Francisco Hortas

Biological invasions are natural experiments for studying the evolutionary and ecological processes underlying colonization success and range expansion. Using genome-wide data—generated via genotyping-by-sequencing (GBS) from 30 populations spanning Europe and South Africa—we investigated the colonization history and successful spread of the invasive buttonweed Cotula coronopifolia, an annual plant introduced into Europe from South Africa about 300 years ago. Our analyses identified three major lineages in Europe distributed across the continent, often co-occurring without evidence of admixture. Phylogenomic dating revealed that these lineages diverged > 2,000 years ago—well before the earliest European records—suggesting divergence within the native range and either multiple introductions or a single introduction with multiple lineages. Mating-system inference shows that reproduction occurs primarily via self-fertilization (∼70% on average), although outcrossing predominates in some populations, revealing a facultative mating system. This high selfing rate has led to extremely low heterozygosity in most populations and a strong genetic structure. Genetic clustering also revealed admixed individuals resulting from rare inter-lineage outcrossing; comparisons of empirical and simulated data indicate that increased diversity after sporadic admixture events decays rapidly under subsequent selfing. Both the introduction history and long-distance dispersal facilitated by waterbirds likely explain the scattered distribution of lineages across Europe. Altogether, these results illustrate Baker's “ideal weed” concept, highlighting the role of a flexible mating system in providing reproductive assurance during colonization and showing how predominant selfing can shape the genomic landscape of an invasive species.

Biological invasions are natural experiments for studying the evolutionary and ecological processes underlying colonization success and range expansion. Using genome-wide data — generated via genotyping-by-sequencing (GBS) from 30 populations spanning Europe and South Africa— we investigated the colonization history and successful spread of the invasive buttonweed Cotula coronopifolia , an annual plant introduced into Europe from South Africa about 300 years ago. Our analyses identified three major lineages in Europe distributed across the continent, often co-occurring without evidence of admixture. Phylogenomic dating revealed that these lineages diverged > 2,000 years ago —well before the earliest European records— suggesting divergence within the native range and either multiple introductions or a single introduction with multiple lineages. Mating-system inference shows that reproduction occurs primarily via self-fertilization (∼70% on average), although outcrossing predominates in some populations, revealing a facultative mating system. This high selfing rate has led to extremely low heterozygosity in most populations and a strong genetic structure. Genetic clustering also revealed admixed individuals resulting from rare inter-lineage outcrossing; comparisons of empirical and simulated data indicate that increased diversity after sporadic admixture events decays rapidly under subsequent selfing. Both the introduction history and long-distance dispersal facilitated by waterbirds likely explain the scattered distribution of lineages across Europe. Altogether, these results provide an empirical demonstration of Baker’s “ideal weed” concept, highlighting the role of a flexible mating system in providing reproductive assurance during colonization and showing how predominant selfing shapes the genomic landscape of an invasive species. SIGNIFICANCE STATEMENT: Understanding how reproductive strategies influence colonization and spread of alien species is central to invasion biology. By combining population genomics and phylogenomic inference, this study provides key insights into the colonization history and successful invasion of the self-fertilizing plant C. coronopifolia , introduced from South Africa to Europe in the 18 ^th century. We show that the species’ invasion success relies on the introduction of multiple lineages and the predominance of selfing, which has drastically reduced genetic diversity yet contributed to reproductive assurance and spread across diverse habitats. Occasional outcrossing and long-distance dispersal by waterbirds or through horticultural transport have further shaped the species’ genetic landscape. These findings illustrate how self-compatibility and ecological generalism can overcome genetic constraints during range expansion and provide the basis for understanding the evolutionary dynamics of selfing plant invasions.

Aquatic ecosystems are characterised by strong trophic links, which can be profoundly altered by the loss or introduction of species. Invasive species that displace key native species may indirectly affect entire communities and ecosystem functioning. Introduced species often show distinctive traits that facilitate their success, such as reduced susceptibility to parasites that commonly infect native hosts. Here, we examine the ecological consequences of the invasion of the American brine shrimp Artemia franciscana on the native clonal species Artemia parthenogenetica and their cestode parasites in a hypersaline ecosystem. We carried out monthly sampling (January-June) in Odiel saltpans to compare Artemia population structure and cestode infections before (2011) and immediately after (2016) the invasion, quantifying host density, biomass and parasitological status. A. franciscana dominated and reached significantly higher densities than the native Artemia by the end of the spring 2016. Although both hosts shared the same parasite species, infection levels were markedly higher in the native species (47.33% vs. 5.30% prevalence). The cestode Flamingolepis liguloides was 18 times more frequent and induced stronger pathological effects (higher rates of castration and red colouration) in native Artemia . Invasive hosts showed a higher proportion of dead F. liguloides cysticercoids. The shift in dominant Artemia species is likely to affect other community members and ecosystem processes, given the keystone role of Artemia in hypersaline ecosystems. Our results highlight the importance of changes in host availability and host-parasite dynamics that can cascade through communities and influence entire ecosystems, particularly in the context of biological invasions.

