Fluorescence chromosome banding and FISH mapping in perennial ryegrass, Lolium perenne L

Plant Tolerance to Drought Stress with Emphasis on Wheat

Environmental stresses, such as drought, have negative effects on crop yield. Drought is a stress whose impact tends to increase in some critical regions. However, the worldwide population is continuously increasing and climate change may affect its food supply in the upcoming years. Therefore, there is an ongoing effort to understand the molecular processes that may contribute to improving drought tolerance of strategic crops. These investigations should contribute to delivering drought-tolerant cultivars by selective breeding. For this reason, it is worthwhile to review regularly the literature concerning the molecular mechanisms and technologies that could facilitate gene pyramiding for drought tolerance. This review summarizes achievements obtained using QTL mapping, genomics, synteny, epigenetics, and transgenics for the selective breeding of drought-tolerant wheat cultivars. Synthetic apomixis combined with the msh1 mutation opens the way to induce and stabilize epigenomes in crops, which offers the potential of accelerating selective breeding for drought tolerance in arid and semi-arid regions.

Polyploidy on islands - concerted evolution and gene loss amid chromosomal stasis

BACKGROUND AND AIMS

Polyploidy is an important process that often generates genomic diversity within lineages, but it can also cause changes that result in loss of genomic material. Island lineages, while often polyploid, typically show chromosomal stasis but have not been investigated in detail regarding smaller-scale gene loss. Our aim was to investigate post-polyploidization genome dynamics in a chromosomally stable lineage of Malvaceae endemic to New Zealand.

METHODS

We determined chromosome numbers and used fluorescence in situ hybridization to localize 18S and 5S rDNA. Gene sequencing of 18S rDNA, the internal transcribed spacers (ITS) with intervening 5.8S rDNA, and a low-copy nuclear gene, GBSSI-1, was undertaken to determine if gene loss occurred in the New Zealand lineage following polyploidy.

KEY RESULTS

The chromosome number for all species investigated was 2n = 42, with the first published report for the monotypic Australian genus Asterotrichion. The five species investigated all had two 5S rDNA signals localized interstitially on the long arm of one of the largest chromosome pairs. All species, except Plagianthus regius, had two 18S rDNA signals localized proximally on the short arm of one of the smallest chromosome pairs. Plagianthus regius had two additional 18S rDNA signals on a separate chromosome, giving a total of four. Sequencing of nuclear ribosomal 18S rDNA and the ITS cistron indicated loss of historical ribosomal repeats. Phylogenetic analysis of a low-copy nuclear gene, GBSSI-1, indicated that some lineages maintained three copies of the locus, while others have lost one or two copies.

CONCLUSIONS

Although island endemic lineages show chromosomal stasis, with no additional changes in chromosome number, they may undergo smaller-scale processes of gene loss and concerted evolution ultimately leading to further genome restructuring and downsizing.

Distribution patterns of rDNA loci in the Schedonorus-Lolium complex (Poaceae)

The Schedonorus-Lolium complex of the subtribe Loliinae (Poaceae) includes several economically important forage and turf grasses. This complex encompasses Lolium Linnaeus, 1753, Festuca Linnaeus, 1753 subgenus Schedonorus (P. Beauvois, 1824) Petermann, 1849 and Micropyropsis Romero Zarco et Cabezudo, 1983. New FISH results of 5S and 18S-26S rDNA sequences are presented for three species and the results are interpreted in a review of distribution patterns of 5S and 18S-26S rDNA sequences among other species in the complex. Micropyropsistuberosa Romero Zarco et Cabezudo, 1983 (2n = 2x = 14) displayed a distribution pattern of rDNA sequences identical to that of F.pratensis Hudson, 1762, supporting a close phylogenetic relationship at the bottom of the phylogenetic tree. "Loliummultiflorum" Lamarck, 1779 accessions sourced from Morocco showed a different pattern from European L.multiflorum and could be a unique and previously uncharacterised taxon. North African Festucasimensis Hochstetter ex A. Richard, 1851 had a marker pattern consistent with allotetraploidy and uniparental loss of one 18S-26S rDNA locus. This allotetraploid has previously been suggested to have originated from a hybrid with Festucaglaucescens (Festucaarundinaceavar.glaucescens Boissier, 1844). However, the distribution patterns of the two rDNA sequences in this allotetraploid do not align with F.glaucescens, suggesting that its origin from this species is unlikely. Furthermore, comparisons with other higher alloploids in the complex indicate that F.simensis was a potential donor of two sub-genomes of allohexaploid Festucagigantea (Linnaeus) Villars, 1787. In the overall complex, the proximal locations of both rDNA markers were conserved among the diploid species. Two types of synteny of the two markers could, to a considerable extent, distinguish allo- and autogamous Lolium species. The ancestral parentage of the three Festuca allotetraploids has not yet been determined, but all three appear to have been sub-genome donors to the higher allopolypoids of sub-genus Schedonorus. Terminal locations of both the markers were absent from the diploids but were very frequently observed in the polyploids.

