Introgression of the avian naked neck gene assisted by DNA fingerprints

Evaluation of quantitative trait loci affecting intramuscular fat and reproductive traits in pigs using marker-assisted introgression

We investigated the effects of previously identified quantitative trait loci (QTL) in an experimental backcross (BC) between Chinese Meishan pigs and commercial Duroc pigs. We performed marker-assisted introgression of two QTL for intramuscular fat (IMF) content (IMF population) and three QTL for reproductive traits (reproduction population) from a donor Meishan pig into a recipient Duroc pig. At the fourth BC generation of the IMF population and third BC generation of the reproduction population, carrier animals were selected for the production of animals homozygous for the QTL. Our previous studies have shown that the presence of a Meishan allele on the IMF QTL is associated with low IMF values, and the Meishan allele on the reproductive QTL is associated with large litters. In this study, the presence of a Duroc allele at the IMF QTL on SSC9 resulted in a 0.27% increase in IMF (additive effect = 0.27 ± 0.08), whereas the presence of a Meishan allele at the IMF QTL on SSC7 resulted in a 0.34% increase in IMF (additive effect = -0.34 ± 0.09). The presence of the Meishan allele at the IMF QTL on SSC7 thus had the opposite effect to our previous studies, that is, increased IMF. In the reproduction population, we observed no differences between the genotypes of the three QTL in regard to number of corpora lutea or litter size. Marker-assisted introgression at these QTL is thus unlikely to result in an associated increase in litter size. These results show that it is possible to introgress alleles from other breeds into a selection population using molecular markers; any unexpected results might be associated with the genetic background.

Introgressing multiple QTL in breeding programmes of limited size

The ability to enrich a breed with favourable alleles from multiple unlinked quantitative trait loci (QTL) of a donor breed through marker-assisted introgression (MAI) in a population of limited size was evaluated by considering the effects of the proportion selected, the size of the marker intervals, the number of introgressed QTL and the uncertainty of QTL position. Informative flanking markers were used to select progeny with the largest expected number of donor QTL alleles over five generations of backcrossing and five generations of intercrossing. In the backcrossing phase, with 5% selected and 20 cM marker intervals for three QTL, there were sufficient backcross progeny that were heterozygous for all markers, and QTL frequencies dropped below 0.5 only because of double recombinants. For higher fractions selected, longer marker intervals, and more QTL, frequency reductions from 0.5 were greater and increased with additional generations of backcrossing. However, even with 20% selected, three QTL, and marker intervals of 5 or 20 cM, mean QTL frequencies in generation 5 were 0.35 and 0.30, sufficient to allow subsequent selection of QTL in the intercrossing phase. After five generations of intercrossing, over 90% of individuals were homozygous for all QTL, and 85% when five QTL were introgressed. The higher the proportions selected, the longer the marker intervals, and larger numbers of introgressed QTL increased the number of intercrossing generations required to achieve fixation of QTL. Location of the QTL in the marked intervals did not affect QTL frequencies or the proportion of QTL lost at the end of the introgression programme. In conclusion, introgressing multiple QTL can be accomplished in a MAI programme of limited size without requiring that all individuals selected during the backcrossing phase to be carriers of favourable alleles at all QTL.

Comparison of Different Foreground and Background Selection Methods in Marker-Assisted Introgression

Three different methods for foreground selection and four different methods for background selection were compared in terms of the efficiency of marker-assisted introgression of a QTL allele from a donor line into a recipient line and also in terms of the recovery of the recipient genetic background. The results showed that for the introgression of a donor QTL allele, a direct selection on the QTL itself (when the QTL genotype can be directly identified) would ensure that the allele is successfully introgressed and rapidly fixed. However, when a direct selection on the QTL is not feasible, an indirect selection using two closely linked flanking markers can be used, which also shows similar results. For the recovery of the recipient genetic background, if the goal is to recover the whole genetic background of the recipient, genomic similarity selection or marker index selection would be the best choice: Only three generations of backcrosses were required to recover over 98% of the recipient genome. Whereas if the goal is to recover certain background traits of the recipient, MBLUP selection would give the best results, which achieved not only over 99% recovery of the recipient QTL alleles for the background traits after three generations of backcrosses, but also showed the best genetic improvement of these traits.

Selection in backcross programmes

Backcrossing is a well-known and long established breeding scheme where a characteristic is introgressed from a donor parent into the genomic background of a recurrent parent. The various uses of backcrossing in modern genetics, particularly with the help of molecular markers, are reviewed here. Selection in backcross programmes is used to either improve the genetic value of plant and animal populations or fine map quantitative trait loci. Both cases are helpful in our understanding of the genetic bases of quantitative traits variation.

Marker-assisted introgression of trypanotolerance QTL in mice

A marker-assisted introgression (MAI) experiment was conducted to use genetic markers to transfer each of the three trypanotolerance QTL from a donor mouse strain, C57BL/6, into a recipient mouse strain, A/J. We used a backcross strategy that consisted of selecting two lines, each carrying two of the donor QTL alleles through the backcross (BC) phase. At the fourth BC generation, single-carrier animals were selected for the production of homozygous animal in the intercross phase. The QTL regions (QTLR) were located on chromosomes MMU1, MMU5, and MMU17. Groups of mice with different genotypes and the parental lines were subjected to a challenge with Trypanosoma congolense. The results show that trypanotolerance QTL was successfully moved into the recipient background genotype, yielding a longer survival time. The mean estimated survival time was 57.9, 49.5, and 46.8 days for groups of mice carrying the donor QTL on MMU1, MMU5, and MMU17 on A/J background. The mean estimated survival time was 29.7 days for the susceptible A/J line and 68.8 days for the resistant C57BL/6 line. The estimated QTLR effects are close to 30% smaller than those in the original mapping population which was likely caused by the difference in the background on which the effects of QTLR are tested. This is the first report of successful marker-assisted introgression of QTL in animals. It is experimental proof of the use of genetic markers for marker-assisted introgression in animal breeding.

The use of molecular genetics in the improvement of agricultural populations

Substantial advances have been made in the genetic improvement of agriculturally important animal and plant populations through artificial selection on quantitative traits. Most of this selection has been on the basis of observable phenotype, without knowledge of the genetic architecture of the selected characteristics. However, continuing molecular genetic analysis of traits in animal and plant populations is leading to a better understanding of quantitative trait genetics. The genes and genetic markers that are being discovered can be used to enhance the genetic improvement of breeding stock through marker-assisted selection.

A model for population growth of laboratory animals subjected to marker-assisted introgression: how many animals do we need?

This study provides methods for calculating the mean and variance of the number of animals with the desired genotype in each backcross generation for a marker-assisted introgression experiment. The ultimate goal is to produce animals which are homozygous for the desired loci. The methods have been developed specifically for experiments with inbred lines. The model assumes a Poisson distribution for litter size, and is similar to that used in stochastic versions of population dynamics models. Certain biological parameters must be specified as well as parameters under the control of the breeder. These methods can be utilized in designing an experiment to determine the number of founder animals required, given the number of animals required at the completion of the backcross process and vice versa. Consideration is given to minimizing the total amount of genotyping over the entire experiment, by varying the number of times each backcrossed male is used. In addition, an outline is given for an adaptive design that allows for changes in male usage to be made during the experiment.