Duckweeds represent a small, free-floating aquatic family (Lemnaceae) of the monocot order Alismatales with the fastest growth rate among flowering plants. They comprise five genera (Spirodela, Landoltia, Lemna, Wolffiella, and Wolffia) varying in genome size and chromosome number. Spirodela polyrhiza had the first sequenced duckweed genome. Cytogenetic maps are available for both species of the genus Spirodela (S. polyrhiza and S. intermedia). However, elucidation of chromosome homeology and evolutionary chromosome rearrangements by cross-FISH using Spirodela BAC probes to species of other duckweed genera has not been successful so far. We investigated the potential of chromosome-specific oligo-FISH probes to address these topics. We designed oligo-FISH probes specific for one S. intermedia and one S. polyrhiza chromosome (Fig. 1a). Our results show that these oligo-probes cross-hybridize with the homeologous regions of the other congeneric species, but are not suitable to uncover chromosomal homeology across duckweeds genera. This is most likely due to too low sequence similarity between the investigated genera and/or too low probe density on the target genomes. Finally, we suggest genus-specific design of oligo-probes to elucidate chromosome evolution across duckweed genera.

• Keywords
• Dual-color oligo-FISH, Duckweeds, Karyotype evolution, Landoltia, Lemna, Microsatellites, Spirodela, Structured illumination microscopy, Wolffia, Wolffiella,
• MeSH
• Araceae classification   genetics   growth & development   MeSH
• Chromosomes, Plant genetics   MeSH
• Species Specificity MeSH
• Phylogeny MeSH
• Genome, Plant * MeSH
• In Situ Hybridization, Fluorescence methods   MeSH
• Karyotyping MeSH
• Evolution, Molecular * MeSH
• Oligonucleotide Probes chemistry   genetics   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Oligonucleotide Probes MeSH

Chromosome numbers have been widely used to describe the most fundamental genomic attribute of an organism or a lineage. Although providing strong phylogenetic signal, chromosome numbers vary remarkably among eukaryotes at all levels of taxonomic resolution. Changes in chromosome numbers regularly serve as indication of major genomic events, most notably polyploidy and dysploidy. Here, we review recent advancements in our ability to make inferences regarding historical events that led to alterations in the number of chromosomes of a lineage. We first describe the mechanistic processes underlying changes in chromosome numbers, focusing on structural chromosomal rearrangements. Then, we focus on experimental procedures, encompassing comparative cytogenomics and genomics approaches, and on computational methodologies that are based on explicit models of chromosome-number evolution. Together, these tools offer valuable predictions regarding historical events that have changed chromosome numbers and genome structures, as well as their phylogenetic and temporal placements.

• Keywords
• chromosome numbers, cytogenomics, dysploidy, genome evolution, phylogenetic models, polyploidy,
• MeSH
• Chromosomes, Plant * MeSH
• Genome, Plant MeSH
• Genomics MeSH
• Chromosome Painting MeSH
• Models, Genetic * MeSH
• Evolution, Molecular * MeSH
• Polyploidy MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

Duckweeds are small, free-floating, morphologically highly reduced organisms belonging to the monocot order Alismatales. They display the most rapid growth among flowering plants, vary ~ 14-fold in genome size and comprise five genera. Spirodela is the phylogenetically oldest genus with only two mainly asexually propagating species: S. polyrhiza (2n = 40; 160 Mbp/1C) and S. intermedia (2n = 36; 160 Mbp/1C). This study combined comparative cytogenetics and de novo genome assembly based on PacBio, Illumina and Oxford Nanopore (ON) reads to obtain the first genome reference for S. intermedia and to compare its genomic features with those of the sister species S. polyrhiza. Both species' genomes revealed little more than 20,000 putative protein-coding genes, very low rDNA copy numbers and a low amount of repetitive sequences, mainly Ty3/gypsy retroelements. The detection of a few new small chromosome rearrangements between both Spirodela species refined the karyotype and the chromosomal sequence assignment for S. intermedia.

• MeSH
• Araceae genetics   MeSH
• Chromosomes, Plant * MeSH
• Genome, Plant * MeSH
• Karyotype MeSH
• Karyotyping MeSH
• Chromosome Mapping MeSH
• Nanopores MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

Genlisea margaretae, subgenus Genlisea, section Recurvatae (184 Mbp/1C), belongs to a plant genus with a 25-fold genome size difference and an extreme genome plasticity. Its 19 chromosome pairs could be distinguished individually by an approach combining optimized probe pooling and consecutive rounds of multicolor fluorescence in situ hybridization (mcFISH) with bacterial artificial chromosomes (BACs) selected for repeat-free inserts. Fifty-one BACs were assigned to 18 chromosome pairs. They provide a tool for future assignment of genomic sequence contigs to distinct chromosomes as well as for identification of homeologous chromosome regions in other species of the carnivorous Lentibulariaceae family, and potentially of chromosome rearrangements, in cases where more than one BAC per chromosome pair was identified.

• Keywords
• BACs, Genlisea, Karyotyping, Multicolor fluorescence in situ hybridization (mcFISH), Reprobing,
• MeSH
• Chromosomes, Plant genetics   metabolism   MeSH
• Genome Size MeSH
• Genome, Plant MeSH
• In Situ Hybridization, Fluorescence MeSH
• Magnoliopsida genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH