Beta maritima (Beta vulgaris ssp. maritima (L.) Arcang.; sea beet), the wild ancestor of cultivated beet, is the most widespread taxon within the genus Beta (Frese and Ford-Lloyd, 2020; Figure 1). Its origin is traced to the Mediterranean region (Romeiras et al., 2016). Its distribution covers nearly all Mediterranean coastal countries, several Atlantic islands, and much of the Atlantic coast of Europe (Frese and Ford-Lloyd, 2020; Veloso et al., 2021; Ben Mahmoud et al., 2025). Following the last glacial period, Beta maritima expanded northward, establishing populations along the Atlantic and North Sea coasts (Doney et al., 1990; Boudry et al., 2002; Fievet et al., 2007; Monteiro et al., 2013). Some populations are large and well-established, while others are small and scattered, reflecting both historical dispersal and recent colonization events (Letschert, 1993; Driessen, 2003; Andersen et al., 2005; Frese and Ford-Lloyd, 2020).

The broad distribution of Beta maritima reflects its evolutionary success and its ecological adaptability. It thrives in highly variable and often harsh coastal habitats, including salt marshes, beaches, and inland ruderal sites (Doney et al., 1990; Stevanato et al., 2001). This environmental heterogeneity, combined with strong local selection pressures, drives high morphological variation within and among populations (Toll and Hendriksen, 1982; Letschert and Frese, 1993; Abdelhameed et al., 2024; Ben Mahmoud et al., 2025). Traits such as growth habit, leaf morphology, and bolting behavior are shaped by adaptation to salinity, drought, and temperature extremes (Letschert and Frese, 1993; El Manhaly et al., 1996; Ben Mahmoud et al., 2025). The species’ ability to colonize diverse habitats and maintain dynamic, in-situ populations preserves adaptive alleles that may be lost in ex-situ collections (Bohra et al., 2021).

Understanding the diversity of wild crop relatives like Beta maritima is essential for conservation and breeding efforts. Sea beet populations provide a reservoir of adaptive genetic variation, contributing valuable traits to sugar beet improvement (Frese et al., 1990; Panella et al., 2020). Over the past decades, numerous studies have documented the morphological variation among sea beet populations. In parallel to morphological investigations, advances in molecular genetics have enabled deeper insights into the population structure and genetic diversity of Beta maritima.

This review provides an overview of current research on the morphological and genetic diversity of Beta maritima populations across Europe and North Africa. It aims to (1) summarize regional patterns of morphological and genetic variation, (2) evaluate the relationship between observed phenotypic traits and underlying genetic structure, and (3) identify knowledge gaps and underexplored populations to support the utilization of Beta maritima as a genetic reservoir for sustainable sugar beet improvement.

Morphological diversity in Beta maritima has been extensively studied, with numerous investigations exploring how geographic and environmental factors shape variation within and among populations (Table 1). Across its native range, sea beet populations exhibit pronounced variability in growth habit, leaf morphology, inflorescence structure, pigmentation, and root traits (Toll and Hendriksen, 1982; Letschert and Frese, 1993; Stevanato et al., 2001; Abdelhameed et al., 2024; Ben Mahmoud et al., 2025).

Morphological differentiation is strongly influenced by environmental factors. Traits such as plant size, leaf thickness, and growth form are strongly linked to habitat conditions. Plants in dry or exposed environments tend to be smaller with thicker leaves, while those in open, resource-rich habitats often develop more expansive, procumbent forms (Toll and Hendriksen, 1982; Abdelhameed et al., 2024; Ben Mahmoud et al., 2025). Exposure to stressors like high salinity, drought, or heat further drives adaptive changes, including reduced leaf size and increased cuticle thickness (Ben Mahmoud et al., 2025). Soil properties, especially organic carbon content, also significantly influence morphological variability (Abdelhameed et al., 2024).

A consistent pattern emerges when comparing inland and coastal populations. Inland types typically have longer, narrower petioles and less succulent leaves, whereas coastal populations display shorter, thicker leaves and a more prostrate growth habit (Letschert and Frese, 1993; El Manhaly et al., 1996). Coastal populations are also more likely to bolt and flower early, while inland types often show delayed generative development. These patterns reflect adaptation to contrasting environmental pressures. Coastal habitats favor compact, robust morphology, while inland environments select for traits that enhance water use efficiency and competitive ability (Letschert and Frese, 1993; El Manhaly et al., 1996).