Buttonweed (Cotula coronopifolia) is native to South Africa but invasive in wetlands in Europe, North America, and Australasia, where it excludes native plants. Despite being dry-fruited, field studies suggest migratory waterbirds can disperse its seeds via gut passage (endozoochory), aiding its expansion. To explore the potential for endozoochory in different regions and habitats, we collected seeds from six populations in Spain, Sweden, and the UK. Germination was tested under different salinity levels (0, 5, 10, 15 g/L) and simulated gut passage treatments: scarification, acidification, or both. No germination occurred at 15 g/L. Higher salinity reduced and delayed germination, but full gut passage treatment (i.e., both scarification and acidification) increased germinability and accelerated germination. Scarification or acid treatment alone resulted in intermediate germination patterns. There were significant salinity × population and gut passage × population interactions on germinability. The acceleration effect of gut passage on germination was stronger at 5–10 g/L than at 0 g/L. This study highlights how migratory birds can facilitate the spread of alien plants introduced by humans. Endozoochory by waterbirds is an understudied mechanism for the long-distance dispersal of dry-fruited alien plants. Further research on C. coronopifolia, including population genetics, is necessary to understand dispersal mechanisms and facilitate management strategies.

Buttonweed (Cotula coronopifolia) is native to South Africa but invasive in wetlands in Europe, North America, and Australasia, where it excludes native plants. Despite being dry-fruited, field studies suggest migratory waterbirds can disperse its seeds via gut passage (endozoochory), aiding its expansion. To explore the potential for endozoochory in different regions and habitats, we collected seeds from six populations in Spain, Sweden, and the UK. Germination was tested under different salinity levels (0, 5, 10, 15 g/L) and simulated gut passage treatments: scarification, acidification, or both. No germination occurred at 15 g/L. Higher salinity reduced and delayed germination, but full gut passage treatment increased germinability and accelerated germination. Scarification or acid treatment alone resulted in intermediate germination patterns. There were significant salinity x population and gut passage x population interactions on germinability. The acceleration effect of gut passage on germination was stronger at 5-10 g/L than at 0 g/L. This study highlights how migratory birds can facilitate the spread of alien plants introduced by humans. Non-classical endozoochory by waterbirds is an understudied mechanism for the long-distance dispersal of dry-fruited alien plants. Further research on C. coronopifolia, including population genetics, is necessary to understand dispersal mechanisms and facilitate management strategies.

Recent field data suggest that migratory gulls disperse many rice field weeds by gut passage (endozoochory), most of which are dry fruited and widely assumed to have no long-distance dispersal mechanisms, except via human activity. We investigated this mechanism with a feeding experiment, in which seeds of five common rice field weeds (in order of increasing seed size: Juncus bufonius, Cyperus difformis, Polypogon monspeliensis, Amaranthus retroflexus, and the fleshy-fruited Solanum nigrum) were fed to seven individuals of lesser black-backed gulls Larus fuscus held in captivity. We quantified seed survival after collecting faeces at intervals for 33 h after ingestion, then extracting intact seeds and running germination tests, which were also conducted for control seeds. All five species showed high seed survival after gut passage, of >70%. Gut retention times averaged 2–4 h, but maxima exceeded 23 h for all species. Germinability after gut passage was 16–54%, and gut passage accelerated germination in J. bufonius and S. nigrum, but slowed it down in the other species. All species had lower germinability after gut passage compared to control seeds (likely due to stratification prior to the experiment), but the loss of germinability was higher in smaller seeds. There was no evidence that the different dispersal syndromes assigned to the five species (endozoochory, epizoochory or barochory) had any influence on our results. In contrast, mean gut retention time was strongly and positively related to seed size, likely because small seeds pass more quickly from the gizzard into the intestines. Non-classical endozoochory of dry-fruited seeds by waterbirds is a major but overlooked mechanism for potential long-distance dispersal, and more research into this process is likely essential for effective weed management.

We report the first record of Linderiella jebalae in the Iberian Peninsula from two temporary ponds in La Línea de la Concep¬ción (Cádiz, southern Spain). This species was described in 2016 in the Moroccan Atlas Mountain range on the other side of the Strait of Gibraltar, 102 km away from this new Iberian location. Sequencing a fragment of cytochrome c oxidase subunit I (COI) showed a higher genetic diversity in Morocco, but only one haplotype in the Iberian populations, suggesting recent dis¬persal from Morocco to Spain, possibly mediated by birds through their annual migrations. This finding increases the diversity of anostraceans in the Iberian Peninsula to 16 species.