Asymmetric Reproductive Barriers and Gene Flow Promote the Rise of a Stable Hybrid Zone in the Mediterranean High Mountain

Reciprocal conversion between annual and polycarpic perennial flowering behavior in the Brassicaceae

The development of perennial crops holds great promise for sustainable agriculture and food security. However, the evolution of the transition between perenniality and annuality is poorly understood. Here, using two Brassicaceae species, Crucihimalaya himalaica and Erysimum nevadense, as polycarpic perennial models, we reveal that the transition from polycarpic perennial to biennial and annual flowering behavior is a continuum determined by the dosage of three closely related MADS-box genes. Diversification of the expression patterns, functional strengths, and combinations of these genes endows species with the potential to adopt various life-history strategies. Remarkably, we find that a single gene among these three is sufficient to convert winter-annual or annual Brassicaceae plants into polycarpic perennial flowering plants. Our work delineates a genetic basis for the evolution of diverse life-history strategies in plants and lays the groundwork for the generation of diverse perennial Brassicaceae crops in the future.

Transcriptome variations in hybrids of wild emmer wheat (Triticum turgidum ssp. dicoccoides)

BACKGROUND

Wild emmer wheat is a great candidate to revitalize domesticated wheat genetic diversity. Recent years have seen intensive investigation into the evolution and domestication of wild emmer wheat, including whole-genome DNA and transcriptome sequencing. However, the impact of intraspecific hybridization on the transcriptome of wild emmer wheat has been poorly studied. In this study, we assessed changes in methylation patterns and transcriptomic variations in two accessions of wild emmer wheat collected from two marginal populations, Mt. Hermon and Mt. Amasa, and in their stable F4 hybrid.

RESULTS

Methylation-Sensitive Amplified Polymorphism (MSAP) detected significant cytosine demethylation in F4 hybrids vs. parental lines, suggesting potential transcriptome variation. After a detailed analysis, we examined nine RNA-Seq samples, which included three biological replicates from the F4 hybrid and its parental lines. RNA-Seq databases contained approximately 200 million reads, with each library consisting of 15 to 25 million reads. There are a total of 62,490 well-annotated genes in these databases, with 6,602 genes showing differential expression between F4 hybrid and parental lines Mt. Hermon and Mt. Amasa. The differentially expressed genes were classified into four main categories based on their expression patterns. Gene ontology (GO) analysis revealed that differentially expressed genes are associated with DNA/RNA metabolism, photosynthesis, stress response, phosphorylation and developmental processes.

CONCLUSION

This study highlights the significant transcriptomic changes resulting from intraspecific hybridization within natural plant populations, which might aid the nascent hybrid in adapting to various environmental conditions.

Spatiotemporal dynamics in butterfly hybrid zones

Evaluating whether hybrid zones are stable or mobile can provide novel insights for evolution and conservation biology. Butterflies exhibit high sensitivity to environmental changes and represent an important model system for the study of hybrid zone origins and maintenance. Here, we review the literature exploring butterfly hybrid zones, with a special focus on their spatiotemporal dynamics and the potential mechanisms that could lead to their movement or stability. We then compare different lines of evidence used to investigate hybrid zone dynamics and discuss the strengths and weaknesses of each approach. Our goal with this review is to reveal general conditions associated with the stability or mobility of butterfly hybrid zones by synthesizing evidence obtained using different types of data sampled across multiple regions and spatial scales. Finally, we discuss spatiotemporal dynamics in the context of a speciation/divergence continuum, the relevance of hybrid zones for conservation biology, and recommend key topics for future investigation.

The puzzle of plant hybridisation: a high propensity to hybridise but few hybrid zones reported

An interesting conundrum was recently revealed by R. Abbott when he found that the number of hybrid zones reported in the literature for plants is very low, given the propensity of plants to hybridise. In another literature survey on hybrid zones performed over the period 1970-2022, we found that the number of hybrid zones reported for vertebrates was 2.3 times greater than that reported for vascular plants, even though there are about six times more vascular plant species than vertebrates. Looking at the number of papers reporting hybrid zones, there are 4.9 times more on vertebrates than on vascular plants. These figures support the relevance of this conundrum. In this paper we aim to shed light on this question by providing a structured discussion of the causes that may underlie this conundrum. We propose six non-mutually exclusive factors, namely lack or deficit of spatial structure, lack or deficit of genetic structure, effects of hybridisation between non-closely related species, lability of plant hybrid zones over time, botanists' perception of hybridisation, and deficit of population genetic data. There does not appear to be a single factor that explains our puzzle, which applies to all cases of plants where hybridisation is detected but no hybrid zone is reported. It is argued that some plant features suggest that the puzzle is not, at least entirely, due to insufficient knowledge of the specific cases, a hypothesis that should be addressed with a wider range of empirical data across different taxonomic groups.