The extent to which morphological variation correlates with geography is variable. Some studies report substantial diversity within short coastal stretches (Abdelhameed et al., 2024), while others find only minor divergence across broader regions (Letschert and Frese, 1993). In the British Isles, morphological variation increases with geographic distance and physical barriers, highlighting the influence of dispersal mechanisms and environmental heterogeneity (Doney et al., 1990).

A clear distinction is observed between Atlantic and Mediterranean populations. Mediterranean populations generally exhibit greater morphological and genetic diversity, with a wider range of growth forms and adaptive traits, while Atlantic populations, especially those in the Baltic and North Sea regions, are more uniform and often display traits associated with recent colonization and founder effects (Frese et al., 1990; Richards et al., 2014; Andersen et al., 2005). This regional contrast is also reflected in the frequency of key adaptive alleles, such as gene B, which controls vernalization requirement and flowering time. The B allele, which eliminates the need for vernalization and promotes early bolting, is frequent in Mediterranean populations but largely absent in northern populations, contributing to the observed differences in life history strategies (Van Dijk et al., 1997; Boudry et al., 2002).

Much of this diversity is attributable to phenotypic plasticity – Beta maritima’s ability to adapt its morphology in response to environmental variation (Ribeiro et al., 2016; Ascarini et al., 2021). Nevertheless, genetic control is also evident, as shown by the identification of major genes (e.g. gene B) and quantitative trait loci affecting flowering and growth (Van Dijk et al., 1997; Boudry et al., 2002). The interplay between plasticity and genetic differentiation complicates the interpretation of regional patterns and underscores the need for integrative approaches.

Beyond basic morphology, few studies also described variation in agronomically relevant traits such as disease resistance and stress tolerance. For example, resistance to important pathogens such as Cercospora leaf spot and Rhizomania has been documented in certain wild populations (Stevanato et al., 2001). Moderate resistance to curly top virus and root aphid has also been identified in populations from Egypt (El Manhaly et al., 1996). However, most findings are based on phenotypic screening rather than genetic confirmation. Nevertheless, these findings underscore the value of Beta maritima as a genetic resource for breeding programs, offering a reservoir of adaptive traits that can be harnessed to improve stress tolerance and disease resistance in cultivated beets.

Following extensive research on morphological variation in wild sea beet, recent studies have increasingly focused on genetic diversity and population structure using molecular techniques (see Table 2 for an overview of diversity measures across studies). These genetic analyses have uncovered both broad-scale patterns and distinct regional differences within Beta maritima populations. Importantly, genetic differentiation across Europe and North Africa reflects a complex, multi-layered process. Rather than being determined solely by geographic distance, population structure in Beta maritima populations is shaped by a combination of historical events, dispersal mechanisms, and environmental heterogeneity.

A key pattern of variation, the Atlantic–Mediterranean divide, originates from glacial history. Mediterranean regions acted as refugia during the last glacial period, preserving high allelic richness and heterozygosity. Northern populations on the other hand were recolonized more recently, resulting in reduced diversity and greater genetic uniformity within these populations (Driessen et al., 2001; Driessen, 2003; Andersen et al., 2005). Comparative studies confirm that Mediterranean populations hold more alleles and show stronger substructure, whereas Northern Atlantic and Baltic groups exhibit high gene flow and low differentiation (Desplanque et al., 1999; Leys et al., 2014; Richards et al., 2014; Veloso et al., 2021; Bertram et al., 2025a).

Specifically Baltic and North Sea populations exhibit genetic homogeneity and low polymorphism, reflecting founder effects and bottlenecks associated with recent colonization and seed dispersal via ocean currents (Driessen et al., 2001; Driessen, 2003). Danish and Swedish populations show high gene flow and no internal structure, consistent with wind pollination and long-distance dispersal (Andersen et al., 2005; Bertram et al., 2025a). These northern groups remain distinct from Mediterranean and Atlantic populations, reflecting their restricted genetic base and recent origin.

Different dispersal mechanisms also influence population structure. Coastal populations showed strong geographic clustering shaped by marine currents (Leys et al., 2014). Inland ruderal populations exhibit more complex genetic structures due to admixture and human activities, such as soil and plant movement and habitat modification (Letschert, 1993; Leys et al., 2014). These actions introduce and mix genetically distinct individuals, increasing gene flow and hybridization, and disrupting clear geographic genetic patterns. Nuclear genes, spread by both pollen and seeds, enable broader gene flow and genetic mixing across regions. In contrast, mitochondrial genes, dispersed only through seeds, are more restricted in their movement, resulting in stronger spatial genetic structuring (Fénart et al., 2008). For example, along the French Atlantic and Channel coasts, an asymmetric gene flow shaped by marine currents and differences in nuclear versus cytoplasmic dispersal was observed (Fievet et al., 2007). This demonstrates how the mode of gene transmission shapes genetic patterns in wild plant populations.