Asynapsis and unreduced gamete formation in a Trifolium interspecific hybrid

BACKGROUND

Unreduced gametes, a driving force in the widespread polyploidization and speciation of flowering plants, occur relatively frequently in interspecific or intergeneric hybrids. Studies of the mechanisms leading to 2n gamete formation, mainly in the wheat tribe Triticeae have shown that unreductional meiosis is often associated with chromosome asynapsis during the first meiotic division. The present study explored the mechanisms of meiotic nonreduction leading to functional unreduced gametes in an interspecific Trifolium (clover) hybrid with three sub-genomes from T. ambiguum and one sub-genome from T. occidentale.

RESULTS

Unreductional meiosis leading to 2n gametes occurred when there was a high frequency of asynapsis during the first meiotic division. In this hybrid, approximately 39% of chromosomes were unpaired at metaphase I. Within the same cell at anaphase I, sister chromatids of univalents underwent precocious separation and formed laggard chromatids whereas paired chromosomes segregated without separation of sister chromatids as in normal meiosis. This asynchrony was frequently accompanied by incomplete or no movement of chromosomes toward the poles and restitution leading to unreduced chromosome constitutions. Reductional meiosis was restored in progeny where asynapsis frequencies were low. Two progeny plants with approximately 5 and 7% of unpaired chromosomes at metaphase I showed full restoration of reductional meiosis.

CONCLUSIONS

The study revealed that formation of 2n gametes occurred when asynapsis (univalent) frequency at meiosis I was high, and that normal gamete production was restored in the next generation when asynapsis frequencies were low. Asynapsis-dependent 2n gamete formation, previously supported by evidence largely from wheat and its relatives and grasshopper, is also applicable to hybrids from the dicotyledonous plant genus Trifolium. The present results align well with those from these widely divergent organisms and strongly suggest common molecular mechanisms involved in unreduced gamete formation.

A simple non-toxic ethylene carbonate fluorescence in situ hybridization (EC-FISH) for simultaneous detection of repetitive DNA sequences and fluorescent bands in plants

The major drawbacks of standard plant fluorescence in situ hybridization (FISH) designed for double-stranded DNA probes include requirement for experimentally determined heat denaturation of chromosomes at high temperatures and at least overnight hybridization. Consequently, processing with chromosomal preparations may easily result in heat-induced deterioration of chromosomal structural details, is time-consuming, and involves the use of toxic formamide and formaldehyde. Here, I have described a simple and appealing non-toxic procedure with ethylene carbonate (EC)-a formamide-substituting solvent and double-stranded repetitive DNA probes. Applying EC as a component of the hybridization solution at 46 °C not only allowed successful overnight hybridization but also gave a possibility to reduce the hybridization time to 3 h, hence converting the technique into a 1-day procedure. Importantly, the EC-FISH tended to preserve well chromosome structural details, e.g., DAPI-positive bands, thus facilitating simultaneous FISH mapping and chromosome banding on the same slide. The procedure requires no formaldehyde and RNA-se treatment of chromosomes, and no heat denaturation of chromosomal DNA. The key condition is to obtain high-quality cytoplasm-free preparations. The method was reproducible in all the plants studied (Allium, Nigella, Tradescantia, Vicia), giving a species-specific signal pattern together with clear DAPI bands on chromosomes. The procedure described here is expected to give a positive stimulus for improving gene-mapping approaches in plants.