New insights into polyploid evolution and dynamic nature of Ludwigia section Isnardia (Onagraceae)

BACKGROUND

While polyploids are common in plants, the evolutionary history and natural dynamics of most polyploid groups are still unclear. Owing to plentiful earlier systematic studies, Ludwigia sect. Isnardia (comprising 22 wetland taxa) is an ideal allopolyploid complex to investigate polyploid evolution and natural dynamics within and among taxa. With a considerable sampling, we concentrated on revisiting earlier phylogenies of Isnardia, reevaluating the earlier estimated age of the most recent common ancestor (TMRCA), exploring the correlation between infraspecific genetic diversity and ploidy levels, and inspecting interspecific gene flows among taxa.

RESULTS

Phylogenetic trees and network concurred with earlier phylogenies and hypothesized genomes by incorporating 192 atpB-rbcL and ITS sequences representing 91% of Isnardia taxa. Moreover, we detected three multi-origin taxa. Our findings on L. repens and L. sphaerocarpa were consistent with earlier studies; L. arcuata was reported as a multi-origin taxon here, and an additional evolutionary scenario of L. sphaerocarpa was uncovered, both for the first time. Furthermore, estimated Isnardia TMRCA ages based on our data (5.9 or 8.9 million years ago) are in accordance with earlier estimates, although younger than fossil dates (Middle Miocene). Surprisingly, infraspecific genetic variations of Isnardia taxa did not increase with ploidy levels as anticipated from many other polyploid groups. In addition, the exuberant, low, and asymmetrical gene flows among Isnardia taxa indicated that the reproductive barriers may be weakened owing to allopolyploidization, which has rarely been reported.

CONCLUSIONS

The present research gives new perceptions of the reticulate evolution and dynamic nature of Isnardia and points to gaps in current knowledge about allopolyploid evolution.

Hybridization and introgression are prevalent in Southern European Erysimum (Brassicaceae) species

BACKGROUND AND AIMS

Hybridization is a common and important force in plant evolution. One of its outcomes is introgression - the transfer of small genomic regions from one taxon to another by hybridization and repeated backcrossing. This process is believed to be common in glacial refugia, where range expansions and contractions can lead to cycles of sympatry and isolation, creating conditions for extensive hybridization and introgression. Polyploidization is another genome-wide process with a major influence on plant evolution. Both hybridization and polyploidization can have complex effects on plant evolution. However, these effects are often difficult to understand in recently evolved species complexes.

METHODS

We combined flow cytometry, analyses of transcriptomic sequences and pollen tube growth assays to investigate the consequences of polyploidization, hybridization and introgression on the recent evolution of several Erysimum (Brassicaceae) species from the South of the Iberian Peninsula, a well-known glacial refugium. This species complex differentiated in the last 2 million years, and its evolution has been hypothesized to be determined mainly by polyploidization, interspecific hybridization and introgression.

KEY RESULTS

Our results support a scenario of widespread hybridization involving both extant and 'ghost' taxa. Several taxa studied here, most notably those with purple corollas, are polyploids, probably of allopolyploid origin. Moreover, hybridization in this group might be an ongoing phenomenon, as pre-zygotic barriers appeared weak in many cases.

CONCLUSIONS

The evolution of Erysimum spp. has been determined by hybridization to a large extent. Species with purple (polyploids) and yellow flowers (mostly diploid) exhibit a strong signature of introgression in their genomes, indicating that hybridization occurred regardless of colour and across ploidy levels. Although the adaptive value of such genomic exchanges remains unclear, our results demonstrate the significance of hybridization for plant diversification, which should be taken into account when studying plant evolution.

