An R2R3 MYB transcription factor determines red petal colour in an Actinidia (kiwifruit) hybrid population

Background

Red colour in kiwifruit results from the presence of anthocyanin pigments. Their expression, however, is complex, and varies among genotypes, species, tissues and environments. An understanding of the biosynthesis, physiology and genetics of the anthocyanins involved, and the control of their expression in different tissues, is required. A complex, the MBW complex, consisting of R2R3-MYB and bHLH transcription factors together with a WD-repeat protein, activates anthocyanin 3-O-galactosyltransferase (F3GT1) to produce anthocyanins. We examined the expression and genetic control of anthocyanins in flowers of Actinidia hybrid families segregating for red and white petal colour.

Results

Four inter-related backcross families between Actinidia chinensis Planch. var. chinensis and Actinidia eriantha Benth. were identified that segregated 1:1 for red or white petal colour. Flower pigments consisted of five known anthocyanins (two delphinidin-based and three cyanidin-based) and three unknowns. Intensity and hue differed in red petals from pale pink to deep magenta, and while intensity of colour increased with total concentration of anthocyanin, no association was found between any particular anthocyanin data and hue. Real time qPCR demonstrated that an R2R3 MYB, MYB110a, was expressed at significant levels in red-petalled progeny, but not in individuals with white petals.

A microsatellite marker was developed that identified alleles that segregated with red petal colour, but not with ovary, stamen filament, or fruit flesh colour in these families. The marker mapped to chromosome 10 in Actinidia.

The white petal phenotype was complemented by syringing Agrobacterium tumefaciens carrying Actinidia 35S::MYB110a into the petal tissue. Red pigments developed in white petals both with, and without, co-transformation with Actinidia bHLH partners. MYB110a was shown to directly activate Actinidia F3GT1 in transient assays.

Conclusions

The transcription factor, MYB110a, regulates anthocyanin production in petals in this hybrid population, but not in other flower tissues or mature fruit. The identification of delphinidin-based anthocyanins in these flowers provides candidates for colour enhancement in novel fruits.

Mapping, complementation, and targets of the cysteine protease actinidin in kiwifruit

Cysteine proteases (CPs) accumulate to high concentration in many fruit, where they are believed to play a role in fungal and insect defense. The fruit of Actinidia species (kiwifruit) exhibit a range of CP activities (e.g. the Actinidia chinensis variety YellowA shows less than 2% of the activity of Actinidia deliciosa variety Hayward). A major quantitative trait locus for CP activity was mapped to linkage group 16 in a segregating population of A. chinensis. This quantitative trait locus colocated with the gene encoding actinidin, the major acidic CP in ripe Hayward fruit encoded by the ACT1A-1 allele. Sequence analysis indicated that the ACT1A locus in the segregating A. chinensis population contained one functional allele (A-2) and three nonfunctional alleles (a-3, a-4, and a-5) each containing a unique frameshift mutation. YellowA kiwifruit contained two further alleles: a-6, which was nonfunctional because of a large insertion, and a-7, which produced an inactive enzyme. Site-directed mutagenesis of the act1a-7 protein revealed a residue that restored CP activity. Expression of the functional ACT1A-1 cDNA in transgenic plants complemented the natural YellowA mutations and partially restored CP activity in fruit. Two consequences of the increase in CP activity were enhanced degradation of gelatin-based jellies in vitro and an increase in the processing of a class IV chitinase in planta. These results provide new insight into key residues required for CP activity and the in vivo protein targets of actinidin.

Trinucleotide repeats as bait for vectorette PCR: a tool for developing genetic mapping markers

Trinucleotide repeats are common within gene coding regions and could serve as beacons to locate genes. Five of the most common trinucleotide repeats in an Actinidia (kiwifruit) expressed sequence tag (EST) database were found to be (ACC)(4), (CAC)(4), (CCA)(4), (CTC)(4), and (TGG)(4). These repeats, with or without an artificial 5'-end tail, were tested by vectorette PCR against genomic DNA from Actinidia chinensis. Eighty-nine randomly selected clones showed an average insert size of 383 bp, with a maximum of 1,151 bp and a minimum of 78 bp. Two-thirds of the clones contained the artificial tail attached to the trinucleotide, showing a slight advantage of possessing such a tail during annealing and amplification. The sequences were searched against the Actinidia EST database and GenBank. Of the 89 clones, 33 had a significant hit (expect value < e(-15)). Twenty-four of those clones matched an Actinidia EST. Twenty-one clones contained one or more simple sequence repeats. This methodology can be applied by conventional cloning and sequencing methods or by high throughput pyrosequencing technologies to develop genetic markers and also for gene mining in species with little or no genetic/genomic resources.

