Biology subjects

Morello, L.

Publications and source records attributed to Morello, L..

6 recordsLinked to original sources

bioRxiv2026

Transposon expansion is associated with reorganization of small RNA and DNA methylation landscapes in the morphologically minimal angiosperm Wolffia brasiliensis

Genome expansion in angiosperms is largely driven by transposable element (TE) proliferation, counteracted by epigenetic silencing. To investigate how TE amplification reshapes silencing landscapes, we compared two closely related, clonally propagating duckweeds, the TE-rich Wolffia brasiliensis, for which we report a draft genome assembly, and the TE-poor Spirodela polyrhiza, that share a broadly conserved silencing and methylation machinery. W. brasiliensis displays extensive and recent TE amplification with pervasive TE-gene interspersion, elevated genome-wide CG methylation, and high levels of both 22- and 24-nt siRNAs. Systematic cross-species comparison reveals that per-element silencing rules are conserved between the two duckweeds: TE length predicting siRNA-producing capacity, inverted-repeat-forming TEs as productive PTGS-associated loci, 24-nt siRNA production predicting non-CG methylation, and intragenic constraint on RdDM-associated methylation. What differs is the genome-wide deployment of these rules across dramatically different TE loads and genome architectures: some divergent outcomes, including elevated 24-nt siRNA abundance and pervasive TE-wide CG methylation, scale directly with TE content, whereas others: near-exclusive 22-nt processing of PTGS substrates, gene-proximity-dependent RdDM at intergenic TEs, and a strong correlation between gene body CG methylation and intragenic TE content that is not detectable in S. polyrhiza; require additional W. brasiliensis-specific contributions. These findings position TEs as central determinants of the epigenetic architecture that emerges from plant genome expansion, and reveal previously underappreciated plasticity in conserved silencing pathways. Significance statementComparing closely related duckweeds with contrasting TE loads but conserved silencing machinery, we show that per-element silencing rules are shared between the two species while the genome-wide outcomes those rules produce diverge profoundly with TE amplification and genome architecture. This reframes TE proliferation as a determinant not only of genome size but of the epigenetic architecture that emerges from it, uncovers a Wolffia-specific coupling between intragenic TE content and gene body CG methylation, and reveals unexpected plasticity in small RNA biogenesis pathways.

plant biology↗

bioRxiv2026

Newly unveiled meiosis elucidates the unreduced gamete frequency and its impact on evolution of the Lemna minor complex

Fusion of gametes possessing meiotically reduced (haploid) chromosome complements is the main pathway of propagation among eukaryotes. However, duckweeds, the smallest angiosperms, propagate mainly vegetatively, and meiosis has not yet been documented in detail for this plant family. The more surprising was the recent evidence of rather frequent interspecific hybrids and triploid clonal accessions which became obvious by genome size measurements, genomic in situ hybridization (GISH) and combined plastid and nuclear DNA markers. These observations indicated sexual propagation involving reduced as well as unreduced male and female gametes in Lemna minor and L. turionifera leading to allodiploid and allotriploid hybrids (MT, MMT, MTT) and autotriploid L. minor (MMM) accessions. Here, we i) documented the meiotic stages of Lemna species for the first time; ii) provided evidence of unreduced male gametes through fluorescent in situ hybridization (FISH) with single locus probes; iii) determined their abundance in different individuals and iv) hypothesized about the reasons of unreduced male gamete formation. These findings open new insights into the modes of sexual reproduction and evolution of duckweeds which may be useful for future breeding efforts in this emerging crop.

plant biology↗

bioRxiv2025

Hybridity of mainly asexually propagating duckweeds in genus Lemna - dead end or breakthrough?

O_LIThe cosmopolitan, mainly vegetatively propagating, organ-reduced monocotyledonous aquatic duckweeds are the smallest and fastest growing angiosperms, distributed world-wide and flower rarely in nature. Recently, we reported intra- and interspecific hybrids and ploidy variants in the genus Lemna. Thus, contrary to the expectation, sexual propagation may occasionally occur within and between Lemna species. Our main goal was to uncover whether the ecologically successful hybrids are evolutionary dead ends, or initiate further speciation and novel sexual recombination. C_LIO_LIWe investigated flower development, pollen viability, seed set and seed germination in hybrids and their parental species and characterized genome size and genetic markers in the progenies. C_LIO_LIIntraspecific crosses yielded fertile progeny, but all dihaploid and triploid interspecific hybrids were male sterile. Only an established allotetraploid hybrid reproduced sexually, while colchicine-induced allotetraploids from dihaploids did not re-gain sexual competence so far. C_LIO_LIWe concluded that only established allotetraploid hybrids represent an evolutionary break-through in duckweeds. Our results regarding sexual traits within the duckweed genus Lemna, and the sexual competence of diverse hybrids i) pave the way for further investigation in this understudied field, ii) provide fundamental data regarding the evolutionary potential of duckweed hybrids and iii) are important for future breeding efforts on this emerging crop. C_LI

