Intraspecific competition reveals conditional fitness effects of single gene polymorphism at the <i>Arabidopsis</i> root growth regulator <i>BRX</i>

• Intraspecific genetic variation for morphological traits is observed in many organisms. In <i>Arabidopsis thaliana</i>, alleles responsible for intraspecific morphological variation are increasingly being identified. However, the fitness consequences remain unclear in most cases. <br> • Here, the fitness effects of alleles of the <i>BRX</i> gene are investigated. A <i>brx</i> loss-of-function allele, which was found in a natural accession, results in a highly branched but poorly elongated root system. • Comparison between the control accession <i>Sav-0</i> and an introgression of <i>brx</i> into this background (<i>brx</i>^S) indicated that, surprisingly, <i>brx</i> loss of function did not negatively affect fitness in pure stands. However, in mixed, well-watered stands <i>brx</i>^S performance and reproductive output decreased significantly, as the proportion of <i>Sav-0</i> neighbors increased. Additional comparisons between <i>brx</i>^S and a <i>brx</i>^S line that was complemented by a <i>BRX</i> transgene confirmed a direct effect of the loss-of-function allele on plant performance, as indicated by restored competitive ability of the transgenic genotype. Further, because plant height was very similar across genotypes and because the experimental setup largely excluded shading effects, the impaired competitiveness of the <i>brx</i> loss-of-function genotype likely reflects below-ground competition. <br> • In summary, these data reveal conditional fitness effects of a single gene polymorphism in response to intraspecific competition in <i>Arabidopsis</i>.