Capsella Rubella: A Model for Plant Evolution and Speciation

Red Shepherd’s Purse, or Capsella rubella, is a plant species in the mustard family. Its similar-sounding name is a coincidence and has no relation to the Rubella virus, which causes German measles. Capsella rubella is a flowering plant, whereas the Rubella virus is a pathogenic agent that causes disease in humans.

Physical Characteristics and Identification

Capsella rubella grows from a basal rosette, a circular arrangement of lobed leaves at the soil level that can be smooth or slightly hairy. From this base, one or more slender stems emerge, reaching a height of 30 to 50 cm. These stems bear a few smaller, pointed leaves that clasp the stem.

The plant produces small, clustered flowers with four petals, a characteristic of the mustard family, that are white to pinkish in color. Following the flowers, the plant develops its most recognizable feature: the seed pods. These pods are flat and triangular or heart-shaped, resembling the purses carried by shepherds in the past, which gives the plant its common name. Each pod contains numerous small, easily dispersed seeds.

It can be challenging to distinguish Capsella rubella from its close relative, C. bursa-pastoris. Both share the characteristic heart-shaped fruit, but there are subtle differences. C. rubella often has smaller petals with a pinkish hue compared to the white, larger petals of C. bursa-pastoris. A technical distinction is that C. rubella has a diploid genome (two sets of chromosomes), while C. bursa-pastoris is a tetraploid with four sets.

Habitat and Global Spread

Capsella rubella originated in the Mediterranean region of Europe. Aided by human activity like agriculture and travel, the plant has expanded its range significantly.

Now a cosmopolitan species, Capsella rubella is found across the globe, including in North and South America and Australia. It shows a strong preference for disturbed habitats like roadsides, agricultural fields, wastelands, and gardens. Its ability to quickly colonize these environments contributes to its widespread presence.

A Model for Plant Evolution

Capsella rubella is a model organism for studying how new species arise and adapt due to a recent shift in its reproductive strategy. It is a self-pollinating species, meaning a single plant can fertilize itself to produce viable seeds without a partner. This trait is known as self-compatibility.

Genetic studies suggest Capsella rubella evolved from an outcrossing ancestor, Capsella grandiflora, within the last 200,000 years. The ancestral species is self-incompatible, requiring cross-pollination to reproduce. The transition to self-pollination in C. rubella allowed it to spread rapidly, as a single seed could establish a new population.

By comparing the genomes of C. rubella and C. grandiflora, scientists can investigate the genetic consequences of this transition. Research has shown that the shift to selfing led to a reduction in genetic diversity, a phenomenon known as a population bottleneck. The plant’s close relationship to Arabidopsis thaliana, the most studied model plant, allows for comparative studies of gene function and evolution.

Practical Considerations

In many contexts, Capsella rubella is considered a common weed. Its rapid life cycle and high seed production allow it to compete with cultivated crops and garden plants. It can establish itself quickly in tilled soil, so its management is a consideration for farmers and gardeners where it is prevalent.

Beyond its status as a weed, the plant is also edible. Similar to its relative C. bursa-pastoris, the leaves, flowers, and seed pods of Capsella rubella are edible, with the young leaves from the basal rosette being the most commonly consumed part. Proper identification is necessary before consuming any wild plant to avoid ingesting a harmful species.