A gene-rich linkage map in the dioecious species Actinidia chinensis (kiwifruit) reveals putative X/Y sex-determining chromosomes

BACKGROUND

The genus Actinidia (kiwifruit) consists of woody, scrambling vines, native to China, and only recently propagated as a commercial crop. All species described are dioecious, but the genetic mechanism for sex-determination is unknown, as is the genetic basis for many of the cluster of characteristics making up the unique fruit. It is, however, an important crop in the New Zealand economy, and a classical breeding program would benefit greatly by knowledge of the trait alleles carried by both female and male parents. The application of marker assisted selection (MAS) in seedling populations would also aid the accurate and efficient development of novel fruit types for the market.

RESULTS

Gene-rich female, male and consensus linkage maps of the diploid species A. chinensis have been constructed with 644 microsatellite markers. The maps consist of twenty-nine linkage groups corresponding to the haploid number n = 29. We found that sex-linked sequence characterized amplified region (SCAR) markers and the 'Flower-sex' phenotype consistently mapped to a single linkage group, in a subtelomeric region, in a section of inconsistent marker order. The region also contained markers of expressed genes, some of unknown function. Recombination, assessed by allelic distribution and marker order stability, was, in the remainder of the linkage group, in accordance with other linkage groups. Fully informative markers to other genes in this linkage group identified the comparative linkage group in the female map, where recombination ratios determining marker order were similar to the autosomes.

CONCLUSION

We have created genetic linkage maps that define the 29 linkage groups of the haploid genome, and have revealed the position and extent of the sex-determining locus in A. chinensis. As all Actinidia species are dioecious, we suggest that the sex-determining loci of other Actinidia species will be similar to that region defined in our maps. As the extent of the non-recombining region is limited, our result supports the suggestion that the subtelomeric region of an autosome is in the early stages of developing the characteristics of a sex chromosome. The maps provide a reference of genetic information in Actinidia for use in genetic analysis and breeding programs.

Analysis of expressed sequence tags from Actinidia: applications of a cross species EST database for gene discovery in the areas of flavor, health, color and ripening

Background

Kiwifruit (Actinidia spp.) are a relatively new, but economically important crop grown in many different parts of the world. Commercial success is driven by the development of new cultivars with novel consumer traits including flavor, appearance, healthful components and convenience. To increase our understanding of the genetic diversity and gene-based control of these key traits in Actinidia, we have produced a collection of 132,577 expressed sequence tags (ESTs).

Results

The ESTs were derived mainly from four Actinidia species (A. chinensis, A. deliciosa, A. arguta and A. eriantha) and fell into 41,858 non redundant clusters (18,070 tentative consensus sequences and 23,788 EST singletons). Analysis of flavor and fragrance-related gene families (acyltransferases and carboxylesterases) and pathways (terpenoid biosynthesis) is presented in comparison with a chemical analysis of the compounds present in Actinidia including esters, acids, alcohols and terpenes. ESTs are identified for most genes in color pathways controlling chlorophyll degradation and carotenoid biosynthesis. In the health area, data are presented on the ESTs involved in ascorbic acid and quinic acid biosynthesis showing not only that genes for many of the steps in these pathways are represented in the database, but that genes encoding some critical steps are absent. In the convenience area, genes related to different stages of fruit softening are identified.

Conclusion

This large EST resource will allow researchers to undertake the tremendous challenge of understanding the molecular basis of genetic diversity in the Actinidia genus as well as provide an EST resource for comparative fruit genomics. The various bioinformatics analyses we have undertaken demonstrates the extent of coverage of ESTs for genes encoding different biochemical pathways in Actinidia.