Within these broader patterns, distinct regional variations introduce additional complexity. Admixture signals, such as clustering between French Atlantic and Corsican individuals, highlight ongoing connectivity despite distance (Richards et al., 2014). Similar dynamics occur in Iberian and Macaronesian systems, where marine currents and isolation create admixture gradients. Northern groups are more differentiated, while southern and insular populations form mixing zones (Veloso et al., 2021).

Environmental heterogeneity also plays a role in genetic differentiation of populations. For instance, the presence of mixed ploidy levels in Portuguese Beta taxa (Castro et al., 2013) and the high genetic diversity found in salt marsh populations (Ribeiro et al., 2016) point to a strong adaptive potential linked to diverse habitats. Such habitat heterogeneity creates a mosaic of selective pressures, fostering local adaptation and maintaining genetic variation. Additionally, the pronounced variability observed in Madeira and Porto Santo demonstrates that environmental selection can sometimes override the effects of geographic distance, leading to distinct population characteristics even within relatively small regions (Ascarini et al., 2021).

Overall, geographic distance alone is an unreliable proxy for genetic differentiation. Population structure results from glacial history, dispersal mechanisms, and ecological selection.

Research on sea beet populations has progressed from early morphological studies to increasingly sophisticated genetic analyses. Initial work focused on phenotypic traits to infer diversity and adaptation, revealing substantial variation shaped by geography, environment, and local selection pressures. With the advancement of molecular techniques, research has shifted toward examining genetic diversity and population structure, offering deeper insights into evolutionary processes and genetic diversity (Desplanque et al., 1999; Andersen et al., 2005; Leys et al., 2014; Bertram et al., 2025a).

A key observation is that phenotypic variation in sea beet populations often exceeds genetic differentiation and is strongly influenced by environmental conditions (Ribeiro et al., 2016; Ascarini et al., 2021; Abdelhameed et al., 2024). The species’ outcrossing mating system, along with high pollen and seed dispersal, promotes gene flow and hence genetic mixing among populations. At the same time, the high phenotypic plasticity of Beta maritima enables individuals to respond flexibly to strong environmental gradients across their range. This plasticity complicates the interpretation of morphological data, especially when environmental variation is pronounced. As a result, morphological traits may not reliably reflect underlying genetic relationships. While some studies have identified major genes, such as the B gene for vernalization requirement, comprehensive integration of phenotypic and genetic data remains limited. Notably, there are successful examples of wild alleles being introgressed into cultivated beet, such as the Rz2 gene for Rhizomania resistance (Capistrano-Gossmann et al., 2017). However, most research has focused on either morphological or genetic variation in isolation, so the extent to which genetic architecture explains morphological patterns is only partially examined. Agronomic traits, especially disease and pest resistances, have been evaluated only minimally across sea beet populations. This gap, likely also due to the difficulty of evaluating such traits in wild material directly (Bertram et al., 2025b), highlights the need for further studies to systematically assess and characterize this diversity.

Comparative studies have consistently identified two distinct genetic groups among sea beet populations – one from the Atlantic and one from the Mediterranean region, with the latter generally exhibit greater genetic and morphological diversity (Desplanque et al., 1999; Richards et al., 2014). This divide has been further confirmed by sequence analyses of 239 sea beet ex-situ accessions from germplasm banks (Sandell et al., 2022; Felkel et al., 2023). This division reflects the influence of both historical events and evolutionary processes on population structure. During the last glacial period, Mediterranean regions acted as refugia and preserved high ancestral diversity. In contrast, post-glacial recolonization toward the north caused genetic bottlenecks and reduced allelic richness in northern populations (Driessen et al., 2001; Driessen, 2003; Andersen et al., 2005).

