Retrotransposon-based S-SAP as a platform for the analysis of genetic variation and linkage in globe artichoke

Use of retrotransposon-derived genetic markers to analyse genomic variability in plants

Transposable elements (TEs) are common mobile genetic elements comprising several classes and making up the majority of eukaryotic genomes. The movement and accumulation of TEs has been a major force shaping the genes and genomes of most organisms. Most eukaryotic genomes are dominated by retrotransposons and minimal DNA transposon accumulation. The 'copy and paste' lifecycle of replicative transposition produces new genome insertions without excising the original element. Horizontal TE transfer among lineages is rare. TEs represent a reservoir of potential genomic instability and RNA-level toxicity. Many TEs appear static and nonfunctional, but some are capable of replicating and mobilising to new positions, and somatic transposition events have been observed. The overall structure of retrotransposons and the domains responsible for the phases of their replication are highly conserved in all eukaryotes. TEs are important drivers of species diversity and exhibit great variety in their structure, size and transposition mechanisms, making them important putative actors in evolution. Because TEs are abundant in plant genomes, various applications have been developed to exploit polymorphisms in TE insertion patterns, including conventional or anchored PCR, and quantitative or digital PCR with primers for the 5' or 3' junction. Alternatively, the retrotransposon junction can be mapped using high-throughput next-generation sequencing and bioinformatics. With these applications, TE insertions can be rapidly, easily and accurately identified, or new TE insertions can be found. This review provides an overview of the TE-based applications developed for plant species and assesses the contributions of TEs to the analysis of plants' genetic diversity.

Construction of a reference molecular linkage map of globe artichoke (Cynara cardunculus var. scolymus)

The genome organization of globe artichoke (Cynara cardunculus var. scolymus), unlike other species belonging to Asteraceae (=Compositae) family (i.e. sunflower, lettuce and chicory), remains largely unexplored. The species is highly heterozygous and suffers marked inbreeding depression when forced to self-fertilize. Thus a two-way pseudo-testcross represents the optimal strategy for linkage analysis. Here, we report linkage maps based on the progeny of a cross between globe artichoke (C. cardunculus var. scolymus) and cultivated cardoon (C. cardunculus var. altilis). The population was genotyped using a variety of PCR-based marker platforms, resulting in the identification of 708 testcross markers suitable for map construction. The male map consisted of 177 loci arranged in 17 major linkage groups, spanning 1,015.5 cM, while female map was built with 326 loci arranged into 20 major linkage groups, spanning 1,486.8 cM. The presence of 84 loci shared between these maps and those previously developed from a cross within globe artichoke allowed for map alignment and the definition of 17 homologous linkage groups, corresponding to the haploid number of the species. This will provide a favourable property for QTL scanning; furthermore, as 25 mapped markers (8%) correspond to coding regions, it has an additional value as functional map and might represent an important genetic tool for candidate gene studies in globe artichoke.

Genetic mapping of the apospory-specific genomic region in Pennisetum squamulatum using retrotransposon-based molecular markers

Pennisetum squamulatum reproduces by apomixis, a type of asexual reproduction through seeds. Apomixis in P. squamulatum is transmitted as a dominant Mendelian trait, and a genomic region, the apospory-specific genomic region (ASGR), is sufficient for inheritance of the trait. The ASGR is physically large (>50 Mb), highly heterochromatic, hemizygous, and recombinationally suppressed. These characteristics have hindered high-resolution genetic mapping and map-based cloning of apomixis genes. In this study, the long terminal repeat (LTR) regions of ASGR-abundant retrotransposons in the genome of P. squamulatum and ASGR-linked bacterial artificial chromosome clones were identified and sequenced for designing LTR-specific primers. Two hundred and ninety single-dose sequence specific amplified polymorphism (SSAP) markers were generated from 38 primer combinations. The SSAP markers combined with two previous ASGR-mapped markers were used for genetic linkage analysis and construction of a genetic map resulting in the formation of 27 linkage groups at LOD 10, one of which contained >60% of the SSAP markers. After removing identical markers (identical band scoring) on the largest linkage group, 46 markers were finally used for genetic mapping at LOD 10. The markers distributed across 10 different loci covering 19 cM; however, 45 markers were distributed within 9 cM. Six markers were recovered and sequenced. Five markers were successfully converted into sequence characterized amplified regions (SCARs). Segregation of SCAR markers was not always consistent with the SSAP markers of origin suggesting a greater level of error in the SSAP map resulting in an inflated map distance for the ASGR. One SCAR marker (Pst 56-1205-400) detected expression of an ASGR retrotransposon in root, anther, leaf and ovary of P. squamulatum, although sequencing of the RT-PCR product failed to find a functional open reading frame for the transcript.

High-resolution melt analysis to identify and map sequence-tagged site anchor points onto linkage maps: a white lupin (Lupinus albus) map as an exemplar

* The provision of sequence-tagged site (STS) anchor points allows meaningful comparisons between mapping studies but can be a time-consuming process for nonmodel species or orphan crops. * Here, the first use of high-resolution melt analysis (HRM) to generate STS markers for use in linkage mapping is described. This strategy is rapid and low-cost, and circumvents the need for labelled primers or amplicon fractionation. * Using white lupin (Lupinus albus, x = 25) as a case study, HRM analysis was applied to identify 91 polymorphic markers from expressed sequence tag (EST)-derived and genomic libraries. Of these, 77 generated STS anchor points in the first fully resolved linkage map of the species. The map also included 230 amplified fragment length polymorphisms (AFLP) loci, spanned 1916 cM (84.2% coverage) and divided into the expected 25 linkage groups. * Quantitative trait loci (QTL) analyses performed on the population revealed genomic regions associated with several traits, including the agronomically important time to flowering (tf), alkaloid synthesis and stem height (Ph). Use of HRM-STS markers also allowed us to make direct comparisons between our map and that of the related crop, Lupinus angustifolius, based on the conversion of RFLP, microsatellite and single nucleotide polymorphism (SNP) markers into HRM markers.

Sequence-specific amplification polymorphisms (SSAPs): a multi-locus approach for analyzing transposon insertions

Multiple copies of transposable elements, inserted at random around the host genome, can be used as molecular markers. Sequence-specific amplification polymorphisms (SSAPs) amplify the region between a PCR primer site near the end of an element and an adjacent restriction site in the flanking genomic DNA. Each amplified insertion is revealed as a band on a sequencing gel or capillary electrophoresis, and a genomic DNA sample produces a characteristic fingerprint of bands. Here, we explain the SSAP marker development method, which can be performed for any species, with recommendations for experimental parameters in several plant species. It takes about 1-2 weeks to complete the whole SSAP marker procedure.