evolutionary biology↗

bioRxiv2025

Triploidy is prominent in the duckweed Lemna minor complex

Duckweeds (Lemnaceae Martinov) are aquatic monocotyledonous flowering plants comprising five genera and 35 recognized species, known for being the smallest and fastest-growing flowering plants on Earth. Many species are morphologically indistinguishable due to their highly reduced structures, yet molecular evidence suggests that visually similar clones may represent distinct species or hybrids. For example, clonal accessions of the globally distributed Lemna minor in the Landolt Duckweed Collection exhibit genome size variations of several hundred megabases (Mb), raising questions about their taxonomic classification and evolutionary origins. We analyzed 58 presumed L. minor clones to resolve these relationships using a comprehensive suite of methods, including whole-genome sequencing (WGS), flow-cytometric genome size measurements, molecular markers, chromosome counting, and genomic in situ hybridization (GISH). Our findings reveal extensive genome plasticity within the "Lemna minor complex," identifying diploid and triploid L. minor clones, as well as di-haploid and triploid interspecific hybrids called L. x japonica (L. minor x L. turionifera), L. x mediterranea (L. minor x L. gibba), and a novel African-clade distinct from known L. minor lineages. Triploidy was prevalent, occurring in 29% of the clones, and was associated with enhanced growth under optimal conditions but reduced performance under high light and temperature. These findings highlight the widespread role of triploidy, cryptic species, and hybridization in the L. minor complex, emphasizing the importance of multiple approaches for accurately classifying duckweed species and understanding their evolutionary trajectories.

plant biology↗

bioRxiv2025

Genome diversity and phylogeny of the section Alatae of genus Lemna (Lemnaceae), comprising the presumed species Lemna aequinoctialis, Le. perpusilla and Le. aoukikusa

O_LIThe Section Alatae of genus Lemna of the monocotyledonous aquatic duckweed family (Lemnaceae) consists of rather diverse accessions with unknown phylogeny and unclear taxonomic assignment. In contrast to other duckweeds, some Alatae accessions, in addition to mainly vegetative propagation, produce readily flowers and viable seeds. We analyzed the genomic diversity and phylogenetic relationship of 52 Alatae accessions. C_LIO_LIFor this purpose, we applied multiple molecular and cytogenetic approaches, including plastid and nuclear sequence polymorphisms, chromosome counting, genome size determination and genomic in situ hybridization in combination with geographic distribution. C_LIO_LIWe uncovered ploidy variation, recurrent hybridization, and backcrosses between diploid species and their hybrids. The latter successfully spread over three continents. C_LIO_LIThe results elucidate the evolution of Alatae accessions (summarized in Fig. 7) and explain the difficult taxonomic assignment of distinct accessions. Our study might be an example for analogous studies to resolve the hitherto unclear relationships among accessions of the duckweed genera Wolffiella and Wolffia. C_LI O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=166 SRC="FIGDIR/small/632899v1_fig7.gif" ALT="Figure 7"> View larger version (23K): org.highwire.dtl.DTLVardef@d05f08org.highwire.dtl.DTLVardef@f33c5org.highwire.dtl.DTLVardef@ff7adorg.highwire.dtl.DTLVardef@4da6e4_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOFigure 7C_FLOATNO Hypothetical scenario of evolution within Lemna Section Alatae. In bold are accessions from America. The classically defined diploid species Le. aequinoctialis and Le. perpusilla sensu stricto were confirmed by the applied molecular methods. The previously named Le. aoukikusa accessions were uncovered as tetraploid interspecific hybrids between Le. aequinoctialis and Le. perpusilla. In addition, further tri- and tetraploid hybrids as well as tri- and tetraploid "Backcross hybrids" were detected from the USA and China, respectively. C_FIG

evolutionary biology↗

bioRxiv2024

A survey of duckweed species in southern Italy provided first occurrences of the hybrid Lemna x mediterranea in the wild

Interspecific hybridisation and polyploidization are two main driving forces in plant evolution, shaping genomes and favouring evolutionary novelty and ecological adaptation. Recent studies have demonstrated hybridisation within the genus Lemna (Lemnaceae Martinov) as well as triploid. L. x mediterranea, a recently described hybrids between Lemna minor and Lemna gibba was identified among only long lasting germplasm collections of in vitro propagated plants, originally collected at different times in the Mediterranean area. We report the first distribution record of L. x mediterranea in the nature, in the Campania region of Southern Italy, the same area where Lemna symmeter was described as a new species about 50 years ago, confirming their synonymy. Eight specimens isolated from five different sampling sites over an area of about 4200 Km2 showed identical genetic profiles by Tubulin-Based Polymorphism (TBP) analysis, suggesting their common origin from the same hybridisation event, followed by clonal dispersal. The L. x mediterranea population of Campania is genetically different from any of the previously analysed clones, suggesting that recurrent hybridisation between the parental species may occur. The natural hybrid clone is triploid, with L. gibba as the plastid donor, and remarkably similar to it by morphology, although the typical gibbosity of this species becomes evident only upon in vitro flower induction. Flowers are protogynous and self-sterile. Ecological factors including competition with parental and invasive species, niche and climate change adaptation, stability in time and space likely played a role in the successful establishment of L. x mediterranea. HighlightsO_LIInterspecific hybridisation within the genus Lemna documented in nature C_LIO_LI{beta}-tubulin intron length polymorphism for tracking duckweed species distribution C_LIO_LIFlower induction in Lemna x mediterranea C_LI

ecology↗