Genetic structure and geneflow of Malus across the Korean Peninsula using genotyping-by-sequencing

This study was to understand the genetic structure and diversity of the Korean Malus species. We used genotyping-by-sequencing (GBS) technology to analyze samples of 112 individuals belonging to 18 populations of wild Malus spp. Using GBS, we identified thousands of single nucleotide polymorphisms in the species analyzed. M. baccata and M. toringo, two dominant mainland species of the Korean Peninsula, were distinguishable based on their genetic structure. However, M. toringo collected from Jeju Island exhibited a different genetic profile than that from the mainland. We identified M. cf. micromalus as a hybrid resulting from the Jeju Island M. toringo (pollen donor) and the mainland M. baccata, (pollen recipient). Putative M. mandshurica distributed on the Korean Peninsula showed a high structural and genetic similarity with M. baccata, indicating that it might be an ecotype. Overall, this study contributes to the understanding of the population history and genetic structure of Malus in the Korean Peninsula.

Tension zone trapped by exogenous cline: Analysis of a narrow hybrid zone between two parapatric Oxytropis species (Fabaceae)

Hybrid zones have been widely highlighted for their interest in understanding evolutionary processes. It is generally accepted that hybrid zones can be maintained in a balance between dispersal and selection. However, the selective forces can either be endogenous (i.e., genetic incompatibilities between parental taxa) or exogenous (i.e., parental taxa are adapted to different environments). In this study, we evaluated these alternatives and determined the maintenance of a narrow hybrid zone between parapatric distributed Oxytropis diversifolia and O. leptophylla in Nei Mongol, China. For 507 individuals sampled from two populations in the hybrid zone, 12 O. diversifolia populations and five O. leptophylla populations, we measured leaf‐morphological characteristics, quantified genetic structure using 11 nuclear microsatellite loci and five chloroplast DNA intergenic regions, collected micro‐ and macrohabitat data, and conducted geographical cline analysis. We found that the two species differed in leaf morphology, and putative hybrids showed either intermediacy or a bias to O. diversifolia. Parental taxa formed two genetically distinct clusters, while populations in the hybrid zone consisted of both parental forms and various admixed individuals, exhibiting a bimodal pattern. The hybrid zone was coupled to ecological transitions of both microhabitat (i.e., the slope) and macroclimatic conditions. However, the genetic clines were significantly narrower than the environmental cline. Our results indicate that endogenous selection can be primarily responsible for maintaining the hybrid zone, while local adaptation accounts for the position of the zone. We further suggest the probable outcome of hybridization could be introgression.

Genetic Variability in Balkan Paleoendemic Resurrection Plants Ramonda serbica and R. nathaliae Across Their Range and in the Zone of Sympatry

The genus Ramonda includes three Paleoendemic and Tertiary relict species that survived in refugial habitats of the Balkan Peninsula (R. nathaliae and R. serbica) and the Iberian Peninsula (R. myconi). They are all "resurrection plants," a rare phenomenon among flowering plants in Europe. Ramonda myconi and R. nathaliae are diploids (2n = 2x = 48), while R. serbica is a hexaploid (2n = 6x = 144). The two Balkan species occur in sympatry in only two localities in eastern Serbia, where tetraploid potential hybrids (2n = 4x = 96) were found. This observation raised questions about the existence of gene flow between the two species and, more generally, about the evolutionary processes shaping their genetic diversity. To address this question, genetic markers (AFLP) and an estimate of genome size variation were used in a much larger sample and at a larger geographic scale than previously. The combination of AFLP markers and genome size results suggested ongoing processes of interspecific and interploidy hybridization in the two sites of sympatry. The data also showed that interspecific gene flow was strictly confined to sympatry. Elsewhere, both Ramonda species were characterized by low genetic diversity within populations and high population differentiation. This is consistent with the fact that the two species are highly fragmented into small and isolated populations, likely a consequence of their postglacial history. Within sympatry, enormous variability in cytotypes was observed, exceeding most reported cases of mixed ploidy in complex plant species (from 2x to >8x). The AFLP profiles of non-canonical ploidy levels indicated a diversity of origin pathways and that backcrosses probably occur between tetraploid interspecific hybrids and parental species. The question arises whether this diversity of cytotypes corresponds to a transient situation. If not, the question arises as to the genetic and ecological mechanisms that allow this diversity to be maintained over time.