Quantitative genetics is the study of the inheritance of traits such as yield, fruit weight, texture, etc. that show a continuous distribution. It provides plant breeders with the theoretical foundation for effective selection, choice of parents, choice of progeny test, relationships among traits and genotype x environment interactions. In particular, it is a useful tool for plant breeders for predicting response to selection for (or against) a given trait, and for calculating genetic gain from multiple trait selection.
A more recent development in quantitative genetics is the analysis of chromosomal regions or quantitative trait loci (QTL) associated with traits of agronomic importance. This particular research area is being developed by our gene mapping group.
Our team has experience in:
We have estimated heritabilities for many traits of importance for fruit quality and quantity as well as genetic correlations among traits. We have used our knowledge of these genetic parameters to predict genetic progress and to develop efficient breeding strategies in our fruit and tree breeding programmes.
All Actinidia species are dioecious – male and female flowers are usually borne on separate vines. Pollen from male vines needs to be transferred (usually by bees) to flowers on female vines to set fruit. In seedling populations, usually half segregate as male and half as female vines. Rarely, other forms are found. These include ‘inconstant males’ (vines with mostly male flowers but with a proportion of perfect flowers that set small fruit) and hermaphrodites (vines that have only perfect flowers). We have studied the inheritance of sex determination in Actinidia and have used the information to breed hermaphrodite lines with large fruit.
Genetic maps which identify QTL (Quantitative Trait Loci) are the modern and very useful tools which molecular geneticists create for use in breeding programmes. Our kiwifruit microsatellite marker map contains many functional genes, which are active in the development of health attributes such as Vitamin C, flavour and colour.
The markers are able to detect the presence of the genes for such characteristics in tiny seedlings, making the development of new cultivars with quality fruit more certain, more rapid and more economical.
A main objective of our pipfruit breeding programme is to breed cultivars resistant to the main pests and diseases of apple and pear: scab, powdery mildew, fire blight, woolly apple aphid, leafrollers. The goals of the research are to identify new sources of resistance in the germplasm, study the genetics and host-pathogen interactions of the resistances, and develop genetic markers for marker assisted selection (MAS).
The host-pathogen interaction studies are focused on both understanding the genetics of pathogenicity and the arrangement of resistance genes on the genomes of the crops. The research supports the development of a breeding strategy to produce cultivars with durable resistances, based on the application of MAS for the selection of breeding lines with the desired combination of resistance genes.
New research into whole-genome MAS has been initiated as part of a ‘fast-breeding’ framework for the accelerated introduction of resistances combined with novel fruit characters. The most recent examples of progress made are the discovery of new scab resistance genes and the identification of scab resistance gene clusters on linkage group 2 of apple.
The ploidy level of an organism refers to the number of complete sets of chromosomes in each cell (except reproductive and some other specialised cells). For example, a plant with two sets of chromosomes is referred to as diploid, one with three sets as triploid and one with four sets as tetraploid. Natural variation in ploidy level is common among many plants including Actinidia (kiwifruit), Rubus and Citrus.
Crosses between species that differ in ploidy level often fail or produce sterile offspring. It is sometimes better to change the ploidy level of one parent so that the ploidy levels are matched before making the cross. In other cases, sterility caused by parental ploidy differences can be an advantage.
For example, if a tetraploid plant is crossed with a diploid plant, the progeny are usually mostly triploid and are often seedless. Seedlessness is a desirable trait in some fruit and plants and we have used this method to produce seedless hops and mandarins.
The ploidy level can be doubled by treating dividing cells with certain chemicals. By treating cells of a diploid plant with colchicine, tetraploid plants can sometimes be retrieved. Somatic hybridisation or protoplast fusion is an alternative technique for manipulating ploidy levels that we have used in our citrus breeding programme.
The technique involves removing the cell walls from cells to produce protoplasts in suspension culture, inducing fusion between protoplasts, selecting 'hybrid' protoplasts and regenerating plants via tissue culture. The ploidy level of the hybrid plants will equal the sum of the ploidy levels of the parent cells.
Flow cytometryFlow cytometry is being used to measure ploidy levels in citrus, hops and kiwifruit and should be suitable for other fruit crops. Very young leaves are chopped with razor blades to free the cell nuclei which are stained with a dye that reacts with DNA in a quantitative manner. The more DNA in the nucleus, the more dye that reacts with the DNA. The nuclei are passed in a stream past a laser in the flow cytometer and by comparison with plant nuclei containing known amounts of DNA, the a DNA in each of the nuclei in the flowing stream can be calculated. In the plants we deal with, as the number of chromosomes in the nucleus increases, so does the amount of DNA. Tetraploid nuclei have almost exactly twice as much DNA as do diploid nuclei. This makes it easy to calculate from the amount of DNA present, the number of chromosomes in the nuclei.
Flow cytometry is generally much faster than chromosome counting for estimating ploidy levels, but we still do some counts to make sure our flow cytometric methods are giving the right results.