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PubMed2001

Mitochondrial disorders: genetics, counseling, prenatal diagnosis and reproductive options.

Most patients with mitochondrial disorders are diagnosed by finding a respiratory chain enzyme defect or a mutation in the mitochondrial DNA (mtDNA). The provision of accurate genetic counseling and reproductive options to these families is complicated by the unique genetic features of mtDNA that distinguish it from Mendelian genetics. These include maternal inheritance, heteroplasmy, the threshold effect, the mitochondrial bottleneck, tissue variation, and selection. Although we still have much to learn about mtDNA genetics, it is now possible to provide useful guidance to families with an mtDNA mutation or a respiratory chain enzyme defect. We describe a range of current reproductive options that may be considered for prevention of transmission of mtDNA mutations, including the use of donor oocytes, prenatal diagnosis (by chorionic villus sampling or amniocentesis), and preimplantation genetic diagnosis, plus possible future options such as nuclear transfer and cytoplasmic transfer. For common mtDNA mutations associated with mitochondrial cytopathies (such as NARP, Leigh Disease, MELAS, MERRF, Leber's Hereditary Optic Neuropathy, CPEO, Kearns-Sayre syndrome, and Pearson syndrome), we summarize the available data on recurrence risk and discuss the relative advantages and disadvantages of reproductive options.

DNA, Mitochondrial↗
PubMed2006

Response to intensity of reproduction in mouse lines resistant or susceptible to fescue toxicosis.

Our objective was to examine whether mouse lines divergently selected for response to fescue toxicosis differed in the impact of increasing reproductive intensity on growth, final weight or first and second parity reproduction. Resistant (R) and susceptible (S) females were never mated (NR), mated only once (low reproduction, LR), mated after their first litter was weaned (moderate reproduction, MR) or paired continuously with a male (high reproduction, HR), allowing concurrent pregnancy and lactation. Final weight was significantly higher in mated than in not mated females (31.6 +/- 0.6, 35.9 +/- 0.6, 36.8 +/- 0.6 and 37.2 +/- 0.5 g for NR, LR, MR and HR females respectively), but the effect of increasing reproductive intensity was similar in both lines. Neither genetic line, reproductive treatment (LR and MR versus HR) nor their interaction affected first parity traits of dams or pups. In second parity, S dams produced larger litters at birth and weaning and heavier litters at birth than R dams, and MR dams produced more pups and heavier litters than HR dams. As with first litters, however, line by reproductive treatment interactions were not significant or important. Concurrent pregnancy and lactation (the HR treatment) had a similar dampening impact on the expected increase in litter size between first and second parity of both lines. Thus, lines divergently selected for toxicosis response did not differ significantly in the impact of increasing reproductive intensity on dam or pup growth, reproductive output through two parities or mature size. This conclusion should not be generalized. Selection responses in livestock should be monitored to ensure that improvement in one trait does not lead to deterioration in others.

Animals↗
PubMed1978

Reproduction in young sheep: some genetic and environmental sources of variation.

"The extent of genetic and environmental variation" in the development of reproduction in sheep is illustrated by examples with particular reference to variation among breeds and to the effects of photoperiod. The interactions between genetic and environmental effects are introduced; these may be so great that genetic groups may reverse their ranking for rate of development in different environments. The "physiology of puberty" is then discussed. The difficulty of separating puberty from seasonal variation is stressed, and a possible contrast is drawn between the physiological characteristics of genetic variation and those of environmental variation in reproductive development. Finally the physiological factors associated with sterility in young females are discussed; most studies, however, have been conducted during the time of year when adult females would also be expected to be sterile, so that conclusions are difficult and a "missing link" cannot be identified.

Animals↗
PubMed2005

Genetic manipulations to study reproduction.

