A blackberry (Rubus L.) expressed sequence tag library for the development of simple sequence repeat markers

[Molecular markers in the genetic diversity studies of representatives of the genus Rubus L. and prospects of their application in breeding]

Род Rubus L. (семейство Rosaceae Juss.), по оценкам разных систематиков, состоит из 12–16 подродов, объединяющих ~750 видов. Самые крупные по числу видов подроды – Idaeobatus (Focke) Focke, к которому относятся малины, и типовой подрод Rubus (=Eubatus Focke), включающий виды ежевик. Представители рода Rubus обладают высокой пищевой и хозяйственной ценностью, а также лекарственными свойствами. Селекционные исследования направлены на расширение генетического разнообразия и создание новых сортов малин и ежевик, устойчивых к биотическим и абиотическим стрессорам и отличающихся высоким качеством плодов. Современные селекционно-генетические программы все шире включают достижения молекулярной генетики и геномики. В данной статье представлен обзор фундаментальных и прикладных исследований генетического разнообразия культивируемых и дикорастущих видов рода Rubus, выполненных на основе методов молекулярного маркирования. Рассмотрены основные типы молекулярных маркеров (RFLP, RAPD, SSR, ISSR, AFLP, SCAR, SSCP, ретротранспозонные маркеры и т. д.) и области их применения в изучении представителей рода Rubus. Приведены результаты работ по использованию методов ДНК-маркирования для решения самых разных задач, включая: исследование межвидового и внутривидового генетического разнообразия, филогенетических связей видов и надвидовых таксонов, выяснение спорных вопросов систематики, генотипирование и уточнение родословных сортов малин и ежевик, изучение сомаклональной изменчивости и др. Наиболее важным результатом в практическом плане является создание насыщенных молекулярно-генетических карт для разных видов малин и ежевик, на которых локализованы многочисленные гены и QTL, детерминирующие различные хозяйственно ценные признаки. В то же время необходимо отметить, что число маркеров, перспективных для проведения эффективного молекулярного скрининга, пока еще недостаточно.

Genetic and genomic resources for Rubus breeding: a roadmap for the future

Rubus fruits are high-value crops that are sought after by consumers for their flavor, visual appeal, and health benefits. To meet this demand, production of red and black raspberries (R. idaeus L. and R. occidentalis L.), blackberries (R. subgenus Rubus), and hybrids, such as Boysenberry and marionberry, is growing worldwide. Rubus breeding programmes are continually striving to improve flavor, texture, machine harvestability, and yield, provide pest and disease resistance, improve storage and processing properties, and optimize fruits and plants for different production and harvest systems. Breeders face numerous challenges, such as polyploidy, the lack of genetic diversity in many of the elite cultivars, and until recently, the relative shortage of genetic and genomic resources available for Rubus. This review will highlight the development of continually improving genetic maps, the identification of Quantitative Trait Loci (QTL)s controlling key traits, draft genomes for red and black raspberry, and efforts to improve gene models. The development of genetic maps and markers, the molecular characterization of wild species and germplasm, and high-throughput genotyping platforms will expedite breeding of improved cultivars. Fully sequenced genomes and accurate gene models facilitate identification of genes underlying traits of interest and enable gene editing technologies such as CRISPR/Cas9.

Developing expressed sequence tag libraries and the discovery of simple sequence repeat markers for two species of raspberry (Rubus L.)

BACKGROUND

Due to a relatively high level of codominant inheritance and transferability within and among taxonomic groups, simple sequence repeat (SSR) markers are important elements in comparative mapping and delineation of genomic regions associated with traits of economic importance. Expressed sequence tags (ESTs) are a source of SSRs that can be used to develop markers to facilitate plant breeding and for more basic research across genera and higher plant orders.

METHODS

Leaf and meristem tissue from 'Heritage' red raspberry (Rubus idaeus) and 'Bristol' black raspberry (R. occidentalis) were utilized for RNA extraction. After conversion to cDNA and library construction, ESTs were sequenced, quality verified, assembled and scanned for SSRs.  Primers flanking the SSRs were designed and a subset tested for amplification, polymorphism and transferability across species. ESTs containing SSRs were functionally annotated using the GenBank non-redundant (nr) database and further classified using the gene ontology database.

RESULTS

To accelerate development of EST-SSRs in the genus Rubus (Rosaceae), 1149 and 2358 cDNA sequences were generated from red raspberry and black raspberry, respectively. The cDNA sequences were screened using rigorous filtering criteria which resulted in the identification of 121 and 257 SSR loci for red and black raspberry, respectively. Primers were designed from the surrounding sequences resulting in 131 and 288 primer pairs, respectively, as some sequences contained more than one SSR locus. Sequence analysis revealed that the SSR-containing genes span a diversity of functions and share more sequence identity with strawberry genes than with other Rosaceous species.

CONCLUSION

This resource of Rubus-specific, gene-derived markers will facilitate the construction of linkage maps composed of transferable markers for studying and manipulating important traits in this economically important genus.