Nevertheless, this does not mean that northern populations lack valuable alleles for breeding. For example, Capistrano-Gossmann et al. (2017) identified the Rz2 resistance gene to Rhizomania, which is of major importance for breeding, in a Danish sea beet population. Other studies have also reported unique alleles and polymorphisms in northern Atlantic populations (Andersen et al., 2005; Bertram et al., 2025a). While it remains unclear which of these alleles hold practical value for breeding, the implications are significant. This highlights the importance of in-situ conservation, specifically of genetically unique micro-populations, to preserve unique genetic variants. It also underscores the need for broad sampling and comprehensive testing to fully uncover useful genetic diversity across all regions. Genomic tools, including high-density SNP arrays and whole-genome sequencing, can aid with the identification of candidate alleles for introgression and the systematic assessment of genetic resources (Andrello et al., 2017; Felkel et al., 2023).

Mediterranean populations, with their high diversity and high amount of unique alleles, surely represent valuable sources for crop improvement. Especially their ability to thrive even under challenging conditions, such as drought or high salinity, makes them especially valuable for breeding programs aiming to develop more resilient cultivars. However, despite evidence of rich diversity, Mediterranean sea beet populations remain notably underrepresented, especially in genetic studies (Figure 2). Notable gaps exist along the southern and eastern coasts of Spain, the Italian coastline and Sardinia, all of Greece, the western coast of Turkey, and other eastern Mediterranean regions. Although genebank accessions sampled from these areas confirm the historical presence of Beta maritima (Andrello et al., 2016), many of these populations have not been genetically characterized. Zucchini et al. (2024) also noted discrepancies between the presence of in-situ populations and the original collection sites of ex-situ accessions. One possible reason for this is the ongoing decline of natural habitats, which threatens the survival of sea beet populations (Doney et al., 1990; Stevanato et al., 2001). However, this does not fully explain the lack of data, as recent studies still report widespread occurrences, for example along the Italian coast (Zucchini et al., 2024). This highlights a significant research gap. Although Mediterranean populations are known to exist and contribute substantial diversity, they remain largely uncharacterized. Future research should prioritize these regions to better capture the full spectrum of sea beet diversity and its breeding potential.

Despite recent progress, the value of many genetic studies on sea beet populations is limited by small sample sizes and a narrow set of genetic markers, restricting insights into the full genetic architecture of these populations (Figure 2). While these studies provide useful estimates of heterozygosity and allelic richness in specific regions, they offer only a partial view of overall diversity. The work by Bertram et al. (2025a) represents a major advancement, applying high-density SNP genotyping across large and diverse populations. This approach enables a more comprehensive analysis of genetic variation, population substructure, and mapping potential, setting a new benchmark for future research. However, while genome-wide SNP data represent a significant improvement over single-marker approaches, SNP panels are often developed based on cultivated material and may suffer from ascertainment bias (Andrello et al., 2017), potentially underrepresenting rare or novel alleles in wild populations. To fully capture the genetic diversity in Beta maritima populations and detect also structural variants such as insertions and deletions (Felkel et al., 2023), whole-genome sequencing would be preferable. Sequencing technologies are becoming increasingly accessible and are likely to become the method of choice for future diversity studies.

Ultimately, the value of sea beet for crop improvement depends not only on the presence of genetic diversity but also on the ability to identify and utilize alleles conferring desirable traits. A key challenge remains: How to evaluate breeding potential without extensive, resource-intensive testing? First studies have begun to tackle this question. Capistrano-Gossmann et al. (2017) demonstrated an approach to identify resistance genes directly within sea beet populations without time-consuming material development. Building on this, Bertram et al. (2025b) used simulation studies to design suitable development schemes for evaluating even complex traits like yield.

Looking forward, addressing current knowledge gaps through integrated genomic, phenotypic, and environmental research is essential to fully harness the potential of Beta maritima for sugar beet improvement. Combining morphological, genomic, climatic, and soil data will provide a more comprehensive understanding. Genome-wide scans and landscape genomics can reveal adaptive variants and clarify the environmental drivers of genetic differentiation. Additionally, systematic phenotyping for stress and disease traits, together with the integration of ex-situ and in-situ datasets, will help resolve inconsistencies and maximize the utility of wild genetic resources for both breeding and conservation.

In summary, sea beet populations exhibit remarkable morphological and genetic diversity shaped by geography, environment, and dispersal dynamics. The transition from morphological to genetic characterization has greatly enhanced our understanding of wild sea beet diversity. Nevertheless, significant gaps remain, particularly in the underexplored Mediterranean populations and in translating genetic variation into breeding value to unlock the full value of Beta maritima populations for resilient and sustainable crop development.