Capsella bursa-pastoris, a Eurasian annual of disturbed areas, has become a cosmopolitan weed of the world's temperate regions; it even gets into cool, elevated areas of the tropics. The species is very variable; different populations differ in features such as height, leaf shape and size, flower and fruit size and seed production.
Since shepherd's purse is self-pollinated, individuals within populations tend to be more similar to each other than to individuals in other populations. A plethora of names, synonymous with Capsella bursa-pastoris, have been given to these populations. Despite its variability, Capsella bursa-pastoris is distinguished from two other members of the genus, Capsella grandiflora and Capsella rubella.
In Britain, shepherd's purse grows rapidly, and flowers and produces seed throughout the year. Experimental studies have shown individual plants are capable of producing more than 59,000 seeds, which can survive in the soil seed bank for long periods. Two types of seeds are produced. The seed coats of some seeds secrete polysaccharide-rich mucilage on contact with water (myxospermy), whilst other seeds are not mucilaginous.
Myxospermy is associated with plants adapted to surviving in arid, well-drained soils, and may play a role in sticking the seeds to the soil, long-distance dispersal by humans or other animals or maintaining a moist environment in which the seeds can germinate. Besides the seedlings of shepherd's purse holding fragile soils together with their root systems, shepherd's purse seeds may help soils to retain water.
The genus Capsella is a close relative of the model plant, thale cress (Arabidopsis thaliana). Furthermore, variation in the biology of different Capsella species means the genus has become an attractive evolutionary model in its own right. Consequently, much recent research has been conducted into understanding the evolutionary relationships among the genus's members.
Capsella bursa-pastoris has four complete sets of chromosomes; the other two species each have two set of chromosomes. Two hypotheses have been proposed to explain this pattern. One hypothesis states that shepherd's purse is a product of hybridisation between the two other species followed by chromosome doubling. The other hypothesis states that there has been doubling of the chromosomes of Capsella grandiflora. Comparisons of the DNAs of all three Capsella species have shown Capsella bursa-pastoris evolved recently but does not support either hypothesis; the origin is more complex.
The plant's popular and scientific names refer to the distinctive, heart-shaped fruits, which look like the pouches traditionally carried by European shepherds.
Aksoy, A et al. 1998. Capsella bursa-pastoris (L.) Medikus (Thlaspi bursa-pastoris L., Bursa bursa-pastoris (L.) Shull, Bursa pastoris (L.) Weber). Journal of Ecology 86: 171-186.
Slotte T et al. 2006. Intrageneric phylogeny of Capsella (Brassicaceae) and the origin of the tetraploid C. bursa-pastoris based on chloroplast and nuclear DNA sequences. American Journal of Botany 93: 1714-1724.
Toorop PE et al. 2012. Co-adaptation of seed dormancy and flowering time in the arable weed Capsella bursa-pastoris (shepherd's purse). Annals of Botany 109: 481-489.