Cross-species amplification of microsatellite loci within the dioecious, polyploid genus Actinidia (Actinidiaceae)

Microsatellite marker transfer across species in the dioecious genus Actinidia (kiwifruit) could offer an efficient and time-effective technique for use during trait transfer for vine and fruit improvement in breeding programmes. We evaluated the cross-species amplification of 20 EST-derived microsatellite markers that were fully informative in an Actinidia chinensis mapping family. We tested all 20 markers on 120 genotypes belonging to 21 species, 5 with varieties and/or chromosome races. These 26 taxa included 16 diploids, 7 tetraploids, 2 hexaploids and 1 octaploid, and represented all four taxonomic sections in the genus. All 20 markers showed some level of cross-species amplification. The most successful marker amplified in all genotypes from all species from all sections of the genus, the least successful amplified fragments only in A. chinensis and A. deliciosa. One species, A. glaucophylla, failed to amplify with all but 2 markers. PIC (Polymorphism information content) values were high, with 14 of 17 markers recording values of 0.90 and above. Sequence data demonstrated the presence of the microsatellite in all the amplified products. Sequence homology was less 5' of the microsatellite and increased toward the start codon of the translated region of the EST from which the marker was derived. The data confirm that EST-derived microsatellite markers from Actinidia species show cross-species amplification with high levels of polymorphism which could make them useful markers in breeding programmes.

Estimation of allele frequencies in polyploids under certain patterns of inheritance

Allele frequencies have long been studied by biologists interested in evolution and speciation. More recently, with the application of molecular markers in human DNA profiling we have also seen the need for reliable population allele frequency estimates for making probabilistic inferences. There is now interest in applying the same DNA profiling technology to identification of plant varieties. HortResearch maintains a large germplasm of horticultural plant species. It is becoming evident that accurate identification of these accessions through DNA fingerprinting is essential for effective utilisation and maintenance of this germplasm. Microsatellites are the markers of choice for this fingerprinting. However, such markers do not reveal the dosage of alleles in a polyploid. Polyploidy is common amongst horticultural plants. Estimating allele frequencies in a polyploid population is, therefore, complicated because of some marker genotypes being phenotypically indistinguishable. For example, in a tetraploid, with four alleles at a locus showing polysomic inheritance, although 35 genotypes are possible, these will fall into only 15 marker phenotypic classes. Furthermore 'null' individuals are rarely detected in polyploids. Furthermore, some polyploids can be cryptic exhibiting disomy, instead of the polysomic inheritance. We will discuss the implications of these factors and present an EM-type algorithm for estimating allele frequencies of a polyploid population under certain patterns of inheritance. The method will be demonstrated on simulated data. We also discuss the nature of some of the additional problems that may be encountered with estimating allele frequencies in polyploids for which other solutions still need to be developed.

EST-derived microsatellites from Actinidia species and their potential for mapping

To increase the speed and reduce the cost of constructing a genetic map of Actinidia species (kiwifruit), for use in both breeding and functional genomics programmes, we sampled microsatellites from expressed sequence tags (ESTs) to evaluate their frequency of occurrence and level of polymorphism. Perfect dinucleotide repeats were the microsatellites selected, and these were found to be numerous in both the 5' and 3' ends of the genes represented. The microsatellites were of various lengths, the majority being repeats with the pattern (CT)(n)/(GA)(n). One hundred and fifty microsatellites, each with more than 10 dinucleotide repeat units, were chosen as possible markers, and when these were amplified, 93.5% were found to be polymorphic and segregating in a mapping population, with 22.6% amplifying more than one locus. Four marker categories were identified. Fully informative markers made up 27% of the total, 36.2% were female informative, 25.8% were male informative and 10% partly informative. The mapping population was an intraspecific cross in the diploid species Actinidia chinensis, with parents chosen for their diversity in fruit and plant characteristics, and for their geographical separation. Linkage was tested using the software 'Joinmap' and a LOD value of 3. The distribution of the EST-based markers over the linkage groups obtained appeared to be random, taking into consideration the small sample size, that the number of linkage groups (31) exceeded the chromosome number of n=29, and that a number of markers were not assigned to any group. Some microsatellite markers which amplified more than one locus mapped to separate linkage groups. According to our study in A. chinensis, EST-derived microsatellites give large numbers of possible markers very quickly and at reasonable cost. The markers are highly polymorphic, segregate in the mapping population, and increase the value of the genomic map by providing some functional information.