Fertility disorders affect approximately 15% of individuals worldwide. With the imminent completion of the human and mouse genome sequence, it will be more feasible to identify the relevant genes underlying many fertility disorders. Already, the mouse has been utilized extensively as a genetic tool for the dissection of gene function, often providing significant insights into the relationship between gene and disease. In fact, there are over 200 mouse models that display reproductive defects. However, the available mouse mutant resources provide functional information for a mere 10% of the total number of genes in the mouse or human genomes at best. The improvement of available genome annotations together with more powerful techniques to manipulate the mouse genome provide substantial improvements in our ability to identify genes involved in reproduction, and in the future will likely benefit patients with fertility problems.

Animals↗
PubMed2006

Allorecognition polymorphism versus parasitic stem cells.

Allorecognition--the ability of an individual to distinguish between self and non-self cells and tissues--ultimately depends on the presence of highly polymorphic gene(s). Allorecognition loci are the most diverse ever described, with tens to hundreds of alleles observed in a population. An unresolved problem in population genetics lies in understanding the origins, accumulation and maintenance of this extensive polymorphism, often over millions of years and across multiple speciation events. Botryllus schlosseri, a primitive chordate, has a life history that links several components of allorecognition from disparate fields that are experimentally accessible. This review outlines these traits and discusses some of the puzzling aspects of allorecognition in Botryllus that might contribute to understanding the evolution of these extraordinary polymorphisms.

Animals↗
PubMed1990

Mutation-selection balance and the evolutionary advantage of sex and recombination.

Mutation-selection balance in a multi-locus system is investigated theoretically, using a modification of Bulmer's infinitesimal model of selection on a normally-distributed quantitative character, taking the number of mutations per individual (n) to represent the character value. The logarithm of the fitness of an individual with n mutations is assumed to be a quadratic, decreasing function of n. The equilibrium properties of infinitely large asexual populations, random-mating populations lacking genetic recombination, and random-mating populations with arbitrary recombination frequencies are investigated. With 'synergistic' epistasis on the scale of log fitness, such that log fitness declines more steeply as n increases, it is shown that equilibrium mean fitness is least for asexual populations. In sexual populations, mean fitness increases with the number of chromosomes and with the map length per chromosome. With 'diminishing returns' epistasis, such that log fitness declines less steeply as n increases, mean fitness behaves in the opposite way. Selection on asexual variants and genes affecting the rate of genetic recombination in random-mating populations was also studied. With synergistic epistasis, zero recombination always appears to be disfavoured, but free recombination is disfavoured when the mutation rate per genome is sufficiently small, leading to evolutionary stability of maps of intermediate length. With synergistic epistasis, an asexual mutant is unlikely to invade a sexual population if the mutation rate per diploid genome greatly exceeds unity. Recombination is selectively disadvantageous when there is diminishing returns epistasis. These results are compared with the results of previous theoretical studies of this problem, and with experimental data.

Animals↗
PubMed1991

Germline selection: population genetic aspects of the sexual/asexual life cycle.

Population geneticists make a distinction between sexual and asexual organisms depending on whether individuals inherit genes from one or two parents. When individual genes are considered, this distinction becomes less satisfactory for multicellular sexual organisms. Individual genes pass through numerous asexual mitotic cell divisions in the germline prior to meiosis and sexual recombination. The processes of mitotic mutation, mitotic crossing over, and mitotic gene conversion create genotypic diversity between diploid cells in the germline. Genes expressed in the germline whose products affect cell viability (such as many "housekeeping" enzymes) may be subjected to natural selection acting on this variability resulting in a non-Mendelian output of gametes. Such genes will be governed by the population genetics of the sexual/asexual life cycle rather than the conventional sexual/Mendelian life cycle. A model is developed to investigate some properties of the sexual/asexual life cycle. When appropriate parameter values were included in the model, it was found that mutation rates per locus per gamete may vary by a factor of up to 100 if selection acts in the germline. Sexual/asexual populations appear able to evolve to a genotype of higher fitness despite intervening genotypes of lower fitness, reducing the problems of underdominance and Wright's adaptive landscape encountered by purely sexual populations. As might be expected this ability is chiefly determined by the number of asexual mitotic cell divisions within the germline. The evolutionary consequences of "housekeeping" loci being governed by the dynamics of the sexual/asexual life cycle are considered.

