Mating system, ploidy and competition in Capsella species

Capsella is a model genus for studying the transition from outcrossing to selfing, with or without change in ploidy levels. The genomic consequences and changes in reproductive traits (selfing syndrome) associated with these shifts have been studied in depth. However, potential ecological divergence among species of the genus has not been determined. Among ecological traits, competitive ability could be relevant for selfing evolution, as selfing has been shown to be statistically associated with reduced competitiveness in a recent meta-analysis.

Mendoza and colleagues assessed the effect of competition on studied three species of the genus Capsella: C. grandiflora (Fauché & Chaub.) Boiss. (diploid outcrosser), C. rubella Reut. (diploid selfer) and C. bursa-pastoris (L.) Medik (tetraploid selfer). They way they studied competition was to compare the performance of the three Capsella species when grown alone and under four competition conditions: intra-specific competition, inter-specific competition with the two other Capsella species, and inter-specific competition with M. chamomilla.

The way they tested competition was to place the plant they wanted to test at the centre of an 11 × 11 × 11 cm pot. Any competitors were placed at the corners of the pot. What counted as success for a plant?

The team measured the diameter of the rosette of the plant. They also counted the flowers as a proxy measure for reproductive success. Why not fruit set? That was because C. grandiflora was self-incompatible, and with an experimental set up without cross-pollination it would not produce fruit. That’s not perfect, but Mendoza et al. note that they were: “…not interested in absolute fitness comparison among species but rather the relative change in fitness due to competition.”

The diploid selfer (C. rubella) was most sensitive to competition, whereas the tetraploid selfer (C. bursa-pastoris) performed the best, with the diploid outcrosser (C. grandiflora) being intermediate. Mendoza and colleagues argue that the difference could be the advantage of having two pairs of chromosomes instead of one, saying: “the observed difference could be partly explained by the masking of deleterious mutations due to genetic redundancy in the tetraploid species.”

The findings could explain why polyploid selfers can appear so successful in the short term. The team conclude: “…[P]olyploidy could buffer the negative effect of selfing and delay the extinction risk. This could contribute to explain the ecological success of polyploid selfing species, benefiting from the reproductive assurance of selfing without paying its full genetic cost, at least temporarily.”