage of alleles potentially favored by selection, the environmental specificity of any selection advantage, the population specificity of any selection advantage, and other problems discussed by Berry (1978). Therefore it is worthwhile to examine heterozygote advantage using systematic inbreeding as an indirect method of detecting it (cf. Dobzhansky et al., 1963), both to examine the incidence of heterozygote selection as an evolutionary mechanism and to broaden our understanding of the inbreeding process itself. Our experiments showed that a delayed the effects of inbreeding beyond what would normally have been expected had there been no selection for heterozygotes. That is if natural selection favors heterozygotes at individual loci, then the rate of homozygote production should lag behind that predicted by F (Wright, 1965), the theoretical coeffecient of inbreeding. We will call this situation a selection-inbreeding equilibrium model since selection and inbreeding occur simultaneously and produce opposing effects (Hayman and Mather, 1953; Reeve, 1955; Li, 1967). This may be either stable (permanent) or unstable (transient): (A) A stable results if balancing selection is sufficiently intense relative to inbreeding and the size of the breeding population is sufficiently large (Hayman and Mather, 1953); as a result the breeding population never becomes genetically fixed. (B) An unstable results if balancing selection is weak relative to inbreeding in a large population, or if the breeding population is small (Li, 1967); as a result, genetic fixation eventually occurs but is delayed relative to an unselected population with the same inbreeding intensity. The small population size (approximately eight breeder pairs per generation) of our own sibmated lines makes an unstable more likely. Accordingly we expected that heterozygote selection would merely delay the eventual fixation of our sibmated lines. Thus we captured a sample from a population of wild housemice, sequestered them in our laboratory and immediately sibmated their progeny. We measured three effects as inbreeding was continued: changes in fertility, changes in homozygosity as measured by skin transplantation techniques, and homozygosity as measured by isoenzyme assay techniques. We hypothesized that inbreeding depression would occur, not at a steady rate, but interrupted by long pauses reflecting resistance by balancing selection. We also hypothesized that the amount of homozygosity produced by 20 generations of sibmating would lag behind the amount

Three mice, two blind.

In a closed colony of approximately one million mice which had been rigidly pedigreed (brother to sister) from a few original mice, three unusual mice have appeared. These three have obviously deviated from what would be expected according to the accepted laws of inbreeding geneity. Thus three unexpected incidences of increased variability have occurred three times in addition to the occasional appearance of new mutations (approximately one in 26,000 mice). The first mouse developed a sudden increase in susceptibility to spontaneous tumors of mammary origin. She became the mother of the well-known C family used extensively in cancer research. The second mouse clearly exhibited, among clear-cut altered biological characteristics, several somatic mutations, thus indicating an unstable genetic condition. The third mouse developed a highly significant capacity to regress and eliminate spontaneous tumor tissue. The conclusion is reached that unstable biological states arise occasionally during the process of inbreeding.

The Value of a "Functional Test" in Selecting Material for a Genetic Study of Mammary Tumors in Mice and Rats

It has been pointed out by Little, Strong, and others that one of the basic weaknesses of the early work in the experimental study of the genetics of neoplasms occurring in mice was that the animal strains used were not of sufficient purity. This objection was met by the long and tedious process of close inbreeding of various stocks coupled with careful tabulation of all tendency to the spontaneous development of neoplasms. As a result lines were produced, on the one hand, which gave as high as 80 to 90 per cent of tumors of the mammary gland, and on the other, such a line as C57, which apparently failed to develop a mammary tumor in ten years of inbreeding. The work of the present writers has been largely with the so-called “non-tumor” or “low-tumor” strains, and their results lead them to conclude that the process of inbreeding alone does not insure a strain free from a tendency to produce spontaneous cancer. Unless some way is devised to make it possible to distinguish those individuals whose constitution is favorable to the spontaneous formation of neoplasms they may remain undetected from one generation to another. Recently we have applied a method to this problem which has successfully located female mice with spontaneous, fully developed mammary carcinoma even in such a “low-tumor” strain as C57 (1, 2). We refer to the so-called “functional test” in which female mice are bred rapidly, starting at an early age, and are not allowed to nurse their young. This procedure subjects the females to a prolonged, vigorous hormonal stimulation of the mammary gland, mainly ovarian, coupled with irritation of the epithelium of the gland due to retained secretions within the ducts. This procedure results apparently in an early “aging” effect upon the breeding females, so that mammary gland tumors may arise in animals only four to six months of age. This method has made it possible to locate female mice with cancer when they are still fertile, so that offspring may be obtained either from them or from their close relations. Conversely the same procedure has resulted in making it possible to detect, at a relatively early age, females whose constitution is apparently resistant to the formation of mammary neoplasms under the conditions of our experiment.