Animals↗
PubMed1992

The clr1 locus regulates the expression of the cryptic mating-type loci of fission yeast.

The mat2-P and mat3-M loci of fission yeast contain respectively the plus (P) and minus (M) mating-type information in a transcriptionally silent state. That information is transposed from the mat2 or mat3 donor locus via recombination into the expressed mating-type locus (mat1) resulting in switching of the cellular mating type. We have identified a gene, named clr1 (for cryptic loci regulator), whose mutations allow expression of the mat2 and mat3 loci. clr1 mutants undergo aberrant haploid meiosis, indicative of transcription of the silent genes. Production of mRNA from mat3 is detectable in clr1 mutants. Furthermore, the ura4 gene inserted near mat3, weakly expressed in wild-type cells, is derepressed in clr1 mutants. The clr1 mutations also permit meiotic recombination in the 15-kb mat2-mat3 interval, where recombination is normally inhibited. The clr1 locus is in the right arm of chromosome II. We suggest that clr1 regulates silencing of the mat2 and mat3 loci, and participates in establishing the "cold spot" for recombination by organizing the chromatin structure of the mating-type region.

Chromatin↗
PubMedDate not supplied

The mutational meltdown in asexual populations.

Loss of fitness due to the accumulation of deleterious mutations appears to be inevitable in small, obligately asexual populations, as these are incapable of reconstituting highly fit genotypes by recombination or back mutation. The cumulative buildup of such mutations is expected to lead to an eventual reduction in population size, and this facilitates the chance accumulation of future mutations. This synergistic interaction between population size reduction and mutation accumulation leads to an extinction process known as the mutational meltdown, and provides a powerful explanation for the rarity of obligate asexuality. We give an overview of the theory of the mutational meltdown, showing how the process depends on the demographic properties of a population, the properties of mutations, and the relationship between fitness and number of mutations incurred.

Biological Evolution↗
PubMed2004

Does heterozygosity estimate inbreeding in real populations?

Many recent studies report that individual heterozygosity at a handful of apparently neutral microsatellite markers is correlated with key components of fitness, with most studies invoking inbreeding depression as the likely underlying mechanism. The implicit assumption is that an individual's inbreeding coefficient can be estimated reliably using only 10 or so markers, but the validity of this assumption is unclear. Consequently, we have used individual-based simulations to examine the conditions under which heterozygosity and inbreeding are likely to be correlated. Our results indicate that the parameter space in which this occurs is surprisingly narrow, requiring that inbreeding events are both frequent and severe, for example, through selfing, strong population structure and/or high levels of polygyny. Even then, the correlations are strong only when large numbers of loci (~200) can be deployed to estimate heterozygosity. With the handful of markers used in most studies, correlations only become likely under the most extreme scenario we looked at, namely 20 demes of 20 individuals coupled with strong polygyny. This finding is supported by the observation that heterozygosity is only weakly correlated among markers within an individual, even in a dataset comprising 400 markers typed in diverse human populations, some of which favour consanguineous marriages. If heterozygosity and inbreeding coefficient are generally uncorrelated, then heterozygosity-fitness correlations probably have little to do with inbreeding depression. Instead, one would need to invoke chance linkage between the markers used and one or more gene(s) experiencing balancing selection. Unfortunately, both explanations sit somewhat uncomfortably with current understanding. If inbreeding is the dominant mechanism, then our simulations indicate that consanguineous mating would have to be vastly more common than is predicted for most realistic populations. Conversely, if heterosis provides the answer, there need to be many more polymorphisms with major fitness effects and higher levels of linkage disequilibrium than are generally assumed.

Computer Simulation↗