Genetic map of the primocane-fruiting and thornless traits of tetraploid blackberry

Blackberry primocane fruiting, fruiting on first-year canes, has the potential to expand blackberry production both seasonally and geographically. The incorporation of the primocane-fruiting trait into cultivars with desirable horticultural attributes is challenging due to its recessive nature and tetrasomic inheritance. Molecular marker-assisted selection has high potential to facilitate incorporation, because breeders already use morphological marker-assisted selection of seedlings without marginal cotyledonary hairs to identify progeny that will be thornless when mature. The development of a genetic linkage map with these two traits is the first step to utilizing molecular markers in breeding for thornless primocane-fruiting blackberry cultivars. A full-sib family segregating for thornlessness and primocane fruiting, from a cross between 'APF-12' and 'Arapaho', was used to construct the first genetic map of tetraploid blackberry. Segregation patterns of several dominant markers and the two phenotypic traits fit those expected uniquely with tetrasomic inheritance (e.g., 5:1, 11:1 and 35:1). Some loci showed significant double reduction frequencies, but genotypes that could have originated only from double reduction were not found. The map consists of seven linkage groups (LG) in each parent, consistent with the basic number of chromosomes (2n = 4x = 28). Naming of LG1-LG6 followed that of the recently revised system for raspberry using SSR markers in common between blackberry and raspberry, and LG7 was tentatively defined by default. The loci controlling primocane fruiting and thornlessness were not linked to each other; thornless/thorny, the S Locus, was mapped on LG4, and the primocane-/floricane-fruiting locus, named in this work the F Locus, on LG7.

Construction of black (Rubus occidentalis) and red (R. idaeus) raspberry linkage maps and their comparison to the genomes of strawberry, apple, and peach

The genus Rubus belongs to the Rosaceae and is comprised of 600-800 species distributed world-wide. To date, genetic maps of the genus consist largely of non-transferable markers such as amplified fragment length polymorphisms. An F(1) population developed from a cross between an advanced breeding selection of Rubus occidentalis (96395S1) and R. idaeus 'Latham' was used to construct a new genetic map consisting of DNA sequence-based markers. The genetic linkage maps presented here are constructed of 131 markers on at least one of the two parental maps. The majority of the markers are orthologous, including 14 Rosaceae conserved orthologous set markers, and 60 new gene-based markers developed for raspberry. Thirty-four published raspberry simple sequence repeat markers were used to align the new maps to published raspberry maps. The 96395S1 genetic map consists of six linkage groups (LG) and covers 309 cM with an average of 10 cM between markers; the 'Latham' genetic map consists of seven LG and covers 561 cM with an average of 5 cM between markers. We used BLAST analysis to align the orthologous sequences used to design primer pairs for Rubus genetic mapping with the genome sequences of Fragaria vesca 'Hawaii 4', Malus × domestica 'Golden Delicious', and Prunus 'Lovell'. The alignment of the orthologous markers designed here suggests that the genomes of Rubus and Fragaria have a high degree of synteny and that synteny decreases with phylogenetic distance. Our results give unprecedented insights into the genome evolution of raspberry from the putative ancestral genome of the single ancestor common to Rosaceae.

Analysis of expressed sequence tags from Prunus mume flower and fruit and development of simple sequence repeat markers

Background

Expressed Sequence Tag (EST) has been a cost-effective tool in molecular biology and represents an abundant valuable resource for genome annotation, gene expression, and comparative genomics in plants.

Results

In this study, we constructed a cDNA library of Prunus mume flower and fruit, sequenced 10,123 clones of the library, and obtained 8,656 expressed sequence tag (EST) sequences with high quality. The ESTs were assembled into 4,473 unigenes composed of 1,492 contigs and 2,981 singletons and that have been deposited in NCBI (accession IDs: GW868575 - GW873047), among which 1,294 unique ESTs were with known or putative functions. Furthermore, we found 1,233 putative simple sequence repeats (SSRs) in the P. mume unigene dataset. We randomly tested 42 pairs of PCR primers flanking potential SSRs, and 14 pairs were identified as true-to-type SSR loci and could amplify polymorphic bands from 20 individual plants of P. mume. We further used the 14 EST-SSR primer pairs to test the transferability on peach and plum. The result showed that nearly 89% of the primer pairs produced target PCR bands in the two species. A high level of marker polymorphism was observed in the plum species (65%) and low in the peach (46%), and the clustering analysis of the three species indicated that these SSR markers were useful in the evaluation of genetic relationships and diversity between and within the Prunus species.

Conclusions

We have constructed the first cDNA library of P. mume flower and fruit, and our data provide sets of molecular biology resources for P. mume and other Prunus species. These resources will be useful for further study such as genome annotation, new gene discovery, gene functional analysis, molecular breeding, evolution and comparative genomics between Prunus species.

Advances in functional research of antioxidants and organoleptic traits in berry crops

Berry species are economically-important crops worldwide and represent an invaluable source of vitamins and other health-related compounds. Species belonging to the families Rosaceae, Ericaceae and Grossulariaceae provide the most popular fruits, showing a strong diversity in natural and breeding populations as to berry traits (fruit type, size, color, flavor, antioxidant capacity), resistance to a/biotic stress, adaptation to different environment/culture conditions. The small genome size of most diploid berry genera is a remarkable feature for last-generation genomics technologies, molecular genetics and functional studies. This review will cover the literature dealing with molecular research in berry crops, focusing on antioxidant- and flavor-related compounds.