Friends or foes? Polyploidy and competition in a grassland geophyte

Abstract

Previous studies have suggested polyploids may possess a competitive advantage over diploids, due to their larger size, increased vigour or ability to better respond to abiotic stress. However, few studies have tested the effect of polyploidy on competitive ability directly, and only relatively recently has this oversight begun to be addressed. Here we assessed potential differences in competitive ability between diploids and tetraploids of a widespread African grassland geophyte, Oxalis obliquifolia. Diploid and tetraploid plants were grown in identical environmental conditions under four different competition scenarios. As a control, diploids and tetraploids were grown under solitary conditions. For intra-cytotype competition, pairs of diploids and tetraploids were planted together. Inter-cytotype competition involved planting individual diploids and tetraploids with one another, and for interspecies competition individual diploids and tetraploids were planted with Themeda triandra seedlings. Thirty-seven morphological, physiological and phenological traits were recorded and assessed for associations between cytotype and competition treatment using ordinations and GLMs. Ordination showed diploids and tetraploids as distinct clusters, with very little separation based on competition treatment. Univariate analyses showed tetraploids have larger but fewer organ structures than diploids, suggesting a tradeoff between these traits. Tetraploids also produce larger seeds. Diploids showed reduced numbers of leaves and were more negatively impacted by interspecies competition than tetraploids, suggesting tetraploids may potentially be the better interspecies competitor. Tetraploids also extended their flowering season in response to inter-cytotype and interspecies competition. Cytotypes respond differently under different competition treatments, and tetraploids may have a competitive advantage over diploids. However, this potential competitive asymmetry does not explain observed cytotype sympatry. Tetraploids also appear to differentiate between intra-cytotype competition, and competition with diploids and other species. The potential for abiotic factors to impact these competition dynamics remains unexplored.